Need Help!- Setting driver amps to suit motor.

[Simon C]

Hi all,

Iv been trying to figure out how to set the amps on my stepper drive to suit my stepper motor,trouble is, its set useing a pot not a dipper switch so i had to kinda guess when i set it. the driver is 4.3A max so i just turned the pot to max then backed it off a little hopeing it was about 4amps which is what my motor is raited at when wired in paralell. the motor runs fine exept it gets very hot very fast, in about 2 mins. i know stepper motors do get hot as i have a hobby cnc pro board that runs the same size motors but they only get hot after about 40 mins, and never get hot enough that i feel the need to turn them off. the motor is 260 oz-in. Could i use my multimeter to check the amps? puting one probe on the drive A and the other on the motor A wire? If that would work,how would i take a readeing? with the motor at max rpm or stationary? also i heard a motor draws more amps than it says on the label. its all very confusing the motor is on the bench so its not even under any load.

Heres some info on the motor and driver im useing Zapp Automation Marchant Dice Knowledge Base :: View topic - SBOX Driver

Any advice on this would be much appresiated,
thanx in advance,
Simon,

[ger21]

There's no manual telling you how to set the current?

[doorknob]

I am not familiar with your drivers, and I have not actually done what I am about to recommend, so I make no promises about whether or not it will work.

That said, it is difficult to measure the current with a multimeter inline with one of the motor leads while the motor is turning, because the winding current will be rapidly changing as the motor is stepped. Also beware that if you connect and/or disconnect the meter leads inline with the motor leads while the power supply is turned on, there is a possibility that you could "smoke" your driver circuit due to inductive voltage spikes as the connection is made or broken.

If I were doing this, I would try a different approach, something like this [Note: if your driver uses a chopper instead of a linear current supply to the motors, see the Addendum below]:

Turn off the power to your system. Disconnect motors, leaving only one connected. Put the multimeter in series between the + terminal of the DC power supply and the heavy wire that currently goes from the + terminal to your driver. Make sure that you are using the high-current terminals of your multimeter, if it has such a thing - some meters have a separate input for a 10 amp current scale, for example. Also make sure that connection is tight so that it doesn't accidentally come loose. Set the multimeter to read current and turn on the meter (if that is a separate switch). Then turn on your power supplies for the driver and the motors, but do not drive the motors to rotate.

I would record the initial current reading, then single step the motor and look at the current again, and maybe single step it a few more times, again recording the current reading at each step. One thing to beware of is that some drivers reduce the current when the motor is idle, and so you want to be sure that that is not being done (otherwise it will confuse your readings).

Depending on the type of stepping that your driver is set up to do (full steps, half steps, or microsteps), and also depending on whether your motor and drive are wired as unipolar or bipolar, the static power supply current reading may represent the current through a single coil (or portion of the coil) or the current through two coils (or portions of two coils). Compare the pattern of currents that you have recorded with what you would expect to be seeing based on the drive pattern that you're using (look, for example, at Jones on Stepping Motors to find the coil on/off pattern that matches the way that your driver and motor wiring are set up). You should be able to deduce the current draw for a single winding and compare that to your intended value.

Remember to power down your power supplies before you remove (or turn off) your multimeter from the circuit.

If you do go ahead and try this method, please post your results here.

Hope that helps, but again I'm no expert, so maybe someone else will chime in and suggest a better method.

Addendum: If your driver uses a "chopper" rather than a linear regulated supply, it is possible that this method will not work, because the driver may not be supplying a steady current, even when the motor is not rotating. If that is the case, then I suspect that it may become necessary to modify this approach by using a dual-channel oscilloscope instead of a multimeter, and by putting a low-resistance, high-current "shunt" in series with the + terminal of the power supply and then you can measure the voltage waveform across the shunt on the oscilloscope, and calculate the instantaneous current based on that reading. I have not actually scoped out the current draw on a chopper-drive unit - the reading may be a bit confusing, because depending on how the chopper is operating (for example, the relationships between the chopped power phase on the separate windings), you may see some strange and wonderful voltage spikes - but my guess is that you will still be able to deduce the value of the current that is flowing in a single winding).

[Simon C]

Originally Posted by ger21 There's no manual telling you how to set the current?

Nope, no documentation came with the driver kit atall. Only a vague set up guide on the net,and makes no mention on setting up the drives

[Simon C]

Doorknob, thanx for your quick and detailed reply.

I dont think its a chopper drive so im going to try the method you suggested. Havent had time to test this tonight but i will tomoro and post the results.

Thanx again
Simon

[Simon C]

Ok i did the test useing the 10amp connecter on my multimeter and the results were a bit erratic like doorknob said they would be. seemed to be bettween 0.8 and 1.5 amps. after i did the amps test i decided to check the voltage bettween the A and A- motor leads and it was showing bettween 10VDC and 5VDC depending on wether i was puting the red meter probe on A and the black on A- or the red on A- and black on A. is this normal? I thought when checking DC voltage if you swiched the red and black probes bettween + and - on the circuit it just showed for example 10V or -10V depending on wich way u put the probes. also i thought 10V for a 260 oz-in stepper was quite high. shouldnt it be more around 2V - 4V? this motor is wired in paralell. Im also runing a 425 oz-in motor from the same type driver with the amps turned up max (4.3 amps) and it also shows the same traits when i check the voltage but it hardly even gets warm no matter how long i run it for. not sure what all this means as my knowledge of electronics is very limited. is it possible to wire a motor incorrectly and it still run fine but get very hot fast? i had a temprature probe on the motor as i was runing it and it got above 50 degrees C so i turned it off. i couldnt put my hand on it for more than 2-3 seconds

[doorknob]

Without a datasheet, it's difficult to know the safe upper temperature limit for a specific motor. However it seems that it's not uncommon for motors to have a max rated operating temperature of 80 C or 90 C. That doesn't explain why some of your motors are cool to the touch and others are hot. The explanation would seem to depend on the details of the operating mode of the driver, which still appears to be a bit of an unknown.

As far as the voltage reading that you're getting, you are correct that the absolute value of the voltage reading should not matter whether you are using the red and black leads in one position or the reversed position. To really understand what you are seeing may require the use of an oscilloscope instead of a multimeter.

[Simon C]

Thanx again for your reply doorknob,


I dont have an O-scope and even if i did i dont think i would know how to use it. I had another look at the data sheet for the motor and it does indeed have a max temp of 80 degrees C so that puts my mind at rest a little. Think what il do is just fit the motors and drivers on the lathe and just see how it goes,the motor was only £19 so im not to worryed about it,the drives were also cheep at £160 for 3 drivers and a break out board. If the drivers do cook the motors il just scrap them and buy a Gecko G540 like i should have done in the first place.


This is a link to the build thread for the cnc conversion of the lathe il be useing thease drivers on incase anyone is intrested http://www.cnczone.com/forums/mini_l...onversion.html