Cool Advice On Hot Motors

Increased operating temperatures damage motors. Protection starts at the source.

By Jim Bryan, Electrical Apparatus Service Association (EASA)

The effects of excessive temperature on motor performance are notorious. After moisture, they are the greatest contributor to bearing and winding failures. Understanding the source of increased temperature is key to correcting the problem and improving the reliability of your facility’s motor fleet.

Figure 1 illustrates the theoretical impact of increased operating temperature on motor-insulation systems. The chart addresses thermal aging and not other conditions that affect motor life. In effect, this graphic indicates that every 10 degree C increase in operating temperature cuts insulation life expectancy by half.

This illustration of the theoretical impact of increased operating temperature on motor insulation systems addresses only thermal aging. As shown, every 10 degree C increase in operating temperature cuts insulation life expectancy by half. Conversely, a decrease of 10 degrees C could double insulation life expectancy.

Conversely, the chart in Fig. 1 also shows that decreasing the temperature by 10 degrees C could double insulation life expectancy. While this is true anywhere on the curve, at some point the rule of diminishing returns dictates that the cost of building and operating a cooler-running motor will outweigh the benefits. This article focuses on several factors that contribute to increased temperature and what to do about them.

Overload and service factor

Overload is a common culprit in temperature problems. Due to load variations in the driven equipment, the condition is sometimes intermittent. At other times, the designer has chosen to let the motor operate above the rated load—which is permissible if the service factor is greater than 1.0.

The NEMA Std. MG 1-2011 definition of service factor says a motor is thermally capable of overload to that point within the insulation class at normal service conditions (rated voltage and frequency). Of course, any overload will increase the operating temperature. Most motors are designed to be most efficient—run cooler and consume less power for the same job—at about 75% of rated load.

Fig. 2. Click to enlarge.

The insulation class determines the maximum allowable operating temperatures that yield “normal” service life (see Fig. 2). If a motor consistently fails prematurely in an application, and little can be done to mitigate the temperature, one solution may be to rewind it with a higher-temperature-class insulation system. Don’t forget the bearings in this attempt. The lubricant is the limiting factor in temperature-related bearing problems, so be sure it will work in your operating environment.

Pulse-width modulated (PWM) adjustable-speed drives (ASD) produce relatively low negative-sequence currents that essentially add load to the motor by trying to make it run in the opposite direction. These negative-sequence currents also greatly increase rotor temperature. A properly designed inverter-duty motor will compensate for this.

Ventilation

Motor designs include a system for dissipating the heat produced by the winding and bearings. Often called the “cooling circuit,” this system can be affected by such factors as fan diameter, shaft speed, the presence and location of air ducts and deflectors, and altitude.

Fan. The amount of air that a fan provides varies as the cube of its diameter and is directly proportional to the speed. In a totally enclosed fan-cooled (TEFC) motor, the fan is often the main source of objectionable noise. The designer must make sure it provides sufficient cooling air without creating too much noise.

In larger open motors, air ducts distribute the cooling air through the rotor and stator cores to improve cooling efficiency. Deflectors may be used in open or enclosed motors to direct the air to locations that need it and to reduce turbulence.

Air ducts and deflectors. In larger open motors, air ducts distribute the cooling air through the rotor and stator cores to improve cooling efficiency (Fig. 3). Deflectors may be used in open or enclosed motors to direct the air to locations that need it and reduce turbulence. Turbulent air doesn’t cool efficiently, so the location of deflectors can be critical for optimizing the cooling circuit (Fig. 4). Clogged ducts or missing or incorrect air deflectors could make the motor run hotter.

Turbulent air doesn’t cool efficiently, so the location of deflectors can be critical for optimizing a motor’s cooling circuit. Clogged ducts or missing or incorrect air deflectors could make a unit run hotter.

Operating temperature. A motor doing a given amount of work will produce a level of temperature increase called temperature rise. This, plus the ambient temperature, equals the motor’s operating temperature:

Temperature rise + Ambient temperature = Operating temperature

Because ambient temperature directly affects operating temperature, NEMA motors list the maximum allowable ambient temperature on the nameplate. By contrast, IEC 60034-1, 5.3 limits the maximum ambient for IEC motors to 40 C. For these motors, the design-temperature rise at rated load plus this maximum ambient should not exceed the temperature-class rating of the motor’s insulation system.

