Induction Motors Duty Cycles

Type of Duty

The following shall be the duty types:

Sl -Continuous duty
The motor works at a constant load for enough time to reach temperature equilibrium.

S2 -Short time duty
The motor works at a constant load, but not long enough to reach temperature equilibrium. The rest periods are long enough for the motor to reach ambient temperature

S3 -Intermittent periodic duty
Sequential, identical run and rest cycles with constant load. Temperature equilibrium is never reached. Starting current has little effect on temperature rise

S4 -Intermittent periodic duty with starting
Sequential, identical start, run and rest cycles with constant load. Temperature equilibrium is not reached, but starting current affects temperature rise.

S5 -Intermittent periodic duty with starting and electric braking
Sequential, identical cycles of starting, running at constant load and running with no load. No rest periods.

S6 -Continuous duty with intermittent periodic loading
Sequential, identical cycles of running with constant load and running with no load. No rest periods.

S7- Continuous duty with starting and electric braking
Sequential identical cycles of starting, running at constant load and electric braking. No rest periods

S8 -Continuous duly with periodic speed changes
Sequential, identical duty cycles run at constant load and given speed, then run at other constant loads and speeds. No rest periods.

American Wire Gauge; AWG calculations

AMERICAN WIRE GAUGE

By definition, No. 36 AWG is 0.0050 inches in diameter, 
and No. 0000 is 0.4600 inches in diameter. 
The ratio of these diameters is 92, 
and there are 40 gauge sizes from No. 36 to No. 0000, or 39 steps.

Using this common ratio, wire gauge sizes vary according to the following formula: 

The diameter of a No. n AWG wire is the gauge can be calculated from the diameter using and the cross-section area is

. Sizes with multiple zeros are successively larger than No. 0 and can be denoted using "number of zeros/0", for example 4/0 for 0000. For an m/0 AWG wire, use n = −(m−1) = 1−m in the above formulas. For instance, for No. 0000 or 4/0, use n = −3.

The ASTM B 258-02 standard defines the ratio between successive sizes to be the 39th root of 92, or approximately 1.1229322. ASTM B 258-02 also dictates that wire diameters should be tabulated with no more than 4 significant figures, with a resolution of no more than 0.0001 inches (0.1 mils) for wires larger than No. 44 AWG, and 0.00001 inches (0.01 mils) for wires No. 45 AWG and smaller.

Table of AWG wire sizes

The table below shows various data including both the resistance of the various wire gauges and the allowable current (ampacity) based on plastic insulation. The diameter information in the table applies to solid wires. Stranded wires are calculated by calculating the equivalent cross sectional copper area. The table below assumes DC, or AC frequencies equal to or less than 60 Hz, and does not take skin effect into account. Turns of wire is on a best-case scenario when winding tightly packed coils with no insulation.

In the North American electrical industry, conductors larger than 4/0 AWG are generally identified by the area in thousands of circular mils (kcmil), where 1 kcmil = 0.5067 mm². A circular mil is the area of a wire one mil in diameter. One million circular mils is the area of a cylinder with 1000 mil = 1 inch diameter. An older abbreviation for one thousand circular mils is MCM.The "Approxismate stranded metric equivalents" column lists commonly available cables in the format "number of strands / diameter of individual strand (mm)" which is the common nomenclature describing cable construction within an overall cross-sectional area. Some common cables are midway between two AWG sizes. Cables sold in Europe are normally labeled according to the combined cross section of all strands in mm², which can be compared directly with the Area column.

Outside North America, wire sizes for electrical purposes are usually given as the cross sectional area in square millimeters.International standard  manufacturing sizes for conductors in electrical cables are defined in IEC 60228.

Note that the area in mm² may differ somewhat from the numbers given in the table, depending on number of strands etc.

AWG chart is as shown below:-

 Rules of Thumb

The sixth power of this ratio is very close to 2, which leads to the following rules of thumb:

• When the diameter of a wire is doubled, the AWG will decrease by 6. (e.g. No. 2 AWG is about twice the diameter of No. 8 AWG.)
• When the area of a wire is doubled, the AWG will decrease by 3. (e.g. Two No. 14 AWG wires have about the same cross-sectional area as a single No. 11 AWG wire.)

Additionally, a decrease of ten gauge numbers, for example from No. 10 to 1/0, multiplies the area and weight by approximately 10 and reduces the resistance by approximately 10.

Full Load Speed of Induction Motor

Slip  and Full-load speed of Motor

The speed at which rated full-load torque is delivered at rated power output is full-load speed. It is

generally given as "RPM" on the nameplate. This speed is sometimes called "slip" speed or actual rotor speed rather than synchronous speed. Synchronous speed is the speed at which the motor would run if it were fixed to the ac power line frequency; that is, if it turned at the same speed as the rotating magnetic field created by the combination of winding pattern and power line frequency. An induction motor's speed is always less than synchronous speed and it drops off as load increases. For example, for 1800 rpm synchronous speed, an induction motor might have a full-load speed of 1748 rpm, this drop in RPM is due to slip of an induction motor.

