Chapter 4 Overload Relays

Objectives Discuss differences between fuses and overloads List different types of overload relays Describe how thermal overload relays operate

Objectives (cont’d.) Describe how magnetic overload relays operate Describe how dashpot overload relays operate

Overloads Designed to protect the motor from an overload condition Must have some: Means of sensing motor current Type of time delay Divided into two separate sections: Current sensing & Contact

Dual Element Fuses Provide short circuit and overload protection Fuse link designed to open quickly under excessive current Solder link connected to a spring designed to melt at a particular temperature

Thermal Overload Relays Operate by connecting a heater in series with the motor Amount of heat produced depends on motor current Two types: Solder melting type or solder pot Bimetal strip type

Thermal Overload Relays (cont’d.) Bimetal strip overload: Operates by converting motor current into heat Heat causes a bimetal strip to bend or warp Warp amount is determined by: Type of metals used to construct the bimetal strip Difference in temperature between the two ends of the strip Length of the strip

Thermal Overload Relays (cont’d.) Three-phase motors must have an overload sensor (heaters or magnetic coils) in each line

Magnetic Overload Relays Sense strength of the magnetic field produced by current flowing to the motor Not sensitive to ambient temperature

Magnetic Overload Relays (cont’d.) Dashpot overload relays use a time delay device that permits the motor to start

Overload Contacts All overload relays contain a set of normally closed contacts Some add a set of normally open contacts Two sets of contacts are either a single-pole double-throw switch or two separate contacts

Protecting Large Horsepower Motors