Stepper motors - unipolar vs. bipolar

[John Eaton]

Douglas W. Jones,201H MLH,3193350740,3193382879 (jo...@pyrite.cs.uiowa.edu) wrote:

: Actually, a unipolar motor should more properly be described as a motor
: operable from unipolar drive electronics, but anyway, for a unipolar motor,
: each of the two windings has a center tap. Usually, the center tap is
: connected to the positive supply (sometimes through a current limiter)
: and each end of each winding is grounded through a switching transistor.
: It takes 4 final stage drive transistors to control the two windings,
: but you pay extra to center tap each winding.

The biggest cost is the fact that you never drive both transistors on a
winding on at the same time. Therefore at any time you will only be using
1/2 of your copper. If you switch to bipolar drive then you get the same
torque for 1/2 the current with the same amount of copper.

John Eaton
!hp-vcd!johne

[le...@fndcd.fnal.gov]

I know that there are two major types of stepper motors - unipolar & bipolar.
What are the selection criteria for them? Why the two different types, and
why would one select one over the other?

Thanks in advance.

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[Douglas W. Jones,201H MLH,3193350740,3193382879]

From article <1994Oct1...@fndcd.fnal.gov>, by le...@fndcd.fnal.gov:

> I know that there are two major types of stepper motors - unipolar & bipolar.

One is easier to make, one is easier to drive.

Limiting ourselves to permanent magnet motors (the distinction
isn't meaningful with variable reluctance motors), the following
descriptions hold:

Actually, a unipolar motor should more properly be described as a motor
operable from unipolar drive electronics, but anyway, for a unipolar motor,
each of the two windings has a center tap. Usually, the center tap is
connected to the positive supply (sometimes through a current limiter)
and each end of each winding is grounded through a switching transistor.
It takes 4 final stage drive transistors to control the two windings,
but you pay extra to center tap each winding.

For a bipolar motor, the windings are simple -- as a result, the two
windings in the motor each have two wires -- your cable to the motor is
smaller, your motor can be smaller, and you pay less to assemble the thing.
On the other hand, the final stage drive circuitry for the motor requires
either eight transistors (2 H bridges) or a bipolar power supply.
Furthermore, if the internal resistance of the winding is low and you
want fast stepping, you'll need a bipolar current limiter (easy if you
just use a resistor, quite messy if you want a transistorized current
limiter).

So, if you already need a bipolar power supply in your system, if your
speed requirements are low, and if you want the least expensive motors
and minimal wiring to the motors, stick with bipolar motors. If you want
ultra fast stepping or if you can afford a few cents more per motor and a
few extra wires per motor, or if the number of power transistors in your
drive circuits is a limiting factor, use unipolar motors.

My stepping motor FAQ (plain text) is stored under my WWW home page at

http://www.cs.uiowa.edu/~jones/index.html

You can get it by FTP from

ftp://ftp.cs.uiowa.edu/pub/jones/stepping

Doug Jones
jo...@cs.uiowa.edu

[brop...@utdallas.edu]

> John Eaton
> !hp-vcd!johne

Could some tell me what a stepper motor is?

Thanks,
Nathan