Need Solid State Switch for 120VDC Motor
Need Solid State Switch for 120VDC Motor
(OP)
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
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





RE: Need Solid State Switch for 120VDC Motor
Omron or Potter-Brumfield online catalog?
Bill
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"Why not the best?"
Jimmy Carter
RE: Need Solid State Switch for 120VDC Motor
RE: Need Solid State Switch for 120VDC Motor
Suggestion to make your life easier (or make your relay's life easier
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If we learn from our mistakes I'm getting a great education!
RE: Need Solid State Switch for 120VDC Motor
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
RE: Need Solid State Switch for 120VDC Motor
Many AC relays depend on the zero crossing to interupt the current.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Need Solid State Switch for 120VDC Motor
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
RE: Need Solid State Switch for 120VDC Motor
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.
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RE: Need Solid State Switch for 120VDC Motor
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
RE: Need Solid State Switch for 120VDC Motor
RE: Need Solid State Switch for 120VDC Motor
RE: Need Solid State Switch for 120VDC Motor
You could make mountains out of molehills, and build a IGFET-based DC "relay", but why create trouble?