Slip speed is difference between synchronous speed and actual rotor speed.

In induction motors Slip is directly proportional to torque of motor, so greater will be the slip greater will be torque generated in motor. So greater will be the induced emf.

When motor is running at no-load then it requires small torque to overcome mechanical, iron and other losses, therefore slip is small. 

Now when the motor is put on load greater torque will be required so slip will increase and correspondingly motor speed get reduced. So slip adjustes itself as per load requirements.


As the size of motor increases this slip keeps on reducing

e.g. for 0.5 KW motor slip is 5%
for 5 KW motor slip is 3 %
for 15 KW motor slip is 2.5 %
For 50 KW motor slip is 1.7%
and for 250 KW motor slip is 0.8%

As soon as motor started slip is 100% and it is keep on reducing as soon as speed increases and is minimum when motor is running at full speed. You can also say that as soon as slip decreased frequency decreases.

Also motor inductive reactance is dependent on slip as inductive reactance is maximum when motor is at standstill, as at that moment slip is maximum.

The inductive reactance will change with the slip since the rotor impedance is the phase sum of the constant resistance and the variable inductive reactance.

When the motor starts rotating the inductive reactance is high and impedance is mostly inductive. The rotor has a low lagging power factor. When the speed increases the inductive reactance goes down equaling the resistance.

There have been conflicting opinions and claims regarding the effect of replacing a "standard efficiency" motor with an "energy-efficient" motor on a centrifugal-type load.

Centrifugal pumps and fans impose what is often called a "cubed-exponential load" on the driver.

For such a pump or fan, torque varies approximately as the square of speed. Because, by definition, power varies directly with torque and with speed, for a centrifugal-type load, power varies approximately as the cube of speed - a small speed change produces a much larger change in power requirement. For example, a 1% increase in speed would bring a 3% increase in load: (1.01)3 = 1.03 Some engineers claim that an energy-efficient motor manifests most of its efficiency improvement at a lower slip speed; that is, as an increase - typically about 1% - in output speed. Because the 1% speed gain equates to a 3% horsepower requirement, they reason, the replacement energy-efficient motor may have to be 1 HP-size larger than the standard motor.

The contention does not fully account for the fact that the power reduction from using an energy-efficient motor is greater than the extra power required by the load - hence, there is a net energy

savings and the motor will run cooler, potentially extending insulation life.

Lighting System Designing Zones

There are following lighting Zones in Designing lighting system of any area:-

Zone Recommended Uses or Areas Zoning Considerations

LZ-0

Lighting Zone 0 should be applied to areas in which permanent lighting is not expected and when used, is limited in the amount of lighting and the period of operation.

LZ-0 typically includes undeveloped areas of open spaces, Parks , Outside area of any industry . Special review should be required for any permanent lighting in this zone.

LZ-1

Lighting Zone 1 pertains to areas that desire low lighting levels.

These typically residential communities with low Population, rural town centers, business parks, and other commercial or industrial/storage areas typically with limited night time activity.

This zone also Includes agricultural zone districts; rural residential zone districts; business parks; open space include preserves in developed areas.

LZ-2

Lighting Zone 2 pertains to areas with moderate lighting levels.

These typically include Highly Populated residential uses, institutional residential uses, schools, churches, hospitals, hotels/motels, commercial and/or businesses areas, playing fields and.

LZ-3

Lighting Zone 3 pertains to areas with moderately high lighting levels.

These typically include commercial corridors, high intensity suburban commercial areas, town centers, mixed use areas, industrial uses and shipping and rail yards with high night time activity, regional shopping malls, car dealerships, gas stations, Petrol Pumps and other nighttime active exterior retail areas.

LZ-4

Lighting zone 4 pertains to areas of very high ambient lighting levels. LZ-4 should only be used for special cases and is not appropriate for most cities. LZ-4 may be used for extremely unusual installations such as high density entertainment districts, and

heavy industrial uses.

Tube lights and its sizes; Tube light diameters

Tube lights are part of every industry and household. LED tube-lights are also coming in same sizes that of traditional lights.

Diameter of tube-light is described in 1/8 of the inch. So a T8 tube light is 1 inch diameter tube-light. This is obtained by simply multiplying 1/8 by numeric letter of description of tube light.  

Sizes of tube lights vary from T2 to T17.

Electronic ballasts, and T5 or T16 (⁵/₈" Ø or 15.875 mm Ø) for very small lamps which may even operate from a battery powered device.

