Submersible Motor 2 or 4 pole, 75 HP

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tom baker

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We are getting bids for a submersible motor, 460V, 3 phase, 75 hp. A 2 pole is $28,000, and a 4 pole is $53,000 - this price is for pump and motor installed. They are both high quality cast iron pumps
The 2 pole motor is 3600 rpm and the 4 pole is 1800 rpm. The difference is substantial in cost.
Any comments on which is better?

By the way we have some Byron Jackson (now Flowserve) submersibles that have 80,000+ hours with no service, and these are 4 pole....
 

Besoeker

Senior Member
Location
UK
We are getting bids for a submersible motor, 460V, 3 phase, 75 hp. A 2 pole is $28,000, and a 4 pole is $53,000 - this price is for pump and motor installed. They are both high quality cast iron pumps
The 2 pole motor is 3600 rpm and the 4 pole is 1800 rpm. The difference is substantial in cost.
Any comments on which is better?

By the way we have some Byron Jackson (now Flowserve) submersibles that have 80,000+ hours with no service, and these are 4 pole....
Generally a higher speed motor will be smaller so lower material cost.
Better? I don't know you would quantify that as a selling point bid. Mechanical parts would likely wear out quicker with a faster motor.
But that win or lose you the bid?

I've been involved in quite a few pumping station projects. I don't recall any 2-pole motors. But my expertise is not on the hydraulics. Maybe that's where you should address your question.
 

Jraef

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Electrical Engineer
This is NOT an "electrical" issue, it's purely mechanical!

Pump speed has to do with flow, pressure, head etc. If someone determined that a pump needs 75HP, that was based on specific values of flow/pressure/head and those ALL depend on the speed of the pump. If a pump required 75HP at 1800 RPM and you put in an 3600RPM motor, that pump is going to need 600HP because power required by the pump varies at the cube of the speed change. So in reality, your 75HP motor will simply overload in a few seconds. It is the PUMP DESIGNER who determines the motor speed, not the electrician.
 

retirede

Senior Member
Location
Illinois
This is NOT an "electrical" issue, it's purely mechanical!

Pump speed has to do with flow, pressure, head etc. If someone determined that a pump needs 75HP, that was based on specific values of flow/pressure/head and those ALL depend on the speed of the pump. If a pump required 75HP at 1800 RPM and you put in an 3600RPM motor, that pump is going to need 600HP because power required by the pump varies at the cube of the speed change. So in reality, your 75HP motor will simply overload in a few seconds. It is the PUMP DESIGNER who determines the motor speed, not the electrician.

Interesting analysis. Presumes the exact same pump is connected to both motors.

I had presumed that the different speed units had appropriately sized pumps to provide equivalent performance.

A pump designer may be offering hydraulically-equivalent alternatives, one operating at 1800 RPM, the other at 3600.
 
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Interesting analysis. Presumes the exact same pump is connected to both motors.

I had presumed that the different speed units had appropriately sized pumps to provide equivalent performance.

A pump designer may be offering hydraulically-equivalent alternatives, one operating at 1800 RPM, the other at 3600.
That would have been my thought, especially for a submersible. They come as units in my neck of the woods and I as an electrician have no choice or say in the matter. Just hook it up after.
 

Jraef

Moderator, OTD
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Location
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Electrical Engineer
Interesting analysis. Presumes the exact same pump is connected to both motors.

I had presumed that the different speed units had appropriately sized pumps to provide equivalent performance.

A pump designer may be offering hydraulically-equivalent alternatives, one operating at 1800 RPM, the other at 3600.

That would have been my thought, especially for a submersible. They come as units in my neck of the woods and I as an electrician have no choice or say in the matter. Just hook it up after.
We are getting bids for a submersible motor, 460V, 3 phase, 75 hp.

For the same flow/head/pressure, they would not be the same HP at the two different speeds... so something is wrong there. Remember, HP is a shorthand expression of torque and speed, so 75HP at 3600 RPM is twice as much torque as 75HP at 1800 RPM. Either one of them made a mistake somewhere in the performance specs, or they are not both 75HP and the two suppliers are bidding on performance, not HP size.
 

