Alternate investment methods

Sunday, May 18, 2014

Why and Where Transformer Tap changer required??

Why Tap Changer required in Transformer??

Every equipment is designed to operate at a particular voltage level. So it becomes extremely important to keep that voltage within limits. To keep terminal voltage within limits transformers are provided with tap changers. Tap changer can be provided either on primary or secondary depending upon requirement.

Principle behind the tap changer is same. For regulating secondary output voltage no. of turns in primary or secondary winding are changed.

e.g. let V(p),N(p) and V(s), N(s) are primary and secondary quantities. If N(p) decreased emf per turn on primary winding i.e. V(p)/ N(p) increases therefore it will leads to increase in O/P voltage as Secondary O/P voltage is {V(p)/ N(p)} XN(s).

Similarly if N(s) is increased and N (p) kept constant then by same formulae we find that same effect on primary and secondary voltages. So we can say that decreasing primary turns and increasing secondary turns have same effect on voltages.

Where Tap Changer to be Provided on Transformer??

Thus tap changers are provided on primary or secondary winding of transformer so as to alter turn ratio. But choice of fixing tap changer on primary or secondary winding is made by keeping in mind that voltage per turn will remains constant. If primary winding voltage per turn decreases the core flux will decreases which will leads to poor utilization of core. On the other hand if voltage per turn will be increased in primary winding then this will leads to magnetic saturation of core and more core losses.

As we know that At generating end Transformers primary voltage will be kept almost constant so that is reason to provide tap changer on secondary winding not primary winding. If there are variable voltages as in case of transmission lines then it is advisable to keep tap changer on primary.

There are other factors which must need to be considered while deciding tap changer position on transformer winding:-

1. Transformers with large turn ratio are tapped on HV side since it enables smoother control of O/P voltage as compared to LV side tap changer as LV side tap changer will leads to vary O/P voltage in large steps which is undesirable.

2. Tap changer on HV side have to handle low currents although it has to handle high voltage or more insulation required.

3. Also it is very difficult to put tap at LV winding as it is placed next to core due to insulation considerations. As HV winding is placed outside of LV winding so it is easily accessible can be easily tapped without any difficulty.

Read about off-load tap changer Click here
http://electrialstandards.blogspot.in/2014/05/transformer-off-load-tap-changer.html

Friday, May 16, 2014

Soft Starter Working Principle and Circuit Diagrams

Soft-Starter is a Solid State device used for starting of motor . Soft- Starters temporarily reduce the load and torque on motor during starting of motor. This will leads to reduced mechanical stress on the motor shaft as well as electrical stress on Power cables and whole electrical systems. In Typical Star-Delta Starter Starting current is about 3-6 times rated current of motor but using Soft-Starters this staring current can be reduced to 2-4 times motor rated current. This is ultimately reduces stress on electrical system which will leads to smooth running of electrical systems. In that Soft-Starter reduced voltage is applied during starting and that reduced voltage will reduce Starting torque. if 50% voltage is applied to a motor, it will produce 25% of it's available torque at that point.

Starting time also feed into soft-Starter parameters , when motor get accelerated Soft-Starter get bypassed by soft-starter through bypass contactor provided inside Soft-Starter. Bypass contactor rating is according to Soft-Starter rated current.

Electrical soft starters can reduces the torque by temporarily reducing the voltage and current I/P to connected motor.

There are two circuits by which soft-starter is connected to motor:-

1. In-line Circuit known as Standard Circuit

2. Inside Delta Circuit.

1. Inline Circuit or Standard Circuit

Circuit by which Soft-Starter is connected to motor is shown below:-

From above circuit it has been clear that Soft-Starter is connected in series with the line voltage applied to the motor. This circuit is known as Soft-Starter within inline circuit.

2. Inside Delta Circuit

There is another circuit by which Soft-Starter is connected is known as Inside delta circuit in which is shown below:-

In that circuit two cables which are going to motor one of them will be connected directly to I/P supply and other cable will be connected through soft-Starter.

One Specialty of that circuit is that Small Soft-Starter can be used for bigger motors. e.g. you have 60KW soft-Starter then that Soft-Starter can be used for motors having rating 90X1.732=103.92 i.e. can be used for 100 KW motors. As phase currents get divided in 2 circuits. But One should remember that in that 60KW soft-Starter 100 KW motor setting parameters should be feed.