Fluorescent tube diameter designation comparison
Tube diameter designations		Tube diameter measurements			Extra
Imperial-based	Metric-based	Inches Ø  (")		Millimeters Ø  (mm)	Socket	Notes
T4	N/A	4/8" Ø		12 mm Ø	G5 Bipin	Slim lamps, tube lengths may vary
T5	T16	5/8" Ø		15.875 mm Ø	G5	Supersedes T8, introduced in the 1990s
T8	T26	8/8" Ø	1" Ø	25.4 mm Ø	G13 bipin	From the 1930s. More common since the 1980s
T9	T29	9/8" Ø	11/8" Ø	28.575 mm Ø		Circular fluorescent tubes only
T12	T38	12/8" Ø	11/2" Ø	38.1 mm Ø	G13 bipin	Also from the 1930s. Not as efficient as new lamps
PG17	N/A	17/8" Ø	21/8" Ø	53.975 mm Ø		General Electric's

T8 tube-lights were most widely used in households were now days replaced with T5 tube-lights.

T5 Tube-Light length available is from 9 inch-21 inch and having power ranging from 2-15 watts.

T8 Tube lights length available are from 18 inch to 6 feet and power range is from 15 watt to 70 watts.

T12 Tube-lights length available are available from 18 inch to 8 feet and power range is from 15 W to 125 Watts.

1/8 of inch= 3.175 mm.

Applications of Tube-lights:-

(i)                T4 tube-lights are very compact and are having very easy installation, These are having very useful application in kitchen counters and worktops. Power consumption of these tube-lights is very low. Life of tube-lights is 9000 hours.

(ii)             T5 tube-lights are installed for efficiently lighting up Schools, factories, offices etc. Life of these tube lights is 20,000 hours. These lights have very low mercury content thus these have very minimum impact on environment. These lights uses specially designed blast which limits current into tube light. So prevents them from overloading. These blasts also enable them working above 20 kHz which gives instant start features to these tube lights.

(iii)           T8 tube-lights are most widely used tube lights and these are useful where there is need to see lot of details. These are useful in Stores, garages, offices, schools etc. These lights are having life more than 15,000 hours.

(iv)           T12 tube-lights are now vanishing out due to popularity of T8 lights. These are useful for lighting large areas such as offices and retail space. 

Fluorescent Lamp and Bulbs nomenclature

Fluorescent Lamp Labeling

For Tube lights:-

There is certain nomenclature which is used while labelling tube lights.

The actual fluorescent tubes are identified by several letters and numbers and will look something like

'F40CW-T12' or 'FC12-T10'.

So, the typical labeling is of the form FSWWCCC-TDD (variations on this format are possible):

·  F - Fluorescent lamp. 

   G means Germicidal shortwave UV lamp.

·  S - Style - no letter indicates normal straight tube; C for Circline.

·  WW - Nominal power in Watts. 4, 5, 8, 12, 15, 20, 30, 40, etc.

·  CCC - Color. 

    W=White,

    CW=Cool white, 

    WW=Warm white, 

    BL/BLB=Black light, etc.

·  T - Tubular bulb.

·  DD - Diameter of tube in of eighths of an inch. T8 is 1", T12 is 1.5", etc.

For the most common T12 (1.5 inch) tube, the wattage (except for newer energy saving types) is usually 5/6 of the length in inches. Thus, an F40-T12 tube is 48 inches long.

For Tungsten lamps:-

There are lamps (bulbs and LED who are available in different sizes and ratings in the market. There are most commonly used bulb sizes with Nomenclature starting with A and thereafter its size. Most commonly used are A15, A17, A19, A20,  A21, A23 . These all sizes are multiple of 1/8th of inch. In Houses most commonly used size was of A19 having 100 W or 125 W rating.

Where A stands for Arbitrary.

There are other models designed according to shapes of bulbs are:-

(i)                 Bulged Reflector:-BR25, BR30, BR38, BR40

(ii)               Candle : C6, C7, C9, C11, C15

(iii)             Globe: G9, G11, G12, G16, G16.5, G19, G25, G30, G40

(iv)             Quartz Reflector Lamp: MR8, MR11, MR16, MR20

(v)               Parabolic Aluminized Reflector (PAR): PAR14, PAR16, PAR20, PAR30 Short or Long Neck

(vi)             Blown Reflector (R): R12, R14, R16, R20, R25, R30, R40)

(vii)           Twist:- CFL’s are  known as Twist shaped bulbs.:T2 Coil, T3 Coil, T4 Coil

As stated above there are so many shapes and sizes available in the market but most commonly used are:-A-19, MR16, PAR38 and 40, R-20 , BR-30, CFL – Twister, G-25 and Candelabras

Now above most commonly used shapes and sizes are having following applications in various fields:-

·                     A-19 is most commonly used in houses. This shape is same as it is first discovered by Edison.

