Need Help!- Problem obtaining speed.

[albertsteptoe]

Hello

I would really appreciate a professional opinion.

Here's the motor spec i'm using :


NEMA 34 Size
Max Current : 4.2A per phase
Coil Resistance : 1.2 Ohms
Coil Voltage : 5v
Inductance : 6.5mH
Holding Torque : 6Nm
Rotor Inertia : 2700gcm

Power Supply : 60v DC at 12 Amps.


I'm currently testing a brand new stepper in Unipolar mode.
The stepping signal to the phases are as follows :

0001
0010
0100
1000

Simply energising one phase after the other to create rotation.
The voltage applied to the coils is 12v at a step rate of 100Hz.
The motor turns perfectly smooth with lots of torque.

However!

I then increased the voltage to 60v and changed step rate to 610Hz.
The software is pre-programmed to give a 20Khz chop to maintain the
motors 4.2A current limit.

Unfortunately, the motor will not turn, it just hums.

Have checked just about everything!
1. The mosfets are switching to ground perfectly.
2. The chop signal is spot on.
3. Have also tried changing the chop frequency and PWM signal.
4. The 60v supply voltage is stable.
5. The back EMF is shunted with fast recovery diodes and a clean signal is observed.

I can only conclude that i need to increase the operating voltage in which
to obtain a step rate 610Hz???
I would have thought that a supply voltage of 60v at 4.2A would have been okay???
If i step at a lower rate of 305Hz, the motor runs ok.

Many thanks for considering my problem.
Really looking forward to your comments.
albertsteptoe

[JonLed]

Are you ramping up to that speed or trying to start out at that speed? You probably need to accelerate or ramp up to speed.

Jon

[albertsteptoe]

Hi Jonled

No, i'm just kicking in the signal from the beginning, not ramping.

Do you mean start from a slow speed and work up to 610Hz???

Good idea though, i will give it a try tonight.
Many thanks for your reply.

In the mean time, if anyone has any other idea's, would really appreciate your thoughts.

Best regards
albertsteptoe

[JonLed]

Yes that's what I mean, start slow and work up to 610Hz. There will be a maximum speed that you can start out from and this will depend on the torque of the motor and the load on it. There will also be a maximum rate that you can accelerate too, again depending on torque and load.

Jon

[albertsteptoe]

Hi Jon

I do appreciate your help.

Unfortunately, the ramping did not work.

I programmed the software to start slow and jump up in frequency.
76Hz then 152Hz then 305Hz and finally 610 Hz where it stops.
Also tried higher frequencies just in case the inductance was resonating at that point but no, it just stayed still.

I'm using the internal timer of a microcontroller to adjust the frequency by jumping to the divided stages.
Perhaps the jump from 305Hz to 610Hz is too much and a more gradual change is required, not sure.
Will write a program tomorrow to make it rise gradually and slowly from 100Hz up to say 800Hz and post the results tomorrow.

With a voltage of 60v, current chopped to 4.2A, i really would have thought that motor would run okay at 610Hz, really supprising.
There's no load at all, its just the motor on its own.

Perhaps the voltage could be higher, after all its only 12 times its operating voltage of 5v.

What you think mate?

Many thanks
albertsteptoe

[albertsteptoe]

Hi Jon and visitors

I did the ramp test again but this time with the speed incrementing very slowly from 100Hz to 1Khz with the standard 20Khz chop.

All was okay until the frequency hit 320Hz, i could hear the motor stalling and then as the frequency hit approx 380Hz the motor could clearly be seen stalling badly.

Below 320Hz the motor runs nice and smooth.

Would really appreciate some more help.
What ya think?, 60v not high enough?


Many thanks
albertsteptoe

[JonLed]

I would think 60V would be enough. Does your motor have a dampener attached? This could be a resonance problem. You said you were slowly increasing the frequency, you might try a faster increase. Say every 10 steps, increase the speed. Does your drive have the capability to drive two coils at once?

Jon

[Crevice Reamer]

Hi Albert. Welcome to the Zone!

Are these 5 wire motors or six wire?

Unipolar motors do not use all 4 coils for one direction. They use one coil from each pair per direction. You need to energise only one coil per pair at a time.

Formula for best/max voltage is 32 times the square root of motor inductance in mH.

This motor could use up to an 81V power supply.

CR.

[Crevice Reamer]

Perhaps this diagram will help:

http://kelinginc.net/KL23H276_30_6A.pdf

It's not YOUR motor, but representative of six wire motors.

CR.

[albertsteptoe]

Hello

Many thanks for everyone's help with this.

Here's a bit more information about the setup :

The motor has 8 wires, two wires per phase and is used in Unipolar Mode.
One wire from each phase is connected to a stable 60v positive supply.
I'm pulsing a negative to the other wires in sequence via Mosfets.
Each phase has a Fast Recovery Diode fitted across the coils and with oscilloscope have observed a very clean signal at each phase.
There is no load, its just the stepper motor and driver.

Have tried energising one coil at a time and also the two coil method but still no joy, the motor stalls at approx 320Hz.

0001
0010
0100
1000
0001 and so on.

Also tried :

1001
0011
0110
1100
1001 and so on

Here's a link to the data sheet for my motor :
http://www.motioncontrolproducts.co....18-4208A-H.pdf

On the data sheet coil diagram, there are 4 coils with the following colors :

Phase.1 (Step.1)
RED (Switched negative signal from Mosfet)
YELLOW (Connected to +60 DC)

Phase.2 (Step.2)
White (Switched negative signal from Mosfet)
Orange (Connected to +60 DC)

Phase.3 (Step.3)
BLACK (Switched negative signal from Mosfet)
BLUE (Connected to +60 DC)

Phase.4 (Step.4)
GREEN (Switched negative signal from Mosfet)
BROWN (Connected to +60 DC)

I simply pulse the Red first, then the White, then the Black, then the Blue and so on to create the rotation.
All runs smooth until i hit 320Hz and then stalling starts.

Once again i really appreciate your help.

Best regards
AlbertSteptoe

[vger]

Albert, is there a load on the motor? You may be hitting the motor resonance and stalling because of that. Try adding just a flywheel to the motor to up it's inertia a bit and see if there is a difference. Take a look at this thread..
http://www.cnczone.com/forums/showthread.php?t=61425

Steve

[albertsteptoe]

Hi Steve

No load at all.
Its just one motor on its own, the driver, a power supply of 60v at 12 Amps.

Its a NEMA 34 (Please spec in post above).

320 Hz just seems so slow.

Help would be really appreciated.
Many thanks
Albertsteptoe