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Air ConditioningAir Conditioning

Starting RelaysStarting Relays

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Starting RelaysStarting Relays

Purpose• Remove the start winding

and/or start capacitor from the circuit

• Compressor start relays are mounted outside the motor

• Centrifugal switch could not be used in hermetic motor

Purpose• Remove the start winding

and/or start capacitor from the circuit

• Compressor start relays are mounted outside the motor

• Centrifugal switch could not be used in hermetic motor

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Types of RelaysTypes of Relays

• These are the three most common• The HP of the motor will

determine what type will be used• Service man may replace not

re-design.

• These are the three most common• The HP of the motor will

determine what type will be used• Service man may replace not

re-design.

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Three types

• Current Coil Relay• Solid State Relay or PTC• Potential Relay

Three types

• Current Coil Relay• Solid State Relay or PTC• Potential Relay

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Current Coil RelayCurrent Coil Relay• Works off amperage to the run winding.• On compressor start up:–Motor draws high current – Induces a strong magnetic field in the

relay’s magnetic coil through the run winding–This magnetic field lifts a plunger with

contacts and a counter tension spring–Contacts to start winding are made–Start winding is energized

• Works off amperage to the run winding.• On compressor start up:–Motor draws high current – Induces a strong magnetic field in the

relay’s magnetic coil through the run winding–This magnetic field lifts a plunger with

contacts and a counter tension spring–Contacts to start winding are made–Start winding is energized

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• Motor now starts because start windingcreates a strong magnetic field

• Motor reaches 75% running capability:• Current drops down• Relay coil is in series with run winding• Not enough current remains to keep coil

magnetized• Spring pushes plunger down• START WINDING IS DE-ENERGIZED

• Motor now starts because start windingcreates a strong magnetic field

• Motor reaches 75% running capability:• Current drops down• Relay coil is in series with run winding• Not enough current remains to keep coil

magnetized• Spring pushes plunger down• START WINDING IS DE-ENERGIZED

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• Used mainly on fractional horsepower compressors for small refrigeration units, not A/C

• Used mainly on fractional horsepower compressors for small refrigeration units, not A/C

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Solid State Relay Solid State Relay• Also works off current• Has materials that changes its

resistance• PTC-Positive Temperature Coefficient• Temp goes up/ resistance goes up• Wired in series with start winding only

• Also works off current• Has materials that changes its

resistance• PTC-Positive Temperature Coefficient• Temp goes up/ resistance goes up• Wired in series with start winding only

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• Uses up voltage (loads in series)• Example:–Voltage drop across relay would be 110v–Voltage drop across start winding 10v

• REMEMBER series law (Et=E1+E2…)

• Uses up voltage (loads in series)• Example:–Voltage drop across relay would be 110v–Voltage drop across start winding 10v

• REMEMBER series law (Et=E1+E2…)

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• Solid State relays have a greater range

• Example: 1 SS relay may be used for–1/16 to 3/4 Hp motor–3/4 to 2 HP motor

• If a current coil was used different relays would be needed.

• Solid State relays have a greater range

• Example: 1 SS relay may be used for–1/16 to 3/4 Hp motor–3/4 to 2 HP motor

• If a current coil was used different relays would be needed.

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Types of Solid State relaysTypes of Solid State relays

• Two wire is relay only• Five wire is start capacitor, relay

and overload• Testing:–When Ohmming them out, a good relay will have a (low) resistance reading–Must cool down before testing

• Two wire is relay only• Five wire is start capacitor, relay

and overload• Testing:–When Ohmming them out, a good relay will have a (low) resistance reading–Must cool down before testing

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Potential RelayPotential Relay

• Operates from voltage created by start winding• Also known as –Back Electromotive Force, or Back EMF

• Operation• Contacts are N.C. and allow full

voltage to start winding

• Operates from voltage created by start winding• Also known as –Back Electromotive Force, or Back EMF

• Operation• Contacts are N.C. and allow full

voltage to start winding

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• Terminals that are used and how to wire potential relay (this is the ONLY way)–Line to start capacitor–Start capacitor to TERM #1–TERM #2 to start winding–TERM #5 to common terminal

• All other terminals are blanks

• Terminals that are used and how to wire potential relay (this is the ONLY way)–Line to start capacitor–Start capacitor to TERM #1–TERM #2 to start winding–TERM #5 to common terminal

• All other terminals are blanks

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Checking the Potential relay with OhmmeterChecking the Potential relay with Ohmmeter

–Term #1 to Term #2•Continuity

–Term #1 to Term #5•Ohm Reading•Ex. 2k ohms

• The same for Term #2 to Term #5

–Term #1 to Term #2•Continuity

–Term #1 to Term #5•Ohm Reading•Ex. 2k ohms

• The same for Term #2 to Term #5

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