·                     MR-16 lamps are used in Kitchens, Architects and have aesthetic looks.

·                     CFL are most widely used and had replaced A-19 bulbs.

·                     G-25 are most widely used in bathrooms

·                     PAR30 and 40 are used in outdoor lighting. They are having higher wattage ratings.

·                     R-20 and BR-30 lights are used in houses and used as flood lights.

·                     Candelabras:- These are  little lights and are used as accent lighting.

Fluorescent Lamp Basics

Fluorescent Lamp Basics

The fluorescent lamp was the first major advance to be a commercial success in small scale lighting since the tungsten incandescent bulb. Its greatly increased efficiency resulted in cool (temperature wise) brightly lit workplaces (offices and factories) as well as home kitchens and baths. The development of the mercury vapor high intensity discharge (HID) lamp actually predates the fluorescent (the latter being introduced commercially in 1938, four years after the HID). However, HID type lamps have only relatively recently become popular in small sizes for task lighting in the home and office; yard and security area lighting; and light source applications in overhead, computer, and video projectors.

Fluorescent lamps are a type of gas discharge tube similar to neon signs and mercury or sodium vapor street or yard lights. A pair of electrodes, one at each end - are sealed along with a drop of mercury and some inert gases (usually argon) at very low pressure inside a glass tube. The inside of the tube is coated with a phosphor which produces visible light when excited with ultra-violet (UV) radiation. The electrodes are in the form of filaments which for preheat and rapid or warm start fixtures are heated during the starting process to decrease the voltage requirements and remain hot during normal operation as a result of the gas discharge (bombardment by positive ions).

When the lamp is off, the mercury/gas mixture is non-conductive. When power is first applied, a high voltage (several hundred volts) is needed to initiate the discharge. However, once this takes place, a much lower voltage - usually under 100 V for tubes under 30 watts, 100 to 175 volts for 30 watts or more - is needed to maintain it.

The electric current passing through the low pressure gases emits quite a bit of UV (but not much visible light). The gas discharge's radiation is almost entirely mercury radiation, although the gas mixture is mostly inert gas and generally around something like 1 percent mercury vapor. The internal phosphor coating very efficiently converts most of the UV to visible light. The mix of the phosphor(s) is used to tailor the light spectrum to the intended application. Thus, there are cool white, warm white, colored, and black light fluorescent (long wave UV) lamps. There are also lamps intended for medical or industrial uses with a special envelope such as quartz that passes short wave UV radiation. Some have an uncoated envelope, and emit short-wave UV mercury radiation. Others have phosphors that convert shortwave UV to medium wave UV.

Fluorescent lamps are about 2 to 4 times as efficient as incandescent lamps at producing light at the wavelengths that are useful to humans. Thus, they run cooler for the same effective light output. The bulbs themselves also last a lot longer - 10,000 to 20,000 hours vs. 1000 hours for a typical incandescent. However, for certain types of ballasts, this is only achieved if the fluorescent lamp is left on for long periods of time without frequent on-off cycles. 

Air Gap Values while designing Machine

Air gap is a necessary evil. Which means you can't avoid air gap in motors as if there is no air gap motor will not start. So you can't avoid air gap instead air gap can be minimized to as low value as possible.
In electrical systems air gap is main cause of concern in designing electrical motors. In an electromagnetic devices there is uniform practice that material selected is such that it will offer low resistance to passage of magnetic flux. This will reduces the electrical energy demand for creating the required flux. But there is always air gap in rotating machines which is unavoidable. This air gap requires increase in magnetizing current as there is high resistance of air gap which will requires more electrical energy to generate required flux.
This air gap will leads to undesirable electrical losses.

There is rule of thumb that higher the motor speed, the larger the gap. 


Why Air Gap Should be Small?

Let's State by taking an example of an Induction motor when Power supply is given to stator than magnetic flux is developed in rotor. There is always an air gap between stator and rotor. Air gap is having very high reluctance.
This high "reluctance" of air means that for every unit length of magnetic flux path, the mmf required to drive flux through the air portion of the path will far exceed what's needed for the magnetic portion. Consequently, the machine's magnetizing current  will be determined largely by the size of the air gap. The larger that gap in an induction motor, the lower the power factor and also lower is the efficiency of motor Or Alternator. 
This air gap is the only reason behind high no load current in Induction motors. In Induction motors there is high no load current is of 30 to 40% of full load current. In Transformers there is no air gap that is why Transformers have low no load current for same rating of machines.

