Electric Motors Voltage and Balance

When we plan to install an electric motor for our application,the one of  important parameter that we MUST verify is about VOLTAGE.Ensure the voltage is balance between phase to phase.

Motor voltages should be kept as close to the nameplate value as possible, with a maximum deviation of 5 %. Normally motors are designed to operate within 10% of nameplate voltage.

What happen if your voltage variations is large than nameplate voltage?The answer is if  voltage variations larger than motor nameplate voltage,it’s can have several damage effects to the motor on torque, slip, current, efficiency,power factor, temperature, and service life.

If a polyphase induction motor faced unbalanced three phase line voltages,it’s may cause unbalanced currents and will causing an overheating of the motor’s stator windings and rotor bars, impact of insulation lifespan, and generate more heat and wasted energy.

A voltage unbalance can magnify the percent current unbalance in the stator windings of a motor by as much as 6 to 10 times the percent voltage unbalance and acceptable value of voltage unbalance is typically not more than 1 %.

When there is a higher than 1% of voltage unbalance, we must take a necessary action to determine and rectify the root cause of the unbalance voltage and not advisable to operate the motor at all.

Below is a simple of voltage unbalance calculation as follows:

Formula :

( Maximum voltage deviation from the voltage average / Average Voltage) x 100

Example of problem :

What is the percent voltage unbalance for a three-phase supply voltage for 1 unit of Induction Motor.From measuring at motor terminal : Red- 435 V, Yellow- 416 V, and Blue -420V

Calculation :
Average voltage deviation = (435 + 416 + 420) / 3 = 423 V

Maximum deviation from the average voltage = 435 – 423 = 12V

Percent voltage unbalance = (12/423) x 100% = 2.8 %

* 2.8% is higher than acceptable limit for unbalance voltage is 1%


Causes of unbalance voltage

Typically,the cause of unbalanced voltages is effected from variations in the load for electrical system.If the load for phases is different between one or three of each others,unbalanced voltage will happen.

Theoretically it happen due to different impedance’s and value of loading on each phase either for single or three-phase system.We need to ensure during phase arrangement for electrical wiring system is always balance and not exceed the limit.

Below are several common causes of unbalanced voltages as follows:

1)  Unbalance incoming voltage from utility supply.Need to install phase sequence relay protection.

2) Miss setting for transformer tap.Need proper setting from expert person.

3) Single phase distribution unbalance on the electrical system

4) Open phase on the primary of a 3 phase transformer on the distribution system

5) Unequal impedance in conductors of power supply wiring

6) Over reactive power for single phase.Normally due to a welding set.

Speak Your Mind


This site uses Akismet to reduce spam. Learn how your comment data is processed.