Dzboyce

Senior Member
Location
Royal City, WA
Occupation
Washington 03 Electrician & plumber
Yes, 4 pole or 1800 rpm motors are usually at least twice the price of 2 pole or 1800 rpm submersible Pump motors. Probably 95% of submersible motors sold are 2 pole motors. For the same performance, the 4 pole motor will require a larger diameter set of bowls. Both motors are nominal 8”. The 2 pole motor might use a set of 8” bowls, shine the 4 pole motor will need a set of 10” bowls.

when you look at line shaft turbine pumps, they are almost exclusively 4 pole motors till you get over 700 hp. Then there’s a lot of 6 pole motors.

you are also probably dealing with a “sole source” supplier that has very high margins. *umptech has done a very good job of convincing engineers that only their products will work.
 

GoldDigger

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Location
Placerville, CA, USA
Occupation
Retired PV System Designer
For the same flow/head/pressure, they would not be the same HP at the two different speeds... so something is wrong there. Remember, HP is a shorthand expression of torque and speed, so 75HP at 3600 RPM is twice as much torque as 75HP at 1800 RPM. Either one of them made a mistake somewhere in the performance specs, or they are not both 75HP and the two suppliers are bidding on performance, not HP size.

For a constant displacement pump the product of head and flow will be very close to the required mechanical power input. Since the volume and stroke are freely variable in the pump design to match the selected motor characteristics. So your statement that the flow/head/pressure would be different if the speeds are different does not hold.

For a centrifugal type pump the power required does not depend critically on the head/pressure but instead is simply proportional to the flow, with the proportionality basically being based on the rotational speed to which the fluid is accelerated in the pump cavity. It seems obvious that for half the rotational speed the linear speed of the fluid will be half and the energy given to it will be one fourth. But this assumes that the dimensions of the pump cavity are the same. That assumption is not valid and doubling the radius of the pump cavity could bring the edge speed back to the same number. Counteracting that for a submersible pump is the possibility that the submersible pump cavity radius is limited by the well bore. If that limitation does not come into play there is no fundamental reason that the required HP for a given flow rate could not be the same for both pumps. The available maximum head/pressure might or might not change.
 

Besoeker

Senior Member
Location
UK
For the same flow/head/pressure, they would not be the same HP at the two different speeds... so something is wrong there. Remember, HP is a shorthand expression of torque and speed, so 75HP at 3600 RPM is twice as much torque as 75HP at 1800 RPM. Either one of them made a mistake somewhere in the performance specs, or they are not both 75HP and the two suppliers are bidding on performance, not HP size.
Or they are two different pumps. The OP's wording certainly reads that way.

"They are both high quality cast iron pumps"

Don't you read it that way?
 

augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
I may misread (one of my habits) but I think its one application with two possible pumps to choose between.
Beyond that I am totally ill equipped to comment.
 

Besoeker

Senior Member
Location
UK
I may misread (one of my habits) but I think its one application with two possible pumps to choose between.
Beyond that I am totally ill equipped to comment.
Yes, two different pump sets. If it's going in a borehole, size may be a constraint as in the diameter to get down the hole.
 
Location
WV
What is it going to do?

What is it going to do?

If you have a hand in this decision you need to be giving the pump folks your expected end results. Expected flow rate at what total dynamic head. That in its self will lead to the need for the difference between 2 and 4 pole motors.
If not hook upp what lands on the site and let the designers worry about the end results.
 

retirede

Senior Member
Location
Illinois
Submersible Motor 2 or 4 pole, 75 HP

If you have a hand in this decision you need to be giving the pump folks your expected end results. Expected flow rate at what total dynamic head. That in its self will lead to the need for the difference between 2 and 4 pole motors.

No
It is plausible that a smaller pump (2 pole motor) running at 2 times the speed of a larger pump with a 4 pole motor could meet the same volume / pressure requirements.

I suspect that the pump supplier is quoting both options. And the faster, smaller pump will cost less.
 

Dzboyce

Senior Member
Location
Royal City, WA
Occupation
Washington 03 Electrician & plumber
In submersible motors, a 4 pole costs WAY more. I just looked at my price list for Hitachi submersible motors. List price for a Hitachi 75 hp 2 pole motor (8” diameter) is $16,666. List price for a Hitachi 75 hp 4 pole motor (which is 10” diameter) is $56,868.
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
In submersible motors, a 4 pole costs WAY more. I just looked at my price list for Hitachi submersible motors. List price for a Hitachi 75 hp 2 pole motor (8” diameter) is $16,666. List price for a Hitachi 75 hp 4 pole motor (which is 10” diameter) is $56,868.

The bearings might end up lasting twice as long at the lower speed. :)
And the downtime and expense for repair or replacement might be great enough to justify the longer lived pump.
Less noise and vibration if that should happen to be important.
 
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