VFD Min speed ?

[BearB]

I am just starting to use VFD's, I have a learning curve. I have an application where I recently installed a small VFD and 1 HP Inverter Duty Motor on a pump. The pump we had in spares and is used to move liquid. The rate Operations needs it to flow came out to be about 1000cc but to get this the drive is only operating the motor at 5hz 1 amp. My question is can the motor operate safely at this slow of a speed?

 

[itsmoked]

Tough question.. Normally if you slow the motor waaay down you need to supply auxiliary cooling.

If the load is very light the point you need this aux cooling may also be very low.

Your best bet is to run a test. With it at your needed flow, if you can rest your hand on it after an hour you are good to go. If you can't anytime before an hour then you will need a temp gun. Use the temp gun to see what the peak temp is. If it exceeds or is close to the RISE rating on the motor's name plate you need aux cooling. Depending on the motor style perhaps a small blower or fan will suffice.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[burnt2x]

Or replace the pump with a smaller output. By doing that, you could run the motor at a higher frequency, hence doing away with motor overheating due to lower cooling fan speed!

 

[NCTHAI]

Too much energy wastage..... Even with using VFD.

Change the pump and motor to suit operation requirements. Be friendly to environment and mother nature.

NC

 

[Phaneas]

We had some problems when we wanted to run a blower on low speeds without using a speed encoder. So that could also be a problem

Kind regards

 

[BearB]

Thanks to all. This site is a great help.

 

[DickDV]

Your answer is going to be about the motor. The drive is fine with your arrangement and the control at 5 hz can be done well also.

You say you have an "inverter duty" motor. That pretty much means that somewhere in the motor data sheet there will be a statement about the lowest speed capability at full torque loading. That will be the piece of data you need for your answer unless you are loading the motor at less than full torque at 5 hz. In that case you can go slower than the spec.

With an inverter duty motor, there is a good chance that the motor has thermal switches inside. Look for P1/P2 leads in the junction box. If the motor has them, connect them to an external fault loop on the drive and turn off the drive's overload calculator. You will get more output from your motor measuring the heat with the thermal switches than by estimating heat with the drive software while getting better protection, as well.

Just to clarify, operation at 5hz is a 10/1 turndown ratio for a 50hz motor and a 12/1 turndown for a 60hz motor.

 

[itsmoked]

DickDV is it common for even a 1HP 'inverter rated motor' to have temp switches in it?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[sow]

You can get problems with the mechanical end of things at too high a turndown. The flow rate turndown will be much more than 10:1 therefore you could be looking at cavitation and thrust issues. Other kit has other problems, e.g. blowers suffer from overheating.

But then again so what? the pumps are tiny - either run it into the ground or buy a new one and put the larger pump back in spares.

If you are having problems you could always throttle the discharge to give you more head and therefore power and speed.