Motor and VFD low frequency "jitters" 1

[toygasm4u]

I've hit a brick wall with a Baldor 1HP motor and a Delta VFD"E" series drive. For a 460V blower application, I want to run the motor at 10~60 Hz. The motor is rated 10:1, and my PWM is set to 15kHz. When this motor is running between 10 and 15 Hz, it cogs. This cogging is translating mechanically through the equipment, making a not so pleasant noise; the carrier is also bumped down to the 4-8 kHz range. Beyond 15Hz, this thing operates as expected; smooth and quiet.

I've tried swapping between Vector and V/Hz mode, and performing an Autotune. For V/Hz mode, I tried setting my midpoint voltage at 230, and my midpoint F at 35Hz, (plus several other combinations). I've also verified that the drive isn't trying to inject any DC braking at the lower frequencies; I just turned those features off. I can't seem to null this cogging out, or even make a dent in it.

Any insight would be greatly appreciated,
~M

 

[toygasm4u]

I want to add that a mechanical resonance has been ruled out; this cogging in the motor happens with or without the load attached.

 

[TurbineGen]

I have experienced similar low frequency issues with some of my smaller VFDs. My application is with a synchronous machine however.

The reason mine resonate appears to stem from the fact that the motor has a set volts/hz ratio. The VFD slams the motor with full voltage at very short bursts wich causes a sudden increase torque on the rotor. At 20% speed the motor is looking for only 20% of the full voltage (which comes in the form of PWM). The PWM signal slams the motor with full input voltage at short bursts which cause a sudden high torque in the machine. Since the machine is rotating so slow, the harmonics of these are audible. Additionally, as each pole passes by the stator pole, the torque on the rotor changes slightly. This is another harmonic influence.

If you can, increase the frequency of your VFD above 15 kHz and it should smooth out the motor. If not, try manipulating your DC braking settings. If that doesn't work perhaps add a choke or saturable core reator?

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[toygasm4u]

I've considered trying a reactor; the only downside to that, even if it works, is that we're trying to keep our enclosure's heat load to a minimum. Our last resort is just changing from a 6:1 speed range to a 4:1. Hate to do it, but.. *shrugs*.

Loving your siggy btw...

 

[itsmoked]

I wonder, based on TurbineGen's experience, if 'field weakening' might help. If your drive allows it, (many now do), you could try that toygasm4u.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Field weakening is not easy in a PM motor. Mostly because the field is produced by the rotor magnets and they are difficult to "weaken" from the stator winding.

I have been told that it is being done in the Toyota Prius, but their inverter is a very special one.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

PM motor.
PM motor?
We aren't discussing no stinnking PM motor!!

Or are we?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[toygasm4u]

No.. this is an AM motor. (insert "haha" here).

What I've got is a Baldor 1 HP 3 phase induction motor. I don't think field weakening is an option Keith; while this little drive has a lot of bells and whistles, V/Hz manipulation or anything else regarding the power section is pretty limited. I think I'm SOL here.

 

[Skogsgurra]

Yes! Where did the PM get in?

I am as confused as anyone here. Forget it. If you can...

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

It may just be a factor of the design of the VFD. Every manufacturer theoretically comes up with their own way of creating a PWM firing scheme and vector algorithm, but some of the manufacturers who specialize in "low-end-high-volume-cheap-labor" just reverse engineer a competitor's and tweak it enough to avoid patent infringement. Those tweaks can end up having consequences.

Borrow a good VFD from a friend or a demo unit from a distributor and see if it does the same thing. I'd be willing to be it doesn't.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[ozmosis]

I have known the Delta VFD's to react to ambient temperature quite oddly. They have a built-in method of derating for higher ambient temperatures and this has an effect of fluctuating the carrier frequency depending on a high ambient and pulsating current. Operating a fan in the lower speed range will sometimes create a level of instability due to the output voltage in this range being relatively low.
Check the following:
#Temperature around the VFD?. You already indicate you are trying to "keep your enclosure's heat load to a minimum" which tells me you could have a small enclosure.
#Try reducing the carrier frequency even lower.
#monitor the current at the low frequency range that is causing the problem. If you see the amps all over the place, it might need a little more volt boost if this is possible.
#Sounds like you have tried a lot of settings. Have you tried doing a factory reset and simply running it from default with no further changes? Might be worth looking at this. Sometimes you can make so many changes in a VFD that you lose track of them.

 

[toygasm4u]

Ozmosis,
The VFD was on the open bench, with a relatively default parameter set (this was done a few times ;-). I also have an Allen-Bradley 4M of the same HP class; I think it's just an inherent property of these little package drives. Guess I need to shop around.

Thanks guys for all the feedback.

 

[ozmosis]

Ok, that answers that one. Sorry, are you saying the A-B vfd does the same thing too?

 

[toygasm4u]

Osmosis,
Yea, the AB 4M and the Delta "E" series both roll off the same manufacturing line. Power sections are identical, with the only difference being aesthetics and some interfacing options.

 

[ScumPunk]

Hi,
What does the drives output current read, compared with the rated current for high PWM frequency? If the drive is derating the pulse frequency it sounds like it's reacting to an overload of current or temperature.
The only other cause I can think of for the problem you describe is a missing phase to the motor, but I'm sure you've ruled that out....
Cheers,
Mort

 

[DickDV]

I would switch to sensorless vector mode, reduce the carrier frequency from that ridiculous 15khz to something around 8khz down to 4khz, and then look for the loop gain setting parameters.

It sounds to me like the autotune is not doing an adequate job at lower shaft speeds. Do a manual tune and see if the cogging can be tuned out.

You say the drive doesn't permit manual tuning of the speed loop?!!! Time to buy a good drive, in that case.

 

[2dye4]

Sounds like a setup parameter. The drive should not be altering its pwm pattern at low frequencies. If it is i suspect the drive is trying to control something and having a problem with an internal control loop.

just a shot in tha dark though.