Whichypolar?

[Chunky]

Thought sure I could avoid this post by searching. Never really found a good answer. For building a router CNC is there a distinct advantage to unipolar over bipolar steppers or vice versa?

[CJL5585]

Chunky:

First: The selection of a Uni-Polar or a Bi-Polar stepper motors may be determined by the stepper drivers that you intend to use on your system.

That said, Bi-Polar stepper motors are more efficient than Uni-Polar.

The difference between the two types are:

Uni-Polar are designed for the +motor voltage to be applied to the motor winding centertap on each phase. The Uni-Polar driver then switches the 2 end wires to voltage common in a pre-programmed sequence. As a result, 6 wires are required to operate each motor.

Now to confuse you further, a Uni-polar stepper can be wired in Bi-Polar mode by insulating the center tap wire and not use it. You would use the two wires on the first winding as A and B. The two windings on the second winding would be C and D. If the motor runs in the wrong direction on your machine, just reverse the wires on one of the windings.

The advantage of Bi-Polar over uni-polar is that it gives you more options without stressing the stepper motor itself. A Bi-polar stepper normally has 8 wires. The internal wiring is almost the same, but each half winding is brought out on a different color coded wire. These windings can be wired in series (like the Bi-polar -not using the center wire), or in parallel (top wire on each winding connected togather, bottom wire on each winding connected togather).

Uni-polar is older technology (I think) than Bi-Polar. I personally would not use a Uni-polar driver. There are too many excellent used Bi-Polar drivers (at $50.00 or less each) on e-bay and other sources, they are almost fool proof to wire up, and seemingly last forever. I have built about 4 different drive systems using used drivers and have yet purchased a defective one.

If you need some used drives and motors (Bi-Polar) with guarantee of not DOA, I know a guy in Idaho that I can put you in touch with.

Hope this helps.
Jerry

[CJL5585]

I forgot to mention that there are some manufacturers that offer 8 wire motors that are specified as Uni-polar. Example: Powermax II by Pacific Scientific. All that means is that the specs are stated if used in Uni-polar mode. In reality the 8 wire Uni-polar motors are Bi-Polar units and are more efficient than the uni-polar ratings.
Jerry

[ger21]

Bipolar drives can run 4, 6, and 8 wire motors. Unipolar drives can run 6 and 8 wire motors. If the drives are chopper drives and microstepping drives, both unipolar and bipolar should give similar performance, with 1 exception. An 8 wire motor on a unipolar drive will only have about 70% of the torque it will have using a bipolar drive.

[Marc]

Wow, the more I read about the subject the more confused I get, I thought most bipolar stepper were 4 wires, I ordered some stepper sold as bipolar but when I got them I noticed they were 6 wire, so I learn later on that I could run them as bipolar or unipolar but should run them as bipolar so I wouldn't loose torque, am I correct on this ?

Marc..

[anoel]

Unipolar runs with 70% less torque at "slow speeds" than bipolar "series" but at high speeds (where we generally run our motors on the CNC routers) the unipolar will perform equally as well if not a smidge better due to Bipolar "series" having a sharp drop of in torque as the speed of the motor increases. Bipolar "parallel" gives both better torque and speed and will outperform both other options at the same voltage but it comes at a price of being very current hungry.