How to Determine if a pump is 240 or 120

[Dennis Alwon]

Curiousity

I was at a new construction job I will be doing and they had an old well pump that has been there for many, many years. There is no visible way to check the pump nameplate etc. The builder wanted to use a generator to run it but I had no idea whether it was 240V or 120V. It is an old enough site that it could be 120V.

Is there a way to determine this thru educated guess knowing resistance etc. Is there a standard resistance one would expect for a 240V motor?

We will not be hooking it up because there is a dead short reading to ground but I was wondering if someone could figure it out.

I said we could hook it up 120V and see how it ran -- that would help me determine if it were 240v or 120V but..... Any help

 

[hillbilly1]

If it is a submersible, I don't recall any being 120 volt, but if its a jet pump, it could be either way. If its a real old submersible, it's probably a three wire, so a control box would have been required.

 

[Dennis Alwon]

If it is a submersible, I don't recall any being 120 volt, but if its a jet pump, it could be either way. If its a real old submersible, it's probably a three wire, so a control box would have been required.

It has an equipment grounding conductor and 2 conductors. You may be correct that submersibles were never 120V-- can't recall.

 

[hillbilly1]

It has an equipment grounding conductor and 2 conductors. You may be correct that submersibles were never 120V-- can't recall.

It may not be an equipment ground, many of the older pumps were not grounded, the third wire could be the start winding. Did they actually use green? Or is it black, yellow and red? You know as well as I do, well drillers will rig just about anything! :lol: ( that may explain why it's reading a short to ground)

 

[Dennis Alwon]

It may not be an equipment ground, many of the older pumps were not grounded, the third wire could be the start winding. Did they actually use green? Or is it black, yellow and red? You know as well as I do, well drillers will rig just about anything! :lol: ( that may explain why it's reading a short to ground)

The green wire was attached to the pressure switch ground screw. I am pretty certain it is an equipment grounding conductor.

 

[hillbilly1]

Too bad you don't know the manufacture, or even the HP, their troubleshooting guides usually gives the resistance values for troubleshooting, looks like the builder is having to pull it anyway.

 

[GoldDigger]

If it is a submersible, I don't recall any being 120 volt, but if its a jet pump, it could be either way. If its a real old submersible, it's probably a three wire, so a control box would have been required.

Low horsepower submersible pumps are available in 120 volt configurations. But they are seldom used for deep wells because of the larger conductors needed to limit voltage drop.
If at the time the well was put into service only 120 volts was available, then they would have chosen a 120 volt pump.

The control box for the pump at the surface, if still there, should allow you to figure out what voltage was being run down to the pump.

 

[hillbilly1]

Low horsepower submersible pumps are available in 120 volt configurations. But they are seldom used for deep wells because of the larger conductors needed to limit voltage drop.
If at the time the well was put into service only 120 volts was available, then they would have chosen a 120 volt pump.

The control box for the pump at the surface, if still there, should allow you to figure out what voltage was being run down to the pump.

Yeah, if this had been a shallow well, (Usually bored, not drilled) I figured they would already have pulled it and known the voltage. Also from what Dennis has said, it's a two wire pump, so no control box was needed.

 

[Dennis Alwon]

If there was a control box I wouldn't have an issue. I was more curious if there was a way to guesstimate what the resistance may be for a 120V or 240V pump. I know it is no guarantee but more an educated guess.

Of course I would probably have to know the hp. If I assume 1/2 hp (9.8 amps) then I would expect around 12 ohms or so if the Voltage was 120. Whereas the ohms would be 48 or so for a 1/2 hp (4.9 amps) at 240v. I did read 10 ohms between the two hot legs but there was also a short to ground so I am not sure how that would affect the reading.

If the 10 ohm reading was close then I suspect it is a 120V motor. Or does ohms law not really work with motor--- I mean to some degree.

 

[GoldDigger]

If the 10 ohm reading was close then I suspect it is a 120V motor. Or does ohms law not really work with motor--- I mean to some degree.

Ohm's law does work with motors, but over much of the starting and operating current range the complex impedance and the reverse EMF generated by motor rotation are more important than the resistance.

If a 1/2 HP 120 volt motor has an FLA rating of 9.7 amps AND, for example, the LRA is three times that, then the DC resistance you read on the windings would have to be 3 ohms or less to allow that to happen!

If you can find an actual 1/2 horse pump (of either voltage setting and any brand) for comparison, you can make resistance readings on it to get an idea of ballpark values. Or look at a pump manufacturer's installation or troubleshooting instructions to see what value to expect.