If a motor is located outdoors, not only is the ambient temperature subject to change, factors such as sunshine come into play. For example, repainting gray pump motors white at an open-pit mine in the Sonoran Desert reduced operating temperatures 10 to 15 C. Building a structure to shade the motors produced a similar result.

Altitude. As altitude increases, air gets thinner, reducing its ability to carry heat away from the motor. If a motor is to be operated at an altitude above 1,000 m (3,300 ft.), its design should be adjusted to accommodate the less-efficient cooling that results.

Voltage variation

Motors are designed to perform optimally when the applied voltage equals the nameplate rated voltage. NEMA Std. MG 1-2011 requires motors to be capable of starting and operating at the rated voltage ±10%; IEC requires ±5%. Although both standards include a frequency tolerance that affects voltage tolerance, for purposes of this discussion, consider the frequency variation to be zero.

The NEMA standard also says motor performance may be affected by voltage variation. Playing on the NEMA requirement that motors be able to operate successfully at ±10% of rated voltage
(230 V – 10% = 207 V), some manufacturers will indicate that their 230/460-V designs are “Suitable for Use on 208 V.” If the 208-V voltage supply varies, however, there’s no margin for the motor, and its performance may suffer. Unless the nameplate or some other communication from the manufacturer indicates “Suitable for Use” on an alternate supply voltage, it’s not a good idea to use your motors in such a manner. That’s because the equipment would produce less torque and higher full-load amps, while running hotter.

Under-voltage. Under-voltage results in higher amperage being required to produce the needed power or work. Ohm’s Law states that P = IE (where P = power, I = current, and E = voltage). If E decreases and P is constant, then I must increase. Since the heat produced varies as the square of the current, this additional current increases resistive losses in the motor winding and, therefore, raises the operating temperature.

Over-voltage. The converse is also true. If E goes up, I will decrease when P is constant. This is one reason motors are designed with a rated voltage of 460 V when the nominal voltage applied is 480 V. The higher voltage helps the motor to run cooler.

The slip of induction motors, however, is inversely proportional to the applied voltage. The higher the voltage, the lower the slip and the faster the motor (and fan) will turn. As noted earlier, the fan will move more air at higher speeds, so more power is required to turn it. This could have an impact on motor current as much as or more than over voltage, offsetting a portion of the decrease in motor current.

Apply this principle carefully, though, because the magnetic flux produced in the core iron also increases with a higher voltage until it reaches the saturation point (maximum flux/cross-sectional area) for that grade of electrical steel. If the voltage is increased beyond the saturation point, additional flux is possible only with a disproportionately large increase in current. This generates more heat, as is discussed in the following electrical steel section.

Unbalanced voltage. Unbalanced voltage in a three-phase motor supply will also result in high temperatures, particularly in the phase that has the highest voltage applied. NEMA Std. MG 1-2011 calculates voltage unbalance as:

% Unbalance = Maximum deviation from average/Average

This equation is used to calculate the average voltage and current. NEMA Std. MG 1-2011 says the percentage of current unbalance may be 6 to10 times that of the voltage unbalance, so accurate voltage measurements (within 0.5%) for all three phases are important. Further, with any voltage unbalance greater than 1%, the rated load should be reduced due to the additional heating.

Electrical steel

Several factors determine the ability of electrical steel to transmit flux, including its thickness, quantity, and type or grade. These properties are defined for the various grades. Some modern grades are capable of handling higher flux levels, which is one reason why higher horsepower ratings can be developed in smaller frame sizes. Of course, these grades are more expensive, which always figures into the design equation.

AC-motor cores are constructed by laminating specially insulated electrical steel tightly together. A core’s length and diameter determine its quantity or volume.

The thickness of each lamination is important in controlling eddy currents, (circulating loop currents induced within the steel by the changing magnetic field), in that piece and in the entire core. The thinner the lamination, the smaller the circulating loops and the lower the current. Eddy currents don’t contribute to work done by the motor, they just produce heat (losses).

The interlaminar insulation also helps control eddy currents. If the insulation is damaged, eddy currents can cross to adjacent laminations and increase in size, causing the magnetizing (no-load) current to increase. Core-loss testing can reveal this increase and indicate whether there is a need to repair or replace the bad steel.

