Servo motor running rough at low speed

[DanishMan]

Dear Friends

I recently bought four Zoje 750W motors with synchronizers.

I have fitted them on three sewing machines.

One thing, that puzzles me is, that when set to low speed (100 RPM), the motors run rough. Is this normal. Also the needle down function appears to be

kind of abrupt.

May be all motors behave like this at low speed - or may be I am not setting up the motor properly.

Any ideas?

Brgds

DanishMan

[Gymnast]

I know a bit about VF-drives for induction motors. When you need low speeds on such motors, then you need to implement more advanced control strategies in the software, to reduce ripple in torque. The answer here gives some indications:

https://www.researchgate.net/post/What_is_the_lowest_possible_speed_that_can_be_practically_achievable_in_a_3-ph_induction_motor_through_VFD

And this is a very technical document on the matter: http://www.how2power.com/pdf_view.php?url=/newsletters/1702/articles/H2PToday1702_design_TeledyneLeCroy_part13.pdf

The strategies for a brushless DC motor is not that much different and they are also described.

So in other words you can say, that it is possible to have a smooth behavior of the motor at low speed by adding development costs to make the software. Possibly they need to change settings in software depending of exsactly what kind of motortype the software is in control of.

However this can also be an error of your servo motor, so I hope other users of the same kind of servo motor can provide you with their observations.

[DanishMan]

Hi Gymnast

Thank you for your advise. My be an option to use a smaller pulley wheen and increase the start up speed.

Brgds

DanishMan

[Matt S]

I have noticed similar roughness at or near the startup speed with several brands of cheaper brushless/induction servos. Usually coincides with the point at which the needle initially contacts the leather, which is the most difficult part of the stitch cycle. I suspect that it's a torque issue -- I believe that torque output is vastly reduced at very low RPMs compared with "normal" speed but @Gymnast can probably talk more knowledgeably about this than I.

Gearing down with a smaller motor pulley or a separate speed reducer, does tend to help.

[dikman]

I agree with Matt. Mine have a noticeable "roughness" at very low speed but fitting speed reducers or using a larger pulley on the head unit tends to smooth things out. The downside to this is that needle positioners may not work when gearing down, but then if you're slowing the stitching speed right down you don't really need a needle positioner.

[DanishMan]

Hi Matt and Dikman

Thank you for your replies. They calm me down a bit.

The Zoje motor is not cheap, but I will try to fit a smaller pulley.

Brgds

DanishMan

[DanishMan]

I do not think, that a different size pulley will disable the synchronizer.

I use the same motor on a PFAFF 1245, which has a bigger pulley, and this one runs much more smooth - and the synchronizer works very well.

Brgds

DanishMan

[Gymnast]

DC motors with brushes are actually much easier to control at low speeds with electronics and software. However I am not sure, that this kind of servomotors was ever made for sewing machines with an internal encoder to provide feed back information about speed to the electronics. To do this will make the production of the "mechanical" part of the motor more expensive, but the electronics and sofware less expensive. In some way, that is what i have done on my Singer 201, if you saw my video on that.

[dikman]

DanishMan, there is another recent post about the issues with NPS and pulley size/ratios. Changing the pulley on the motor shouldn't have any effect on the NPS but a much larger head pulley might (depending on the motor software). I have Lishui Skyrit motors and none of them would work properly once I changed the pulley ratio (about 3:1), they all threw error codes with the NPS connected. When I added the extra magnet, as per the posting, it stopped the error code but still couldn't be set reliably to either needle up or needle down. Some brands seem to work ok, others don't. What is the pulley ratio on the one that works?

As I also mentioned there, however, in my opinion once you slow a machine down that much an NPS is a bit redundant.

Edited October 10, 2019 by dikman

[DanishMan]

Thank you Dikman. I will try and see, what happens.