[Beefy]

Hello Everyone,

This is my very first post on cnczone so first of all I'd like to thank the cnczone people themselves and all the members who've gave out their hard earned information.

The following is not me saying this is how it works. It is more me thinking out loud and asking everyone have I got this right.

I think I'm slowly getting closer to understanding the riddle of steppers and their supply requirements. Having bought 2 chunky nema 34 motors knowing not much about stepper specs, I've been informed by the guys at Gecko they are very high inductance and need nearly 140 volts to get max speed out of them. This was the first I knew of inductance being a factor in what max voltage you'll need.

On the Mechmate forum found a formula for working out the max voltage needed for a stepper motor. Take the square root of the phase mH (milli-Henrys) and multiply that by 32. In my case square root of 18mH multiplied by 32 equals 136 volts. Chinese motors by the way. An 80 volt supply should give me at least half speed (300 rpm ???) so maybe I'll be OK because they're for a plasma table and I should be more interested at maintaining constant velocity at lower speeds. Correct ???

Now your typical microstepping drive seems to have a max volts of about 80v so if you require max speed from your steppers your motors can't have more inductance than about 6.2 mH

Now I've got a BIG QUESTION regarding this inductance / max. voltage relationship. When you wire the motors in series as opposed to parallel or unipolar the inductance quadruples. Yes multiplies by four.

I'm using series because it's supposed to give more low end torque which is what I want to maintain constant velocity when the gantry is doing sharp changes in direction. Because the inductance multiplies by 4 which in my case takes it to 18x4 = a whopping 72 mH, would I now need a max voltage of 271 volts to run them at max speed. Again 271 volts came from square root of 72 mH multiplied by constant of 32.

A simpler way of asking my question would be do you need 4 times the max voltage to get max speed of stepper motors in series, as what you need to run them in parallel or unipolar.

I curiously await anyones reply. Thanks in advance and for anyone else who all of this is news for I hope it helps you.

[port_huon]

There's a good explanation of the issues you've raised over at the Gecko website.
Check out the FAQ.

[langwadt]

the physics is straight forward:

magnetic force is related to current * turns
so for serial you have twice as many turns as for parallel and the max current for one coil, in parallel you the max current for two coils, i.e. the end result is the same if you can supply the current

the voltage across an inductance is L*di/dt, so for higher inductance you need higher voltage when you want to change the current quickly.

inductance in series is L = L1 + L2 ..., inductance in parallel is L= 1/(1/L1 + 1/L2 ...)

[ger21]

I'm using series because it's supposed to give more low end torque which is what I want to maintain constant velocity when the gantry is doing sharp changes in direction.

Series will give you a little more torque at VERY low speeds, but parallel will give you nearly the same torque at very low speeds, and much more torque at higher speeds.

I believe that you'll see better performance with parallel wiring than series wiring.

[Beefy]

Thank you all for your replies.

I actually made a mistake when I asked if I need 4 times the voltage to run a stepper connected in series, seeing as it has 4 times the inductance. The formula to calculate the voltage from the phase inductance actally gives exactly twice the voltage for a series connected motor even though the inductance is 4 times the amount. Now that makes sense to me. Just like having two resistors in series, pushing the same current through each resistor requires twice the voltage.

Gerry your advice about parallel connection was like gold to me. Nowhere have I read that the extra torque with series connection is marginal at VERY low speeds. I will now change over to parallel connection. At least then I should be able to get 56% of the motors max speed / voltage with an 80v Gekko drive and supply. Hopefully that will be all I need for a plasma table. Just have to remake my power supply now.

Thank you all.

Regards,

Keith.

[Crevice Reamer]

Hi Keith. Welcome to the Zone!

Maybe some of this basic CNC info will be helpful to you:

http://www.cnczone.com/forums/showthread.php?t=78713.#3

CR.

[Mariss Freimanis]

Beefy,

Actually there is no difference between series and parallel in low-speed torque. Here's why:

Torque = ampere-turns. Fancy way of saying torque is proportional to current times the number of turns of wire in a winding. In series, current passes through twice as many turns of wire as it does in parallel. Current is set to 1/2 of the parallel rating. Twice the turns, half the current and torque is exactly the same as in parallel.

Mariss