Servo Overheat

[dachopper]

Hello guys,

I've been trying to diagnose a skipped steps problem, that I seem to be having part way through operations, after around 20 - 30 minutes of machining.
The X axis will randomly jam for 3-4 seconds, performing an identical cut that it just completed 70-80 times successfully, and then resume moving partway through the cut, now displaced be a few cm's.

I thought maybe it was caused by a lack of torque, and now I am wondering if it is the case, due to motor overheating causing a loss in torque?

Aside from reducing the peak current from the driver, what other things reduce heating?

Reduce acceleration? or speed?


Nick

[Neale]

Not sure that speed and acceleration make much difference to heating with stepper motors. Might be worth seeing if your drivers have a "half current" option for when the motor is stationary, though, as this can help reduce heating. I don't think that heat effects in the motor would cause it to suddenly stop and start either. Might be driver overheating and faulting, though? Does your machine monitor fault output from the drivers? What drivers/motors/supply voltage are you using? Any obvious signs of binding/mechanical stickiness (motor trying to move but just making noises)?

Last time I saw this effect was on my 3D printer and in that case it was excessive motor current causing the drivers to overheat and go into shutdown until they cooled and started up again - having lost sync.

[dachopper]

Hi,
Am882 drivers , 68 Volts, KL23H2100-35-4B motors.

I had peak current set to 4.5 then 4.9 Amps, but now I've lowered it to 3.5 Amps for the next cut. I emailed the company to try and find out if 3.5 amps was RMS or Peak, and they said try both and see if the stepper gets hot. At 3.5 Amps it was cold, so I increased to 4.9 assuming the rating was 3.5 RMS..... but the motors got bloody hot to touch ( as in almost could not hold hand on there for more than 3-4 seconds ) !, so I have backed them back to 3.5 for now.

If the motor stalls at low speed ( when reversing direction for example where it is fighting the machine inertia + cut resistance, ) then at low speed no fault is displayed by the AM882, and the motor will pickup position once it decelerates and commands a reverse, or after a couple seconds of being stalled, it will just start moving again and continue to the new incorrect position.

The Motor coupling is definitely not slipping, and there is no binding, so for some reason at times the motor torque required, is too much for the torque output sometimes, and the only factor I could think of is that over time the stepper gets hot, and that in turn reduces torque. So therefore decreasing speed will decrease cut resistance, or decreasing acceleration will decrease the torque required to overcome inertia in a given time period.

[Neale]

General view seems to be that motor case temp of about 100C is about the limit, and that's based on manufacturer recommendation that wire insulation temp should not exceed 130C and there's about 30C difference between wire and case temps. Manufacturers do not seem worried about magnetism. My own steppers run at about 60-65C (based on the "just to hot to keep hand on" test) and that seems to be pretty normal.

I have two motors driving X and the only stalling has been when I've had too-rapid "rapids" configured - max X speed is at about the corner speed for my steppers so they are starting to lose torque and tend to stall. The driver concerned tripped with a stall fault. Low-speed stall is clearly something else. What kind of moving mass are you talking about, and what acceleration settings? Odd that it only happens after running happily for a while, though.

[dachopper]

I only have 1 motor driving the X axis , I would estimate the weight to be around the 30kg mark. My max speed with low acceleration was around 18,000, so I set 8000, which is about 70% torque figure.
acceleration settings around 1500. It takes about 1 to 1.5 secs to stop from rapid speed max acceleration is about 3000-4000 at low speeds, and it will stall on reversing ( or on accel / decel ). I've not seen evidence it stalls mid move at constant speed ( I'm doing 3D profiling so there are other speed csonstraints beside my X max speed

[dachopper]

My specs below...

If I reduce the microstepping amount, say from 3200 steps/rev to 1600 or 800 steps/rev, that will increase resolution, but will it provide any greater torque using the AM882?

100 RPM 190 N cm 1000mm/min
300 RPM 174 N cm 91% 3000mm/min
500 RPM 150 N cm 80% 5000mm/min
700 RPM 120 N cm 63% 7000mm/min
750 RPM 105 N cm 55% 7500mm/min
800 RPM 85 N cm 45% 8000mm/min

RAPIDS / ACCEL SETTINGS
X 8000, Accel 1200, single motor ~30 kgs....... 10mm pitch
Y 7500, Accel 1000, dual motor ~ 60-70kgs..... 10mm pitch
Z 3500, Accel 1200, single motor ~ 15-20 kgs.. 5mm pitch

Bottom Chart is my motor