8.3
The ultimate goal of motor selection is to find the most cost-effective motor that will meet the performance and service life objectives of the application. This must be done while keeping the motor from exceeding its thermal limits. Motors for many applications are selected by the methods described previously, and while the average losses cause heating that does not exceed the insulation class of the motor, yet failures from apparent overheating occur. Typically, these motors exhibit charred winding insulation with shorted turns. However, the rest of the motor does not appear to have seen excessive heating.
In many cases, the failure occurs when the motor sees high currents for short durations, such as on acceleration, deceleration, plug reversal, or momentary overloads. When the current densities are calculated under these conditions, they exceed the range of the normal running conditions, which are shown in Table 8.3. Keep in mind that these current densities are guidelines and may not be appropriate in all cases and that motor temperature must be kept within the insulation class of the motor. These densities are commonly exceeded, but the wire insulation grade must be capable of handling the heating.
TABLE 8.3 Current Densities
| Motor type | Density range, A/in2 |
| Enclosed | 2000-4000 |
| Ventilated | 6000-8000 |
These high currents force the electrons to the outer surface of the conductor, causing localized excessive heating at the magnet wire insulation and copper junction. This results in insulation degradation and failure. The insulation in these spots is likely to flake off, resulting in shorted turns.
The solution to this problem is to reduce the current density by increasing the diameter of the magnet wire being used. There are times when the motor’s slot fill is already too high to allow for a larger wire. In this case, the higher-temperature magnet-wire insulation may be adjusted to solve the problem.
