Motor control circuit help

Thread Starter

piefairy

Joined Aug 19, 2021
10
I'm working on a circuit to control 2 motors, and I'm running into an issue with the relay at the relays. I know I will need a snubber for each relay, but I do not know the values of the motor. They are old, but work well. 1 is an AC hydraulic pump, the other is a reversible AC motor, there is also a solenoid for the hydraulic pump. The exact specs on each motor are unknown other than their voltage and amperage rating. The hydraulic motor is 120v at 7 amp, and the electric is 120v at 4 amp.

Attached is the circuit thus far, if you all don't mind taking a look. Its been years since I took classes for this. The snubber will be across the load on each relay. Also, the up relay is suppose to operate the solenoid and motor relay, but down is only the solenoid. The controls to operate the relays will be connected via 7 foot long RJ-11 cables. I believe they will support the relay coil load, correct?

I know its crude, but before I got further
 

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MaxHeadRoom

Joined Jul 18, 2013
23,805
If no electronics is involved, I would normally not install snubbers, What issues are you running into?
What relays are you using?
For motors, it is often best to use contactor style.
Also on new designs it is usually preferable to use DC coil variety where possible.
 

Thread Starter

piefairy

Joined Aug 19, 2021
10
I'm trying to avoid the wear and tear on the contacts when kicking on such a large load. The inrush current on these old motors is huge. Its a 7 amp at 120v AC motor when running. The relays are DC coil power relays, I'm running a low voltage 12v dc circuit to run the control switches.

I am trying to avoid the size and cost of the older contactor style relays. This is a retro fit into an older piece of equipment and space is a bit limited. The motors are turned on in short cycles sometimes less than a second. From my understanding when switching a high amperage relay load, the contactor can take excessive and shorten the life of the system.
 
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MaxHeadRoom

Joined Jul 18, 2013
23,805
What are the relays you intend using now?
Usually a retrofit implies some improvement or upgrading, the latest DIN style contactors are much smaller than their older counterpart.
 

MaxHeadRoom

Joined Jul 18, 2013
23,805
There is no way I would use those to control AC motors even of those sizes..
They may be rated for 10a, but that is for a resistive load, at best.
 

LowQCab

Joined Nov 6, 2012
1,189
For repetitive Jogging / positioning / short-cycle usage, those Relays won't last long.
For about twice the cost of the Relay solution, You can use SSRs (Solid-State-Relays),
but, they do have the requirement of needing to be mounted to
some sort of a Heat-Sink,
a large Aluminum-Box, or a flat, bare, Cast-Aluminum-Surface may be just fine.
They never wear-out or create Arcing like an old-school mechanical-Relay.

Using Data-Cables is really "iffy" with Relays, but no problem at all with SSRs.
A single 9-Volt Battery will operate the SSRs for probably ~20-hours or more of "on-time".
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Thread Starter

piefairy

Joined Aug 19, 2021
10
I've looked at the SSRs, it is what the more current version of what I am trying to retrofit uses. I was going towards physical relays due to the cost and supply limitations on the SSRs currently.

Your saying using the data cables is iffy, is that because of the voltage drop over the distance of the cable?

Also, if I do go to SSR's, is there anything that would need to be added to the circuit for them? I mean like a sub circuit, or can I just put them in place of the current relays?

Thanks
 

LowQCab

Joined Nov 6, 2012
1,189
DigiKey has ~82 part numbers in stock that are appropriate for your application,
starting at ~$20.oo each for a ~20-Amp Model, no Socket required.
They have locations World-wide.
This is a filtered list .............
https://www.digikey.com/en/products...PjIBWPgvGI0ANAAEyUeCVkwy4QE4eQAE9ZH5VwuWQVigA

If I remember correctly, You don't need Motor Reversing,
so generic SSRs are a drop-in replacement.

You can also get slim DIN-Rail-Mounted versions as well.

They will work with your existing ~12-VDC Power-Supply too,
but they only require about ~2 or ~3 percent of the
Coil-Energizing-Current of a standard Relay.

And,
they don't make any noise.
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Thread Starter

piefairy

Joined Aug 19, 2021
10
Thanks for the link.

One of the motors will need to operate in forward and reverse. What will I need to look for in that specific SSR?

Question for the future. What do I need to look at in the data sheet that will dictate whether or not a relay, or other part, will support the load of whatever I'm trying to control? I had been going with voltage and amperage support for the given load being enough. Obviously not given my first relay choice.
 

MaxHeadRoom

Joined Jul 18, 2013
23,805
Question for the future. What do I need to look at in the data sheet that will dictate whether or not a relay, or other part, will support the load of whatever I'm trying to control? I had been going with voltage and amperage support for the given load being enough. Obviously not given my first relay choice.
Contactors generally have a HP rating on them, Other than contactors, use the 30amp rated versions at min. for the motors you are using.
 

LowQCab

Joined Nov 6, 2012
1,189
I would recommend an SSR with at least twice the normal running Current of the Motor, actually measured with an Amp-Clamp-Meter under full normal Load.
A 20-Amp SSR controlling a Motor that draws ~7-Amps running, should be fine.

SSRs do produce some Heat, this should be addressed first.
Some of the DIN-Rail models have built-in Finned-Heat-Sinks,
most other models must be mounted to
something flat that can dissipate some Heat.

Reversing will depend on the Wiring of your Motor,
do You have a diagram, or
a picture of the Motor-Plate,
showing the Wiring-changes required for Reverse ?
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LowQCab

Joined Nov 6, 2012
1,189
I wasn't aware of the space limitations so I purposefully selected
models that had easy to connect Screw-Terminals.
Sorry about that.
The Model You referenced makes me worry about how much Heat it can Dissipate,
but they rate it at 25-Amps, so I suppose it's adequate.
(A small Fan would be good insurance)
Maybe a larger Box would be in order.
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Thread Starter

piefairy

Joined Aug 19, 2021
10
Its no problem. I appreciate the help. Heat dissipation isn't much of a concern given these never run longer than 30 to 45 seconds straight, and sit idle 95% of the time.

Is there any kind of control voltage protection I need with this type of relay? I know they are sensitive, do I need to put a drain on one of the control legs to make sure it drops back to open?
 

Thread Starter

piefairy

Joined Aug 19, 2021
10
I need to break down the wiring on the reversible motor, but it is a 4 wire setup with 1 of the 4 being ground.

I hadn't associated HP to an electrical calculation. The larger of the two motors does have a stamp of 1/8th HP on it, but I'm not sure its the same thing or if it helps.
 

MaxHeadRoom

Joined Jul 18, 2013
23,805
I need to break down the wiring on the reversible motor, but it is a 4 wire setup with 1 of the 4 being ground.
Sounds as though it is a PSC motor, where the two windings are equal?
These are very easy to reverse just by switching the two windings, to the start cap, These motors are usually restricted to 1/2HP at most.
 

Irving

Joined Jan 30, 2016
2,197
If the motor has an HP rating this is usually nominal output and crudely 1/8HP = 750/8W = ~100w out or roughly 120 - 140W input at rated rpm. That suggests a run current around 1 - 2A so a startup current/stall current of 7A isn't unusual/unreasonable.
 
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