In  case of other machines other than Induction motors such as synchronous and d-c machines. There are two separate magnetic fields interact in the air gap. The alternating current magnetic field created by the armature which is stationary in case of synchronous machines and rotating in DC machines, distorts that supplied by the d-c field, reducing its effectiveness and degrading machine performance thus air gap is required to be increased to reduce the effect of that "armature reaction." 
This is reason why Synchronous and DC machines have air gaps several times larger than those in induction motors.

That creates an unavoidable contradiction in the design process. 

For mechanical reasons, we want to avoid too small an air gap; for electrical reasons, we don't want it too large

So we always have to make a compromise between the two, For a given rotor diameter, the slower an a-c machine the smaller the gap. Horsepower output will be lower, and the power factor lower as well, so that the electrical effect of a large gap is relatively less acceptable than for a high-speed, higher-horsepower machine having that same rotor diameter. As polarity decreases and both speed and horsepower go up, the trend reverses.

Below Formula will give us the required value for air gap in our machines:-

Air gap, inch = 0.005 0.0003D 0.001 L 0.003V

in which D = rotor O.D., inches

L = core stack length, inches
V = rotor peripheral velocity in thousands of ft/min.= D(RPM/12,000)
Air gap must be uniform.. A non-uniform air gap also tends to increase noise.

Usually Air gap is kept between 0.2 MM to 5 MM of motors rating from ¾ Kw to 750 KW.

Precautions while working with Gas Discharge Lamps & Fixtures

                Safely Working with Gas Discharge Lamps and Fixtures

Fixtures for gas discharge lamps may use up to 30,000 V while starting depending on technology. And, they are often not isolated from the power line. Neon signs are powered by transformers or electronic ballasts producing up to 15,000 V or more. Thus, the only safe way to work with these is to assume that they are potentially lethal and treat them with respect.

Hazards include:

• Electric shock. There is usually little need to probe a live fixture. Most problems can be identified by inspection or with an ohmmeter or continuity tester when unplugged.
• Discharge lamps and fixtures using iron ballasts are basically pretty inert when unplugged. Even if there are small capacitors inside the ballast(s) or for RFI prevention, these are not likely to bite. However, you do have to remember to unplug them before touching anything!

Neon signs using iron transformers are also inert when unpowered - just make sure they are off and unplugged before touching anything!

• However, those using electronic ballasts can have some nasty charged capacitors so avoid going inside the ballast module and it won't hurt to check between its outputs with a voltmeter before touching anything. Troubleshooting the electronic ballast module is similar to that of a switchmode power supply. 
• The pulse starters of some high intensity discharge lamps may produce up to 30 kV during the starting process. Obviously, contact with this voltage should be avoided keeping in mind that 30 kV can jump over an inch to anyplace it wants!
• Nasty chemicals: Various toxic substances may be present inside high pressure discharge lamps (sodium and mercury) and neon signs (some phosphors). Contact with these substances should be avoided. If a lamp breaks, clean up the mess and dispose of it properly and promptly. Of course, don't go out of your way to get cut on the broken glass! WARNING: Metallic sodium reacts with water to produce hydrogen gas, an explosive. However, it is unlikely that the inner tube of a sodium vapor lamp would break by accident.
• Ultra-Violet (UV) light: High intensity discharge lamps generate substantial UV internally, often the particularly nasty UV-B variety. Unless designed to generate UV (for medicinal purposes, photoengraving, or whatever), the short wave radiation will be blocked by the outer glass envelope and/or phosphor coating. However, should the outer envelope break or be removed, the lamp will still operate (at least for a while - some have a means of disabling themselves after a few hours or less of exposure to air). DO NOT operate such a lamp preferably at all but if you do, at least take appropriate precautions to avoid any exposure to the UV radiation. 

Gas Discharge lamps Basics

                                                    Gas discharge lamp basics

The use of electrically excited gas discharges significantly predates the invention of the incandescent lamp. Physics labs of yesteryear as well as today have use of a variety of gas filled tubes used for numerous purposes involving light generation including spectroscopy, materials analysis, studies of gas dynamics, and laser pumping. Look through any scientific supply catalog and you will see many different types of gas filled tubes in all shapes and sizes.

Gas discharge lamps are used in virtually all areas of modern lighting technology including common fluorescent lighting for home and office - and LCD backlights for laptop computers, high intensity discharge lamps for very efficient area lighting, neon and other miniature indicator lamps, germicidal and tanning lamps, neon signs, photographic electronic flashes and strobes, arc lamps for industry and A/V projectors, and many more. Gas discharge automotive headlights are on the way - see the section: "HID automotive headlights".

Because of the unusual appearance of the light from gas discharge tubes, quacks and con artists also have used and are using this technology as part of expensive useless devices for everything from curing cancer to contacting the dead.

