120vdc H-bridge Relay Dead

Hello all and thanks to those who helped me in my last project about ac to dc conversion for A0 read (in this same General Electronics forum). I have another issue...a more pressing matter...

I am driving a 120vdc motor (yes 120vdc). The motor is made to do a job in one direction/movement. If the motor gets stuck or bogged down too much, it is to reverse direction for a half second to try and free itself, then, go back to its original forward movement. The problem is that I'm blowing out my H-bridge reversal relay (to reverse direction of the motor). It seems to be a power issue / something going wrong when the motor tries to reverse. First though, this is the logic:

  1. switch goes on, supplying 120vdc to h-bridge relay

  2. smaller, 5v spdt, 10A 250vac relay also turns on allowing current flow from h-bridge relay setup, through this relay and to the motor (motor is now turning)

  3. motor stops rotating or becomes stuck

  4. cut power to the motor completely for half second via the single 5v spdt, 10A 250vac relay.

  5. switch h-bridge relay and reverse motor poles.

  6. Return power to the spdt 5v relay, motor now powered in reverse for half second.

  7. SPDT relay cut power again for half second

  8. H-bridge relay off

  9. SPDT on again forward motion once more.

Those are the steps which may seem more complicated than reality. Just goes forward, gets stuck, cuts power completely, reverses, reinstates power, go reverse, cuts power briefly again, then forward again. Rinse - repeat.

Again, my problem is that I keep killing the h-bridge relay (I see a bright flash at the relay itself). Should I get a more robust 12v or 24v relay? I should say as well that the motor sometimes is still spinning when winding down, meaning that reversing the motor while this happens could cause a brief, but huge current spike, yes? Also, I'm using a diode on the relay (see picture) as well. Could this also somehow cause an issue?

I am driving a 120vdc motor

And

Should I get a more robust 12v or 24v relay

No you should get a relay that is rated for at least 120V and twice the stall current of your motor. Most relays have AC current ratings, these are not the same as the DC current rating. Normally any given set of relay contacts can only handle a small fraction of the AC rating at D.C.

Also, I'm using a diode on the relay (see picture) as well. Could this also somehow cause an issue?

No, you would have issues without it but you should have a diode ( well four ) on the motor itself.

I probably would have used two SPDT relays in a H-bridge configuration,
with a diode bridge to kill back-emf (flashing contacts).

Motor to the two CO (common) contacts of the two relays.
The two NC (normally closed) contacts to ground (supply-).
The two NO (normally open) contacts to supply(+).
Bridge rectifier +and- to supply +and-
Bridge rectifier AC/AC to the two motor terminals.

No relays active: motor brakes.
Relay#1 active: motor turns one way
Relay#2 active: motor reverses
Both relays (accidently) active: motor brakes

That kill switch could be in the supply(+) line instead of in the motor line.
Leo..

You need a strong protection diode on motor - Flyback diode - perhaps 10A/1000V or even higher voltage.

The spike can be 100 times of supplied voltage.

ted:
You need a strong protection diode on motor - Flyback diode - perhaps 10A/1000V or even higher voltage.
The spike can be 100 times of supplied voltage.

The spike will only be 0.7volt higher than the power supply if you use a back-emf diode.
Diode peak current depends on motor size (which OP did not post).
Leo..

So you again teaching me something, I told you I don't care about your opinion.

UsernameD:
3. motor stops rotating or becomes stuck

  1. cut power to the motor completely for half second via the single 5v spdt, 10A 250vac relay.

You're switching the motor when it's stalled, so that's at stall current. Also you're switching DC rather than AC which causes much more severe sparking and/or arcing.
Your 10A AC relay can probably handle about 2A in DC, so your motor should not be rated more than about 0.2-0.4A for this to work without burning up your relay. This are ballpark numbers only.

ted:
You need a strong protection diode on motor - Flyback diode - perhaps 10A/1000V or even higher voltage.

The spike can be 100 times of supplied voltage.

This voltage spike is in the forward direction of the diode; the 1000V rating is for reverse voltage. For a 120V DC supply, the diode should never see (much) more than 120V in its reverse direction while the motor is running, and the reverse current (which without diode may indeed be produce a voltage spike that high) is simply passed through. A 250V rated diode would be just fine, as long as it can handle the peak current. A 10A rated diode can typically handle peaks of 40-60A or even more which is the important rating here (check data sheets on specific diodes).

I mention high voltage diode just to be more safe.

