effect of motor change out 2

[mel127]

Good day,
We received an order to replace a 300Hp,460V,3 phase,1750 RPM with a 150Hp,460V,3 phase,1185RPM.My question is what will be effect of these motor change out in terms of pressure and some other parameters.By the way the motors are being used in refuelling tank and distributing to other fuel farms.Im kind of lost because with the 300Hp motor the operator told me that at the end of the line sometimes he can get as low as 1psi pressure.I imagine a lower or maybe neglible pressure using the 150Hp motor or maybe im wrong.

Thanks in advance.

 

[electricpete]

If you keep the pump the same and keep the speed approx the same (i.e. same number of poles for fixed-speed non-vfd induction motor), then the 150hp motor should do just as well as the 300hp provided that it doesn't reach a breakdown torque limit during transient operation and doesn't reach it's steady state current limit during steady state operation.

Was the old motor operating at low load (<50%)? I would think that would be the case, or else I'd be asking the person who replaced the motor why they selected this one.



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[electricpete]

There is one more caution: the new motor of course needs to be able to safely start the load as well.

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[jraef]

???
Pumping is all about 3 things: pressure and/or head and/or flow. HP affects (or is affected by) any and all of these things. If, as E-pete says, the existing pump was running at reduced capacity, then the new smaller pump could work OK. If the old pump was running at capacity, I don't see how using a pump with less HP is going to work without reducing the pressure/flow/head.

 

[alehman]

I'm not sure I understand the question. The pump will performance at 1185 RPM will be vastly different than at 1750 RPM. Pump manufacturers publish curves that will help you determine the effect of the reduced speed and the load on the motor. You'll need to know the model of the pump and the specific impeller installed (if it's centrifugal type).

You will need a mechanical engineer to help with the pressure/flow question once you have the pump data.

 

[electricpete]

Good points.

I didn't notice the change in speed in my posts above, so you can throw my response out the window.

IF they are re-using the pump and it's a centrifugal pump and there is no change in the system flow resistance, then the change in performance can be predicted by the pum laws: changing speed to approx 2/3 will decrease flow rate to 2/3, dp to (2/3)^2 and power to (2/3)^3. So you would likely have a decrease in pressure and/or flow. There may be valves to open to increase flow but if the system was struggling before, it will struggle more at a lower speed.

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