Motor Speed Turn Down Ratio

[steven17]

I have a 40 HP, inverter duty, 1800 rpm motor fitted with VFD for sewage pumping application. Can this motor speed be turned down 6:1, i.e. down to 300 rpm without overheating issues?

 

[ScottyUK]

How does the load torque vary with speed?

 

[DRWeig]

ScottyUK, here's a typical vs theoretical curve:

http://www.engineeringtoolbox.com/pumps-speed-torque-d_1114.html

steven17, you need to find the 16 percent speed characteristic for your specific pump and your specific head, and then determine the flow at that point. You might find it's way lower than you would expect. You'll have to find the minimum safe flow from the pump manufacturer. Don't forget motor cooling and bearing oil movement.

From my experience, 30 percent flow is the very lowest I have seen for a pump. Most often I see designs specify 40 to 50 percent flow minimum.

My opinion is that 6:1 is stretching it pretty far.

Best to you,

Goober Dave

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[jraef]

Addressing ONLY the motor issue, in general an "inverter duty" motor is designed for at least a 6:1 turn down ratio. But unfortunately, there is no accepted standard for what "inverter duty" means, so it is up to the motor mfr. Therefore, the only valid answer must come from them.

"Will work for (the memory of) salami"

 

[rotw]

to add to DRweig comment, not sure if is totally safe also with regard to critical speed of the driver and driven equipment.
another point to possible think about is the bearing lubrication capabilities at much lower speed than design.

 

[ScottyUK]

Thanks Dave,

The reason for the question was that sewage is a mix of all kinds of, erm, crap and if it has a lot of solids and rags etc then it won't necessarily follow the normal pump laws. If it is a macerator type pump then characteristic different again.

 

[steven17]

Thank you all for good comments and responses.
The pump in consideration is a rotary lobe (positive displacement) pump that has flow ranges from 20 gpm (at low rpm) to 200 gpm (at high/full rpm of 60 Hz). The pump is direct driven by a motor through a gear box. The pump supplier says that the pump output can be varied without problem as long as the motor and VFD can handle it. The pump rpm at 20 gpm is approximately 1/6 of the pump speed for 200 gpm. The pump horsepower will, of course, increase with higher rpm. The motor will be sized based on the high/full rpm. So far, I am not sure if I got the answer I am looking for. I am not an electrical engineer, so I might not have provided all the information you may need. If I don't get an answer from this thread, I may have to contact the motor manufacturer. It appears that the answer to my question is not simple and straightforward.

Thanks,

Steven

 

[ScottyUK]

The key to the answer lies in how much current the motor is demanding at reduced speeds, which is a function of torque demanded by the pump.

My opinion based on the handful of lobe pumps I've encountered, and in the absence of a speed-torque curve for the pump, is that you won't get a 6:1 turndown without adding an auxiliary blower because that design of pump requires more torque at low speed than a centrifugal type. If your pump produces 1/10 flow at 1/6 speed then it isn't a PD pump - flow is proportional to speed in a PD pump.

 

[ozmosis]

steven17
For rotary lobe pumps we typically advise no more than 1:4 downturn but it also depends what you're pumping; i.e, on the viscosity of the stuff you pump. Lower the viscosity, the less you can turn down and vice-versa.

 

[DRWeig]

Thanks for pushing the clarification, ScottyUK. I wasn't thinking of the potential chunkiness of sewage.

Best to you,

Goober Dave

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[ScottyUK]

That's a lovely description. [flush]

 

[macmckim]

PD pumps are constant torque (CT) application. You need to check with your motor supplier to find out what your 40HP CT turn down ratio is. If motor is a newer premium efficiency VFD duty motor, most brands offer a 10:1 CT ratio.
Also check your VFD drive mfg to confirm the type of VFD you have. Is VFD rated for varible torque (VT for fans and centrifical loads) or it a constant torque VFD. Many VFD brands / mfg's sell two types, VT and CT.

 

[steven17]

Thank you for more useful responses. Based on responses to date, I am thinking of getting a premium efficiency, VFD duty, constant torque motor, which seems to provide the answer to my question. I don't need 10:1 turn down. I will be happy with 6:1 turn down capability without overheating issues. I will check with the motor supplier before proceeding with my plan. Thanks.

 

[ScottyUK]

Avoid the constant torque / variable torque marketing BS. Size the drive based on current rating and overload margin, which are the main parameters hidden behind the marketing weasel words.

 

[iop995]

With a V/f=ct inverter can't have constant torque at 1:6 downturn ratio. Torque will be about 36 times lower than nominal. You need a vectorial control inverter and maybe need aditional forced cooling.

 

[electricpete]

iop995 - you lost me on the factor of 36.

=====================================
(2B)+(2B)' ?

 

[iop995]

Sorry, my mistake. Forgot my last post...