Undersize motor 2

[LeitoGM]

Good Day All,

I've been trying to research the effects of having an undersized motor on a pump with some luck, but I would like a second opinion. Would the pump maximum flow/pressure capacity decrease? or would the motor run at a higher capacity but draw a lot more current (than practical) as a result.

If the motor is at overload, what would be some of the common consequences?

Regards

 

[bimr]

Centrifugal pumps are said to be overloaded when the driving motor draws excess power consumption. This overloading of the pumps can be caused by various reasons but one reason is operating far to the right of the BEP.

Centrifugal pump operation to the right of the BEP causes various problems such as cavitation may occur, shaft may undergo deflection, mechanical seals and bearing can be stressed. This can cause a drop in pump efficiency and an increase in power consumption by pumps, which may cause the motor to trip.

If the motor was overloaded, more current will flow through the motor than the wiring was designed for.

The result of the excess current is that the wiring may overheat. If the overheating is excessive, the wiring insulation will melt and the motor will short out.

 

[Artisi]

If the motor is undersized and the pump input power exceeds the maximum motor output, the motor will either trip on overload (assuming over current protection is available) or it will run until as such time as it fails from excess heating of the windings.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[BigInch]

I think he wants to know how you can possibly get 47HP out of a 25HP rated pump motor, even if it's just for a few seconds. Am I right?

 

[LittleInch]

A fixed speed motor, which is what I assume you're talking about, will draw or consume energy at whatever it needs to try and maintain that speed. If the shaft power required at the rotational speed of the motor is say 20kW, then the motor will try and supply that 20kW. This is regardless of whether it is "rated" for 10kW or 30kW. This can happen when a pump changes duty and no one checks, e.g. it goes from pumping Gasoline to pumping crude oil.

There are issues when you overload motors in terms of "slip" whereby the speed of the motor goes down even thought he input frequency stays the same, which also leads to higher temperatures.

Overloaded motors heat up more than their cooling system can cool them down or too much heat is generated in the windings. Most motors will trip either on excess amps or the thermocouple in the windings will trip the motor, providing they are set and fitted. If not eventually your motor will either catch fire or simply fuse as the insulation on the windings melts. Neither is a good thing.

why do you ask?

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

 

[TenPenny]

"If the motor was overloaded, more current will flow through the motor than the wiring was designed for. The pump may will overheat."

More likely the MOTOR will overheat.

 

[gempump]

If you use an undersized motor on a pump,the pump will not run normally,let alone the maximum Flow rate and Head.

Heavy-Duty Centrifugal Slurry Pump Manufacturer

http://www.gempump.com

 

[ScottyUK]

That really depends on just how undersized the motor is, and the shape of the motor's speed-torque curve. It will almost certainly run at increased slip unless the motor and pump are so absurdly mis-matched that the motor's locked rotor torque can't turn the pump over at all. The final speed reached depends on the shape of the motor speed-torque curve and the pump curve. If the final speed is less than the motor nominal speed then the motor will draw abnormally high current from the supply and will trip on overload.

 

[moltenmetal]

Wow, all these responses and yet the OP has not properly described what they mean by "undersized motor", or even what type of pump it is!

ASSUMING that it is a centrifugal pump with a fixed speed motor (no VFD), the motor is only undersized IF the desired operating FLOW is above the flow at which the motor's design power is reached. It's the flow that matters, not the pressure, and the flow is determined by the frictional loss of the system. If the frictional losses of the system are such that the pump cannot generate enough head to cause that much flow, then the motor is not undersized for that duty. Some people think you must always size a pump's driver for "full run-out", meaning the flow you would get with a flooded suction with no frictional loss and a fully open discharge, i.e. operating the pump to the far right hand side of the head/flow curve, well to the right of BEP. That is definitely not always the case!

Large pumps start to have durability problems if not operated close to their best efficiency point, and of course you want that anyway because they consume a lot of power. But smaller pumps (say less than 10 hp) are often run nowhere near their BEP for their entire lives- often far to the left of the BEP at a high head, low flow and hence low power case, and as long as their minimum flow is met, they do so without significant ill effect unless something else (cavitation, surging etc.) is causing the damage.

If the motor is operated beyond that max driver power (flow) point, it will pull more current than it is allowed in design, and in a properly designed system the motor will trip the overload.