Pump motor size

[Smacky]

I have a situation that requires a vertical turbine pump. The pump will need a motor to provide 67-70 horsepower, depending on head. However, the available motor sizes jump from 60 hp to 75 hp, and the cable supplying electricity to the building can't be made any bigger, so 70 hp is the limit. Is it OK to install a 75 hp motor but only supply it with enough electricity to run at 70 hp? Is there a special controller that will allow this?

 

[electricpete]

The steady state power drawn by the motor depends primarily on the pump/fluid system, not on the motor.

The larger motor will likely have higher starting current and may have different power factor and efficiency.

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(2B)+(2B)' ?

 

[Artisi]

Guess you can limit the current to the motor by setting the overloads to below maximum available from the supply but the problem with this is if the pump demand exceeds this setting it will force a shut-down.
As electricpete has pointed out, the system hydraulics dictates the power requirements and to reduce power demand you need to reduce the pump duty which will lower the power requirement, this can be achieved by 1. reducing impeller diameter or 2. imposing more head on the pump.

Remember, power reduce by the cube of the diameter change.

 

[Chance17]

To me your statement "67-70 HP depending on head" raises a red warning flag.
The motor size should be evaluated at "end of curve".
You want the pump to operate throughout its entire dynamic range.
Consider pump start and flowing into an empty pipeline.
The pump starts at "end-of-curve" and backs up the curve as pipe friction is encountered.

 

[chief]

You can control your power requirement by either throttling you discharge valve or fitting an orifice in the discharge. This of course affect your flowrate.

Offshore Engineering&Design

 

[NGLENGR]

Another method (more expensive) is to use a small VFD to limit your amp draw. For a 460V installation, there are NEMA cabinet sized VFDs that can be used to give you exactly the limit of what your utility or lines can handle.

We've used this method in a couple installations where the total draw from a system could be no more than X amps. The amp limit on the VFD prevents the pump from ever hitting overload. Another bonus is improved control and starting characteristics.

Get a good system curve and a good pump curve and see if a variable speed drive could work in your situation.