VFD problem to run at low frequencies

[nado59]

Hi,

I am a mechanical/automotive engineer with limited knowledge in Electrical/Electronic engineering.

I have a machine driven by a 2.2kW motor with an VFD, both the motor and the VFD made by Watt, the VFD is model L2500, it is a basic model, no Vector Control.
I changed the Watt VFD to a Bonfiglioli Vectron Synplus one because this one has Vector Control and I have a special software to control it. (The problem has nothing to do with the control software, it is the same if I set the VFD manually.)

I have problem running at very low speed i.e. at 1Hz. I am aware that this is not recommended but it was working fine with the original Watt VFD. Now with the Bonfiglio the motor does not start to rotate under 7Hz.

When I turn it on there is no error message indicating overload and the monitor shows the following values:

Output freq: 1Hz
Output volt: 5.1-5.2V
Output amp: 0 – 0.5A
Output power: 0 – 2.55W
Output torque: 0 – 3.3%
Bus volt: 305V

Where I used a hyphen, the values fluctuates between the two values.

If I take off the load the motor would run. Because the original Watt frequency drive was able to start up and run the machine at 1Hz under the same load, I assume that I would need to alter the settings of the new VFD. I tried everything but no luck so far.

I run the auto tuning and I also tried the V/F control mode. Also I have a second machine which slightly different mechanically but uses the same VFD I have the problem with and a cheap no-name motor and works ok.

I would greatly appreciate any comment!

Many thanks in advance!

Nado

 

[itsmoked]

All drives are not the same. Especially with regard to very slow motion. Could be the Bonfiglioli Vectron Synplus needs speed feed back to work well.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[DickDV]

nado59, you said you ran the autotune on the new drive but sometimes this is simply a motor ID run.

You then have to tune the speed loop separately or the performance will be really sluggish, like you describe.

Look for parameters that mention speed loop tuning, particularly P and I gains that need to be adjusted either manually or with another autotune.

 

[Skogsgurra]

"Output freq: 1Hz
Output volt: 5.1-5.2V"

Bus volts 305 says that your mains is probably 220 V. That means that your motor should be a 220 V motor, which I hope it is. And that it is connected for that voltage.

At 1 Hz, you should have a voltage proportional to frequency, that is 2.2 V. You have around 5 volts. And that means that you have boost activated. Good. You could try some more boost so that torque goes up a bit more. But make sure the protection is set correctly before testing that. More boost means more current and more heat.

As Smed says, all drives do not perform the same and some do not perform at all below five Hz. It all depends on how the control algorithm works. Some are much better than others. Some don't work at all.

Gunnar Englund

http://www.gke.org

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

 

[nado59]

Dear All,

Thank you to everybody, I am going to answer to all of you in this post.

DickDV

The manual of the Bonfiglioli VFD says under the other functions: PID control. The following parameters can be set:

11-0: PID Operation Selection
0000: PID disable
0001: PID enable (Deviation is D-controlled)
0002: PID Feedback D-controlled
0003: PID D Reverse characteristic controlled
0004: PID Feedback D characteristic controlled
0005: PID, Frequency command + D controlled
0006: PID, Frequency command + Feedback D controlled
0007: PID, Frequency Command + D reverse Characteristic controlled.
0008: PID, Frequency Command + Feedback D reverse Characteristic controlled.

The Bonfiglioli VFD has three operation mode:
1. Vector – General Purpose
2. Vector – Variable Torque
3. Volts/Hz

If you could help further, my main question is: Am I wrong to think that to utilise the PID control I have to wire in a feedback signal into the VFD and I have to run it in Volts/Hz mode instead of Vector Control?

Regarding the rest of your answer:

I tried to input the rated motor parameters as well (Voltage, Current, Power, RPM, Frequency)

Running the autotune supplied the following parameters: Stator Resistance, Rotor Resistance, Equivalent Inductance, Magnetizing Current, Ferrite Loss Conductance.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

itsmoked and Skoqsqurra

Yes, both inverters are one phase 220V feed types. The motors wired ok. As I indicated in my original post, the machine works fine at 1Hz with a non vector control type (bottom range model) VFD. BUT I have a second machine which works fine with this new Bonfiglioli VFD, the main difference is that the motors are different made (but same parameters 2,2kW 4-poles)! I am sorry if I failed to explain this clearly. I am going to have a second go so I have two machines:

1. Watt motor + Watt VFD
2. No-name motor + Bonfiglioli VFD

The second machine works fine and as I have a software developed for the second machine. I would like to use this software on the first machine as well that is the reason why I want the first machine to run with a Bonfiglioli VFD instead of the Watt. And this is where the problem started. On the second machine the Bonfiglioli VFD works fine in Vector Control mode, I run the autotune function but otherwise all the parameters are default. Hence I feel that the Bonfiglioli should be able to run the first machine with the Watt motor.

