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HVACR317 – Core Refrigeration

Motor Protection

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Overloads

• The overload is designed too:– Shut down the motor circuit due to heat or high

amperage draw.

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Motor Starter Types

• Part of the motor starter package (start components).

• The contactor has built in overloads.• The overload consists of:– Ratchet wheel– Solder– Heater– Spring

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Motor Starter Types

• Overload Operation:– The overload is activated by a heater wire.– Excessive current which produces excess heat,

melts the solder and the ratchet wheel opens the contacts.

• Heater is line duty – is in the main circuit.• Contacts are pilot duty, is in the control

circuit.

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4 Wire Overloads

• Consists of:– A current sensing coil– Contacts

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4 Wire Overloads

• Operation:– Coil senses amperage– Excessive amperage will open contacts.– Contacts are usually wired in series with motor

contactors.– Used mainly with three phase motors.

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Bimetal Overloads

• A bimetal strip opens and closes contacts.

• Commercial type:– Use a heater– The heater and contacts may be in series or

parallel.

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Bimetal Overloads

• Domestic Type– Mounts to compressor– May be bimetal only or bimetal with heater.

• Operation– Current flow and unit temperature combine to

open contacts.– Contacts automatically reset– Overload is line duty – directly in circuit.

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Sizing Overloads

• All are rated by amperage and horsepower.

• All have normal amperage draw of 115%-125% of full load amperage for a set amount of time.

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Newer Methods

• Solid State Thermistors– As heat is produced resistance changes and

changes the current flow to a micro processor.– Information is received by the microprocessor

and in turn shuts down the unit.