Motor Speed Turn Down Ratio
Motor Speed Turn Down Ratio
(OP)
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?





RE: Motor Speed Turn Down Ratio
RE: Motor Speed Turn Down Ratio
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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RE: Motor Speed Turn Down Ratio
"Will work for (the memory of) salami"
RE: Motor Speed Turn Down Ratio
another point to possible think about is the bearing lubrication capabilities at much lower speed than design.
RE: Motor Speed Turn Down Ratio
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.
RE: Motor Speed Turn Down Ratio
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
RE: Motor Speed Turn Down Ratio
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.
RE: Motor Speed Turn Down Ratio
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.
RE: Motor Speed Turn Down Ratio
Best to you,
Goober Dave
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RE: Motor Speed Turn Down Ratio
RE: Motor Speed Turn Down 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.
RE: Motor Speed Turn Down Ratio
RE: Motor Speed Turn Down Ratio
RE: Motor Speed Turn Down Ratio
RE: Motor Speed Turn Down Ratio
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(2B)+(2B)' ?
RE: Motor Speed Turn Down Ratio