Motors should have protection for themselves, the branch circuit, and the feeder line. Other protection, provided by fuses and circuit breakers, guards against fault conditions caused by short circuits or grounds and overcurrents exceeding locked-rotor values.
Selecting motor protectors: Line current to the motor and internal motor temperature determine motor protection. Many protectors respond to only one of these parameters. Others are both current and temperature responsive. However, use of one protector of each is not uncommon.
Current-responsive protectors are normally located in the motor or between the motor controller and the motor. In fhp and small ihp motors, temperature-responsive protectors are located within the motor and include contacts to interrupt the motor circuit. For larger motors, pilot-circuit protectors located inside the motor open the holding coil circuit of the motor-controller contactor.
Temperature-responsive protectors are assembled as integral parts of motors in order to protect against dangerous overheating from overload or failure to start. This thermal protection is provided by line-break devices or by control-circuit systems.
There are several suitable methods for motor protection. Current-responsive protectors provide safeguards against common causes of overload where the line current increases appreciably. However, they do not respond to overtemperature caused by hot ambient conditions or blocked ventilation. On the other hand, temperature-responsive devices protect against running overloads that produce a gradual increase in winding temperature.
Line-break thermal protectors are prevalent in smaller motors, while control-circuit systems are common on larger motors. Thermistor systems provide complete protection with small sensors in the motor. In many high-slip induction motors, the critical temperature occurs in the rotor. Temperature measurement requires slip rings and brushes, making current relays a better choice. Special motors may be designed for continuous locked-rotor current, but are still susceptible to burnout from blocked ventilation. Temperature-responsive devices are best used here.