# Single phase motor start relay



## gottspeed (Mar 8, 2010)

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?


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## just the cowboy (Sep 4, 2013)

IIRC it is a current relay.
Check out this
https://www.sensata.com/sites/defau...tor starting relay current type-datasheet.pdf


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## emtnut (Mar 1, 2015)

gottspeed said:


> 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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## emtnut (Mar 1, 2015)

just the cowboy said:


> IIRC it is a current relay.
> Check out this
> https://www.sensata.com/sites/defau...tor starting relay current type-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 :biggrin:


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## gottspeed (Mar 8, 2010)

Thank you both very much.


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## gottspeed (Mar 8, 2010)

*Coil Voltage*



emtnut said:


> 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)
> ...



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


This is a 230V motor.


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## emtnut (Mar 1, 2015)

gottspeed said:


> 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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## glen1971 (Oct 10, 2012)

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.


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## 460 Delta (May 9, 2018)

glen1971 said:


> 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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## CMP (Oct 30, 2019)

*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/documen...ss-reference-instruction-sheet-en-4209026.pdf


https://www.marsdelivers.com/wps/wc...&CACHEID=1dca933e-44c4-4dad-b6ae-9acd7c6d82dd


https://climate.emerson.com/shop/Se...&pageView:grid&minPrice:&maxPrice:&pageSize:&


https://marsdelivers.com/wps/wcm/co...&CACHEID=0f4c2377-9b43-415c-b2eb-a89019251d2c


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## 460 Delta (May 9, 2018)

I’m not seeing a RPC here, am I overlooking something?


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## emtnut (Mar 1, 2015)

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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## dronai (Apr 11, 2011)

I don't see it either.


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## FaultCurrent (May 13, 2014)

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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## Galt (Sep 11, 2013)

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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## gpop (May 14, 2018)

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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## micromind (Aug 11, 2007)

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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## Cow (Jan 16, 2008)

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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## glen1971 (Oct 10, 2012)

460 Delta said:


> 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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## Funksparky (Nov 8, 2017)

Good thread!


Sent from my iPhone using Tapatalk


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## CMP (Oct 30, 2019)

The OP'ster said that the motor was 1Φ. But if you look at the motor specs on the drawing, the amperage and HP listed seems to correspond to a 3Φ 2.3HP motor @ 208-220V.


If it is a 1Φ motor, the amperage listed seems low for a 2.3HP unit. We cant know for sure unless we see a motor tag, especially since the terminal labels seem odd. It may be a European or Asian 50Hz unit with labels like that. Hard to tell from here.


It can be a 1Φ motor, where the auxiliary winding serves start and run functions. Start mode would use both capacitors, and the run mode would have just the run cap in the circuit. But the amperage listed seem low for that scenario.


The circuit shown could work for either scenario, 1Φ or 3Φ, depending on what the capacitor values were, but the print does not indicate what they are.
The RBM potential relay appears to be a special, since it is not listed on the cross reference chart, leading me to believe it is calibrated for a voltage other than standard US one.


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## bill39 (Sep 4, 2009)

I will chalk this up to “what I learned today.”

Good stuff here. Thanks.


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## emtnut (Mar 1, 2015)

glen1971 said:


> 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.



I don't have any real life experience with RPC. Any time I've needed 3 phase, it was there.

When you posted about the RPC, I googled it and did find many diagrams extremely similar to the OP. I thought I fugged up :biggrin:

The contactor had me confused a bit, but then micromind posted about the 3 phase overload, and I remember coming across that once ... thanks micro:vs_cool:




micromind said:


> 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.


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## micromind (Aug 11, 2007)

CMP said:


> The OP'ster said that the motor was 1Φ. But if you look at the motor specs on the drawing, the amperage and HP listed seems to correspond to a 3Φ 2.3HP motor @ 208-220V.
> 
> 
> If it is a 1Φ motor, the amperage listed seems low for a 2.3HP unit. We cant know for sure unless we see a motor tag, especially since the terminal labels seem odd. It may be a European or Asian 50Hz unit with labels like that. Hard to tell from here.
> ...


The drawing is pretty standard for a 1Ø motor that does not have a built-in centrifugal switch to disengage the start winding. Most submersible pump motors and refrigeration compressors use this starting system. 

I thought the current seemed a bit low too but looking in the Leeson motor spec book, most of their 2HP 1Ø models draw anywhere from 8.2 to about 10 amps. 

Capacitor-run motors will draw less current than their induction-run counterparts because the run capacitor will result in a higher power factor and thus, lower current.


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## glen1971 (Oct 10, 2012)

emtnut said:


> I don't have any real life experience with RPC. Any time I've needed 3 phase, it was there.
> 
> When you posted about the RPC, I googled it and did find many diagrams extremely similar to the OP. I thought I fugged up :biggrin:
> 
> The contactor had me confused a bit, but then micromind posted about the 3 phase overload, and I remember coming across that once ... thanks micro:vs_cool:


I haven't worked on an RPC style converter in quite a few years, and the ones I did seemed to be unique, each with it's own twist, to the area they were in. I assume that is like everything else, where it's the electrician or vendor in the area at the time doing the work that determines what products are installed.

The ones I've hit lately either have a step up transformer in the same enclosure or separate and are of 3 different manufacturer's, again all similar, but also unique. I've been after them for years to change them our when they fail to a step-up transformer and a VFD, but so far one has been done.


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## dronai (Apr 11, 2011)

I've furnished and installed one RPC in my life. It was for one specific large pump. I've never seen one used for multiple motors, or a whole shop.
Have any of you guys seen this ?

The one guy that had a thread going, and continious problems, is the only place I heard of doing that.


