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