AB PF70. What parameters for allowing full V below nominal f ?

Hi.

I have an application where some drive motors needs some extra "oomph" for a short stint when running below nominal frequency.
Normally, the voltage is reduced to keep V/f constant, but I want to override this. It will be for a short period of time, and the motors have thermistors so it should be OK.

What parameters do I have to change for this to happen ?
The drive is an AB PF70.
I think I have to set parameter 53 Torque Perf Mode = 2 (for Custom V/Hz). But then what ?

Yes, set 53 t ocustom v/hz, and then use parameters 69 and/or 70 to set the start boost voltage level and the run boost voltage level.

Thanks Paul.

It says in the manual that the limits for parameters 69 and 70 are from 0 V to nameplate volts * 0.25.
Does it mean I can get max 0.25+0.25 boost combined ?
There is a small diagram in the manual, but it doesnt really say clearly what is the difference between Start and Run boost.

The Start Boost will only operate up to a specific frequency (generally around 20hz). The Run Boost operates over the whole frequency range.

A word of caution here: There is no free lunch, not even with motors. Overvoltaging a motor can add a little torque to the output in some motors (these are described as being "voltage starved") but, for most motors, you can't do much of that before the magnetics start to saturate. When that happens, more voltage will get you less torque and lots of extra heat.

So, approach this with caution and watch for a peak in output torque. As soon as it starts to fall, back off to the point where it peaked. That's all the motor will give you.

Thanks Dick. I will heed your advice.

Would you say that the 0.25 + 0.25 are safe limits or that I should start much lower ?
The "low speed" where I need the boost will be somewhere between 10 and 20 Hz. And it shall run at this speed for 15-30 seconds. There is definitely some sticktion to overcome at the very start.

I would only use the Start Boost and start near zero going up slowly.

Actually, this is a PF70 so why not switch to sensorless vector control. That was its all managed automatically at all speeds. You will notice a nice (not huge) increase in available torque especially in the 1-20Hz range.

I am just trying to learn so in what applications would you use this?

how would you overvolt hte motor if the range is only nameplate + 0.25?

is that 25%? i.e max is 600 volts?

is that what dickdv means by overvolting?

Please excuse so many dumb questions this is my first yaer of industrial electrical.

Kid
Do not apologize for questions - they are how you overcome ignorance.

GENERIC VFDs control voltage to line volts divided by frequency. So for 240 volt it is 4 V per Hz. This ratio is maintained from 0 to "base speed" ie max freq and RPM in USA this would be 60 Hz and 3600 RPM (2 pole). Over base the voltage is held constant as freq is increased. We will assume 2 pole 3600 RPM and 60 cycle motor.

Torque boost is done to allow motor a little more torque at start
FREQ 0 1 2 4
VOLT 0 4 8 16
the above is normal -- now we will do boost I assume 25% added
SO boosted
VOLT 0 5 10 20
Values are for example and not set in stone. You would only need to do this at the lower end of the motor speed range say from 0 to 10 (600 RPM) or maybe 20 Hz (1200 RPM).

Dan Bentler

Dan thanks for the example that helps clear things up.

In dickdv post I see a caution about overvolting and voltage starving? That seems to be contradicting to me so what am I missing?

On boost if I understand your example correctly you give more volts per Hz than normal but how could you over volt and damage the motor if the motor is rated for 240 volts or 480 or whatever it may be?

I assume this is for starting a heavy load like a loaded conveyor or something? Any other type of apps this is useful in?

I will check back here after lunch. I have graduation rehearsal in the morning.

Dan you are a great teacher thanks for all you input. I hope I can be as knowledgeable as the people on this forum someday.

Thanks guys.

Dan thanks for the example that helps clear things up.
REPLY YOu bet

In dickdv post I see a caution about overvolting and voltage starving? That seems to be contradicting to me so what am I missing?
REPLY He did not mention undervoltage. In normal across line motors are rated for max volts and min volts (often 480 +5% and - 10% for example) I have noted that the overvoltage seems to frequently be a smaller value than the under voltage.
DICK DV -- can you explain this to both of us please.
Either over or under can damage motor.

On boost if I understand your example correctly you give more volts per Hz than normal but how could you over volt and damage the motor if the motor is rated for 240 volts or 480 or whatever it may be?
REPLY look back at my message. V/f ratio for 240 V motor is 4V/Hz. I added 25% voltage. But you would normally stop this at 20 Hz or less. At the low frequency yes you are probably going to overheat motor if you operate in the low freq area constantly. The extra voltage is done only in this are to get the motor to develop a little more torque to break the load loose. This should be a matter of seconds and when motor speeds up the fan will be able to cool it. If you went with 25% all the way to 60 Hz you would be at 300 VAC which would be outside motor design tolerance. At low freqs even without torque boost a separate fan may be requied to cool motor - intenal fan is not pumping enough air at low RPMs.

I assume this is for starting a heavy load like a loaded conveyor or something? Any other type of apps this is useful in?
REPLY Any hard starting load ie cranes, compressors, truck on a hill or as you say a loaded conveyer.

I will check back here after lunch. I have graduation rehearsal in the morning.

Dan you are a great teacher thanks for all you input. I hope I can be as knowledgeable as the people on this forum someday.
REPLY Well keep going to school and asking questions both there and here. NAG NAG NAG did you do your homework in the other post ie "band heaters" was it?
Thanks guys.

DickDV said:

Actually, this is a PF70 so why not switch to sensorless vector control. That was its all managed automatically at all speeds.

Click to expand...

Aha. I have no experience with sensorless vector. If it it the smartest way to do it then why not. I'll have to read up on it.

A little detail that complicates things is that I am not on-site myself. I have to guide another person in changing settings. So I prefer to tweek in small steps and not change something drastically.

Update.

The shuttle is now working as expected, starting - running - braking back and forth with no problems.

It has been running with sensorless vector control the whole time. Switching to custom v/f only made it worse. With sensorless vector it would start occasoinally. With custom v/f never.

The problem that I was looking for:
The 6 motors had been exchanged for a type with higher In without my knowledge. When that was corrected in the setup of the PF70 it immediately worked as it should.
I am troubleshooting by email, so that is my excuse.
But I am kicking myself in the b**t for not asking the motor name plate details as the first thing.

Thankyou all for your time.