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.

 

[DanishMan]

When I measure the shaft diameter, it appears to be 15 mm.

The pulley wheels, I have located seems to have a hole, that is 16 mm.

I suppose, they will fit.

[Matt S]

1 hour ago, DanishMan said:

When I measure the shaft diameter, it appears to be 15 mm.

The pulley wheels, I have located seems to have a hole, that is 16 mm.

I suppose, they will fit.

15mm straight bore pulley wheels are very common for sewing machines. Most industrial dealers, and Ebay, will have them for a few €. Certainly College Sewing sells them if you get stuck.

[DanishMan]

I just ordered 4 x 40mm pulleys at College Sewing.

Thank you all  for your advise.

Brgds

DanishMan

 

[Matt S]

6 hours ago, dikman said:

In my opinion once you slow a machine down that much an NPS is a bit redundant.

I see your point and don't disagree with you per se but would say more that a NPS largely eliminates the need for a speed reducer. So long as you have a fairly low starting speed on your servo motor, a single stitch isn't difficult to achieve reliably. I've built huge reduction ratios into machines before -- largest I think was a 8:1. It certainly increases torque at the machine, and lowers the effective starting speed for crappy servos that aren't really suited to leather sewing (had a couple that start at 500RPM, that wasn't fun). The problem is that with a typical maximum motor speed of 4500RPM that's only what, 560SPM? Pretty slow. Maybe if you're only sewing small things like knife sheathes, holsters and wallets that's okay but it gets old real fast the greater the length of stitching you try to do in a day. Even with a 3:1 reduction you're limited to 1,500SPM, which is half the top speed of a good quality modern triple-feed upholstery-class machine. Or to put it another way you're halving the number of items you can sew in a day.

If the purpose of putting in a reduction gear is to be more precise, a £20 needle positioner achieves this at lower cost, takes up far less space on/under the table, is easier to set up, and doesn't limit the speed I can sew. I have good quality modern machines with relatively high top speed, partially because I was getting fed up with the limited top speed of the lower quality machines I used to own. I run my machines full-bore whenever I can. Even on a 3" straightaway. Every second I shave is another morsel of profit. Those seconds add up. Even for a hobbyist I don't think that spending precious free time sitting in front of a sewing machine crawling along a seam is going to be that much fun.

Just my humble opinion of course and it's worth exactly how much you paid for it  But I think a counterpoint from someone who's been on both sides of the debate is useful.

[dikman]

You're quite right of course, Matt. If you sew things for a living (or even partial income) then time can be important and speed reducers can be an issue. For anyone who sews at speed then an NPS is definitely an asset. For me, as a rank amateur and hobbyist, slow speed is far more preferable for the control I get as it reduces the chance of making a mistake!! I also don't do long seam runs so slow speed isn't an issue there.

One of mine does one stitch every 2 1/2 secs. at its slowest. A bit extreme, but impressive, I thought!

[Gymnast]

In the comments of this video, I asked Stephen Feldmeier regarding the slow speed behavior of the Sailrite Workhorse servo motor. He did not feel a vibration or "rough" behavior: https://youtu.be/SOyQtt3eDLw . I know that this is subjective evaluation.

On ‎10‎/‎10‎/‎2019 at 12:02 AM, dikman said:

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.

A speed reducer in general will cause a higher speed of the motor. A higher speed causes an inchreased frequency of the ripple torque and then the speed is better smoothed by the inertia of the motor.

[Pterytus]

With my aerostar 750W, I had a similar problem at low speeds. The roughness/vibration problem was much less bad when I disabled the sensor in the software. Although this does not solve the problem when you actually want to use the sensor, it might be interesting to see if your servo behaves the same.

In the end, my solution was to add a speed reducer. The motor now runs at higher speeds without roughness and top speed is still fast enough for me, but that is personal choice.