What is voltage variation?

Voltage variation is the deviation of voltage from the rated voltage; NEMA MG1 Section 12 allows a plus or minus 10% variation from rated voltage. That rating assumes balanced voltages and acknowledges that motor performance will not necessarily be the same as the rated voltage.

Table 1: Typical Effects of Voltage Change
This table explains numerically the relationship between various motor performance parameters and a dynamic power supply subject to variation.

Operating characteristicsEffect of voltage change
90% Voltage110% Voltage120% Voltage

Starting & Max running torque

Synchronous Speed

Percent Ship

Full Load Speed

-19%

No Change

+23%

-1.5%

+21%

No Change

-17%

+1.5%

+44%

No Change

-30%

+1.5%

Efficiency
Full Load, high eff.
  T frame
0.75 load, high eff.
  T frame
0.5 load, high eff.
  T frame


-1 to -2 pts
+0.5 to 1 pts
Pract. no change
+1 to 2 pts
+1 to 2 pts
+2 to 4 pts


+0.5 to 1 pts
-1 to -4 pts
Pract. no change
-2 to -5 pts
-1 to -2 pts
-4 to -7 pts

Small Increase
-7 to -10 pts
-0.5 to -2 pts
-9 to -12 pts
-7 to -20 pts
-14 to -16 pts

Power Factor
Full Load, high eff.
  T frame
0.75 load, high eff.
  T frame
0.5 load, high eff.
  T frame


+1 pt
+9 to 10 pts
+2 to 3 pts
+10 to 12 pts
+4 to 5 pts
+10 to 15 pts
 
-3 pts
-10 to -15 pts
-4 pts
-10 to -15pts
-5 to -6 pts
-10 to -15 pts
 
-5 to 15 pts
-10 to -30 pts
-10 to -30 pts
-10 to -30 pts
-15 to -40 pts
-10-30 pts 

Full Load Current
  High eff.
  T frame  

 
+11%
+3 to 6%
 
-7%
+2 to 11%
 
-11%
+15 to 35%

Starting current

-10% to -12%+10 to 12%+25%
Temperature Rise
  Full load, high eff.
  T frame

+23%
+6 to 12%


-14%
+4 to 23%

-21%
+30 to 80%
Mag. Noise, any loadSlight DecreaseSlight IncreaseNoticeable Increase

Figure 1: Shape of the curves determined by the design's flux density


This figure is a graphical representation of the values represented in Table 1. it must be noted that these values are intended to be representative. Each motor's design will affect the shape and location of the curves generally represented here. We can infer that as the voltage is increased or decreased, change will occur in each parameter.

Understanding the Chart

The key to understanding Figure 1 will be the full load amps curve. Note that as the voltage is increased, the current decreases. We expect this because P = IE (P = Power in Watts, I = Amps, E = Volts) and therefore, to maintain constant power developed by the motor, the current required will decrease as the voltage increases. We see on the chart that the power factor is also affected by the variation in voltage. But for the sake of simplicity, we will not consider it in our power equation, and we expect the curve to be linear, but it's not.

Power output is not the only result given by applying voltage. To develop torque, magnetic flux must be produced in the iron core. The core's magnetization varies as the square of the voltage applied, giving our curve a quadratic shape. As the core approaches magnetic saturation, more voltage yields more current. The motor designer will take advantage of this magnetizing current curve by locating his design point before the minimum point on the curve. This is why motors are more sensitive to a low voltage than too high. As the terminal voltage increased slightly above design voltage, the current continues to drop until magnetic saturation is reached.

Effects of Core Loss

One of the contributors to inefficiency is core loss. These are the losses produced by magnetizing the core steel. It is represented by the efficiency curve and is the opposite of the full load amps curve. As long as the full load amps are decreasing as the voltage increases, the efficiency will increase. When we reach saturation, the increasing core losses decrease efficiency. We should not confuse this factor with the increase in efficiency exhibited when the load is reduced. Most motor designs will reach peak efficiency at about 3-quarter load. Here we are only considering the effect at constant load.

NEMAMG-1 requires that the motor design performs at voltages +/- 10% of the rated voltage. It also warns that the performance of the motor will not be the same through that range. One major factor is the heat produced. The higher the current, the greater the I2R losses; in other words, heat. Low voltage requires the current to increase to maintain the power to drive the load. By operating at higher voltages that result in magnetization beyond the saturation point, the heat produced will increase, thus shortening the life of the insulation system.

Care in Selecting Voltage

Table 2: Some Common System and motor voltage ratings
System VoltageMotor Voltage Rating

Frequency

20820060
24023060
38038050

400

40050

415

40050

480

46060

600

57560
 
2400230060
3300330050
4160400060
6900660050/60
 
110001100050
138001320060

Some motors are name plated 208-230 / 460V, indicating that the motor is "suitable for use" at 208 volts. Extreme care should be exercised if using this motor at 208V. 230V is the "rated" voltage, and 208V falls within the minus 10% range. So theoretically, the motor will work at 208V. This logic has some danger. The motor will draw higher amps, run hotter, and have lower efficiency. It should also not be confused with a motor rated at 208V with the requisite +/- 10% operating rate of that voltage. Operating a 208-230 / 460V motor at 208V minus 10% (or 187V) will damage the motor. The conservative system designer will require a motor rated for 208V operation (typically 200V) to provide optimum performance and service life.

Other Factors

Other performance factors generally follow the curve on the graph. Although they are depicted as linear, they rarely are. For instance, as the voltage goes up, the slip decreases, thus increasing the motor's speed. If the motor is coupled to a variable torque load such as a fan or a centrifugal pump, the load will also increase, thus decreasing the load amps. There are similar interactions with all the parameters listed.

 

 

 

 

 

 

 

 

Conclusion

Motors are designed with the applied system voltage in mind (see Table 2). Detrimental effects can be avoided by understanding the interrelations of voltage with various motor performance parameters and by maintaining the system voltage as near nominal as possible. The chart in Figure 1 can help develop this understanding.