Austin Bonnett
Austin Bonnett Engineering, LLC
Gallatin, Missouri
EASA Education and Technology Consultant

The three-phase squirrel cage in­duction motor is a very robust machine that can operate over a wide range of voltage conditions;  it is the workhorse of the industry. However, for optimum performance and life expectancy, the voltage supply should be a balanced three-phase sine-wave voltage, and its magnitude should be as close to the nominal voltage stated on the motor nameplate as practical. 

Any deviations from these condi­tions may cause a depreciation in the motor performance and expected life, and increase energy costs. Benchmarking the service entry voltage and the motor perfor­mance may be a useful tool to as­sist in knowing when reasonable limits have been exceeded and that corrective action needs to be taken. 

The following detrimental conditions may alone or in combination exist at various times or continuously:

1. Voltage variation (+/- nominal) 
2. Voltage unbalance
3. Transient voltage (surges)
4. Voltage waveform distortion (harmonics)

Voltage variation
NEMA MG1 Part 12.44 states that the motor shall operate successfully under running conditions with a varia­tion in the voltage up to plus or minus 10%. IEC 600034-1 allows for two dif­ferent zones. The first is a plus or minus 5% variation for continuous operation; the second is plus or minus 10% varia­tion for intermittent operation. The IEC Standard for continuous operation is half of the NEMA allowance and there­fore a 5% voltage variation will have a lesser effect on the motor performance than the NEMA allowance. Under the NEMA allowance, motor performance may vary, and it is possible for the motor heating to increase and the ef­ficiency to decrease depending on the motor magnetic circuit. 

The worst of these conditions is op­erating at under voltage. Some amount of overvoltage (less than 5%) may actually decrease the heating as the motor slip decreases and improves the efficiency at the expense of a decrease in power factor. Generally, conserva­tive designs (lower flux densities or higher permeability steels) are less affected by overvoltage, while higher flux density designs are able to handle under voltage conditions.

Operating at overvoltage can have an additional “side” effect that is not often considered. As the voltage increases above the nominal level, the slip of the rotor decreases by the square of the voltage. 

For many pumps, fans, blowers, and some compressors, the load increases as the cube of the speed in accordance with the affinity laws of physics. Figure 1 provides an example of this vari­able torque condition. The motor may operate at a higher efficiency level, but the cost to operate will increase significantly on systems that actively control the flow rate. 

The starting current, sometimes re­ferred to as the inrush current, is directly proportional to the voltage. Hence, as the voltage increases so does the starting cur­rent. The EPAct, NEMA Premium and IE3 motors tend to draw more current during starting than previous genera­tions of motors, so when operating at the allowable upper voltage limits it is more likely that the motor may trip off the line during starting. Under-voltage conditions will normally not cause a trip­ping problem unless the motor cannot accelerate to full speed. 

The starting torque is proportional to the square of the voltage. Hence there is a significant increase in accel­erating torque as line voltage increases. This is not normally a problem for over voltage conditions. However, it can be a serious problem for under voltage conditions, and the motor may not be able to come up to full speed depend­ing upon the load characteristics. 

There are a number of motors rated as tri-voltage (208-230/460)  motors. Simply stated, at 208 volts, these motors are always operating at an under-voltage condition and at the extreme allowable lower voltage limit. These motors will run at much higher than normal temperatures, reduced efficiency and will have a much lower thermal life expectancy on 208 volts. Since these effects vary from design to design and system-to-system, it is recommended to not operate near the allowed NEMA limits for an extended period of time. 

Unbalanced voltage
 Unbalanced voltage is another condition that is harmful to the motor and is defined as follows: 

When the line voltages applied to the motor are not equal, an unbal­anced current in the stator will exist. A small change in voltage unbalance will result in a much larger amount of current unbalance. Consequently, the temperature rise of the stator and rotor will increase significantly.

A good rule of thumb for the amount of increase in stator temperature is that the increase in tempera­ture rise is equal to two times the percent voltage unbalance squared.
Increase in temperature rise = 2 x (% Voltage Unbalance)^₂.

Hence, a 2% voltage unbalance will result in an 8% increase in winding tem­perature, and a 3% change will result in an 18% increase in winding tempera­ture. Keep in mind that for every 10° C increase in winding temperature, the theoretical thermal insulation life is cut in half. Hence if the total winding temperature was 120º C at a balanced voltage, a 2% unbalance would result in an increase in temperature of ap­proximately 10º C and thereby reducing the thermal life of the winding by half. 

The effect of operating with an unbalanced voltage is to create a negative sequence torque that has a direction of rotation opposite to the positive sequence three phase voltages. The negative effects results of unbal­anced voltage include increased stator current, increased winding tempera­ture, a significant increase in the rotor heating, in­creased motor slip, additional motor losses and a major reduction in the motor efficiency. 

The key point is that although NEMA allows for operating a motor with an un­balanced voltage if the load is reduced, there are serious consequences in doing so: mainly a major reduction in insulation thermal life of the winding, a reduction in the motor efficiency and an increase in cost to operate under these conditions. 

When using a de-rating factor (see Figure 2), care must also be given to the selection of current overload devices. According to NEMA MG1 Part 14.36, “this as 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 devices.”  Failure to address this issue can result in premature motor failure. 

Voltage surges or transients
A number of different conditions can result in harmful voltage surges being imposed on the stator winding; some of the most common sources are:

1. Line-to-line or line-to-ground faults
2. Repetitive re-strikes on unground­ed systems
3. Interruption of current limiting fuses
4. Rapid bus transfers (reclosures)
   See “Potential damage to motor that can result from reclosure” in the December 2013 issue of Currents.
5. Opening and closing circuit breakers
6. Capacitor switching
7. Insulation failures in the distribu­tion system
8. Lightning strikes
9. Variable frequency drives
10. Standing waves

When there is an expectation that the motor may be exposed to dam­aging surges, it may be wise to add surge protection as close to the motor terminals (within 3 feet/1 meters) as practical. In the case of adjustable frequency drives, output filters or line reactors between the drive and motor have proven to be very effective in suppressing the peak voltage gener­ated by the drive. 

The motor manufacturer can pro­vide the surge voltage limits that the winding was designed to withstand, and EASA service centers can normally duplicate or improve the insulation systems of the motor. Most manufac­tures of line filters or chokes (reactors) can recommend what size to use for a given motor application.

Voltage waveform distortion (harmonics)
It is beyond the scope of this article to discuss the impact of voltage distor­tions or harmonics that may exist on a specific application. However, when the motor is operated on a voltage waveform that has significant harmon­ic content, the motor will experience some increase in losses. In those cases where this condition exists, line filters can be added to minimize the effect of these types of harmonics. The recom­mendation is that harmonic distortion in excess of 5% at the service entry requires corrective action. 

Summary and conclusions
For optimum motor performance and life expectancy, the quality of the voltage power supply is critical. Efforts should be made to maintain balanced voltages as close to the nominal as practical, and with a minimum amount of harmonic distortion. It is recognized that NEMA and IEC allow the opera­tion of a motor over a wide range of voltage options but each of them may have a negative effect on optimum motor performance. And, yes, in some cases such as overvoltage, the motor performance may even be improved somewhat up to the point of magnetic saturation, but there is an additional cost of energy associated with oper­ating at the consequential increased speed for many applications.

Categories: AC motors, Design/theory/application

Tags: Voltage unbalance Voltage variation Voltage surges

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