Voltage unbalance can be more detrimental than voltage variation to motor performance and motor life. When the line voltages applied to a polyphase induction motor are not equal in magnitude and phase angle, unbalanced currents in the stator windings will result.
Some of the causes of voltage unbalance are the following:
An open circuit in the primary distribution system.
A combination of single-phase and three-phase loads on the same distribution system, with the single-phase loads unequally distributed.
An open wye-delta system.
a) Variation in ground supply impedance
An increase in primary ground impedance increases the voltage and current balances. Maximum unbalance occurs with overloaded transformers, and the large single-phase load is in the lagging phase. The motor serves to balance the system voltage better when the motor is loaded than when it is unloaded.
b) Transformer loading varied 50 to 150%
The greatest unbalance occurs when a smaller transformer is lightly loaded and a larger transformer is overloaded.
c) Impedance of lines to the single-phase loads
The voltage and current unbalance ratios increase with the line impedances. Again, the unbalance ratios decrease as the motor is loaded more heavily.
d) Impedance of the supply line to the motor
The voltage and current unbalance ratios decrease with an increase in the line impedance to the motor. However, this results in lower voltage at the motor and decreased motor torque and speed.
e) Other parameters
Variations in the magnitude of transformer impedances, the power factor of single-phase loads, and primary line impedances have minor effects (not more than 3%) on the phase currents and unbalance ratios.
An open delta-delta system:
When the two transformers are supplied by three-phase conductors, the only differenee is in the lack of neutral impedance. Therefore, under usual conditions, the open delta-delta configuration will show superior performance to the open wye-delta configuration.
However, when there are unequal line impedances or unusually long supply lines, there are additional observations.
- There are mixed effects with variation of the lines supplying the single-phase loads.
- An increase in the common primary supply line impedance results in increased voltage and current unbalances.
NEMA Standard MG1 defines the percent voltage unbalance as follows:
These unbalanced voltages will result in unbalanced currents on the order of 6 to 10 times the voltage unbalance. Consequently, the temperature rise of the motor operating at a particular load and voltage unbalance will be greater than for the motor operating under the same conditions with balanced voltages. In addition, the large unbalance of the motor currents will result in nonuniform temperatures in the motor windings.
An example of the effect of unbalanced voltages on performance is illustrated in Table 1 for a 5-hp, 1725-rpm, 230-V, three-phase, 60-Hz motor.
TABLE 1 – Effect of Voltage Unbalance on Motor Performance
|Percent unbalanced voltage||0.3||2.3||5.4|
|Percent unbalanced current||0.4||17.7||40.0|
|Increased temperature rise, °C||0||30||40|
Voltages should be evenly balanced as closely as possible.
In recognizing the detrimental effect of unbalanced line voltage on electric motor performance, NEMA Standard MG1 recommends derating motors that are applied to unbalanced systems, in accordance with Figure 1 (NEMA MG1-14.35):
When the derating factor is applied, the selection and setting of the overload device should take into account the combination of the derating factor applied to the motor and the increase in current resulting from the unbalanced voltages.
This is a complex problem involving the variation in motor current as a function of load and voltage unbalance in addition to the characteristics of the overload device relative to Imaximum or Iaverage.
In the absence of specific information, it is recommended that overload devices be selected and/or adjusted at the minimum value that does not result in tripping for the derating factor and voltage unbalance that applies. When unbalanced voltages are anticipated, it is recommended that the overload devices be selected so as to be responsive to Imaximum in preference to overload devices responsive to Iaverage.*
The order of magnitude of the current unbalance is influenced not only by the system voltage unbalance but also by the system impedance, the nature of the loads causing the unbalance, and the operating load on the motor.
Figure 2 indicates the range of unbalanced currents for various motor load conditions and system voltage unbalance.
The effect on other electric motor characteristics can be summarized as follows:
- Torques – The locked-rotor and breakdown torques are decreased. If the voltage unbalance should be extremely severe, the torques might not be adequate for the application.
- Full-Load Speed – The full-load speed is reduced slightly.
- Locked-Rotor Current – The locked-rotor current will be unbalanced to the same degree that the voltages are unbalanced, but the locked-rotor kilovolt-amperes will increase only slightly.
- Noise and Vibration – The unbalanced voltages can cause an increase in noise and vibration. Vibration can be particularly severe on 3600-rpm motors.
Reference // Energy Efficient electric motors by Marlin O. Thurston; Department of Electrical Engineering The Ohio State University Columbus, Ohio