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The Basics: Stepper Motors

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tom osselton:
A 6 pack of Guinness?  :coffee:

Will_D:
My next question:

I have ordered another (dual shaft) stepper for the Y axis.

It is an 8 wire version and so there are 3 different ways to connect it up to a 4 output controller:

1. Uni-Polar  (Rated @ 4.0 Amp)

2. Bi-Polar Series  (Rated @ 2.83A)

3. Bi_Polar Parallel (Rated @ 5.66A)

The spec says that the Holding torque is 2.83 Nm for Bipolar and only 2.0 Nm in Unipolar

The rated currents are as above

Which should I use? and why?

Thanks in advance

Joules:
Bipolar series against parallel, how quick do you want the machine to run.  Parallel gives you the torque at the top end speed and can run faster (hotter).  Series gives you the torque low down the speed range but has a lower top speed (my preference).  Unipolar operate 0 - X+ volts where as bipolar work -X -0- X+ volts, swing plus and minus (bipolar) and have more torque as they make better use of the windings.  Unipolar have centre taps on the windings, in effect only using half the winding.   So all you really need to know, is how to wire them bipolar.

Will_D:
Cheers Joules, yes I have the wiring diagram with the colour codes so no problem there.

Don't need hi-speed its a mill not an engraver.

So its Bi-polar series.

Will

PK:
Joules was explanation was empirically correct, but perhaps didn't do justice to "Why you always want to wire the motors in parallel".
First off. The rated current of the motor is fairly meaningless on its own. Multiply it by the rated voltage (which is even more meaningless on its own) and you have a figure for motor power that is a little bit useful in comparing motors.

The reason the rated current is meaningless is that the motor will only actually draw that current at its rated voltage at 0 RPM. The instant the motor starts to turn, the motor current (and hence torque) will drop away. How fast it drops away depends on the most important characteristic of the motor, its inductance.

Now stepper drives do a clever thing, they take your motor with its (say) 5V rated voltage, and apply 50V to it. You would think this would instantly blow up the motor, but the inductance of the coils means that the current ramps up (fairly) slowly. When the current reaches the set point of the drive, it turns the voltage off and the current starts to drop, on, off, on  this is what makes that squealing/hissing noise you hear with some drives...  Effectively the drive will increase the applied voltage to the motor as it begins to turn maintaining the set current until the motor is turning at a speed that the drive can't provide enough voltage for, at which point the current t (and hence torque) will drop away.

So you end up with a torque curve that looks like this:


What determines the point at which torque drops off are drive voltage and motor inductance..  This is why the more expensive drives can work up to around 100VDC, and why you always want to wire your bipolar motors in parallel.

PK

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