Energized electric motor testing is an expedient test method for identifying a considerable number of fault mechanisms that can cause motor failure. The most important parameter that is monitored is voltage. It should be evaluated before all other parameters and if voltage is out of tolerance, in any way, further power quality evaluation should not be conducted until the voltage anomalies are corrected.
Voltage must be balanced in both magnitude and phasing as well as being free of distortion. It is recommended that phase to phase and phase to neutral voltage be observed for RMS, Peak, and Crest Factor. Unbalance and harmonic distortion effects should also be determined as they may require derating a motor.
A voltage unbalance or the presence of harmonics can cause a motor to run significantly warmer than design allows. When they are present at varying levels, motor loading can be reduced to minimize temperature increase. NEMA provides guidance for Derating and IEEE has recommended levels for harmonics. (NEMA MG-1, IEEE 519)
Poly phase voltage balance can easily be calculated by utilizing the above formula.
Unbalance voltage results in unbalanced current and torque. This causes motor operating temperature to increase. NEMA provides a derating chart to compensate for temperature increase, by load reduction. Using the derating chart at left; if you have a 5% voltage unbalance, you need to reduce motor loading by 25%.
As voltage unbalance increases load must be reduced. A Harmonic Voltage Factor also derates motor load with increased harmonic distortion.
Voltage phase to phase and to neutral should be of equal magnitude with 1200 of phase separation.
On ungrounded systems, grounds on a common bus will affect the balance of Phase to Neutral Voltage. The lower the ground resistance, the greater the unbalance.
The phase with the lower neutral voltage is the phase on a common bus with the ground. If a zero ground was present the unaffected phase would indicate 480V and the grounded phase would indicate 0.00V.
Most Common Causes of Voltage Unbalance:
- Unbalanced incoming utility supply
- Unequal transformer tap settings
- Large single-phase distribution transformer on the system
- Open phase on the primary of a 3-phase transformer on the distribution system
- Faults or grounds in the power transformer
- Open delta connected transformer banks
- A blown fuse on a 3-phase bank of power factor improvement capacitors
- Unequal impedance in conductors of power supply wiring
- Unbalanced distribution of single-phase loads such as lighting
- Heavy reactive single-phase loads such as welders.
For more information on evaluating voltage and power quality, consider attending one of the Snell Groups' electric motor training courses. Don’t forget to enquire about our new Energized Electric Motor Data Analysis Wall Chart, a 36” x 48” laminated wealth of information, that will make energized testing fault identification a breeze. We also have a De-Energized Electric Motor Testing Data Analysis Wall Chart available to assist with your data analysis.