PD Pump Low Speed Operation Motor Stall 2

[KenAlmon]

We have a PD Pump that we are just commissioning. The pump is intended to boost pressure for a Meg line from 14 MPa to approx 17 MPa. The turndown is supposed to be 10-1. The Variable Speed Drive has been programmed for 6-60Hz operation. The high pressure to provide suction to the pump is not available so 140 kPa was used. At 60Hz, approx 6000 kPa was generated. At 20Hz, approx 2000 kPa was generated. At 6 Hz, the motor stalls and no output pressure achieved.

My question is:

Is this an adequate test or should we expect at a higher suction pressure, it will be "easier" for the motor to pump and therefore not stall?

Thanks

 

[bulkhandling]

I suspect that it's not because the suction pressure, it's the low frequency that makes the mpump stall. When the frequncy is too low, the check valve at suction side may not be able to be well-shut. If it is the case, then your pump may still stall at low frequncy even the suction pressure is high as your designed. Then you need to review if your pump has to be programed for the low speed operation or not.

 

[smckennz]

This is a tough duty for a VSD/motor combination.
I expect the field windings are not producing enough magnetism to provide the required linear torque demand.
With most of the standard VSDs that I have used, 5:1 is about it and the torque collapses.
However if you really need it, by using rotary encoder feedback and a fixed speed cooling fan on the motor, I have got better than 20:1 (on a belt feeder, if you are wondering). The rotary encoder measures the angular response of the output shaft and adjusts the driving frequency/current accordingly.

An outfit called PDL used to be really good at wide range frequency control.

If you have purchased the motor and VSD separately, you are in for an argument with the suppliers.

Good luck

Steve

 

[jraef]

Actually you an get a 10:1 speed range out of a good Open-Loop Vector drive (no encoder feedback). That PDL drive mentioned is good for that as well (you can get it in the US under the Motortronics brand name). If you have a standard "V/Hz" or "Scalar" VFD, you cannot operate effectively beyond a 6:1 speed ratio. That is more likely your problem.

"Venditori de oleum-vipera non vigere excordis populi"

 

[tonyh]

You need to check that the design is actually capable of doing what you want. If you size a VFD and motor combination for its maximum duty - it will not necesarily have the torque capacity at minimum speed - and this sounds as if it may be your problem. You should get the pump vendor to tell you the power requirement for your worst minimum speed duty, and then calculate what torque you need. Then find out from the VFD provider what torque it will provide at that frequency.

 

[bingopin]

As a side comment to this subject (not that it is related to stalling) better be sure the pump lubrication system will work properly at these very low speeds. I've seen this problem in variable speed PD pumps trying to run at low speeds.

 

[CARF]

That's a smart solution smckennz !

Another idea maybe putting a small by-pass pipe from the pump outlet to the feed tank (with a throttle valve to fine tune). This will force the pump-motor to run a higher frequency while the pump is producing the same pressure/flow.

C.

 

[CARF]

Hi smckennz,

Do you have a manufacturer of this rotary encoder?

Thanks !
CARF

 

[smckennz]

Hi Carf
I think you can get through PDL (Schneider) and also SEW (gearmotor manufacturers). From memory, there are several different types and selection is important; especially ensuring suitable interface with the VSD to close the loop.
If you have no luck, gert back to me and I will look more thoroughly.

Cheers

Steve