UsernameD:
Again, my problem is that I keep killing the h-bridge relay (I see a bright flash at the relay itself). Should I get a more robust 12v or 24v relay? I should say as well that the motor sometimes is still spinning when winding down, meaning that reversing the motor while this happens could cause a brief, but huge current spike, yes? Also, I'm using a diode on the relay (see picture) as well. Could this also somehow cause an issue?

Reversing while spinning will cause upto twice the normal stall current to flow, so yes, a huge current
spike, but that's not the flash in the relay - the double stall current flows after the relay closes, on opening
you get the existing current flow only to cause arcing.

Once you get to high power DC equipment ordinary relays and switches become unusable, the current
cannot be broken if there's enough power to hold a DC arc (fireball!), unless a properly rated contactor
is used. You'll find plenty of 240V ac relays rated for 12 or 24V dc only.

You are interrupting the motor current at near stall conditions with one of those little blue relays? Oh my! Better buy them in bulk. :slight_smile:
You never did say what the motor current was.

outsider:
You are interrupting the motor current at near stall conditions with one of those little blue relays? Oh my! Better buy them in bulk. :slight_smile:
You never did say what the motor current was.

First, thank you folks for the reply help. So..

The motor spec is 6A with load. Load torque I'm not sure - this particular motor maker has a bunch of different speed/geared motors. All their motors say 6A at the particular motor's run speed(gearing and strength) with different torque applied to each motor. Enough of this though...it's 6A with load so that's that...

So if it is 6A with load, that reverse current spike can be up to 12A? WAWA....I get what you're saying with the two relays, and it sounds like a good configuration as i can break (break meaning stop) the motor this way. What does breaking do though...what happens to all that current at the moment of breaking?

Also, why would I need the diode on the motor at all? Since the point of the motor is to travel in both directions, wouldn't I need..well...2 diodes (which doesn't sense)?

I need an electrical theory class :wink:

I forgot about reversing.
The back-emf is occurred in moment of disconnecting power after that ( 0.5sec. ) you can reverse diode connection using another relay then connect connect reversed 120V.
Show picture of your H-bridge Relay .

when you drive a car you not put gear in reverse at 120km/h

UsernameD:
Also, why would I need the diode on the motor at all?

Reversing or not google - motor flyback diode

I drew out WAWA's idea for the H-bridge with breaking. Can we apply the diodes to this?

Also, I think what I''m looking for in a relay doesn't exist. Ultimately, this should be something that could be transferred/applied to a PCB. I might be up a creek...

I'd like a relay that is either 12 or 24v input, 15amp switching current and 120vdc switching voltage.

p.s. I'm a low voltage, logic person (hence the Arduino Forum) - this is not my cup-o-tea...

h-bridge with break.png

This should work but not cheap

40A
https://www.aliexpress.com/item/SSR-40a-solid-state-switching-relays-3-32V-DC-to-5-220V-DC-relay-module-KS1/32659755841.html

Thanks TED, I'm familiar with these relays (I have a few of this type for other things/projects).

Unfortunately, these large relays are unrealistic for my application. Like I said, I hoped to (apparently dreaming) have this made into a PCB (like a real, manufactured one with the logic chip and all the fixins). With real space constraints and money being an issue, I would need to solve this with much much cheaper hardware solution - it seems I may be barking up the wrong tree...

My real problem now seems to be the fact that I am working with a 120VDC motor. I should probably be looking into a more realistic 24v motor with the correct torque, however, I will undoubtedly loose rotational speed which I don't want, and I can't go with a physically larger motor either. Ugh...

Thank you anyway to those who tried to help and answered my questions, even if it ended with an unfortunate realization about the motor.

*If anyone does however have any other, small scale solution to switch/drive this 120dc motor, I'll keep checking back here. Thanks again.

try this

I do have one other question though, what physical aspect of a mechanical relay makes it rated for 120vac , but only 28vdc? (these are common ratings I am seeing for DPDT mechanical relays)

I'm sure the rating has to do with the physical terminal sizes (just like gauge of wire), but it must also be the hold strength of the electrical current vs the spring force (when releasing the relay/cutting power?

I assume as well that increasing voltage potential means a longer (physically longer) arc could happen when releasing the relay. So...the higher the dc voltage, the more physical space you would require between the actual poles on a mechanical relay, that way your arc has less chance of bridging that gap - yes?

what physical aspect of a mechanical relay makes it rated for 120vac , but only 28vdc

No physical aspect but Physics.
AC goes to zero 100 or 120 times a second, DC dosn't.