1. Chapter 4
Overload Relays

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

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

4. 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

5. 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

6. 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

9. 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

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

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

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

17. 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

20. Protecting Large Horsepower Motors