Unlike incandescent lamps, gas discharge lamps have no filament and do not produce light as a result of something solid getting hot (though heat may be a byproduct). Rather, the atoms or molecules of the gas inside a glass, quartz, or translucent ceramic tube, are ionized by an electric current through the gas or a radio frequency or microwave field in proximity to the tube. This results in the generation of light - usually either visible or ultraviolet (UV). The color depends on both the mixture of gasses or other materials inside the tube as well as the pressure and type and amount of the electric current or RF power. (At the present time, this document only deals with directly excited gas discharge lamps where an AC or DC electric current flows through the gas.)

Fluorescent lamps are a special class of gas discharge lamps where the electric current produces mostly invisible UV light which is turned into visible light by a special phosphor coating on the interior of the tube. 

LUMENS Selection

LUMENS SELECTION

Brightness or Lumens

Light output can vary greatly from one model of projector to another. ANSI lumens are a standard for measuring this light output.

When there's light in the room, or when you're trying to project onto a wall or other atypical surface, you need a strong light source.

In general, the higher the lumens rating, the more light you've got—and the better the projected image.

Less than 1000 Lumens

The lowest output projectors are typically the least expensive—but under non-ideal conditions the image may not be satisfactory.

Low light output means that presentations will need to be made in a dark or dimly lit room so that the image on the screen is not washed out by ambient light.

1000 - 2000 Lumens

Most standard-use projectors, suitable for normal business conference room and small classroom use, fall into this category.

Although a totally dark or dimly lit room is usually not necessary, presenters should still expect to reduce room lighting somewhat for best screen viewing.

2000 - 3000 Lumens

These projectors are typically found in the high-performance range of the portable and semi-portable projectors, and are suitable for large conference rooms and classrooms.

They offer more flexibility in terms of ambient lighting, since the image is bright and less likely to wash out.

They also offer more flexibility in terms of audience size, since they can illuminate a larger screen without much loss of image quality.

More than 3000 Lumens

Ultra-bright projectors produce more than 3000 lumens and are typically expensive.

They are used in a variety of large venue applications, including very large board rooms, conference rooms, training rooms, auditoriums, churches, concerts, and nightclubs.

The air gap in rotating machines Necessary Evil

But it often gets overlooked when it should be getting close attention

NOT TOO BIG, NOT TOO SMALL. THE Essential air gap separating rotor from stator in any electric motor or generator has several important influences on machine performance-nevertheless, in any design discussion, few essential details get less attention than the air gap.

Air offers much higher resistance to the passage of magnetic flux than the so-called "magnetic materials." Such materials allow a far more intense field to pass through a given space with a relatively lower driving potential (what we call magnetomotive force or mmf). That force is measured in ampere-turns within the electrical circuit causing the magnetization.

Naturally, then, we want to minimize the intervention of air into the magnetic field path. Yet it can't be omitted. It can't be too small. Discontinuities or joints in the magnetic core material are necessary also in a transformer or other "solid-state" electromagnetic apparatus, but are kept small or in staggered locations, to minimize their influence.

Special demands of motors and generators

The electric motor (or generator) presents the unique necessity for an air gap fully separating a rotating magnetic structure from a stationary one. Not only must the separation be complete; it must be large enough so that manufacturing tolerances will not allow the separated components to come into damaging contact during machine operation.

As a further complication, these components exert a strong magnetic attraction on each other. The mechanical structure-including the shaft of the rotating component-must be stiff enough to maintain separation despite that pull. In large machines, the magnetic pull is calculated, then carefully compared to the restraining stiffness, to ensure the assembly's "magnetic stability." (see Figures 1 and 2.)

The high "reluctance" of air means that for every unit length of magnetic flux path, the mmf required to drive flux through the air portion of the path will far exceed (as much as tenfold) what's needed for the magnetic (steel) portion. Consequently, the machine's magnetizing current (and the associated PR loss in the winding) will be determined largely by the size of the air gap. The larger that gap in an induction motor, the lower the power factor.

That creates an unavoidable contradiction in the design process. For mechanical reasons, we want to avoid too small an air gap; for electrical reasons, we don't want it too large.

Attempts to generalize about this are liable to be wide of the mark, by failing to account for all the influences at work. For example, when "energy efficient" motors were being introduced, the claim was often made that they used "smaller air gaps" to reduce losses. In its 1992 report (TR-101290, Vol. 2) on energy efficient motors, the Electric Power Research Institute cited a decreased air gap as one of seven basic ways to increase motor efficiency.

The reasoning is that a smaller gap meant less magnetizing current and therefore a lower stator copper loss. That's true. However, in the theoretical calculation of stray load loss for an induction motor, some component losses vary as the inverse square of magnetizing current. By increasing that current, a larger gap reduces those loss components. Some "energy efficient" motors have therefore used larger air gaps for the best overall effect. In this, as in many other design issues, the answer to the question "what's best?" is, "that depends."

