Slowing a Pump; VFD or static?

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  #1  
Old 06-22-12, 09:53 PM
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Slowing a Pump; VFD or static?

Working on a premise that slowing down the frequency of the power will slow down a 'capacitor start pump', but how?

A VFD might not work as Capacitor-start motors have a centrifugal switch that opens at about 75% speed which will damage the drive. (A supplier's) 1-phase VFD product is only suitable for permanent split capacitor (PSC) or shaded-pole motors on variable torque applications (generally, fans & pumps).

The pump manufacturer says "This pump can be controlled by a VFD, but I would not use above 60Hz, as you can overload the motor, and cause damage".

Is there a way to slow the pump at a set speed? The application does not need variability.
 
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Old 06-23-12, 06:30 AM
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Is there a way to slow the pump at a set speed?
The pump will always operate at the same speed (RPMs) as the motor. Slowing the pump can only be accomplished by slowing the motor. That being said, is it the pump speed or the flow you are wanting to slow down? What is HP of the motor?
 
  #3  
Old 06-24-12, 02:32 PM
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The pump motor is a 2/3 hp tsurumi, 230v, 1ph, 60hz.

The flow is higher than needed...
 
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Old 06-24-12, 04:42 PM
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How about installing a metering valve downstream of the pump discharge, with a valved bypass before the metering valve, returning flow to the intake side of the pump? You should be able to dial in the exact flow you need in this way without overloading the pump. Hope my explanation made sense.
 
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Old 06-24-12, 05:40 PM
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How about installing a metering valve downstream of the pump discharge, with a valved bypass before the metering valve, returning flow to the intake side of the pump? You should be able to dial in the exact flow you need in this way without overloading the pump. Hope my explanation made sense.
In addition, I would put an clamp around ampmeter on the motor leads and watch the amp draw as you throttle the flow up or down and compare it to the listed FLA on the motor nameplate.
 
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Old 06-25-12, 12:05 AM
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Would a method like this reduce power consumption? It seems like the watts used would only go upward if throttling the flow/which would increase the head pressure/psi.
 
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Old 06-25-12, 12:49 PM
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If this is a single stage centrifugal pump then throttling the output IS the correct method of decreasing the flow. Throttling the flow will decrease the power used by the pump even though the discharge pressure rises. This is because centrifugal pumps are what are classified as dynamic pumps and the power input is directly proportional to the flow. Minimum power input will be achieved with no flow through the pump and maximum power input with maximum flow through the pump.

Large centrifugal pumps are always started with the discharge valve closed to reduce the starting load on the driving motor.

Now with multi-stage pumps the procedure may be a bit different as the pump will have a certain minimal flow rate that must be maintained to keep the pump from overheating. This is usually accomplished with a small bypass pipe and valve configuration.
 
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Old 06-25-12, 01:37 PM
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If the motor was three phase, it seems there are more options for slowing the pump such as a
three phase motor combined with a single phase to three phase converter when combined with a
[h=4]OPTIDRIVE E2 - 115VAC 1-PHASE IN, 230V 3-PHASE OUT[/h](My power supply is @ USA, 1ph, 60hz)
 
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