Jim Bryan
EASA Technical Support Specialist
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 at rated voltage. Note: The tolerance for voltage unbalance is only 1%, an order of magnitude less than the 10% voltage variation tolerance. For more information on this subject, see the December 2007 edition of Currents for an article on voltage unbalance titled “Unbalanced Voltages and Electric Motors.”
Table 1 explains numerically the relationships between various motor performance parameters and a dynamic power supply subject to variation. It was featured in a paper by P. Pillay at an IEEE Petroleum and Chemical Industry Committee (PCIC) conference in 1995 and is shown on Page 24 in EASA’s Electrical Engineering Pocket Handbook. The table was a subject of discussion at the recent meeting of the EASA Technical Services Committee in St. Louis. One purpose of this article is to better utilize the information in this table.
Figure 1 is a graphical representation of the values represented in Table 1. It must be noted at this point 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, and E = Volts), and therefore in order 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 here. Since there are no exponents in the power equation, we expect the curve to be linear, but it is not.
Power output is not the only result given by applying voltage. In order to develop torque, magnetic flux must be produced in the iron core. The magnetization of the core 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 low voltage than to high. As the terminal voltage is 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 opposite 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 three-quarter load. That is a separate discussion for another time; here we are only considering the effect of voltage variation at constant load.
NEMA MG-1 requires that the motor design be able to perform 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; or, 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
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 (230 – 23 = 207). So theoretically, the motor will work at 208v. Based on the discussion in the previous paragraph, this logic has some danger. The motor will draw higher amps, run hotter, and have lower efficiency. Also, it should not be confused with a motor rated at 208v with the requisite ±10% operating range 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 that is rated for 208v operation (typically 200v) to provide optimum performance and service life.
Other factors
Other performance factors generally follow the curves 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 speed of the motor. If the motor is coupled to a variable torque load such as a fan or centrifugal pump, the load will also increase, thus increasing the load amps. There are similar interactions with all the parameters listed, which increase the difficulty in tabulating them.
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 be helpful in developing this understanding.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
EASA Technical Manual
More information on this topic can be found in EASA's Technical Manual- Section 2: AC Machines
- Section 7: Electrical Testing
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