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What is the difference between Relay and Contactor
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Answer Posted / wleob
1) All of the previous answers are mostly correct.
2) In general, a relay and a contactor have at least the
same two basic parts:
a. A coil that makes the contacts close or open. The coil,
whether energized by AC or DC always uses much less current
(amps) to operate than the current, AC or DC that the
contacts can carry. Coil ratings can be from 12 VDC to 600
VAC, for example.
b. Contacts, rated DC or AC that open or close a circuit to
a load (resistance or inductance) which demands more current
(amps) than the coil does. Relay contacts are usually
applied to lower amperage circuits and Contactor / Motor
Starter contacts usually carry higher voltages and amperage.
3) AC and DC coils are built differently. AC coils have
steel plates to break up the eddy currents inherent in AC.
As AC goes "through zero" 60 times per second, no
discharging diode is required. The rating on the coil will
state whether it is for 60 Hertz (cycles per second) , 50
Hertz (CPS) or 60/50 Hertz, meaning it can be used in the
USA or Canada. DC coils usually have a diode and/or a
capacitor for the inherent discharge of the coil when the
voltage is removed. The current going through the coil must
be discharged as the field in the DC coil collapses.
4) AC contacts in low power applications don't require any
special arc chutes as AC, again goes through zero 60 times
per second (50 times per second in Canada). DC contacts in
low power applications do not require arc chutes. However,
as the current being carried gets larger and if the load is
not pure resistance, the discharge power must be dissipated
or "blown out" in an arc chute.
5) In summary, relays are usually applied to control
circuits and in lower power applications while contactors
usually imply that they are controlling larger loads with
higher current (amp) requirements. Industrial relays
typically can carry 5, 10 or 20 amps as do the relays in
your cars fuse box for your headlights, blower motor, or
your trailers electric brakes. However, though called a
starter "relay" it is contactor that will carry at least 100
amps to run your starter motor. AC Motor "starters"
/contactors typically have 120 VAC coils or 460 VAC coils
and can run motors that require from 5 amps to 100s of amps.
Hope this helped,
WLeoB
BSEE '71
Canton, MI
| Is This Answer Correct ? | 7 Yes | 2 No |
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