# What causes motor current unbalance?

## What causes motor current unbalance?

Current unbalance can be due to the motor or the supply line. If the high current and low current readings follow the same line leads, the supply is the cause of the problem. If the high and low readings follow the motor leads, the motor is the source of the problem.

## What is acceptable current imbalance?

RE: Acceptable current unbalance in three-phase motor NEMA MG1 standard gives a maximum allowable voltage imbalance of 1%. If current imbalance exceeds 10%, the motor must be derated.

**How do you calculate motor current imbalance?**

There are three steps in the calculation:

- Determine the voltage or current average.
- Calculate the largest voltage or current deviation.
- Divide the maximum deviation by the average voltage or current and multiply by 100 % unbalance = (Max deviation from average V or I/average V or I) x 100.

**Can a motor be unbalanced at full load?**

Yes, at full load. At no load current unbalance can be up to 20 times voltage unbalance. A common cause of current unbalance on a perfectly balanced supply is a slight eccentricity of the rotor. In practical terms, I would be happy to see the currents match so closely on a motor in the field.

### How can you tell if a motor voltage is unbalanced?

The first step in testing for unbalanced voltages is to measure line-to-line voltages at the motor terminals, following all applicable safety precautions. Likewise, measure the current in each supply line, because the current unbalance is often 6 to 10 times greater than the voltage unbalance.

### How to calculate horsepower and amps for a motor?

Amps = HP × 746 V × η × PF. Thus, the current in amps is equal to horsepower times 746 divided by the voltage times the efficiency η times the power factor. For example, let’s find the current draw in amps of a 1 horsepower motor that runs at 120 volts with a 90% efficiency. Amps = 1 HP × 746 W 120 V × .9. Amps = 746 120 V × .9.

**How much horsepower does a 120 volt motor have?**

Current measurements in amps for common horsepower values at 120 volts for a motor with 90% efficiency. Horsepower Amps Voltage Efficiency .25 HP 1.73 A 120 V 90% .5 HP 3.45 A 120 V 90% .75 HP 5.18 A 120 V 90% 1 HP 6.91 A 120 V 90%