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Old 02-10-2020, 02:40 PM   #1
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Default Single phase motor start relay

Hi guys, I'm dealing with a single phase motor that requires a start relay to switch the start capacitor. I'm coming up empty handed on a replacement for the relay when I search the numbers on the nameplate, so now I need to understand how they work so I can select a new one.


I've attached drawings and a photo of the existing relay.


How would I select a new relay for this system?
Attached Thumbnails
Single phase motor start relay-start-relay.png   Single phase motor start relay-start-relay2.png  

Last edited by gottspeed; 02-10-2020 at 02:54 PM. Reason: wording
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Old 02-10-2020, 02:53 PM   #2
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IIRC it is a current relay.
Check out this
https://www.sensata.com/sites/defaul...-datasheet.pdf
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Old 02-10-2020, 02:56 PM   #3
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Quote:
Originally Posted by gottspeed View Post

Do these use potential to pull in the relay? A timer? What's the philosophy on this and how do I select a new relay?

Potential start relay is what I've always called them.


No timer, they just open when the motor is up to speed (from back emf)


Just get a 120V potential start relay, and make sure the contacts are rated for your HP.
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Old 02-10-2020, 03:07 PM   #4
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Quote:
Originally Posted by just the cowboy View Post
IIRC it is a current relay.
Check out this
https://www.sensata.com/sites/defaul...-datasheet.pdf


That may be it too.
From what I remember, the current relay was in series and a NO contact, and the potential relay was across phase and a NC contact.
If I remember correctly then, that would make it a potential start.


But I wouldn't go to the bank on my memory
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Old 02-10-2020, 03:12 PM   #5
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Thank you both very much.
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Old 02-10-2020, 03:16 PM   #6
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Quote:
Originally Posted by emtnut View Post
Potential start relay is what I've always called them.


No timer, they just open when the motor is up to speed (from back emf)


Just get a 120V potential start relay, and make sure the contacts are rated for your HP.

I'd be looking for a coil with motor nameplate voltage, no?


This is a 230V motor.
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Old 02-10-2020, 03:24 PM   #7
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Quote:
Originally Posted by gottspeed View Post
I'd be looking for a coil with motor nameplate voltage, no?


This is a 230V motor.

230V motor, but the relay is only tied to one phase (I'm assuming the 'W' is white for neutral) .. so 120V
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Old 02-11-2020, 09:09 AM   #8
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Umm.. Unless I'm reading it wrong, you don't have a single phase motor. You have a phase convertor, and are looking for the control relay for the start capacitors. The manufacturer of the convertor will help get the right replacement part. If I remember right, it's called the voltage sensing relay, and makes sure the voltage is correct, before dropping out the start caps.

Last edited by glen1971; 02-11-2020 at 09:27 AM.
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Old 02-11-2020, 12:16 PM   #9
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Quote:
Originally Posted by glen1971 View Post
Umm.. Unless I'm reading it wrong, you don't have a single phase motor. You have a phase convertor, and are looking for the control relay for the start capacitors. The manufacturer of the convertor will help get the right replacement part. If I remember right, it's called the voltage sensing relay, and makes sure the voltage is correct, before dropping out the start caps.
According to the diagram he’s posted he has a single phase motor using a three phase contactor looped through. SUPCO makes a universal potential relay that’s voltage adjustable that works well. A decent HVAC store should have one on the shelf.
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Old 02-11-2020, 01:15 PM   #10
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Default Potential Start Relay

What you have there is a White Rodgers / RBM potential relay. Normally used on a refrigerant compressor. Looks like in your case used as a phase converter to run a 3Φ motor, from 1Φ with the addition of a start and run capacitor.


There has been much consolidation with manufacturers of potential relays. You likely will not find that exact model, but there are several manufacturers that offer similar replacements and cross references.


But before I would condemn the relay, I would first check or replace the start capacitor and run capacitor. The start capacitor is usually the first thing to go.


Seems you have a second unit in the panel for another motor. If the other unit is working you may want to substitute its parts for testing before trying to get replacement units.


You will likely need more information on the system voltage and the cutout voltage to obtain a similar replacement, from several other manufacturers. But check the caps first.



Some supplemental information that may be helpful.


https://climate.emerson.com/document...en-4209026.pdf


https://www.marsdelivers.com/wps/wcm...e-9acd7c6d82dd


https://climate.emerson.com/shop/Sea...ce:&pageSize:&


https://marsdelivers.com/wps/wcm/con...b-a89019251d2c
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Old 02-11-2020, 08:31 PM   #11
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I’m not seeing a RPC here, am I overlooking something?
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Old 02-11-2020, 08:50 PM   #12
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I'm not seeing where the phase converter is here ??


From the drawing OP posted, just a single phase 230V motor fed from 3 phase supply.
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Old 02-11-2020, 09:01 PM   #13
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I don't see it either.
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Old 02-11-2020, 09:30 PM   #14
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I see 2 incoming conductors feeding a 3 pole switch. The second and third poles of the switch are connected in series. The first pole feeds the B terminal. The third pole feeds the R terminal of motor. A relay connects the start capacitor to the starting winding through NC contact, and then opens leaving the run capacitor connected to terminal W.

No phase converter.
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Old 02-11-2020, 10:33 PM   #15
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Start the motor and measure the voltage across the start winding that is 2 and 5 be careful cause it can be much higher than the line get a potential relay with a pick up voltage about 80 percent of what you measured.
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Old 02-12-2020, 12:14 AM   #16
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weird it uses B,R,W

when it looks like B = run, R = common and W = start (unless that just the wire color rather than a terminal name)
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Old 02-12-2020, 12:47 AM   #17
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The reason for the 3 pole contractor and 2 poles connected in series is cause it most likely has a 3 pole electronic O/L relay. These relays need to see roughly the same current on all 3 poles, if there's current on only 2 poles, it'll trip on single-phasing.

I'd check the capacitors first, especially the start one. In my experience the caps will fail more often than the start relay.

This is a potential start relay. It looks at voltage across the start winding, when the motor first starts, the voltage is low. As speed increases, voltage rises. When it has risen to the point that the start winding is no longer needed, it removes the start capacitor from the circuit and the start winding becomes an auxiliary run winding via the run capacitor.

If either cap is bad, the voltage will not rise to the point of removing the start cap. from the circuit and either the O/L will trip or the motor will burn up.

One nice thing about the electronic O/Ls is they are, or can be set, to class 10. This means that it'll trip in about 10 seconds at 6X the full-load current setting (roughly normal starting current). Submersible pump motors and refrigeration compressors are pretty high-strung and won't take starting current nearly as long as a normal motor.

Normal O/Ls are class 20. 20 seconds at 6X the current setting.
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Old 02-12-2020, 10:05 AM   #18
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I just worked on the exact same deal for a small single phase lift pump on Monday. It wouldn't start, only hum.

It had a bad start cap.
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Old 02-13-2020, 12:35 AM   #19
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Quote:
Originally Posted by 460 Delta View Post
I’m not seeing a RPC here, am I overlooking something?
Never said it was a Rotary Phase Converter. I had said it looked like the OP had a phase converter. I drew that conclusion as the drawing looked very similar to one I had worked on the previous day. Apparently I am mistaken.
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Old 02-13-2020, 03:12 AM   #20
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Good thread!


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