Need Help!- Steppers motors pulse with increased current

[holyjewsus]

I'm having a bit of an issue with three of these motors:
Vexta PK266-02A NEMA 23 stepper motor Interinar Electronics

I'm running them in bipolar parallel, and when increasing the current all the way to max on my stepper driver boards (2amps) with the potentiometer they start to make a vibrating whining noise, and then eventually start to pulse on and off slower and slower.

When I run they machine in this state the motors will actually pulse on and off, move stop moving, start again, adjusting the current changes the frequency of the pulse.

these are the drivers I'm using:
Pololu - A4983 Stepper Motor Driver Carrier with Voltage Regulators

I'm driving them with a 24volt, 6.5amp supply

any hints?

[holyjewsus]

Pololu believes the stepper driver boards are overheating and turning themselves on and off when I turn the current up to two amps. This seems very probable as they are extremely hot to the touch. I have added small heatsinks with thermal tape, but will be attempting to add additional heatsinking to the bottom of the boards, or upgrading to a nicer driver board.

[neilw20]

When current regulated drivers are working properly they should not get very warm at all.
Does your power supply have good size filter capacitors, and power wiring separated from the logic wiring?
Too much power supply ripple can cause the drivers to get hot.

[holyjewsus]

thanks for the reply, my drivers have no filter caps between them and the power supply,

I do have my wiring separated pretty well, I think the max ripple on my supply was 120mv, is this poor?

What size caps do I add on the supply line?

thanks for your help

also, these drivers are pretty small, and my heatsinking is minimal at this time.

[neilw20]

120mV would seem OK.
Look closely at the driver specs.
To make these work you need at least 4W worth of heatsink.
I believe this driver is designed for smaller motors - like 1.2A max.
And that is coil current, not supply current as noted below in BLUE.

Try running at 12v, and if this works OK, I think you need a more powerful driver or smaller motor.

direct from the PDFs:
Current limiting

To achieve high step rates, the motor supply is typically much higher than would be permissible without active current limiting. For instance, a typical stepper motor might have a maximum current rating of 1 A with a 5Ω coil resistance, which would indicate a maximum motor supply of 5 V. Using such a motor with 12 V would allow higher step rates, but the current must actively be limited to under 1 A to prevent damage to the motor.


The A4983 driver IC has a maximum current rating of 2 A per coil, but the actual current you can deliver depends on how well you can keep the IC cool. The carrier’s printed circuit board is designed to draw heat out of the IC, but to supply more than approximately 1 A per coil, a heat sink or other cooling method is required.

Quite a nice little chip, but I think you are expecting more than it is capable of.

Please note that measuring the current draw at the power supply does not necessarily provide an accurate measure of the coil current. Since the input voltage to the driver can be significantly higher than the coil voltage, the measured current on the power supply can be quite a bit lower than the coil current (the driver and coil basically act like a switching step-down power supply). Also, if the supply voltage is very high compared to what the motor needs to achieve the set current, the duty cycle will be very low, which also leads to significant differences between average and RMS currents.

Note: your coil resistance is 1.8 ohm per phase.

The vibrating whining noise is the current limiting action.
Single point star earthing is VERY IMPORTANT.

Initially get it going with one driver and stepper, and once it all works add extra drivers and steppers.

Any erratic operation will then be related to wiring problems/supply filter problems.

The motor should only rotate while inputting step pulses.

[eSilviu]

from the A4983 datasheet, the "output ON resistance" of power transistors is about 0.3ohm. Let's do a simple math:
2A/coil = 2A in the power transistors;
there are 4 transistors/coil, but
only 2 transistors/coil conduct simultaneous
so we have: P [heat] = 2A*2A * 0.3ohm * 4 ON_transistors = 4.8 W.
In my math I have neglected loses due to commutations, but I expect them to be 1...2W
That tiny board will not handle properly this 5W only with copper from bottom as the heatsink.

[holyjewsus]

Thanks so much for all the replies and calculations guys. Sounds like I'll need to snag some bigger drivers or to really do some effective heatsinking... maybe thats not worth the money for a dodgy solution:

but just to ask first, before I move on to larger drivers and deplete the last of my funds,

does anyone have recommendations for heatsinking solutions for a board this small, or drivers that can accept step and dir signals from a microcontroller instead of a parallel port.

Thanks again guys!

[neilw20]

Find a 5W resistor, put he correct voltage on it for 5W.

volts = sqrt( watts / ohms )

Now you have 5W, you can see how much surface area of aluminum you might need.

That will give you a guide.

Or maybe you do this with a 20W resistor and see how much for 4 drivers.

As long as it stays below about 70 C it will be OK.