The induction motor is most vulnerable if the voltage is high or low than its rated voltage. The performance of the motor depend on the voltage. The speed torque characteristics changes if the voltage is unbalance. The unbalance voltage cause unbalance stator current and finally it increase the losses and the net torque. In this post, we will discuss effect of unbalanced voltage on induction motor.
The major points of discussions are as follows.
- How does voltage & impedance unbalance creates problems in the motor?
- How the torque gets reduced with current unbalancing in the motor?
- Troubleshooting of the slip ring induction motor in case of unbalance in rotor impedance caused due to GRR- Grid Rotor Resistance
The balanced current drawn by the induction motor is a very important criterion for evaluating the efficiency of the induction motor. Any deviation in the voltage and current waveform from the perfect sinusoidal voltage or current waveform in terms of magnitude or phase shift is known as voltage or current unbalance.
Reasons of unbalance current in Induction Motor
The main reasons for the unbalanced current in the stator winding are because of variation in the following parameters.
1. Unbalance supply Voltage
2. Distorted Supply Voltage
3. Unbalance impedance of the stator or rotor winding
4. Unbalance resistance of the Grid Rotor starter
When the three-phase voltage, the impedance of the stator, rotor, and the externally connected resistances to the rotor is perfectly balanced, the stator current of all three phases will be balanced.
Any difference in the phase voltage or impedance of the motor circuit will lead to the problem of an unbalanced current in the stator. The unbalancing of the stator current is a matter of serious concern in view of the operational reliability of the motor.
The single phasing either due to the supply phase missing or winding open may burn out the motor if the single phasing protection relay does not operate.
What is voltage unbalance?
Definition as per NEMA:
The voltage unbalance is defined as the maximum deviation from the average of three-phase voltage or current, divided by the average of the three-phase voltage or current. The voltage unbalance is expressed in percent. According to NEMA (National Electrical Manufacturers Association of USA) standard equation is as given below.
Example
The unbalanced voltage and current in the electrical network can be resolved into three balanced systems a positive sequence, negative sequence, and zero sequence components. If an electrical system is balanced, the negative sequence and zero sequence components would be absent. In case the phase current is unbalanced, the unbalanced current can be resolved into positive and negative and zero sequence current. The unbalance system voltage and circuit impedance causes the unbalanced current in the induction motor.
The unbalance current has the positive sequence, negative sequence, and zero sequence components. The negative sequence current has the phase sequence just opposite to positive sequence components.
Why the torque delivering capacity reduces with unbalance current?
The positive sequence current produces a positive torque which rotates the motor in the forward direction. The negative sequence current produces the backward rotating magnetic field and torque in the motor. Thus, the torque produced by the negative sequence current tries to retard the motor by exerting opposite torque. The net torque produced in the motor is;
Net Torque= Tp-Tn
Tp and Tn is the torque produced by the positive and negative sequence current.
The net torque of the motor reduces with unbalance in voltage and the total motor current increases that cause a temperature rise in the winding. The winding insulation of the motor gets halved with every temperature rise of 10 degrees centigrade.
The following graph shows the relation between motor loss and temperature rise with unbalance in voltage. With an increase in voltage unbalance the heat loss in the motor increase and consequently, the efficiency of the motor gets reduced.
The operation of a motor with above a 5% unbalance condition is not recommended, and will probably result in damage to the motor. The efficiency of the motor gets reduced with an increase in voltage unbalance.
Problem of unbalanced current tripping in slip ring motor
A 450 KW,6.6 KV slip ring motor used for fan application was tripping with the unbalance current fault. The motor was checked thoroughly and found that the following circuits were alright.
1. Motor Breaker
2. The Stator and Rotor Winding
3. Carbon Brushes and slip rings
4. Grid Rotor Resistance
The motor was started and found that the motor again tripped with unbalance current. Again the motor and Grid Rotor Resistance Starter were checked and found that the connection point of one of the phases in the GRR was loose and the connection point got slightly carbonized and it created additional resistance. The unbalance resistance in the GRR circuit caused unbalance current in the stator.
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