Using a relay on DC motor field

Not my field but I would be concerned as to how you would ensure that the relay never opened when power was on. For example, if the relay power supply failed or was momentary interrupted. It is a bit different with something that is mechanically locked, so it cannot poosibly open

-except by a postive, not a negative, action. Of course, if the relay coil itself failed with the contacts made, the results would be dramatic, to say the least.

So, I might consider doing this with a latching relay or one with a mechanical interlock. But not otherwise.

I would probably go for a thyristor. It cannot be left "on" with the supply off and will latch until the power is taken off.

*Theoretically*, yes. The low DC voltage rating is because *interrupting* a DC current is much more difficult and interrupting an AC current. When you look in your DC controller, you'll see that the line contactors in there are quite a bit larger/different than your typical AC motor controller.

That's the *theory*. Now, in practice, how are you going to make sure the relay doesn't open while the motor is running? If it opens you quite likely will have an arc that won't go out at all. It'll burn the relay contacts and melt any plastic housing. And still the arc will continue. About the only thing that will stop it is if the arc expands to involve some grounded metal in the cabinet and then faults the line to blow a fuse or other protective device.

Another issue to at least think about is how to be sure the relay picks up and actually applies field current. If it fails to pick up, you'll be starting the motor with no field. That's a guaranteed trip of the overloads.

I'd recommend against this. I've worked with a lot of DC machinery of all shapes and sizes (350VDC and below, not as high as 500V). It's a 'different animal' in many ways from AC motors. Proper sequencing of starting relays, full-field current start, and separate/special contactors. Most of the controllers I've seen are configured that the main contactor's don't pick up unless the field and starting contactors are aligned correctly. Even the low current field and auxilary contactors have a larger travel (upwards of

3/4 inch) than AC contactors, just to interrupt the arc.

daestrom

The little relay would work for a while ... but some memorable stuff will happen the first time power fails while the field is energized. If you really must switch a highly-inductive load on the DC side, get a big contactor with blowouts designed for the job ... or be prepared to find another job somewhere far away.

Is the the field supply constant-potential? If so, you'd be better off putting in field exciters (diode bridges) for each motor and put the contactors on the A/C side: much easier to load break and the exciter acts like a free-wheeling diode to suppress the inductive voltage.

Roby Also, be sure you've provided overspeed protection.

Thanks for your input sue. You are right. What will protect it if power fails. Something came to mind and that is adding a heavy back diode across the field to absorb it since I will not be reversing the field current. I've noticed some field supplies have this diode across the output. This should protect it. I have an interlock that will not allow the relay to engage if the drive is on first. Thanks.

Thanks Roby. I was just thinking of the same thing of adding a back diode. The drive runs on tach FDBK, so it does have overspeed protection. Thanks guys.