How does the magnetic overload relay operate?
A solenoid connected in series with the supply contains a plunger whose movement is damped by a dashpot. When the safe current is exceeded, the solenoid pulls the plunger up – disconnecting the supply. The damping provided by the dashpot prevents unwarranted tripping on short-time overloads. 7. How many overload relays are required in the control gear? On three-phase supplies where the neutral point of the system is connected to earth, as is usually the case, three overload relays (one in each line) are necessary for complete protection. For 2-phase 3-wire and 4-wire supplies, two overload relays are required, one in each phase line, none being connected in any neutral or earth conductor. With single-phase motors one overload relay in any conductor except an earthed conductor or neutral. 8. What happens when one of the three lines supplying a three-phase induction motor becomes open-circuited? The motor, if already running, will continue to run as a single-phase motor on the remaining
