I have Glycol Pumps installed at my plant for glycol circulation. The pump motor is inverter duty motor rated 40HP, 480V, 1775 RPM & 48.2 Amps. The motor is run through a VFD as the circulation rate is not fixed but the speed is controlled manually (not a feedback control loop). The motor is currently running at 800 rpm but the rpm needs to be decreased. Up to what minimum rpm I can run the motor safely?

Dear Ahmed,

First determine what is the minimum Q (flow) you require in your process flow(to be determined by the process engineer)

Second, from the pump curve (furnished by the pump manufacturer), determine
the pump BHP and the RPM to attain minimum Q.(to be determined by the mechl engr.)

Third, reconcile the data from step 1 and steo 2 with the VFD manufacturer RPM and
TORQUE /KW/HP table or curve.(to be reconciled by the plant engineering lead team)

I would guess 800 rpm would be your minimum.At any rate if your only aim is to throttle
down a bit the discharge Q of this pump, you just throttle a little bit your discharge or
inlet valve.

Be careful in the adjustment using this method, as throttling down the pump
inlet may cause cavitation and throttling down too much the discharge valve, would cause
heat build-up in the pump casing.

Happy troubleshooting!

Please log in your results!
Thanks!
Happy troubleshooting!

Your main concern will be heat development in the motor windings and in the motor stator pack.
The main heat source will be caused by the stator current, and the current again will be a function of the torque required from the motor.
Thus, if your pump is of centrifugal type, you can go virtually as slow as you want, but for practical reasons I would limit it to around 20% of full speed (around 350RPM).

If your pump is of screw type, or displacement type, or any other type where the torque does not reduce dramatically as the speed reduces, you will have a different type of limitation. For most air-cooled Induction Motors the cooling efficiency is proportional to the speed cubed, so you must ensure that the current drawn reduces at least proportionally with the speed (frequency), otherwise you may have to install separately powered cooling fan(s).

Ensure that your VFD reduces the output voltage together with the frequency.

Farhan, besides the process/pump concerns, if the pump/motor is installed in a hazardous area so the motor is a certified equipment. I recommend you to look at the motor certificate where should have a minimum RPM. This value is not only to protect the motor but to avoid reach the auto ignition temperature of the local area.

Mr. Knudsen

VFD only reduces the frequency, output voltage remains the same.

Though you already have the correct answer and your problem should have been solved. Reducing the speed of motor also reduces the cooling through shaft mounted cooling fan of the induction motor, you will have to use a induction motor with separate mounted cooling fan. The speed can safely go upto 10 - 20 % of rated speed, The torque requirement is of main concern and thereby the running current, take care of motor cooling and reduce the speed to your requirement.

If that is the case, you better get it re-programmed. Otherwise your motor will draw far too much current at the lower speed. I hope you are mistaken, as the normal setting is for a constant V/f ratio through most of the range, up to "full" frequency - in your case 60Hz, judging from the nominal motor speed. Above that, most VFDs have their output voltage limited by the nominal supply voltage.

If you obtained the "constant" output voltage by measurement, you are using an un-suited voltmeter. The output voltage from the VFD consists of a number of individual pulses, that each have a voltage equal to the supply voltage, but of varying duration, according the so-called PWM, or Pulse Width Modulation, so it for the motor "looks" as if the voltage varies in proportion to the output frequency.

As Farhan mentioned, if the installation is in a hazardous area, the motor and the VFD must be certified together as one unit. (At least in the jurisdictions where I work).

minimum speed depends on torque, many answers detailed that. But also pay attention that reducing speed in pumps will drop the flow in a cubic relationship and you will lose also the head on linear relationship.
according to one answer saying you can go down to 20% means you reduce speed by 80% and in this case your new head will be 20% from the previous, and flow will be only 4%.... No flow! Power losses will be incredible 
ih pumps it doesn't they'll to go less than 75%. That's personal opinion