Current density

Another derivative of Ohm’s Law says P = I²R (where I = current and R = resistance). In the case of a motor winding, P = wasted power (losses).

As wire size decreases, the resistance per foot increases so, for a given current, a smaller wire produces higher P (losses). Since these losses are manifested as heat, in an AC motor it is always best to use the largest total cross section of wire per turn that comfortably fills the slot.

The resin used in the winding process also conducts heat better than does air, but it won’t bind the wires together effectively if the stator slot is less than about 45% full. The resulting voids will cause higher operating temperatures.

When designing a motor, one trade-off involves the number of turns per coil versus the pitch of the winding. The pitch of the motor winding is the number of winding slots in the stator core that are spanned by the wire coil.

Generally, as the pitch is increased (up to and including full pitch minus one slot), the number of turns may be reduced. With fewer turns in each coil, a larger cross section of wire per turn is possible.

The trade-off here is the length of the end turns, especially in two-pole motors. As the coil spans more slots, the width of the coil increases, making the coil and the end turns longer. This increases the coil resistance and thereby increases the losses. Also, the wider the pitch, the more difficult the winding process becomes. To optimize the design, it’s important to use the longest pitch practical, while keeping in mind windability and total length of turn.

Circulating currents

Circulating currents are produced in the winding under certain conditions. These don’t contribute to the work being done by the motor; they are losses that produce additional heat.

Beware of coil groups in parallel that don’t contain the same number of turns. Circulating currents will produce high temperatures in circuits with fewer turns or coils. In the case of odd grouping (where the number of slots per phase isn’t equally divisible by the number of poles) the uneven number of coils must be distributed equally through all phases. Rewinders should count the total number of coils in each phase to confirm they are the same.

Uneven coils. Beware of coil groups in parallel that don’t contain the same number of turns (Fig. 5). Circulating currents will produce high temperatures in circuits with fewer turns or coils. In the case of odd grouping (where the number of slots per phase isn’t equally divisible by the number of poles) the uneven number of coils must be distributed equally through all phases. When rewinding, the service center should count the total number of coils in each phase to confirm they are the same.

Two-speed, two-winding motors. Two-speed, two-winding motors can also produce circulating currents. If one or both windings are connected delta or multiple parallel wye circuits, a closed circuit will be present when that winding is not energized. A special connection with four leads can open this circuit on motors connected with a one-delta circuit. For this approach to be effective, the motor starter must have four contacts rather than three. Energizing the other winding will induce a voltage in the un-energized winding, and the closed circuit may allow current flow. This unintended current flow produces additional heat in the motor. For this reason, it’s always advisable to use a one-wye connection, since it does not have this type of closed circuit. Where this isn’t possible, a reputable service center can help identify the connections with the highest probability of success.

The odd harmonics of the fundamental AC waveform (except multiples of three) will produce negative torques when the rotor speed is above the synchronous speed for that harmonic. By opposing a motor’s fundamental torque, negative torques add load and increase heat. As shown here with effects of the 5th and 7th harmonics, total harmonic distortion (THD), expressed as a percentage, can be measured with a power quality analyzer.

Harmonics

The odd harmonics of the fundamental AC waveform (except multiples of three) will produce negative torques when the rotor speed is above the synchronous speed for that harmonic. Negative torques oppose a motor’s fundamental torque, adding load and, thus, increasing heat. Figure 6 shows the effects of the 5th and 7th harmonics on the fundamental waveform. Such effects can be measured using a power-quality analyzer to find the total harmonic distortion (THD), expressed as a percentage. IEEE 519 states that THD should not exceed 5% at the point of common coupling (the facility service entrance).

These harmonics are produced by non-resistive loads supplied by the same power feeder as the motor. Motors themselves are a source of harmonics since they are mostly inductive loads. Ballasts, rectifiers, and power-factor-correction capacitors are a few examples of other sources.

The higher the motor operating temperature, the shorter its expected life. Anything that can be done to lower the temperature, whether it be improving the ventilation or optimizing the design, will provide better service life and reliability in your operation’s motor fleet. MT

Jim Bryan is a technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis. EASA is an international trade association of more than 1,900 firms in 62 countries that sell and service electrical, electronic, and mechanical apparatus.

To learn more, visit
— easa.com
— motorsmatter.org
— ptda.org

Note: This article was updated August 21, 2015.