SPST vs SPDT

[Newbie2016]

Going to have relays starting/stopping pumps and opening/closing valves.

Can i use SPST relays and if not, why?

 

[gclshortt]

An electromechanical relay will work. However, a solid state relay would be my choice for a quick solution. You don't have any contacts to corrode, arc, etc., and degrade.

Check the specifications on the pump and valve for the amount of current you want to switch. Note: DC valves sometimes have a high inrush current if they do not have a freewheeling diode. Your relay must be motor rated.

Regards,

 

[Phil Buchanan]

SPST simply means single pole single throw which means it has one pole and has one throw or one change of state in it's contacts so if the relay is SPST N.O. this means you only have one pole which is set of contacts and its single throw so it has only one state change if it's an N.O. contact it closes when the coil is energized. There is no separate set of N.C contacts to open in this config.

The most common relay is DPDT double pole double throw so you have 2 sets of contacts in the relay and each has 2 state changes so when the relay is energized the N.O. contacts will close and the N.C. contacts will open all at the same time.

So tom answer your question yes you could use SPST If you design your system to use them but I would not choose them myself because they are much less common that DPDT and having something more common may make it easier to source a replacement when something fails.

 

[DwSoFt]

I use a common SPDT relay as it takes up half the space than a DPDT relay.

 

[jraef]

There is no way to answer that with the information you gave.

IF the motor is single phase AC, AND the relay in question has a motor RATING that is at or greater than the motor kW / HP in question, then it would not matter if the relay was single pole or double pole. For a single phase motor, all that is necessary to start and stop the motor is to break one of the line power leads. Protection for the motor is not part of what a relay does, so all of that is separate.

IF the relay does not have a rating in kW or HP at least equal to or greater than the motor rating, you cannot use it. This is not as simple as just matching the motor FLC to the maximum current rating of the relay, the MOTOR rating is different because it is much harder of the relay contacts. Without that induction motor rating the contacts could weld and the motor may not be able to be stopped.

IF the motor is 3 phase, then you must break at least TWO of the 3 power leads, and you MUST use at least a double pole relay.

If the motor is DC, the relay must have a DC rating at the voltage you are using, which in the type of relays described as SPST or DPST, is unlikely.

Valves are less complicated, but the same issues of ratings apply because a valve coil is still an INDUCTIVE load, so the relay contact rating must be in watts, not just amps.

 

[Tim Ganz]

If the motor is DC, the relay must have a DC rating at the voltage you are using, which in the type of relays described as SPST or DPST, is unlikely.

Valves are less complicated, but the same issues of ratings apply because a valve coil is still an INDUCTIVE load, so the relay contact rating must be in watts, not just amps.

Why do you say the relay must be rated in watts and not amps? I have been following this but not quite sure I am getting the concept?

Also I have seen a lot of relays and I have never seen one marked up rated in watts it's always a 5 amp 8 amp 10 amp or something like that on the AB and potter brumfield relays we use.

 

[just the cowboy]

Just looked at some relays

Why do you say the relay must be rated in watts and not amps? I have been following this but not quite sure I am getting the concept?

Also I have seen a lot of relays and I have never seen one marked up rated in watts it's always a 5 amp 8 amp 10 amp or something like that on the AB and potter brumfield relays we use.

I just looked at some ice cube relays, AD AB and potter Brumfield they all had HP ratings. Double check the ones you use.