I found your comment about the boost and voltage level helpful. Am I right that generally a VFD outputs a voltage according to user demanded frequency and the current is a consequence of the resistance/inductivity of the motor AND the load?

When the VFD is in Vector Control mode, does it monitor the current and modify the voltage accordingly?

If there is no Vector Control then – apart from the protection modes – the current is what the motor state dictates and that it is?

Many thanks to all of you again!

Nado

 

[Skogsgurra]

Without having all details of the Bonfiglioli VFD, I would say that anything involving a PID controller needs a feedback signal to work with. So, your assumption that you need an encoder to use the PID controller is probably correct. I say "probably" because some high-end VFDs have an internal speed estimator that can be wired as the feedback to a PID controller. I do not think that the Bonfiglioli has that possibility.

I think that you should use mode 1. General purpose vector control. V/f is very sluggish and torque control is not what you are after in this application.

Your assumption about constant volt/hertz ratio in V/Hz (AKA Scalar mode) mode is correct. It is also correct that the voltage is adapted to actual load and frequency when in vector mode. You will see a lower voltage at low loads and a higher voltage at higher loads. There is also a flux optimizer built into some drives, which does the same thing - more or less.

And, yes, current is load dependent. But less so than one thinksa. The reason is that magnetizing current is a large proportion of total current. Have a look at an Ossanna diagram (google) to see more details about the asynchronous induction motor.

BTW 'Smed' was supposed to be 'Smoked' - looks like my keyboard was blind for a split second.



Gunnar Englund

http://www.gke.org

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

 

[itsmoked]

That explains it. I couldn't line up any Freudian slips.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DickDV]

nado59, first, I am in complete agreement with skogsgurra and his recommendations.

Second, as to the PID control choices you list, it isn't real clear just what those are but they look like configuration parameters for the feedback signal. If that is correct, then I think 0002 or 0004 would be the right choice since all you want for a speed feedback signal is the encoder signal, not the signal mixed with anything else.

But, none of those represent the PID loop tuning. Just as I suspected, the tuning you did is to create a model model. Somewhere there is another tuning for the PID loop. That will tighten or loosen the speed performance at all speeds including near zero.

 

[nado59]

Dear All,

Thank you for the further info. I got some idea from your post.

DickDV - You wrote: “Just as I suspected, the tuning you did is to create a model model.” I am not sure what you meant by this.

Actually I started to wonder if my problem could be something to do with the motor and not primarily the VFD. This seems to me the only logical thing. Could it be that the auto tuning got it wrong?

If the VFD uses these parameters it established during the auto tuning, could I start to play with them to influence the Vector Control? If yes, which parameter and to up or down?

Thank you again!

Nado

PS: Yours help is already greatly appreciated but if someone would have further time and interest, the Bonfiglio manual could be found under the following link.

http://www.bonfiglioli.com/pdf/synplus/vec205/man_synplus_uk.pdf

 

[DickDV]

nado, do not change the motor data in Group 14 as the instruction manual warns. These are the result of the motor autotune that you did.

When you select the PID mode, you create a new external closed loop feedback system. This loop also needs tuning. Look at Group 11. There are gains and offsets for this external loop tuning. That's your problem as the default values for the gains are way too low to get any kind of speed stability. If you do not know how to do this loop tuning, you better get someone involved who can. It is a bit of an art and if done wrong, can result in instability and all kinds of other problems.

 

[nado59]

Dear Skogsgurra and All,

Finally I have found the source of the problem. The motor has 12 wires and it was wired wrongly, it was in delta but wiring it to delta-delta, solved the issue.

I am sorry! Thank you for your time! At least I learnt a lot from you comments. I will get into the PID control and open a new thread.