Measuring ohms on a stepper motor

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I just bought an old excello mill that has slo-syn stepper motors. rated at 1.3V 20 amps. Typically you can measure ohms on the coil to determine the center tap however measuring these I get .1 and eventually 0.00 ohms.

I made a stepper driver and I am able to drive the motor but only at 100 steps per second maximum. Of course the tiny wires on the breadboard don't allow much current so that is not surprising.

Looking at the scope I used 2 12 volt motorcycle batteries in parallel and a 1 ohm power resistor, can't believe the voltage on the center taps goes to about 1 volt, these things must take a huge power supply.

Question, should I see at least some resistance when measuring these coils? I connected the motor with some diodes and a capacitor and a hand drill produces about 20 amps using a 2 ohm resistor the scope shows 9 to 12 volts sine wave while spinning (voltage over resistor) so the windings don't seem to be shorted. Other steppers (new ones) I see resistance when measuring but these 25 pound 1100 oz beasts show little or no resistance.

Also when I did find the center (I think) there is no difference in current when I connect ground to both of the windings on that center tap, i would think the current should be double when both coils on one phase are connected, funny thing is I get the same current connecting any of the three on either phase.

I guess I will make a driver with some beefy wires and give the 20 amps a go but I still would like to understand why I see 0 ohms and the stepper can generate so much power as a generator and does step at 100 steps per second full steps with some amount of torque, A proper driver may make these work but they may just become generators for a windmill or I may want to sell them on ebay but I wan't to know if they actually are any good first.
I have 4 of them.

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The average multimeter is not good at measuring low ohms. In the real world a resistance of zero ohms is impossible! In order to get speed from the motor, you need low inductance coils, so that is why you have low resistance and high current. Nowadays with technoligy you use pulse width modulated current control and higher voltages. So, get yourself a decent motor driver.

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Yes it is that motor. I had a friend of mine that is a very good analog motor hvac man. He brought over his fluke that measures to .1 ohms, we did see better readings however not stable but not necessary anyway. He took the back cover off and pulled up a datasheet on the motor, 1,2,3,4,5,6 very nice.

Now I went back home and hooked up a keiling 36V 9.5amp power supply, found 4 one ohm power resistors to put in series and a 15 amp meter inline. Pulling 7 amps across one coil from the center tap with a 2 inch gear on the shaft I could not move the shaft. The voltage drop on the power supply went from 36.2 to 35.7 so only 1/2 volt drop.

By the way, the tape drive he has in the picture I got with the xlo mill still works! (no mouse turds in this one) point to point wirewrap on all the boards, must be 30 logic boards all fit in one atmel tiny45 now!

I am thinking I may be able to use the power supply however since I am going to convert this mill to use mach3 like my other cnc I built.

I am going to lay out the dalton stepper board with big holes to solder big wires to all points. I need to order the IRFS3810PBF mosfets. Avnet has them in stock for $4.81 and wow $1.26 for the other mosfet dalton used. Pic attached of the rotating 200lb table I got with the mill.

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So, get yourself a decent motor driver.

So do you have any in mind that can handle 20 amps? The bipolar drivers at 7 amps will not get the job done, I don't think you can buy a driver that will handle these motors at rated amperage not at a hobby price range.

Yes, I could buy new motors with 1200oz in and new drivers for $250.00 a pair times 4 axis but I have to pace myself and I may just sell some drivers if they work well on ebay and pay for this mill. Lots of those motors still around.

KL34H2120-42-8B (1/2" Dual shaft with flat) Specification Price: $129
Torque –Speed Curve
1200 oz In. Hybrid Motor

1: Digital Stepper Driver KL-8070D, $129.95, Special sale: $119.95/pcs

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I haven't had much luck with the beaver mill - probably something to do with the beaver dam.

My solution was to go out and pay those $$$ for the motors, drivers and psu.

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I tested this circuit on a unipolar stepper, a small one rated at 1amp. I used 2n7000 fets instead of the transistors. I got about 10,000 half steps per second about 1500 rpm at 30V. The diode protection and the way they drive the steppers mosfets is what I have been missing in other designs. I have had problems with the voltage spikes messing up the avr at higher voltages on the motor. this design is very nice and very smooth. I hope using the fets Dalton talks about will make it even better.

http://www.zilog.com/docs/z8enco...

Dalton says:
"One big reason for not using unipolars is because you have little or no control over current decay. To get reasonable speed out of the motor, you need to have the magnetic field collapse asap when you turn off the winding. Adding snubber diodes to the FETs will seriously hamper this decay so that is not an option."

and then recommends the IRFP4410Z that has the avalanche design.

Is the mosfet taking the place of the diode snubber? If I used those fets in the design linked above from www.zilog.com can i eliminate the D5 diode in the schematic on page 6, and the fet will take care of of the kickback?

can I add a chopper current limit circuit to the schematic from zilog to replace the resistor R13 using a fet to disconnect the ground source? I can't seem to find a circuit design for a chopper circuit that replaces a resistor in the L/r stepper design drivers.

What frequency would one use to chop?

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I wonder if I can use these monster power transformers for my DC supply?

The unipolar driver I have working. I am waiting for some 5 watt .01 ohm shunt resistors for the chopper circuit.

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Is the mosfet taking the place of the diode snubber?

Yes :) The diode in the Zilog schematic will let the kickback rise to the value of whatever your PSU provides. In my case the avalanche breakdown will let it rise even further. In the order of 100-150V. This will speed up stepping, but create LOTS of heat.

The limiting factor in my case is heat. hence the seriously overspecd fets and cooling.

We have run this mill for approx 2 years now and it runs just fine. With forced cooling (fan) the temp hovers at 40C.

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metron9 wrote:
I just bought an old excello mill that has slo-syn stepper motors. rated at 1.3V 20 amps. Typically you can measure ohms on the coil to determine the center tap however measuring these I get .1 and eventually 0.00 ohms.

It is possible to measure very low resistance by the aid of a power supply and two multimeters:

http://www.allaboutcircuits.com/...

Just put some small(!) voltage on the low resistance device you want to test, and monitor the current. When the current is for example 1A, use another multimeter to measure the voltage across it. Suddently R=V/I.