Form A and Form B relay

[jason619]

Form A relay is single pole single throw with a default state of normally open.

Form B relay is single pole single throw with a default state of closed open.

So can we say that Form B relay can be equivalent of Form A relay when the coil of Form A relay is energized?

i mean whts the main difference between the two other than their contacts being open/closed in their default state.
Say if we have to control a motor ( i.e start/stop) and we have to select between Form A and Form B relay.
we can use both of these relays for the start/stop purpose...but my concern is whts their main difference.

Thanks

 

[GoldDigger]

The practical difference is that in one case the contacts are held together by spring force and in the other they are held together by the coil/armature force. Makes a difference for weak coil drive.

 

[Jraef]

Actually, the difference is simply the state of the contacts in the DE-energized state of the coil. So can you use a Form B contact as a seal-in contact on a motor starter with a momentary start/stop push button? No.

 

[gar]

150301-2128 EST

jason619:

Very big difference for safety or security. In a given circuit ask what you want to happen when power to the relay coil is lost?

As an example consider a permanent magnetic DC motor on a hoist. Connrected to the motor is a spring loaded brake. On loss of power to the brake the brake goes into brake mode. Also there is a normally closed relay across the motor armature. On loss of power the relay contacts close and short the motor armature. Suppose the brake does not engage, then there is still some dynamic braking from the shorted armature that at least slows down the falling rate of the hoist's load.

You can find many similar needs for a normally closed contact.

.

 

[gadfly56]

150301-2128 EST

jason619:

Very big difference for safety or security. In a given circuit ask what you want to happen when power to the relay coil is lost?

As an example consider a permanent magnetic DC motor on a hoist. Connrected to the motor is a spring loaded brake. On loss of power to the brake the brake goes into brake mode. Also there is a normally closed relay across the motor armature. On loss of power the relay contacts close and short the motor armature. Suppose the brake does not engage, then there is still some dynamic braking from the shorted armature that at least slows down the falling rate of the hoist's load.

You can find many similar needs for a normally closed contact.

.

Great example, gar.

 

[jaggedben]

If safety or functionality in power loss isn't a primary issue then you consider the energy use of the coil. The 'normal' position should whatever position the relay will be in most of the time, i.e when the device is idle. Especially if the the relay will spend long periods in one position and only rarely go to the other position when used.

 

[tom baker]

Another example is to fail safe a circuit. Take an overlflow switch on a water tank. If the switch closes the circuit, it may not work due to corrosion on the connections. But if the switch is normally energized, it opens to send a alarm. If the connections are corroded, then a bad connection will cause an alarm. Its a matter of determining the safest way for the relay or switch to operate.

 

[Jraef]

If safety or functionality in power loss isn't a primary issue ...

True, but those conditions are rare. Lighting relays are one example I can think of, because if the lights are On more time than they are Off, you might be better off using Form B contacts and ENERGIZE the coil in order to turn the lights off. But other than that, there are very few applications where safety and/or primary state is of no concern.

 

[JoeStillman]

Form A relay is single pole single throw with a default state of normally open.

Form B relay is single pole single throw with a default state of closed open.

So can we say that Form B relay can be equivalent of Form A relay when the coil of Form A relay is energized?

i mean whts the main difference between the two other than their contacts being open/closed in their default state.
Say if we have to control a motor ( i.e start/stop) and we have to select between Form A and Form B relay.
we can use both of these relays for the start/stop purpose...but my concern is whts their main difference.

Thanks

Physically there is very little difference, only the state of the contacts in the de-energized state.

For a typical 3-wire stop-start control circuit, you would use a type "A" auxiliary contact on the motor contactor. If it's a type "A" aux relay instead, the coil is in parallel with the contactor coil, and the NO contacts are in parallel with the momentary-contact pushbutton. It looks like this;


Traditionally, the sense of a contact, normally open or normally closed, is defined by whatever state the relay is in when it's coil is not energized. they are always drawn that way on the diagrams, whether or not it's coil is energized in the picture. Here are the three forms;