Residual Current detector principle of operation and uses

Residual Current Detector:-

Residual current detector works on the principle that amount of current flowing through phase should return through neutral as both phase and neutral are connected to RCD’s . RCD detects any mismatch between two currents that is through phase and neutral. RCD tripping value is usually in milliamp and mostly available in 30mA and 100 mA tripping range.

If a house has an earth system connected to an earth rod and not the main incoming cable, then it must have all circuits protected by an RCD

A current flow of 30 mA is sufficiently small that it makes it very difficult to receive a dangerous shock. Even 100 mA is a relatively small figure when compared to the current that may flow in an earth fault without such protection.

The residual current device – principle of operation

The residual current devices monitor the current flowing in a circuit by means of a torroid, which is a small current transformer specially designed to detect earth fault currents. All live conductors will pass through this coil, the currents flowing in the live conductors of a healthy circuit will balance and therefore no current will be induced in the torroid. Live conductors of a circuit include all phase and neutral conductors. When an earth fault is present on a circuit, current will be flowing to earth through an abnormal or unintended path. This earth fault path could be through a person in contact with live parts .

The Technology behind the RCD’s

There are two types of technologies available in residual current devices:-

1. Electromagnetic and

2. Electronic

These both offer very reliable performance.

Electromagnetic devices use a very sensitive torroid, which operates the trip relay when it detects very small residual currents. These devices generally require no reference earth lead and are unaffected by temporary loss of supply, as the power to trip the device is derived directly from the fault current.

Electronic devices do not need such a sensitive torroid as electronic circuits within the device amplify the signal to operate the trip relay. However, these devices often require a safety earth reference lead to ensure that the device will continue to operate in the event of the supply neutral being lost. The power to trip the device is taken from both the fault current and the mains supply, enabling the overall size of the devices to be reduced.

Limitations of RCD:-
1. RCD are use-full only where there is standard form of waveforms and can't be used where non standard waveforms are generated from loads.
2. RCD can't provide protection against overloads.
3. RCD's can't protect against overheating.
4. There is always problem of Nuisance tripping in RCD's . Sudden changes in load causes current flow in earth which will leads to tripping of RCD's.

Thursday, May 15, 2014

Induction Motor Name Plate details

Every Induction motor has given a Nameplate which describe some important aspects about motors. There are standards sets by NEMA for Nameplate marking that following is the Minimum information shall be given on all nameplates of single-phase and polyphase induction motors.

Following are the abbreviations for the same:-

1. Manufacturer's type and frame designation

2. Horsepower or KW output.

3. No. of poles

4. Maximum ambient temperature for which motor is designed.

5. Insulation system designation.

6. RPM at rated load.

7. Frequency.

8. Number of phases.

9. Rated load current.

10. Voltage

11. Power factor

12. Efficiency of Motor

13. Motor Weight

14. Bearing size

There is brief discussion about above abbreviations:-

Manufacturer Type and Frame Designation:-

Manufacturer must describe their name on the name plate. Frame size of the motor also to be described on motor name plate. Usually manufacturers will give same frame size of motors with same rating so that any manufacturer motor can be replaced with existing motor.

This nameplate block can offer a lot of information if the motor is nearly standard. The frame size sets important mounting dimensions such as:-

(i) Foot hole mounting pattern

(ii) Shaft diameter

(iii) Shaft height

NEMA standards do not set some dimensions that can turn out to be important if the motor must fit into a confined space.

These include maximums of overall height and length, and maximum conduit-box extensions.

The data in the "Frame" block can be hard to interpret when special shafts or mounting configurations are used.

Horsepower or KW O/P

Motor KW and Horsepower is also recommended to be described on motor name plate so that even a non technical person can easily read and select the motor of desired rating and their load requirement. Shaft horsepower is a measure of the motor's mechanical output rating, its ability to deliver the torque required for the load at rated speed. It is usually given as "HP" on the nameplate.

In general:

HP = (Torque) x (speed)/5,250

Here Torque is in lb-ft Speed is in rpm

No. of Poles

No. of poles will tell about speed of the motor as Speed= 120X Frequency/ No. of poles this is known as synchronous speed. But there is slip also in induction motor which is usually around about 4% in most of induction motors. If Slip is 4% that means induction motor speed is lower by 4%.

More the no. of poles lower will be the speed. So when poles are given speed can be easily calculated.

If there are 4 poles on motor then synchronous speed will be =120X 50/4= 1500 RPM and if there is 4% slip then Induction motor speed will be = 1500- (4/100X1500)= 1440 rpm.

