Relay selection

[alexdouglas1969]

When is a potential relay used?
When is a current relay used?

[Cofreth]

When is a potential relay used?
When is a current relay used?

1. Potential relay is use to disconnect the start capacitor after a motor get started.(Motor design with a start & run capacitor)

2. A current relay is use to disconnect the short together between start & run windings after a motor get started.(Motor design without a capacitor)

[Electrocoolman]

Hi Alex,
You will see several different single phase compressor motor types.
RSIR (Resistive[or Reactance] Start Inductive Run) ...this uses a CURRENT relay.

CSIR (Capacitor Start Inductive Run) ....this uses a CURRENT relay.

CSR (Capacitor Start [ and Capacitor] Run) .....this uses a POTENTIAL relay.

PSC (Permanent Split Capacitor)...no relay required

A Current relay switches ON due to the high current through the Run winding and relay when the motor is starting....as the motor speeds up the current reduces and the relay 'drops out' disconnecting the start winding or capacitor.

A Potential (or Voltage relay) operates as the induced voltage on the start winding increases as the motor speeds up. At a couple of hundred volts the relay switches the start capacitor out of circuit.

[monkey spanners]

When took to bits , a current relay has thick wire wound a few time around an armature. As the motor current passes through it, the high start current causes the relay to pull in powering the start winding, as the current drops as the motor reaches speed, the relay opens again.
The potential (voltage) relay has a thin wire wound many times around an armature. As the motor reaches speed the voltage increases on the start winding causing the relay to disconnect the start capacitors from the start winding.
Current relay = on only during starting
Potential relay = on all the time once motor has started

Cheers Jon

[alexdouglas1969]

Thanks for the help!!

On some equipment I have seen some little 'black boxes', I assume relays, on the PCB in addition to the main compressor relay. Presumably these are for other motors, i.e fan motors ?

Or are relays of some kind used to switch the control circuit?

Thanks again

Alex

[Quality]

it is worth mentioning that the limitation of a current relay is down to the run winding current drawn through it