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## 460 Delta (May 9, 2018)

dronai said:


> I've furnished and installed one RPC in my life. It was for one specific large pump. I've never seen one used for multiple motors, or a whole shop.
> Have any of you guys seen this ?
> 
> The one guy that had a thread going, and continious problems, is the only place I heard of doing that.


I’ve a r-mix plant that runs entirely off of 1 of 2 50hp Arco RPC’s. The original owner/builder believed that overkill is underrated. The service is a 1000 amp 240 single phase service. The voltage balance was way out of shape when we took over, ac was ~ 275 and bc was ~ 285 volts. I moved some caps around and eliminated a few also. Putting the caps in the combo units to be switched in and out was the key. The plant works well, but the constant errrrrrrr all day long can grate on your nerves.


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## JRaef (Mar 23, 2009)

1000A 1 phase 240V service... that's nuts!


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## 460 Delta (May 9, 2018)

JRaef said:


> 1000A 1 phase 240V service... that's nuts!


Like I said, old owner, rest his soul, was a certified agricultural engineer, figure out how heavy it needs to be then double it for safety. Truth be told, there was plans for a small screening plant to make their own aggregate for the concrete and it never got past the gathering up the equipment stage. The old guy was a ball of energy, but there is only so much daylight in a day.


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## dronai (Apr 11, 2011)

460 Delta said:


> I’ve a r-mix plant that runs entirely off of 1 of 2 50hp Arco RPC’s. The original owner/builder believed that overkill is underrated. The service is a 1000 amp 240 single phase service. The voltage balance was way out of shape when we took over, ac was ~ 275 and bc was ~ 285 volts. I moved some caps around and eliminated a few also. Putting the caps in the combo units to be switched in and out was the key. The plant works well, but the constant errrrrrrr all day long can grate on your nerves.


746Wx50=37,300W/240V=155A each


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## 460 Delta (May 9, 2018)

dronai said:


> 746Wx50=37,300W/240V=155A each


746 watts in a perfect world with no losses.lain:


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## dronai (Apr 11, 2011)

So about 300A conversion on a 1000A service ?


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## 460 Delta (May 9, 2018)

dronai said:


> So about 300A conversion on a 1000A service ?


Probably about 1000 watts per hp, so 200 amp a piece or so. Both running with a 100 amp single phase load in the control room = 500 amp. With the farm engineer multiplier, 1000 amp.:smile:


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## CMP (Oct 30, 2019)

*Submersiable Pumps*



micromind said:


> The drawing is pretty standard for a 1Ø motor that does not have a built-in centrifugal switch to disengage the start winding. Most submersible pump motors and refrigeration compressors use this starting system.
> 
> I thought the current seemed a bit low too but looking in the Leeson motor spec book, most of their 2HP 1Ø models draw anywhere from 8.2 to about 10 amps.
> 
> Capacitor-run motors will draw less current than their induction-run counterparts because the run capacitor will result in a higher power factor and thus, lower current.



Been a busy week, so a little late on the reply. I haven't been involved with 1Φ submersible pumps in the past, just above ground units, so I made time to look at some Franklin Submersibles for context. 

You were right on the money with the amperage and power factor of these type units. They list the currents and resistances of each winding in their catalog specifications, as well as capacitor values. Very handy when troubleshooting. See the attached PDF catalog page below.


Since they run fully loaded at all times, the power factor is easily adjusted for one point of load, with capacitance, and theirs is shown near .95PF. They even list, additional maximum capacitance the can be added to quiet a noisy motor.


Another interesting fact was that they require a class 10 overload protection, to not void the warranty. Which is what we saw in the OP setup.
Pretty efficient setup when you can control the load point at one setting. Not so much true with other pump setups.


Refrigerant compressors would be very similar. Sealed unit, with internal fluid cooling, and one point of load.


Pump motor data specification page
View attachment Franklin Submersible Motor Data.pdf



Franklin Manuals Page for those interested in more information.


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## CMP (Oct 30, 2019)

dronai said:


> I've furnished and installed one RPC in my life. It was for one specific large pump. I've never seen one used for multiple motors, or a whole shop.
> Have any of you guys seen this ?
> 
> The one guy that had a thread going, and continious problems, is the only place I heard of doing that.



It's pretty common for shops in the rural areas to use them for machine shops and wood shops. Far less cost than stringing primary lines for miles and upgrading services. RPC's have a sine wave output, and that can be an advantage on multi motor equipment, with some across the line motors and some driven by drives, such as a CNC machine. 

Most users of equipment don't have the skills to convert a machine to a VFD drive or 1Φ operation, so a RPC is the easier option for them, and less expensive.


In the RPC thread with Mobius87, they weren't having continuous problems with the RPC setup, just the one Asian bar twisting machine. It was pretty small compared to the RPC it was powered from. And that can create problems. A smaller RPC idler motor and capacitor bank would likely run that small motor with no issues, but in this case a small VFD, would likely be the best solution.


For an example of a machine shop powered by a RPC, have a look at Keith Rucker & Vintage Machinery, and few videos from a well re-guarded RPC vendor about his shop.



A typical forum discussion on RPC vs VFD from several perspectives, would give you a better understanding of the need.


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## dronai (Apr 11, 2011)

CMP said:


> It's pretty common for shops in the rural areas to use them for machine shops and wood shops. Far less cost than stringing primary lines for miles and upgrading services. RPC's have a sine wave output, and that can be an advantage on multi motor equipment, with some across the line motors and some driven by drives, such as a CNC machine.
> 
> Most users of equipment don't have the skills to convert a machine to a VFD drive or 1Φ operation, so a RPC is the easier option for them, and less expensive.
> 
> ...



Thanks ! Good information

His new meter socket, and first panel has pass through lugs,but I don't see a main disconnect in there like he says ?


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