Maximum ambient temperature for which motor is designed.

The nameplate lists the maximum ambient temperature at which the motor can operate and still be within the tolerance of the insulation class at the maximum temperature rise. It is often called "AMB" on the nameplate and is usually given in degrees C.

Insulation Class used

Insulation class is also mentioned on Motor name plate and is often abbreviated "INSUL CLASS" on nameplates. Insulation class specifies about thermal tolerance of the motor winding.

Insulation class is a letter designation such as "A," "B," or "F," depending on the winding's ability to survive a given operating temperature for a given life. Insulation classes of a letter deeper into the alphabet perform better.

RPM of Motor

Although we can calculate motor speed by knowing no. of poles of motor but motor RPM are also described on Motor name plate as slip of motor in not mentioned on Name plate. By knowing RPM of motor and no. of poles we can easily find slip of an Induction motor.

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

Frequency of motor

Motor frequency at which motor will operate or ratings are given should be inscribed on motor name plate. Input frequency is usually 50 or 60 Hz. When there are more than one frequency is given on Motor name plate other parameters that will differ at different input frequencies must be defined on the nameplate.

Phase

This represents the number of ac power lines supplying the motor. Single and three-phase are the norms.

Current

Motor Rated load current in amps is given along with motor Horsepower or KW and voltage and frequency. Usually motor rated currents in both star and delta are given on nameplate. Also different Currents according to different voltages should also be given.

Voltage

The voltage at which the motor is designed to operate is an important parameter.

It is common for manufacturers to nameplate a wide variety of voltages on one motor nameplate.

A common example is a motor wound for 230 and 460 V (230/460 V) but operable on 208 V. This 208-230/460 V motor will have degraded performance at 208 V. Another common misconception is to request a motor rated at network voltage; for example, at 480 V. The NEMA standard is 460 V. The voltage rating assumes that there is voltage drop from the network to the motor terminals. Thus, the 460-V motor is appropriate on a 480-V network.

Power Factor

Also given on the nameplate as "P.F." or PF," power factor is the ratio of the active power (W) to the apparent power (VA) expressed as a percentage. For an induction motor, power factor also varies with load. The nameplate provides the power factor for the motor at full load.

Active power is the power is the actual power consumed and apparent power has a reactive component. This reactive component is undesirable - the utility company must supply it, but it does not work.

A power factor close to unity is most desirable. But due to air gap power factor is low and usually of 0.80 to 0.91.

Efficiency of Motor

Efficiency is defined as output power divided by input power expressed as a percentage:

(Output / Input) x 100

There are losses known as Windage losses, Iron losses and Stray losses. This will reduce motor efficiency. NEMA has established the maximum variation allowed. Generally motor efficiency remains between 85% to 95%.

Motor Weight

Motor weight is specified on motor name plate.

Bearings

Bearings are the important factor for maintenance in an ac motor. Bearings information is usually given for both the drive-end bearing and the Non driving end.

Some manufacturers use a simplified designation simply indicating the bearing size and type –for example, 6309 for a size 309 ball bearing. This brief information can leave questions like: Is the bearing sealed, shielded, or open? Still, some manufacturers may use special bearings and elect to display their own bearing part numbers on the nameplate. Many special bearings are applied in motors for reasons such as high speed, high temperature, high thrust, or low noise. It pays to understand your motors' bearing requirements.

Wednesday, May 14, 2014

Variable Frequency Drive (VFD) disadvantages

Although there are various advantages of VFD's but still there are some disadvantages which must be considered while selecting a VFD.

1. Very High Initial Cost
Cost is the main factor for using VFD’S as compared to other speed controls. Cost of VFD’S is usually high which obstacles in installation for the same in industry although it saves energy but initial cost is very high. It has been found that payback period for the cost of VFD is around about 1- 2.5 years depending upon application where it is used and amount of energy saving during process.

2. Maintenance and Troubleshooting requires Special Skills and costly
Starters like star- Delta Starters and DOL starters maintenance is very easy and very much cost effective . A Thorough inspection of the above starters is enough to see whether and why devices fail. VFDs, like any solid state device, require special troubleshooting practical and theory knowledge. Special trained personal is required for the maintenance and troubleshooting. Also if any part get failed then repair cost for the same is very high. Also it’s easy to repair the VFD’s for everyone as special skills required for the same.

