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# Minimum Speed of AC Motor2

## Minimum Speed of AC Motor

(OP)
Hi Guys,

I’m a mechanical engineer new to electric motors. Could you please reply to following question.

1. We have a centrifugal pump that required AC motor size was 2 KW but 7.5 KW selected to account for future pressure drops due to changing the line. Is it a problem to use that big motor for small applications except energy loss issue?
2. If we want to reduce the speed of the motor by VFD, how much down we normally can go in conventional electric Motors Like WEG W22 or TECO (without forced cooling)?

Thanks,

### RE: Minimum Speed of AC Motor

1) No problem. Just not very efficient and it will lower your facility power factor some if that's a problem at your facility.

2) This is a more complex question to answer. It depends on the pump style. Centrifugal pumps drop their load rapidly with slow down. Very rapidly! So you often don't need to add auxiliary cooling because you're unloading the motor by the cube of the speed reduction. If you have some other kind of pump then not-so-much.

Because of this same turn-down a centrifugal pump won't pump hardly anything much below 60% speed so even worrying about aux cooling at something slower is pretty pointless.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: Minimum Speed of AC Motor

(OP)
Thank you very much Itsmoked. Apart from pump performance, what is generally minimum speed can be achieved by AC motors? Are you saying that an Electric motor speed can be decreased to 60%?

### RE: Minimum Speed of AC Motor

Electric motors? In general.

A VFD used in V/Hz mode can be turned down to about 5Hz.
A VFD used in Sensorless Vector Mode can be turned down to about 2Hz.
A VFD used in Sensored Vector Mode can be held at full torque a 0Hz.

The second above requires that the VFD be closely matched to motor.

Depending on the load aux cooling could of course be needed.

If you're talking about your pump I can't say because I don't have any information.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: Minimum Speed of AC Motor

msii,

Your described design is quite odd and difficult to understand without data. Pumps don't normally have that sort of range between phases of design.

Your 7.5kW motor will only consume the power it needs, but at low % will be less efficient. This thread has some data https://www.eng-tips.com/viewthread.cfm?qid=475196

But everything else on the motor is base don max power or FLA (Full Load Amps). So the starter is bigger, the cables are bigger, the volt drop when you turn the thing on is bigger, all of which cost money.

I've normally seen VFDs go to about 40 htz to 70 htz with 50 being the "neutral" position. They can go lower but lose torque and power.

The key is matching pump demand to the motor. Provide some details and maybe post in the pump forum and you'll get some answers, but what you're trying to do doesn't sound cost effective or good long term for either pump or motor.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Minimum Speed of AC Motor

Dear Mr. msii (Mechanical)
Q1. " ... We have a centrifugal pump that required AC motor size was 2 KW but 7.5 KW selected to account for future pressure drops due to changing the line. Is it a problem to use that big motor for small applications except energy loss issue?
A1. Electrically, no problem to ( use a motor operating at say 30-40% full-load), except [energy loss] as you are aware of. No specific action need to be taken if it is for a short term... "to account for future..."
Attention: The motor will run at about (or very slightly higher) than the rated speed, at low load.
A1.1 You can control the delivery by mechanical means " except [energy loss] as you are aware of.
A1.2 Is it possible to change the pump [impeller size] ? This would be within your field of expertise.
Q2. If we want to reduce the speed of the motor by VFD, how much down we normally ....
A2. VFD can do the job by lowering the motor speed. It is expensive! It would be far too expensive for short term use, if you are not going to control the delivery in the future.
A2.1 Consider whether above A1.2 would be feasible? It could? be (lower in cost) and [extremely reliable/trouble free].
Che Kuan Yau (Singapore)

### RE: Minimum Speed of AC Motor

You did know that with the existing centrifugal pump, greater pressure drops may require less power, not more power.

Bill
--------------------
"Why not the best?"
Jimmy Carter

### RE: Minimum Speed of AC Motor

Dear Mr. msii (Mechanical)
Q1. " ... We have a centrifugal pump that required AC motor size was 2 KW but 7.5 KW selected to account for future pressure drops due to changing the line. Is it a problem to use that big motor for small applications except energy loss issue?..."
A1. In addition to my earlier post suggesting of a smaller pump impeller, I can think of another possible? solution in the deduction of the speed instead of the use of "fanciful" VFD; which would be " beyond? " what you can handle [without expensive advice/service from the VFD agent/manufacturer].
A1.1 If the motor is NOT direct coupled to the pump, you may? change the pulley ration. This would also be within your expertise. It could? be (lower in cost) and [extremely reliable/trouble free].
Che Kuan Yau (Singapore)

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