Shorted turns in a winding will cause overheating. When just a few coil turns are shorted, they form a closed loop. A circulating current is transformed into the loop. The current is usually high enough to melt a wire, opening the closed loop.
Before this happens, a pole forms from the circulating current in the shorted turns. This pole doesn’t conform to the surrounding poles. Its magnetism causes a ringing sound, unique to motors with shorted turns.
Shorted turns may not slow the motor at all. A line-to-line ampere comparison will show a substantial difference between motor leads, and will identify the problem as shorted turns.
Ground in Winding
If a winding is grounded (shorted) to the stator or frame, it will get hot (from the increased ampere flow). This condition usually causes a fuse or breaker to open. If there are enough coils in the motor’s circuitry between the line and the ground, the motor will still run, but with increased (and unbalanced) amperes.
Worn Bearings and Uneven Air Gap
Worn sleeve bearings cause overheating. A three-phase motor’s torque is so smooth that it may be necessary to move the shaft to detect a worn sleeve bearing. It’s common for a rotor to drag on the stator before the problem is detected.
Uneven air gap from worn sleeve bearings will cause internal heating in some motors. Motors that use an internal circuit connection (that balances the current path through the poles) are less affected by uneven air gap.
Although uneven air gap should be avoided, it won’t cause immediate damage to a motor’s winding. In the past, most three-phase motors had sleeve bearings. Many ran for years with worn bearings and an uneven air gap—with no electrical problems.
Wrong Service Factor
The service factor (found on the motor’s nameplate) is the amount of overload a motor can handle (without overheating) for a limited time. The service factor number is a multiplier. The multiplier number times the motor’s nameplate amperes is the amount of overload a motor can handle. Service factor numbers are 1, 1.15, and 1.2.
A high service factor usually indicates a well-designed motor. A totally enclosed motor will have a service factor of 1, meaning it can’t be loaded higher than nameplate amperes.
Many air compressors are deliberately designed to use the service factor.
(They are in a highly competitive market.)
In one case, an air compressor had a 75-horsepower motor with a 1.2 service factor. The compressor was designed to use the full 1.2 service factor value. (When a compressor operates normally, it will cycle for a time unloaded so the motor has time to cool down.) Unfortunately, this motor never ran unloaded because the air volume demand was so high. The motor failed after a few months.
Connected for the Wrong Voltage
If a dual-voltage motor connected for high voltage is connected to low voltage, it will produce only one-fourth of its rated horsepower. It will start much more slowly than normal. Some loads allow this motor to reach nearly normal speed. If an induction motor’s RPM is below its nameplate rating, the high slip will cause it to overheat.
If a dual-voltage motor connected for low voltage is connected to high voltage, the results with any type of load are immediate. The motor develops many times its normal starting torque, and it draws so many amperes that its winding is destroyed in a matter of seconds. NEMA standards allow ± 10 percent of nameplate voltage. If a fully loaded motor, rated for 220 volts, is connected to 250 volts, it will run hotter than normal (a 12 percent difference). Motors with frequent start cycles will have extreme overheating problems when voltage is this high.
A motor rated for 208 volts but connected to 250 volts will overheat without a load. (The connection is 20 percent over its rated voltage.)
A motor rated for 250 volts but connected to 208 volts can’t pull its rated load. It may not start a load requiring high breakaway torque. The motor will work if the load is reduced. (A tachometer should be used to make sure the RPM isn’t below the nameplate rating.) Any departure from rated voltage greater than + 10 percent will result in extra heat.
Motors designed for 50 Hz power most of the machinery manufactured in Europe. Problems can occur when this machinery is used in the United
States on 60 Hz. A four-pole 50-Hz motor runs 300 RPM faster on 60 Hz. The motor will be overloaded if its load is air or liquid. Conveyer belts and augers will also overload this motor. (Changing the pulley dimension ratio solves the problem for some applications.) Direct-driven loads require major redesigning or replacement.
In one case, the power for an entire facility was converted to 50 Hz, because so much of the equipment used 50 Hz. When failed motors were replaced with 60-Hz motors, they ran hotter than normal on 50 Hz. (The 60-Hz motors have fewer turns per pole than 50-Hz motors.) If the supply voltage is lowered for the 60-Hz motors, they won’t run as hot, but power output is lessened.
Internal Motor Problems
Internal motor problems can cause overheating. The problems that follow were covered earlier, under “Assorted Rotor Problems,” but are reviewed here briefly.
Rotor/Stator Alignment If the rotor and stator iron aren’t aligned properly, the result is high amperes (loaded or no load) and loss of power. This problem can’t be detected with an ohmmeter, or limited current and turning of the shaft.
Open Rotor Bars Open rotor bars cause power loss. With a normal load, the rotor will run more slowly than the nameplate RPM, resulting in high amperes in both the rotor and the stator windings. (Too much slip increases rotor hertz, which causes higher amperes.)
Cracked End Ring Cracked end rings cause uneven torque and loss of power. The result is similar to that of open rotor bars.
Air Gap Too Large If the rotor becomes “out of round,” it may drag on the stator core. This condition is corrected by skimming some of the iron off the rotor with a lathe.
Skimming the rotor increases the air gap. Air gap should be kept at a minimum because it’s a break in the magnetic circuit. A large air gap creates a large increase in the motor’s magnetizing amperes. The motor will run hotter, and there will be a slight power and efficiency loss.
Whether to skim the rotor should be decided on a case-by-case basis. If the motor has more power than needed or its duty (frequent starts, etc.) doesn’t cause above-normal heating, skimming the rotor does no harm.
A large air gap can be similar to a misaligned rotor. (It can’t be detected with an ohmmeter, or limited current and turning of the shaft.)