Single Phase but 3-wires to motor 2

[Bluenoser337]

Looking for interpretation of the attached drawing. Power source is 2-wire, single phase. Wondering if the pump motor is 3-phase (hence the 3 wires going to the motor) using the capacitors for start and run phase shifting, or ??

 

[jraef]

Yes, that's what you have there. SR1&2 are likely voltage relays that change state when the motor reaches close to slip speed. It's a common trick, but results in a significant derate of the motor power, usually 20-30% depending on the nature of the load.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[LionelHutz]

It's a single phase motor with the start winding being switched externally instead of internally via a centrifugal switch. This seems to be relatively common on submersible pumps.

 

[Bluenoser337]

so, LionelHutz...this pump couldn't simply be connected to 3 phase power without changing stator connections if possible? Thanks!!

 

[LionelHutz]

I wouldn't count on it being a 3-phase motor. As I already posted, I would expect that it is a 1-phase motor with the speed switch and capacitors mounted externally and remotely from the motor.

 

[ScottyUK]

Looks like it is a single-phase machine as Lionel suggests, and the switching out of the start capacitor is done using a current relay.


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If we learn from our mistakes I'm getting a great education!

 

[Compositepro]

It looks to me like a static phase converter set-up. Used to operate a three phase motor from a single phase source.

 

[waross]

I'm with Scotty on this. The wires to the motor are run, start and common. The start capacitors may be switched out by a voltage relay when the back EMF comes up on the starting winding.
No three phase.
No phase conversion.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

SPC wiring looks just like that too. An SPC is often called a "Phase Splitter" "that makes a 3 phase motor into a capacitor start single phase motor". The principle is the same, the execution inside of the motor is of course different.

Look at the diagram again more closely. Why would you have the resistor in a cap-start single phase motor with an external start circuit? It's not there on a centrifugal switch version is it? In some SPC designs, they use the resistor to cause a slight delay in the output of that starting cap which creates a phantom rotation to start the motor spinning.

The only thing about that diagram that bothers me is the R, W, B terminal numbers, which looks like Red, White and Black. That seems on the surface to indicate single phase. But that may just be some home-made concept because they planned on using SO cord for the connections and those are the colors you would find in something like 12-3 W/G cord, Black, Red, White and Green.

By the way, I had a brain fart and had to use the term "voltage relay" in my earlier post, I meant "potential relay".

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[ccjersey]

A resister is frequently applied across the start capacitor terminals to reduce contact erosion when the relay opens. This is done more on motors that start frequently and have to operate reliably for long periods. Refrigeration compressors and submensible pumps are two applications where I have seen these used.

 

[jraef]

OK

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[OperaHouse]

If you got shocked by the case, the motor is now toast. Probably from overheating while testing which broke down the insulation. None of those wires should be connected to the case. Time to find another motor.

 

[jraef]

Operahouse, I think you meant to reply to another post that is likely gone now because it was a DIY homeowner post.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[OperaHouse]

So thats why I got the error message when I first posted. Anyway, I put external an timer on a motor that had a fatigued centrifical switch I couldn't find a replacement for. That may be one of the reasons they don't put them on some pumps with the high number of starts they have to do.

 

[electricpete]

The diagram doesn't make sese to me.

The capcitors are labeled SC and RC, which might suggest stand for cap and run cap as I think was suggested.

But look at RC1 for example. It is connected (in series with relay coil) between terminals R and B. But R and B are connected to the input power. So we have RC1 and the relay in parallel with the input power. Does not seem like a run cap which should be in series with a motor winding. Maybe I'm missing something... someone can explain in more details what the heck that circuit is supposed to accomplish?

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(2B)+(2B)' ?

 

[electricpete]

I left out a word... added in bold below:
"The capcitors are labeled SC and RC, which might suggest stand for startcap and run cap as I think was suggested."

=====================================
(2B)+(2B)' ?

 

[ScottyUK]

ePete,



Look at the connection from the junction of the relay coil and RC1 which goes to motor terminal W. I am pretty sure that W is the start winding. It's a really goofy drawing, it could have been drawn a lot more clearly.


I withdraw my earlier remark about it being a current-sensing relay. I blame the goofy drawing affecting my eyes.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[waross]

I withdraw my earlier remark about it being a current-sensing relay. I blame the goofy drawing affecting my eyes.

You are forgiven. Not an easy diagram to follow.
As I read the diagram W is the start winding.
The relay is across the start winding and operates when the back EMF comes up.
The run capacitor is in circuit with the start winding to increase the HP somewhat.
Motor capacitors are generally discharged through the motor winding. The resistor may be a discharge resistor (or not).

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Thanks Scotty. It makes sense now if we imagine start winding between R/W and run winding between R/B. It must have been that darned goofy drawing throwing me off. (it couldn't be that I'm getting old and too many pre-Thanksgiving baking scraps caused blood sugar transients that affected my brain)

=====================================
(2B)+(2B)' ?