chapter 4 overload relays. objectives discuss differences between fuses and overloads list different...
TRANSCRIPT
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