Protection equipment for three-phase induction motors includes the following:
Circuit breaker
It has overload and short circuit protection functions.
The former is a thermal function and has on-delay characteristics, while the latter is an electromagnetic function and has instantaneous characteristics.
Thermal relay
Changes bimetal with Joule heat of overload current, opens or closes the contact, and performs on-delay operation.
Comprehensive motor protection relay
This unit measures the current from all the three phases and checks for single-phase, unbalance and overload.
The measurement and comparison of these three factors provide protections against short circuit, single-phase, earth fault, phase sequence and thermal protection to the motors.
Temperature sensing thermistors are also embedded in the stator winding of HT motors during manufacture and connected to relays to monitor winding temperature and raise alarm when needed
Dry-run protection
In addition, dry-run protection is also provided by water level sensors in the sump, which sense any low level of water and prevent dry running, thereby protecting the pump and motor.