High Harmonics:-

It has been observed in VFD’S that O/P waveform generated by VFD’s is non linear which will create harmonics the motors. These harmonics will leads to create heat in motor. It has been found that VFDs create between 5 to 8% more heating in a motor when compared to that same motor running on a sinusoidal waveform from the power line. This is the reason inverter rated motors are used for the VFD applications. Inverter rated motors are nothing but motors with insulation class F.

4. Lower Speed might leads t Overheating
Induction motors which require constant torque there is a possibility that the motor will overheat during low speed operation. As constant torque loads will draw the same current regardless of the motor operating speed. VFD’S will reduce motor frequency in order to decrease motor speed which will leads to lower speed of motor but same torque requirement will leads to overheating of motor. If the motor produces a high level of heat while operating at low speed, overheating results.

It has been found that a fully loaded motor with Class B insulation running at 50% rated speed on a constant torque load will not overheat. If the motor is run below 50% speed continuously, it will overheat.

If motors are used having Class F insulation then the speed of a fully loaded motor may be decreased to approximately 20% speed without overheating. The lower the continuous operating speed below the motor overheating point, the more the motor and VFD must be derated

Tuesday, May 13, 2014

Variable Frequency drive (VFD) types

There are three types of Variable frequency drives available in Market. These VFDs differ primarily in the type of rectification by which they convert AC power to DC power.

1. Voltage Source Inverters
2. Current source inverters
3. Pulse width modulated inverters

1.Voltage Source Inverter VFD’S

These types of VFD’s are most widely used in industries as they have multi-motor control facility. The VSI was the earliest solid state VFD. It is also known as "six-step" drive because of the voltage sent to the motor.

Variable source inverter operation is relatively simple. In that drives AC input voltage and frequency is converted to DC by rectifiers, then converted back to AC through the inverter section. Desired O/P Voltage and frequency is generated at VFD O/P so as to meet the volts per hertz ratio at the VFD output.

2. Current Source Inverter VFD’S

These types of VFD’s are also called current-fed inverter these behave like a constant current generator which produces an almost square-wave of current.

These type of VFD’s are used instead of Variable source inverters for large VFD’S about 200 HP as these VFD’S have simplicity, regeneration capabilities, reliability and lower cost.
But there are some implications with these VFD’s are that they have poor power factor at low speeds, and are not suitable for multi-motor operation.

There is another disadvantage of CSIs is "cogging," or jerky start/stop motions or pulsing shafts while running. Now days VSI type drives come even upto 500 HP as they have multi-motor operation facility.

3. Pulse Width Modulation VFD’S

These VFDs are used where constant V/f ratio required as they deliver a constant voltage hertz ratio with no line notching and very stable current input for the motor. These VFD’s have main advantage over Voltage and current inverters is that they have high efficiency, constant power factor which doesn’t depend on speed as in case of current inverters. These VFD’s also have multi-motor operation.

These VFD’s have pulse width modulation converter section which consists of a diode bridge to rectify AC power, rather than an SCR bridge. As the input to the inverter section is constant, the inverter controls both voltage and frequency. Inverter section might consists of transistors, GTOs, or SCRs.

But the main disadvantage is that these VFD’s require extra parts for line regeneration capabilities and also have more noise. PWM units create significant audible noise.

Sunday, May 11, 2014

Benefits of VFD'S other then Power saving

Lower Starting Current

VFD has an application that it acts like a starter with reduced voltage starting which will limit high in-rush current. If A motor started using DOL Starter or Star-Delta Starter then motor starting current remains to be very high approx 4-7 times rated current of motor but in case of VFD’S maximum starting current is limited to 150% of rated current of motor.

VFD’S also provide high starting torque by which motor will be able to start even heavy loads. E.g. Gas compressors needed to be drained during starting if other starters used but if we install VFD’S for the same then compressors needed not to be drained.

Multi-Motors Control

A single VFD has the capability to control multiple motors. But other speed control starters don’t have that capability.

Lower System Maintenance
VFD’S use reduces maintenance cost both electrically. In Case of Star-Delta and DOL Starters high starting current will reduce life of Cables, Contactors and motors. VFDs also help to reduce wear on belts, sheaves, gearboxes and couplings.

Synchronization of motors
By using VFD's it is possible to synchronize the motors. VFD's help to start all motors simultaneously with speed control and stop them simultaneously. VFD's help in precise speed control of motors.

High and lower speed control:-
VFD's help in running the motors below and above rated speed of motor. With VFD's it will also be possible to run motors in inching mode which will not be possible with other starters.