Solid-State relay for DC

[walker1]

Someone just asked me, if it would be possible to use a solid-state relay to disconnect a 115-130 VDC line with an electrical motor as load ?

As the motor will be inductive, I may have to add a fly-back diode with or without an additional resistor in series.
But what else should I look out for?

What kind of voltage should the relay be rated for?
Nearly all the standard ones I have found so far are specified at 250 VAC or so. Only one has a DC rating at max. 60V.

 

[ScottyUK]

Have a look at Magnecraft's 6DDX range: 200V DC, up to 40A.

http://www.magnecraft.com/products/e104_section2_01.pdf

Page 5 has some commentary on SSR's for motor applications.

I've always liked Magnecraft's mechanical relays - I have no experience of their SSR's, although they appear to have bought out Grayhill's range which was also not bad.

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I don't suffer from insanity. I enjoy it...

 

[felixc]

Crydom makes quite powerful solid-state relays with "zero-crossing". I've used one to control a compressor in a temperature chamber and it never failed.

 

[itsmoked]

Felixc he needs DC. No zero crossing!!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[walker1]

Yes, I have also stumpled over that zero crossing. Yet another thing to keep an eye out for.

On the other hand it might turn out, that the guy is using 24VDC, which of course makes it somewhat easier.

 

[coconutalley]

As for what to look out for:
It depends on how your motor is connected and if it is just going one direction. Are you using the SSR as an ON/OFF switch?
Normally, if you are using a drive, the output stage H-bridge has those "fly back" diodes. You might want to put a flyback diode across the solid state switch if it is an igbt or fet and it doesn't already have it. Normally, drives don't need SSR. They can be shut down automatically due to faults. A fuse or cb is used to interrupt the supply in case there is a short in the drive.
The SSR also needs a minimum driving current [usually not much] and the gate has limited voltage. Since they used either FET or IGBT, it would have similar characteristics, e.g RDSon or Vsat. The AC SSRs use SCRs that's why they have zero crossing shut off issue. If the driving voltage source is on the high side [above motor] then the driving voltage must float and its return connected to the emitter or source of the IGBT or FET, not the Motor supply return.

 

[walker1]

The load, as I understand it, is more or less standard household appliances (fridge, freezer etc, but running on DC!) that needs to be turned on and off remotely.

 

[waross]

That type of appliance will probably already have control equipment in place to start and stop the motor. If the control scheme is sophisticated, you may be able to use the existing controller to stop the motors.
If the appliances are using simple thermostat controls this won't work.
Don't forget that as well as interupting the current to stop the motor you may have a significant starting surge with a DC motor.
yours