Need Solid State Switch for 120VDC Motor 3

[mwtucker]

I have a 120 VDC brushed motor that we have been switching on and off using an Omron G2RL-24-DC12 relay (contacts rated at 8A 250VAC, 8A 30 VDC). The relay contacts have been "sticking" occasionally, presumably from arcing as a result of switching an inductive load. The motor is being driven by unfiltered, unregulated 120VDC which is obtained by rectifying the 120VAC line with a full-wave bridge. There is a 0.47uF suppression cap across the motor terminals along with an MOV for surge suppression. The motor draws about 2 amps when running normally, but its current could rise to 4-5 amps when it is laboring (and possibly higher when it is first switched on, but no measurement on that yet).

Would a solid state switching scheme be appropriate here? Perhaps replace the mechanical relay with an SCR or Triac?

We do not need to control the motor speed, just on and off. The direction is controlled by another DPDT relay that flips the polarity of the motor excitation voltage. This relay, however, is not sticking because it is never energized/de-energized with power on its contacts.

A microcontroller output is presently driving a transistor that switches the G2RL-24-DC12 coil voltage on/off. So the solution will require interface to a microcontroller output pin...

Thanks for your input.

Regards,
Mike

 

[waross]

Google?
Omron or Potter-Brumfield online catalog?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mwtucker]

Relay is an Omron G2RL-24-DC12.

 

[ScottyUK]

Check out Crydom's range of SSRs.

http://lmgtfy.com/?q=crydom+relay

Suggestion to make your life easier (or make your relay's life easier ) : put the rectifier after the relay, so the relay switches AC. Relays hate DC.



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[mwtucker]

Thanks, ScottyUK...

I'm not sure that there is much difference between 120 VAC and full-wave 120 VDC that is un-filtered, pulsing at 120 Hz??

I'll look at the Crydom products.

Mike

 

[waross]

With a coasting motor you may not have a zero crossing with full wave DC.
Many AC relays depend on the zero crossing to interupt the current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

PS.
There is a reason why your 250VAC relay is only rated at 30 VDC.
"contacts rated at 8A 250VAC, 8A 30 VDC"


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

DC is MUCH harder to switch than AC. Know what happens when you exceed the DC voltage rating of a relay? The contacts weld! Sound familiar?

I suggest taking ScottyUK's advice, switch the AC ahead of the rectifier. SSRs for DC are typically for much lower voltages and currents or much larger budgets. SCRs can switch DC on, but not off. You need GTO thyristors or transistors, both of which need more complex control boards etc. You are going to solve one problem ad add 6 more new ones.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[mwtucker]

So you guys think that switching the 120 VAC will be an improvement, even though the 120 VDC is full-wave rectified? (And does get to zero 120 times per second, I think).

Bill: I guess that the coasting motor will not feed back through the bridge since it would be generating DC?

Jref: Thanks for the advice about solving one problem and adding 6 more! It is easy to forget that....

Even though I have an MOV across the motor terminals, I think I will also look into an RC network across the relay contacts...

Thanks!
Mike

 

[burnt2x]

So you guys think that switching the 120 VAC will be an improvement, even though the 120 VDC is full-wave rectified? (And does get to zero 120 times per second, I think).

Full-wave rectified output does not cross zero voltage. The waveform is "DC" with ripples at the tops. Even a half-wave rectified output is hard to switch-off as your inductive load amps does not coincide with the zero-volt point, i.e. there is still a reasonable amount of current even when your half-wave voltage zeroes - and your solid-state switch does not go "off" even when not gated.

 

[LionelHutz]

If you are already building the control logic yourself, then look at using 2 pulse transformers and changing 2 of the 4 diodes in the rectifier to SCR's so you can just switch the rectifier on and off directly.

 

[fsck]

I concur that a suitable SSR on the AC would be a better approach.

You could make mountains out of molehills, and build a IGFET-based DC "relay", but why create trouble?