The effect of unbalanced voltage on induction motor includes increased current, reduced torque, overheating, and efficiency loss. Learn the causes and fixes.
How Voltage Affects Induction Motor Performance
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 reduction in net torque.
What is Current Unbalance in Induction Motors?
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.
Voltage unbalance causes a rise in motor current, reducing efficiency and increasing heating.
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(slip ring motors)
Each of these factors can create an imbalance in the magnetic field and torque output.
Voltage Unbalance vs. Load Unbalance
It is important to distinguish between voltage unbalance and load unbalance, as they originate from different causes and require different solutions.
- Voltage unbalance is usually a supply-side issue caused by unequal phase voltages from the utility or faulty wiring.
- Load unbalance, on the other hand, results from uneven distribution of loads across the three phases, often within a facility.
Below is a comparison for better clarity:
| Factor | Voltage Unbalance | Load Unbalance |
| Source | Utility or wiring issue | Uneven load on phases |
| Affects | All motors connected to the supply | Only the local motor or load |
| Causes | Transformer tap settings, cable faults | Unequal phase connection of equipment |
| Solution | Voltage correction, tap changes | Load redistribution or balancing |
Understanding the difference helps diagnose motor issues more accurately and implement the correct corrective action.
Balanced Voltage Condition in Induction Motors
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.
This imbalance leads to heat buildup and premature failure of motor insulation.
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?
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.
According to NEMA MG-1, voltage unbalance should not exceed 1%. Operating motors with higher unbalance leads to accelerated damage.
Example of Voltage Unbalance Calculation
Read detailed article on: What is Voltage Unbalance and How to Calculate It?
Symmetrical Components in Unbalanced Voltage Systems
The unbalanced voltage and current in the electrical network can be resolved into three balanced systems: positive sequence, negative sequence, and zero sequence components.
This method helps analyze the impact of each imbalance component on motor behavior.
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 Due to Voltage Unbalance
The torque-delivering capacity of the motor reduces if the stator of the motor is fed unbalanced voltage.
Even a 2–3% voltage imbalance can lead to 6–10% current imbalance, increasing temperature and reducing lifespan.
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
Here,Tp and Tn are the torque produced by the positive and negative sequence currents.
Current Increase and Temperature Rise
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.
Thus, the temperature rise stresses insulation and shortens motor life drastically.
Maintaining voltage balance is critical to avoid overheating and premature motor failure.
Heat Loss and Efficiency Drop
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.
Higher voltage unbalance leads to exponential loss increase and accelerated thermal aging.
Operating a motor above a 5% unbalanced voltage condition is not recommended and will probably damage the motor.
This, the efficiency of the motor gets reduced with an increase in voltage imbalance.
Unbalanced Current Tripping in Slip Ring Motor – A Case Study
Problem Overview
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.
Even minor connection faults can introduce significant resistance, leading to current imbalance.
The unbalanced resistance in the GRR circuit caused an unbalanced current in the stator.
Protection Devices for Voltage Unbalance
Voltage unbalance can have a significant impact on motor performance, leading to overheating, torque reduction, and insulation damage. To prevent these issues, modern induction motors are equipped with protective devices that monitor electrical parameters and trip the motor when unsafe conditions are detected.
Common protection devices include:
- Single-Phasing Preventers – These devices detect the loss of one phase and stop the motor from operating in a single-phasing condition, which can otherwise cause severe damage.
- Voltage Monitoring Relays – These relays continuously monitor the voltage levels across all three phases and detect any imbalance beyond the permissible limit.
- Overload Relays with Phase Failure Protection – These relays protect the motor from excessive current drawn due to voltage imbalance or phase failure.
- Motor Protection Relays (MPRs) – Advanced relays that provide comprehensive protection by monitoring parameters such as phase voltage, current imbalance, thermal overload, and phase sequence errors.
Using these protection systems ensures early detection of voltage imbalance and prevents potential motor failures. It also improves system reliability and reduces unplanned downtime.
Conclusion – Effect of Voltage Unbalance on Motor Performance
The effect of unbalanced voltage on induction motor operation is critical and far-reaching. Voltage unbalance leads to current imbalance in the motor windings, which directly affects the performance and reliability of induction motors. It causes increased heating, reduced torque output, and overall efficiency loss.
Prolonged operation under unbalanced conditions may result in frequent tripping and insulation damage.
Monitoring voltage levels, maintaining balanced impedance in stator and rotor circuits, and ensuring proper GRR (Grid Rotor Resistance) connections can help prevent unbalanced current faults in motors.
It is recommended to keep the voltage unbalance within the acceptable limit defined by standards such as NEMA to avoid long-term damage and ensure reliable motor performance.
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