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8 wire stepper confusion
Can someone help please?
I am in the very early stages of designing my gantry mill, I have purchased my Geko 210's and also my stepper motors which are 8 wire, this is were my confusion begins, I am trying to workout the various parts to build my power supply, the data sheet for the steppers is as follows Voltage 2.5
Phase Current 4.6A . After reading various threads on the forum I have used the following formula:-
20 x 2.5v = 50vdc
50 / 1.4 = 35vac (approx)
3 x 4.6A = 13.8A Total for 3 motors
67% of 13.8 = 9.2
35vac x 9.2A = 322VA
The winding resistance is 0.5 ohm.
If my understanding of joining a resistance in series is correct will this make my windings 1 ohm each?
Likewise if I join my windings in parallel will this make my windings 0.25 ohm each?
Will this affect the current drawn from the circuit?
What value current resistor will I use on the Geko's
I am probably missing something simple here, so I appologise in advance for my ignorance
With an 8 wire motor, there would be 3 different currents depending on how you connect it. A lot of the motor manufacturers provide the different current specs for unipolar, bipolar-serial, and bipolar-parallel. If only one is provided, it is most likely the unipolar current. Usually the bipolar-parallel current is 1.414 times the unipolar current, and the bipolar-serial current is .707 times the unipolar current.
The reason for changing the current is to keep power disipation(ie heat) the same. Power is equal to resistance times the square of the current. If you half the resistance by connecting in parallel, you need to multiply the current by the sq root of 2, to keep the power dissipation the same. Likewise when you double the resistance by connecting in series, you need to divide by the sq root of 2.
Since torque is proportional to the number of turns in the coil times the current, the static torque for bipolar is usually about 1.4 times the unipolar rating.
The bipolar ratings might be lower than the 1.4 and .7 to avoid magnetic saturation of the core, but usually this is not the limiting factor.
8 wire stepper confusion
I will do some more number crunching then post my findings again. If you don't mind will you have a look and make sure I have understood everything, I want to get this bit right as I do not want to damage my Geko's or stepper motors
I have been experimenting with a 1.5v dry cell battery and my stepper motor, to try and make some sense of how my motor is wired and the manufactures specifications.
No of Phases 4
Current Per Phase 4.6 amps
Resistance 0.55 ohms
Inductance mH 2.5
Holding Torque 3.05 Oz/in
Detent Torque 0.15
Rotor Inertia 1.3 kg.m2
Excitation Mode 2
The wiring on the motor, as regarding pairing, seems to be straight forward, red/red with white tracer black/black with white tracer and so on, but just to make sure, I connected the dry cell battery across a "pair" and the rotor locked. The next thing I tried was to connect my amp meter in series with one pair of the motor wires and my 1.5v battery my meter showed a current of 2 amps, using ohm's law the resistance of the winding works out to be 0.75ohm's which roughly is 50% more than the manufactures spec's. I then joined two windings in parallel and carried out the same test, this time my meter was showing 3.2 amps, using the same calculation, this shows a winding resistance of 0.46 ohms which is much closer to the spec's.
Do you agree with what I have done and found? I did try and just take the ohmic reading of the windings, but my readings were not consistent enough for me to use.
Do you think it will be safe enough for me to go ahead and assume that the manufactures spec's are based on two winding's in parallel.
Hope I havenít waffled on to much
Thanks for your help so far
From what you have done, I would say that the specs you posted are the unipolar specs, and each of the 4 windings is .55 ohms. When using a battery like that, you have to remember that an alkaline battery has an internal resistance of around .2 ohms. If you subtract that .2 ohms from the .75 you calculated from your current measurement, you get the .55 ohms specified. To get a precise measurement, you need two meters, one to measure current, and one to measure voltage right across the coil.
If you connect the windings in parallel, you would be very safe in running 4.6 amps, and you would get the rated unipolar torque. You could in all likelyhood run currents up to 6.5 amps with the parallel conection with a proportionate increase in torque.
That makes sense.
What I will do is build the power supply and start off with the unipolar 4.6 amps and see how I go. I will post my findings when completed.