 

[hillbilly1]

Here is a link to the resistances. http://www.deanbennett.com/testing-submersible-pump-motors.htm

 

[GoldDigger]

Here is a link to the resistances. http://www.deanbennett.com/testing-submersible-pump-motors.htm

Extrapolating from that, Dennis has a 1/4 horsepower 240 volt pump with a winding to ground short.
Pull it!

 

[hurk27]

I have only seen a control box on Franklin pumps, with the start/run caps in it, all the rest I have seen don't have a control box and are connected directly to the pressure switch, if it is a 240 volt it had both current conductors on the pressure switch, and if it was 120 volts it only had the hot, sometimes the pump installer would parallel the two sets of contacts on the pressure switch when they used a 120 volt pump, I have also seen where both the hot and neutral was landed on the pressure switch, but around here both voltages were used in submersible pumps?

Remember when you read the leads to the motor you will be reading the start circuit also but luckily since there is a cap in series with the start winding you will only have to know what the resistance of the run winding should be as it will be the only one you see after the cap bleds off.

 

[Dennis Alwon]

Extrapolating from that, Dennis has a 1/4 horsepower 240 volt pump with a winding to ground short.
Pull it!

Yes, the won't even pull it because they will dig a new well as that is very old. They were hoping they could use the pump since it was there but that ain't going to happen.... . I was just curious about it.

 

[GoldDigger]

They were hoping they could use the pump since it was there but that ain't going to happen.... .

Two more comments on the way out the door:
If the well is old enough, they may not even have been able to pull the pump without damage if they tried.
Newer pump designs and materials make it false economy to try to reuse an old pump unless it is already in the well and you know exactly what was installed and when.

Make that three: The owner/GC will need to follow local codes on abandoning the old well. Here in CA abandoned wells have caused serious problems when the casing eventually failed and allowed pollution from surface water or upper aquifer to get into a lower aquifer. Not an electrical problem but I feel strongly about the subject.

 

[hurk27]

Yes, the won't even pull it because they will dig a new well as that is very old. They were hoping they could use the pump since it was there but that ain't going to happen.... . I was just curious about it.

Even if you had good readings and knew the correct voltage, if the pump hasn't been used in a while it's probably silted in and the pump would be froze up with silt in the impeller.

 

[hillbilly1]

Two more comments on the way out the door:
If the well is old enough, they may not even have been able to pull the pump without damage if they tried.
Newer pump designs and materials make it false economy to try to reuse an old pump unless it is already in the well and you know exactly what was installed and when.

Make that three: The owner/GC will need to follow local codes on abandoning the old well. Here in CA abandoned wells have caused serious problems when the casing eventually failed and allowed pollution from surface water or upper aquifer to get into a lower aquifer. Not an electrical problem but I feel strongly about the subject.

Yeah, there starting to require concrete pads in a 4' circumference around the well to prevent surface water penetration now, the have probably been doing that out west for a while, but it just now hitting the southeast. My sister is trying to adopt a kid, and they made her do it to an existing drilled well. I noticed it being done recently on an old bored well, and wondered what was going on. (By the way, I'm slap dab in the middle of gold country!)

 

[kwired]

Too bad you don't know the manufacture, or even the HP, their troubleshooting guides usually gives the resistance values for troubleshooting, looks like the builder is having to pull it anyway.

If there was a control box I wouldn't have an issue. I was more curious if there was a way to guesstimate what the resistance may be for a 120V or 240V pump. I know it is no guarantee but more an educated guess.

Of course I would probably have to know the hp. If I assume 1/2 hp (9.8 amps) then I would expect around 12 ohms or so if the Voltage was 120. Whereas the ohms would be 48 or so for a 1/2 hp (4.9 amps) at 240v. I did read 10 ohms between the two hot legs but there was also a short to ground so I am not sure how that would affect the reading.

If the 10 ohm reading was close then I suspect it is a 120V motor. Or does ohms law not really work with motor--- I mean to some degree.

You also need to know how long the leads are to the motor and consider the resistance of them.

Motors have low impedance when no A/C current is flowing. The expected 12 ohms you mention is correct, but it is total imedance which will include resistive and inductive components at rated voltage and current.

 

[Haji]

Here is a link to the resistances. http://www.deanbennett.com/testing-submersible-pump-motors.htm

Finding the status of 'degree' of grounding of phase leads of the (submersible) motor with an ohm meter may not be correct. An insulation tester is more appropriate for that.

 

[kwired]

Finding the status of 'degree' of grounding of phase leads of the (submersible) motor with an ohm meter may not be correct. An insulation tester is more appropriate for that.

If resistance is low enough to read without an insulation tester what more are you going to learn about the situation by using an insulation tester?