Commonly misunderstood/misapplied terms, part 12.
The thermal protector was, for a long time, the only motor control device that provided some thermal protection; thus, its name. When subjected to sustained high current, but not high enough to trip the breaker, the protector (typically a bimetal strip) would open. A common cause of sustained high current is excess load (e.g., a gearbox with dirty oil). Low voltage is another cause.
While thermal protector devices are effective in countering excessive motor heat due to this "under the trip threshold" current problem, they don't completely protect the motor from excess heat. Here are some reasons why they don't.
- Due to huge differences in mass, the strip cools long before the motor does. So you can keep restarting the motor until you burn it up. That's the reason for administrative policies that require specific minimum times between starts.
- A small amount of voltage imbalance causes a large amount of motor heating. The bimetal strip won't protect against this kind of heating.
- Poor ventilation can result in a higher ambient temperature than the motor can withstand over time. The motor does a slow bake and eventually fails.
