switching direction with softstarter

[ILCML]

I want to change direction on a 3ph motor using a soft starter....I would like to make the construction more simple and the cost lower by adding a relay between the motor and ss rather than adding 2 contactors before the ss. I do not have a lot of experience with soft starters, and I'm not sure if this is such a good idea....I would rather not use a VFD if possible.....

 

[waross]

You will have basically the same current after the soft starter as ahead of it. Any contactors suitable for installation between the SS and the motor should be suitable for installation ahead of the SS.
The most economical would probably be a manual transfer switch if you don't need automatic operation.
You can reverse with a pair of two pole contactors.
respectfully

 

[Marke]

Hello ILCML

The soft starter comprises an AC solid state switch is series with two or more phases to the motor. As such, the standard soft starter is not able to reverse the motor. (unlike the VSD)
The only way that you can reverse the motor when using a soft starter, is to swap two phases. This can be done before or after the soft starter. You can use two contactors, or a changeover switch. Either way, you should stop the starter before you switch the phases.

It is possible to manufacture a soft starter with five (or four) controlled solid state switch assemblies and use these to reverse the phases to the motor. While these have been manufactured, they are not common and would probably be a more expensive option.

If the motor is small, there are solid state reversing modules available such as the IC Electronic unit, but these are only suitable for low currents.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[ILCML]

This particular motor will be running near 10A....Ideally, I'd like to just use a 3PDT relay...incidentally, why aren't there contactors with NO and NC contacts for reversing? At least I'm not familiar with one.

 

[Marke]

Hello ILCML

The problem with using relays, is that you are controlling a very inductive load that can regenerate as well. You do not have arc extinguishing capabilities in the relay and if you wish to switch with any current through the relay, you will end up with a phase to phase short circuit.
If you could guarantee that there would be no voltage on the relay before you switch it, you may get away with it, but what happens in the case where there is a supply problem that causes the relay to drop out with voltage applied??

I personally do not think that this is an option.
I believe that you need to use a three stage switching where there is an ON - OF - ON sequence to ensure that you do not have arc induced short circuits.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[ILCML]

right.....thanks, this is the best place there is to get a sanity check....

 

[dpc]

If you're in the US, motor contactor must be horsepower rated and UL listed for switching motor loads.

 

[jraef]

If it's 3kW 7.4A or smaller, Siemens has a reversing solid state contactor (SSC) that would work and look like a single device (the crossover circuit is internal). Probably similar to the one mentioned by Marke. That size is about the limit of that technology however.

I'm also not entirely sure it would function correctly with a soft starter either in front of or behind it. Solid state switchers need current to start flowing immediately, so either the soft starter or the SSC is going to be deprived of current flow for a brief moment when initialized.

Are you adverse to just doing with a reversing contactor like everyone else has?

 

[ILCML]

Not adverse, just takes up a lot of space....that's what I've decided to do however. Thanks for all of your inputs!