Technical Info

The effects and impact of voltage variation and slip speed on electric motor performance.

As realised, a three phase electric motor is a robust machine and can operate over a wide range of voltage conditions, depending on its design.

The voltage supply to a three phase motor must be a balanced sine wave voltage, for optium life expectancy. There are four (4) detrimental conditions that can be individual or a combination at various times or being continuous.                                             Voltage variation: +/- the nominal. Volatage unbalance. Transient voltage (surges). Voltage waveform distortion (harmonics).                                                                 We are only addressing slip speed and voltage unbalance in this article.

Although IEC 600034-1 allows for two (2) different balanced voltage operations. Continuous can be up to 5%. Intermittent, can be up to 10%.                           Operating at over voltage can have an additional effect that is often not considered. Voltage increases above the nominal level cause the slip of the rotor to decrease by the square of the voltage increase.

For many pumps, fans, blowers and some compressors, the load increases as the cude of the speed in accordance with the laws of physics. The motor may operate at a higher efficiency level, but the operational cost will increase on systems that actively control flow rate. it also should be realised that when replacing an old large horsepower/kilowatt motor with a high efficiency motor, which has a lower slip speed, the above will apply.

All electric motors from 0.3kW up to 185kW must comply with MEPS 3 to be sold in Australia. You will find that motors over 185kW will be designed for high efficiency.         An example is a 355kW WEG 4 pole motor has an efficiency of 97%.

The lower slip speed can cause a rise in full load current. The supply system contactors, thermal overloads and supply cables might not be rated high enough.                           An example we had recently was a possible replacement of a 175HP 8pole motor with a full load speed of 735 r.p.m. A new WEG High Efficiency 132kW 8 pole motor (equivalent to 175HP) had a full load speed of 745 r.p.m a 10 r.p.m lower slip speed.                           Motor efficiency was 95.8%. The above would apply, as it was driving a pump.

It should be noted that any changes to a belt driven load i.e increasing the speed of a pump, fan or some types of compressor, by changing the pulleys, will have the same effect, and overload the motor and controls.

Turning to the unbalanced voltage being applied to a motor is harmful, and is defined as follows:                                                                                                                                            (maximum voltage deviation                                                                                           from average voltage)                                                                     Vu (%) = 100 x ----------------------------------                                                                                             (average voltage)

When, an unbalanced line voltage, is applied to the motor. An unbalanced current in the stator will exist. A small change in voltage unbalance results in a much larger amount of current unbalance.

A 5% unbalance would actually require a de-rating factor of 25% to have an acceptable unbalanced current.

With this occurring, the temperature rise of the stator and rotor will occur.                         A rule of thumb for an increase in stator temperature is that the increase in temperature rise is equal to two (2) times the percentage voltage unbalance squared.

Increase in temperature rise = 2 x (% voltage unbalance)²                                           Therefore a 2% voltage unbalance will result in an 8% increase in winding temperature.     A 3% change will result in an 18% increase in winding temperature.

It should be remembered that for 10ºC rise in winding temperature the theoretical life of the thermal insulation life is cut in half. if for example the thermal winding temperature was 120ºC at a balanced voltage, a 2% unbalanced voltage would result in an increasae in temperature of approximately 10ºC, thereby redusing the life of the winding by half.

The negative effects results in unbalanced voltage include increased stator current, increased winding temperature, a significant increase in the rotor heating (can be transferred to the bearings, causing premature failing), increased motor slip, additional motor losses and major reduction in motor efficienty, leading to an increase in costs to operate under these conditions.

Les Parsons                                                                                                             Managing Director