Simple question about motor current draw and power 2

[Glenfiddich]

I'm wondering about the current draw from a constant speed pump motor. If the discharge control valve from a pump discharge is throttled more, would that make the motor draw more current and therefore increase the power? Or is the motor taking constant current regardless of what is happening with the discharge control valve?

Thanks

 

[ScottyUK]

Depends on the type of pump. For example, a positive displacement pump will behave very differently to a centrifugal type when throttled.


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

 

[rovineye]

Also depends on how much the discharge is throttled. Need pump curves to determine.

 

[Glenfiddich]

This would be a centrifugal pump. Let's keep it general and not too specific. Hydraulic power is a function of pump head. When the pump head is increased, the hydraulic and driver power increases. Would that cause the pump to draw more current? Or is the current flow always constant?

 

[dpc]

More power required by the pump = more current required by the motor.

 

[jraef]

Generally, Current (I) in a centrifugal pump follows the flow rate (Q) of the pump. But there is also a minimum amount of current that will be drawn even if there is no flow, such as when the head is insufficient to raise the column to where it needs to go, or a flow control valve is closed off completely.

So assuming that when you first said "If the discharge control valve from a pump discharge is throttled more..." you meant throttled DOWN, as in closed off more, then the motor current would in fact go DOWM as the Q is reduced.

 

[Glenfiddich]

Great. That's what I thought happened. A colleague is under the impression that current and, thus, power (P=IV) is always constant in a pump motor. Really, current/power changes with respect with flow.

 

[itsmoked]

Absolutely.

BTW are you sure the hydraulic pump isn't positive displacement though? I'm not sure how you could ever get 3,000psi out of a single stage centrifugal pump!


I suspect it is a positive displacement pump and would indeed draw more current if throttled.

It could be a pump with a built-in relief valve. In which case it always puts out so much pressure and hence the relief valve(bypass)is always open and as the load is throttled the relief just carries more. This would not behave in the classical manner described previously.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Glenfiddich]

Woah, where did the 3000 psi come from?

 

[jraef]

Keith,
Methinks he was using the term "hydraulic" as in general liquid flow, not as in Hydraulic Power Unit.

 

[itsmoked]

I guess you're right Jeff.

Normally when you say "hydraulic" and "power" in the same sentence it means "hydraulic power equipment".

But that's cool.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Here is a demo I did to make the point.

The system running with the ball valve wide open.
The pump is consuming about 100W of power doing maximum work, which is moving about 1100gals/hr of water. Note the solid column of water returning to the tank. 103.8W

Now for some throttling. The ball valve is about half closed.
Notice the flow has dropped substantially. Notice also the power has not risen. The pump is not working harder.. It is working LESS. The power has decreased. 72.7W

Now the valve has been closed about 25%. Power has dropped still further.
57.9W

Here I have closed completely, blocked, the flow. Notice the power has dropped further yet! Of course it is not zero as there is a fan being run, bearing friction is present. Windage is occurring in the motor. There is resistance in the windings and there is some recirculation occurring in the pump head. Over time - minutes - the pump head would get warm. If there was the slightest trickle of water allowed thru the pump there would be no measurable heating as water can carry a great deal of heat away.
47.7W

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Did you just build that for this thread?

 

[itsmoked]

Yes..












I kid.. but I did it in Feb. Funny how many times it has come in handy. It should be in any engineering text book that talks about pumps.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

Could also be a good illustration of my prostate problems....

 

[electricpete]

A pure radial flow centrifugal pump behaves as described above over most of its range.

A pure axial flow type pump (propeller type pump) behaves the opposite. Decreasing flow by throttling down on a flow favle increases the power draw over most of the range.

A multi-stage mixed flow pump can be a combination. Results depend very heavily on where you are on the range.

Rightly or wrongly, axial flow pumps are often called centrifugal (they follow the centrifugal pump laws).

So, I agree with Scotty: "it depends". Even if you are talking centrifugal pumps.

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Eng-tips forums: The best place on the web for engineering discussions.

 

[jraef]

I have wondered about that with axial flow pumps actually. I observed that on an application once but the pump guy convinced me at the time that an axial flow pump is NOT a centrifugal pump. Should have paid more attention in those classes....

 

[Glenfiddich]

To give you an idea of the details, the pump is supposed to pump out two different fluids. One fluid is half as dense as the other, so the control valve should ensure that the flow and TDH is the same for both fluids.

When both fluids are set for the same flow and TDH from the pump, the power draw is higher with the denser fluid.

 

[itsmoked]

Makes sense!

Keith Cress
kcress -

http://www.flaminsystems.com