Electric Motor Protection in Case of Voltage Unbalance and Single Phasing

 

Voltage Unbalance

When the voltage between all three phases is not equal, the current values in each phase will also become unbalanced. According to NEMA, the maximum voltage unbalance is limited only to 1% both for electric motors and generators. When the voltage unbalance occurs, the current increases gradually in the motor winding which if it continues, the motor will be damaged. Therefore it is necessary derate the motor according to the expected voltage unbalance. 

If in case, the derating is not possible and the voltage unbalance still persists, the loading in this case must be reduced accordingly. This method must be considered to avoid the damage of the equipment.

Causes of Unbalanced Voltage 

• Connecting unequal single-phase loads. This is why many consulting engineers specify that loading of panelboards be balanced to ± 10% between all three phases.
• Open delta connections. 
• Transformer connections open - causing a single-phase condition. 
• Improper tap settings on transformer banks.  
• Transformer impedances (Z) of single-phase transformers connected into a “bank” not the same. 
• The capacitors used in power factor correction capacitors are not the same or some of them are off the line

Insulation Life The effect of voltage unbalance on the insulation life of a typical T-frame motor having Class B insulation, running in a 40°C ambient, loaded to 100%, is as follows:

Note that motors with a service factor of 1.0 do not have as much heat withstand capability as do motors having a service factor of 1.15. Older, larger U-frame motors, because of their ability to dissipate heat, could withstand overload conditions for longer periods of time than the newer, smaller T-frame motors.

Insulation Classes 

The following shows the maximum operating temperatures for different classes of insulation. 

• Class A Insulation = 105°C 
• Class B Insulation = 130°C 
• Class F Insulation = 155°C 
• Class H Insulation = 180°C

Reference: 

• Cooper Bussman