How can the motor avoid getting extremely hot?

[Ola.L]

Hello Chad,

now I finally have some answers to you:

I made a picture of the following error and DAC output measured synchronously attached to this message. Even when both of them are zero the servo sends out a lot of power. When measuring these parameters I manually moved the motor axis to show how the system handles it.

I have tried different parameters but there are no big difference about how the servo behaves when the motor is standing still. The parameters I'm currently using are: Kp 100, Kd 1000 and Ki 0 (default parameters).

The PWM voltage I measure just over the motor plus and minus using a scopemeter. I've added a picture showing the scopemeter measuring voltage when there is no following error and the DAC output is zero.

The current I have measured using a multimeter connected in serial between the servo and motor. I have used AC current as range of measurement.

//Ola

[Ola.L]

I had problems when trying to attach two pictures in one message so I send them again.

Here is the following error and DAC output measured synchronously attached.

[Ola.L]

... and here is the picture showing the scopemeter measuring voltage when there is no following error (0) and the DAC output is zero.

It seems that the pulse width is far too long.

//Ola

[Ola.L]

I have found out what the problem is:

This motor type has extremely low inductance. I measured it to 90-120 microH. A Swedish motor sales agent thought that the servo current loop starts to self oscillate and suggested me to put a choke coil in serial with the motor and it helped. So now the problem seems to be solved.
But what inductance do you recommend in this case? The one I tried was 1000 microH but it just handles 2,5A so I will need another one. The motor has a max current of 8A and it would be good to be able to use that current.

What inductance of the choke coil do you recommend? Is it good to add as low inductance as possible (maybe about 100-160 microH) or is it better to increase the total inductance (motor + choke coil) to a higher level?
The sales agent recommended 15-30 mH to put in serial with this motor but it seems very much and a choke coil like that is hard to find.

In the Servo user guide it says 110-440 microH if I set the inductance dip switch to low. What is actually done in the servo when I set the dipswitch to low?

//Ola

[ChadE]

Hi Ola,

I am happy to hear that you were able to get things working. For our MID-7654 drives, I recommend an inductance of at least .5 mH.

Regards,

[Ola.L]

Hi Chad,

is that the total inductance, motor + choke coil that should be 0.5mH?

Thanks everyone for all help.

//Ola

[ChadE]

Hello Ola,

Yes. I recommend a total inductance of at least 0.5 mH. The MID-7654 relies on this inductance to smooth or average out the Pulse Width Modulated output so that it acts more like a DC voltage.

Regards,