A larger air gap can also reduce electromagnetic noise. Although slot combination and winding configuration exert major influences on noise production, they're seldom subject to change after the machine is built. But studies have shown that doubling the air gap can reduce overall sound pressure level by at least 10 decibels. Although that large a gap increase is seldom possible, significant noise reduction can occur with much smaller gap increases.

A necessary compromise

The resulting compromise is a familiar one. For a given rotor diameter, the slower an a-c machine speed (meaning the larger the number of magnetic poles in the winding), the smaller the gap. Horsepower output will be lower, and the power factor lower as well, so that the electrical effect of a large gap is relatively less acceptable than for a high-speed, higher-horsepower machine having that same rotor diameter. As polarity decreases and both speed and horsepower go up, the trend reverses.

In published material on basic design, the most common guidelines relate power and speed to two basic dimensions: core diameter squared, and core stack length, or DL in shorthand notation. There are a couple of variations. Theoretically, the diameter should be the stator inside diameter, what's often called the bore or gap diameter. Some designers, however, have used the core outside diameter instead. Also, the exponent (the power to which the diameter is raised) has been quoted as either 2.0 or 2.5.

Given those properties, the design practices usually go on to recommend various approaches to selecting the number and size of slots, the winding configuration, and arriving at estimated heat dissipation and performance. Seldom, however, is any firm guidance offered on the selection of air gap.

In most textbooks on electric machinery design and performance, the air gap is considered a "given." Much attention is paid to calculation of losses and performance involving air gap flux density and ampere-turns, reactances, excitation requirements, and so on. But mention is almost never made of either the rationale for choosing air gap size, or the range of sizes typically used in design.

One of the more popular texts, first published in 1936, offers only this advice:

". . . it is necessary (for low magnetizing current) to use small (but not too small) air gap length. . . . The [air gap] must be so selected that the exciting current and machine reactances conform to the performance desired. Reduced gaps may increase motor noise and tooth-face losses. . . ."

According to another design textbook, "It is impossible at the present time to derive a satisfactory equation, directly from theoretical considerations, for determining the proper length of gap." This "empirical" (based on experience) equation is often considered accurate enough:

Air gap, inch = 0.005 0.0003D 0.001 L 0.003V

in which D = rotor O.D., inches

L = core stack length, inches

V = rotor peripheral velocity in thousands of ft/min.= D(RPM/12,000)

Figure 3 illustrates typical design air gaps for four-pole polyphase squirrel-cage induction machines that are fairly consistent with that equation. How these will vary with speed is evident from Figure 4.

Whatever the nominal gap, it must be uniform. Eccentricity drives up stray loss. Other characteristics are also affected. One 800 hp four-pole motor developed a vibration problem (up to 0.3 inches/second at running speed) that could not be solved by rebalancing. The vibration disappeared when the rotor was machined to correct a 7 mil runout. A non-uniform air gap also tends to increase noise; Figure 5 illustrates the relationship developed experimentally by one motor manufacturer.

Anyone dealing with other types of motors will recognize that these air gaps are much smaller than in either a-c synchronous machines or d-c motors. Here's why: In a synchronous motor or generator, two separate magnetic fields exist in the air gap, supplied from two separate sources. The d-c excitation field, produced by the winding on each rotor pole, is concentrated along the central axis of the pole itself. The "armature field," created by the polyphase stator winding, has its axis half a pole pitch away. The net effect is a distortion in the overall air gap field, weakening its effect and (in a motor) reducing the available torque.

This is referred to as armature reaction-the reaction of the stator field against the rotor field. To minimize this, the air gap must be large enough to keep the ratio of air gap ampere-turns to armature reaction ampere-turns per pole above an empirical limit, often taken as 1.0. The higher that ratio, the less the influence of armature reaction, and the more sinusoidal the flux waveform under load. For a highly stable machine, a ratio of 2 to 3 may be needed. Hence, for any given rotor diameter, a synchronous machine's air gap may be two or more times that of an induction machine (see Figure 6

As in the induction machine, of course, the increased gap means more rotor field ampere-turns (and current) are required to drive the desired flux across that gap. That increases both power usage and cost of the field winding, as well as the short-circuit current the machine will supply to a fault on the power supply circuit.

In a d-c motor, armature reaction of the same sort takes place, the difference being that the main field is stationary while the armature rotates. Again, achieving the desired motor performance requires a relatively large air gap. Typical values in industrial d-c motors range from 1/8 to 1/4 inch. (Contrast that with Figure 4.)

