Operators often cause motors to overheat. If paid by the unit, operators often overload the machine. The mining industry is a case in point.
An overloaded induction motor can be quickly checked with a tachometer. Compare the tachometer reading with the motor’s nameplate.
FIGURE 6.40 Two lead lugs that can cause motor failure.
Just a few RPM slower than the nameplate RPM will cause overheating. The design A (or a high-efficiency) motor will run hotter than a standard induction motor under this condition. This can be seen in the torque curve that the design A motor develops near its full-load speed (Fig. 6.41).
Frequent starting and stopping often causes excessive heating in motors above 25 horsepower. The physical mass of a large motor doesn’t dissipate heat easily.
Most motors will cool better while running with no load than when shut off. If a motor is required to start often forced ventilation or the use of a clutch should be considered if motor failure is frequent.
An abrupt speed change is harmful to all motors, but especially to brushtype (DC and wound-rotor three-phase) motors. The design D induction motor should be considered if its torque curve is compatible with the load requirement.
with the load requirement.
FIGURE 6.41 The design letter table used for choosing a motor to fit the load’s torque demands.
Duty-cycle loading requirements (that cause overheating) can be relieved by forced ventilation. Some conditions helped by forced ventilation are overheating from prolonged slow speed at maximum torque, frequent startstop cycle, and a duty cycle that includes overloading for a short time before shutdown. Operating a motor for a time with no load will cool the winding and core uniformly. Running with no load on reduced voltage is an even better way to cool a motor.
The part-winding start method (of starting a motor) isn’t recommended for frequent starting. Part of the winding (one-half or two-thirds) starts the load and will heat excessively when started frequently. Switching from part winding to full winding too slowly (over 3 seconds) will also overheat the motor
If the motor has concentric-wound coils, it won’t start on part of its winding. Figure 6.42 compares the concentric- and lap-shaped coils. This motor needs a special internal connection before it can be used on a part-winding start control. Many motors (up to 300 horsepower) use
FIGURE 6.42 A motor with concentric-shaped coils won’t start using the partwinding start method. However, it will start with lap-shaped coils.
the concentric type of winding. When an old part-winding start motor is replaced, the new motor must be designed for this starting method.
A poorly designed variable-hertz control can cause a motor to overheat. (Voltage should change with the hertz change.) Not all motors are suited for variable-hertz drives. In many cases, older T-frame motors have not worked as well as high-efficiency motors or U-frame motors on variable hertz.
If a motor is labeled inverter duty (variable-hertz duty), it doesn’t mean the motor is designed for slow speed at maximum torque or any other extreme deviation from the motor’s speed. Inverter duty means the motor’s coil wire has special insulation. This insulation postpones breakdown from voltage spikes caused by abrupt voltage and current changes. It does not prevent breakdown. Reactors, filters, or surge capacitors should be installed to reduce this problem.
Heat Developing from Reduced-Power Starting
The amount of internal heat a motor accumulates in across-the-line starting compares to that produced in the wye-delta or reduced-voltage starting methods. Any reduced-power starting method that applies all the motor’s winding is better for the motor than a part-winding start method.
Reducing the motor’s starting amperes lessens the amount of voltage drop caused by across-the-line starting. In some cases, reduced-power starting is done to protect a load that is sensitive to quick starts.
If a load that requires frequent starting causes the motor to heat excessively, forced ventilation will lengthen a motor’s insulation life. Ventilation should continue for a time after the motor is offline. (The amount of time required depends on the motor’s size, etc.)
For maximum motor life, it’s best to start a large motor with no load. (A clutch can be used to apply the load after the motor has reached full speed.)