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Motors larger than 1 horsepower must be provided separate motor overload protection devices. |
C The most common devices typically used include: |
1) magnetic or thermal overload devices 2) electronic overload relays 3) fuses |
Magnetic & Thermal Overloads |
Overload devices are usually located in the motor’s starter and connected in series with the motors electrical supply circuit and can be operated by either magnetic or thermal action. |
C The same amount of current passes through the overload relay and the motor. |
C If the current or heat through the overload device is higher than the device’s rating, it trips |
and shuts down the electric power to the motor. |
Magnetic Overload Relays |
A magnetic overload relay is an electro-mechanical relay operated by the current flow in a circuit. |
C When the level of current in the circuit reaches a preset value, the increased magnetic field |
opens a set of contacts. |
• Electromagnetic overload relays |
operate on the magnetic action of the load current flowing through a coil. |
C When the load current becomes |
too high, a plunger is pulled up into the coil interrupting the circuit. |
C The tripping current is adjusted by altering the initial position of the plunger with respect to |
the coil. |