Obviously, with a given stator and rotor, decreasing the original air gap is impossible, except in d-c machines, where pole shims can be used. Increasing the gap, however, is possible by either boring out the stator or turning down the rotor. Regardless of the reasons for doing so, either procedure presents problems for squirrel-cage machines.

The first is the tendency for adjacent laminations to "smear" together at the surface during machining. That effectively short-circuits the interlaminar insulation, and can escalate surface losses enough to bring the steel to red heat. Grinding, rather than turning, is less likely to cause trouble.

However, machining the stator in any way risks damage to the slot wedges. That's not a concern for a squirrel-cage rotor-where two other related problems may arise. If the rotor contains a cast aluminum cage in partially open slots, the machining operation may not only smear the laminations themselves, but also spread a film of aluminum over the finished surface, forming an even more troublesome conductive layers.

If the slots are closed or bridged, even a small reduction in rotor diameter may remove part of the steel bridge from the laminations (see Figure 7). When that increases the width of rotor bar exposed at the rotor surface, some stray loss components may increase considerably. The effect on temperature rise may not be great, but efficiency suffers, especially if the number of rotor slots exceeds that for the stator.

In addition, the sharp feather edges of lamination resulting from the severed bridge are liable to eventually break loose to damage the stator winding. Predicting when a "skim cut" on the rotor can safely be made without that result is not possible without design data on the rotor laminations, because the bridge cannot be measured beforehand.

If the rotor is perfectly concentric with the stator, the air gap will be the same throughout its 360° circumference. The magnetic attraction between stator and rotor will therefore be the same throughout; at any point the pull will be identical to that in the opposite direction 180° away. The rotor will therefore remain centered.

Some degree of unbalance inevitable

That ideal situation never exists, of course, because of manufacturing tolerances, bearing clearance, and shaft deflection caused by gravity. Some degree of unbalanced pull always exists, therefore, and the designer's task is to make sure the mechanical stiffness of the system prevents that unbalance from causing the rotor to move into destructive contact with the stator. That requires calculation of the amount of pull involved, which varies inversely with the air gap itself. The smaller the gap, the greater the pull, and the greater the required mechanical resistance.

How does the air gap influence locked-rotor current (and torque)? For gap values within a range that's practical in view of the other limitations discussed here, the answer is "not much."

The relationship is not simple. Several components of both stator and rotor reactance are inversely proportional to the air gap, while other components are unaffected. But the amount of change is highly variable depending upon slot dimensions and stator winding configuration. In any event, an air gap variation of even 20% is unlikely to make a significant change in locked-rotor performance.

"Increasing the air gap" often implies simply machining an existing rotor to a smaller diameter. That decreases the rotor slot reactance (and, as we've seen, several other consequences can be expected as well). If, however, a machine is designed and built for a larger gap, the slot configuration-and the associated reactance-can remain unchanged despite a smaller rotor diameter.

A common limit on measured air gap variation in an assembled motor is ±10% of the average. However, close examination of that requirement reveals that it is not as simple as it seems. To see why, consider some readings for a hypothetical machine. Suppose the gap intended by design is 0.06 inch. Now suppose a series of measurements yields these actual values:

0.050, 0.052, 0.061, and 0.064

What is the "average"? Adding all four readings together and dividing by 4, we get 0.059 (quite close to the design figure). The 10% variation allows readings to be anywhere from 0.054 to 0.065. Two of the measurements fall below the minimum, which is unacceptable.

An obvious solution is machining, to enlarge the gap. Assume the possibility of an increase by 0.006. We would then get this new set of measurements:

0.056, 0.058, 0.067, and 0.070

Now, the average is 0.063. The 10% limits become 0.057 and 0.069. One reading is still below the minimum, and now one other reading exceeds the maximum.

What's happening is that any action taken to bring readings closer to a numerical average will also change that average. To get past this problem, we can go to the extreme of increasing the gap by 0.015. The measurements then become:

0.065, 0.067, 0.076, and 0.079

The average is 0.072; limits are 0.065 and 0.079, and all readings are now acceptable (barely)-except that the average gap has been increased by 25%, which is unacceptable regardless of eccentricity.

Additional complications

A further complication is the variation in gap that may occur from one end of the machine to the other. Still another is the way measurements are made. Actual feeler gauge readings of the gap in an assembled machine are possible only for the larger horsepower ratings, and only at certain locations.

An out-of-tolerance value may exist over only a small area, and therefore have little influence. No single set of four or eight measurements can establish the condition of the total peripheral gap area.

In a large, high-speed machine, with an air gap of 1/8 inch or more, slight changes are seldom troublesome-but neither are they greatly effective in achieving some desired performance change.

Ignored in most redesign or costing procedures, seemingly not a direct influence on temperature or torque characteristics, the air gap in a motor or generator is nevertheless clearly important to machine performance. It should not be overlooked.