VSD Operating Frequency Range 1

[BlackOilsandBitumen]

Hi Guys,

In the very early stages of looking at a VSD for a PD gear pump and my knowledge is purely theoretical (and limited). Basically what is the practical frequency range I can operate a electric motor at to get my flowrates.

Normal supply is 415V 50Hz. Motor is 4 pole. so approx 1500RPM. Pump is PD so flow vs speed is approx linear and insensitive of backpressure.

I have heard that if you back the speed off to far the motor can overheat as the cooling fan is on the shaft.

I would operate say 70% of the time at 46Hz (1380 RPM), 25% of the time at 58Hz (1740 RPM) and 5% of the time at the lowest practical speed (?) for 2 min intervals.

Any guidance on what frequency I could spec the low speed at?

 

[dbaird]

Check:
faq237-1062

David Baird
Sr Controls Designer
EET degree.

http://www.afctool.com

 

[jraef]

Generally, it is only in the lower speed ranges that you need to worry about cooling issues, but there is no hard and fast rule. The reason is that it will depend a lot on the efficiency of your motor and where the losses are. A lot of the losses are what are called "copper losses", which are based on loading, so as your load drops with speed, so do those losses and although the cooling efficiency drops, so does some of the need for cooling. But another portion of motor losses are magnetization losses, which are fixed. So as you lower the speed, those losses represent more of the power used by the motor, and begin to overtake the reduced cooling capacity. That crossover point can be anywhere from 30% speed to 50% speed, it depends on the motor design.

One simple solution is to just use a separately powered cooling fan on the motor if you need one. Whenever the drive is running, the fan is running at full speed because it gets it's power from the line, not the drive. It doesn't hurt the motor to have more cooling air than it needs when losses are lowest (unless you are at sub zero ambient maybe). You will find that a lot of "inverter duty" motors come with separately powered fans, among other benefits. I would look into insisting on that for your PD pump. Also, make sure that you order a motor with embedded thermistor temperature sensors (or RTDs), most modern VFDs will provide an external sensor input, so match the type it needs.


"Will work for salami"

 

[macmckim]

Your PD gearpump from my experience is a constant torque application.
You need to check with the motor supplier - Is your motor suitable for VFD?
What is moktor's possible speed range (50HZ and below)when used on a constant torque application?

As for overspeeding your 1480RPM motor up to 1740RPM - you will need to find out from the pump supplier what kind of torque is required at 1740RPM because when a motor goes into overspeed condition it becomes a constant horsepower unit and will produce approx 15% less torque at 1740RPM.

Macmckim

 

[mikekilroy]

There are old fashion 'rule of thumbs' for min speed or a generic shaft mounted fan cooled motor: typical values is 1/6th base speed for nameplate rated torque. so your 50 hz motor should be fine down to 8hz or 250rpm. No motor should be so bad to not allow 1/3rd base speed (17hz or 500rpm).

To be sure in your case, just watch it at the lowest speed you want to run and make sure it does not overheat.

As for overspeeding, of course torque rating goes down linearly with increased speed, but I have never seen a motor NOT capable of 1.5x base speed, and most are at least ok to 2x base speed. Above that and you risk running into the motor's breakdown torque rating that goes down with square of the speed increase.

 

[BlackOilsandBitumen]

Hi Everyone,

Thanks for input. I have got the workshop manual for the electric motor now and this holds my answers. For this motor in constant torque application motor is good for 60Hz and down below 25Hz will have cooling issues. An electric cooling fan is avaiable from the manufacturer to alow below 25Hz.

Thanks again to all for input.