Last Updated on February 1, 2024 by Electricalvolt
The induction motor is most vulnerable if the voltage is higher or lower than its rated voltage. The performance of the motor depends on the voltage. The speed torque characteristics change if the voltage is unbalanced. The unbalanced voltage causes an unbalanced stator current, increasing the losses and the net torque. In this post, we will discuss the effect of unbalanced voltage on induction motors.
The significant points of discussion are as follows.
- How does voltage & impedance imbalance create problems in the motor?
- How does the torque get reduced with current unbalancing in the motor?
- Troubleshooting of the slip ring induction motor in case of unbalance in rotor impedance caused by GRR- Grid Rotor Resistance
The balanced current drawn by the induction motor is a crucial 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 magnitude or phase shift is known as voltage or current imbalance.
Reasons for unbalanced current in Induction Motor
The main reasons for the unbalanced current in the stator winding are variations 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 externally connected resistances to the rotor are perfectly balanced, the stator current of all three phases will be balanced.
Any difference in the motor circuit’s phase voltage or impedance leads to an unbalanced current in the stator. The unbalancing of the stator current is a matter of serious concern because 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. It is expressed in percent. According to NEMA (National Electrical Manufacturers Association of USA), the standard equation is expressed below.
The unbalanced voltage and current in the electrical network can be resolved into three balanced systems: positive sequence, negative sequence, and zero sequence components. The negative and zero sequence components would be absent if an electrical system is balanced. If the phase current is unbalanced, the unbalanced current can be resolved into positive, negative, and zero sequence current. The unbalanced system voltage and circuit impedance cause the unbalanced current in the induction motor.
The unbalanced current has positive, negative, and zero sequence components. The negative sequence current has the opposite phase sequence to the positive sequence components.
Effect of Unbalanced Voltage on Induction Motor- Reduced Torque
The torque-delivering capacity of the motor reduces if the stator of the motor is fed unbalanced voltage.
Why does the torque-delivering capacity reduce with an unbalanced current?
The positive sequence current produces a positive torque that rotates the motor in the forward direction. The negative sequence current produces the backward-rotating magnetic field and produces the opposite torque to the positive sequence component. 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 are the torque produced by the positive and negative sequence currents.
The net torque of the motor reduces with an unbalance in voltage, and the total motor current increases, causing a temperature rise in the winding. The life of 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 an unbalanced voltage. With an increase in voltage imbalance, the heat loss in the motor increases, and consequently, the motor’s efficiency gets reduced.
Operating a motor above a 5% unbalanced voltage condition is not recommended and will probably damage the motor. The efficiency of the motor gets reduced with an increase in voltage imbalance.
The problem of unbalanced current tripping in slip ring motor
A 450 KW,6.6 KV slip ring motor used for fan application tripped with the unbalanced current fault. The motor was checked thoroughly, and the following circuits were all right.
1. Motor Breaker
2. The Stator and Rotor Winding
3. Carbon Brushes and slip rings
4. Grid Rotor Resistance
The motor was started, and it was found that the motor again tripped with an unbalanced current. Again, the motor and Grid Rotor Resistance Starter were checked, and it was found that the connection point of one of the phases in the GRR was loose. The connection point got slightly carbonized, and it created additional resistance. The unbalanced resistance in the GRR circuit caused an unbalanced current in the stator.