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.

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:-

1. 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.

2. 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

3. 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.

4. 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.

5. 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.

6. 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

7. 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.

8. Phase

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

9. 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.

10. 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.

11. 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.

12. 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%.  

13. Motor Weight

Motor weight is specified on motor name plate.

14. 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.

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.

3. 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

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.

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.

Power Saving by Using Variable frequency drives (VFD'S)

As VFD’S are used for speed control specially in Fans, Centrifugal pumps, compressors etc. In that cases flow is proportional to current so whenever flow to be reduced it will leads to lower power drawn by induction motor which leads to lower power consumption depending upon the flow requirement.

There are some facts which you must know:-

According to Laws of Affinity:-
1. Speed is proportional to current flow.

2. Torque is proportional to the square of the speed change

3. Horsepower is proportional to the cube of the speed change.

VFD’S will be power savings upto 33% below that we can’t achieve power saving by reducing the speed. VFD’s speed can be either controlled by either connecting Potentiometer or directly from VFD.

Also it should be noted that if you don’t require speed control or flow control means induction motor will always run at full load then there is no use of connecting VFD as that will not leads to any power saving at all.

Mostly everyone got confused that VFD’S will always lead to power saving even sellers misguide consumers about VFD’S.

But Just think that if VFD’S will always saves power then everyone will install VFD’S and save power.

No Power Factor correction required:-

There is another advantage of VFD is that there is no need of capacitor bank for induction motors if VFD’S are installed as VFD’S has inbuilt capacitor . So no need of power factor correction required. This is leads to savings leading to installation and maintenance of capacitor bank.

Lower load applications:-
There are certain cases where load is lower and needed to de-rate the motor so as to keep MDI in limits than in such cases VFD's are best to be used. In such cases frequency is reduced so this will leads to lower power requirement of motor. This is also lower rpm of motors .

Direction change in VFD'S

In starters such as Star- Delta Starters, DOL Starters, Soft Starters whenever there is need of changing the direction of motor than that will be done by changing I/P leads, output leads of supply. This will leads to lot of effort making. This will also leads to loose connections. 
There are also phase change-over's available in market for changing direction of motor. 
In VFD connected motors if motors direction is need not be changed then there is no need to changing input and output connections. Below will explain the same:-

Most often question comes in mind that is how to change direction of motor in VFD’s??

In VFD’S we can’t change the direction of motor by changing phase sequence from I/P supply. As in VFD’S motor I/P get converted into DC supply thereafter AC O/P will be generated through inverter. As any phase sequence of AC supply will get converted into DC thereafter that will converted into AC supply so Phase sequence will always remains same even after changing phase sequence.

In VFD’S direction can be changed by changing option available in VFD’S which will change the direction i.e. direction can be changed by simply changing programming in control circuit. Option may b available in VFD’S depending upon VFD manufacturer it may either ABC and ACB or RYB or YRB.

During initial startup of motor and reversing the motor there is often need of changing the direction of motor than same will be done as per method stated above.

But this doesn't imply that you can install VFD's for keeping the direction same for VFD's, as VFD's are very costly than other starters.