Bipolar chopper drivers - tradeoffs of various connection options
Two phase stepper motors are supplied in four main wiring
formats: 4, 6, 8 and 5 wires. The first 3 are usable with
bipolar drivers, and we will discuss each in turn.
A (two phase) bipolar stepper driver has 2 pairs of bi-directional
power circuits (I'll call them phase A and phase B). Therefore,
phase A has a pair of terminals, and phase B has another pair of
terminals, for a total of 4 motor connections.
A 4 wire stepper motor is manufactured specifically for
bipolar driver applications, and is the simplest to explain
and hook up.
Each coil (of two) is rated (we call this the "sticker rating")
for a maximum amperage, and a voltage (when continuously applied,
gives rise to the stated amperage). If the voltage rating is not
given, it can be calculated by measuring the phase resistance with
an ohm-meter an multiplying by the amperage(I) rating (since V=IR).
More current gives rise to more torque, up to the maximum sticker
amperage rating. Currents above the amperage rating will cause the
motor to overheat and fail.
However, voltages above the sticker voltage ARE allowed, if they
they are applied through a duty cycle ("chopped" or "chopper driver").
Due to the inductance of the wires in the motor coil, there is a time
delay between the application of the voltage and the onset of current.
Therefore it is easy to see that applying a higher voltage with a
chopper driver (5 to 25 times the sticker rated voltage) results in a
faster current rise (and thus faster accelerations and motor speeds).
Getting back to a 4 wire stepper motor, the current & voltage
"sticker" rating may be used to select the appropriate power supply
voltage (to power the driver, to drive the stepper).
For example, if the current (I) rating was 6.4A and the phase
resistance (R) was 0.5 ohms, and the inductance was 2.5mH(milliHenrys)
then the phase voltage could be calculated to be V=IR or 3.2V = 6.4A *
Using a Gecko G201 (7A max drive, 80V maximum) would be a great fit
for this motor, as an 80V (power supply) divided by the phase voltage
gives a 80/3.2 = 25 ratio. This is optimimal in our target range
of 5-25X sticker voltage. The driver would be configured to trim
maximum current output back to 6.4 Amp.
Six wire motors have 3 wires per phase, each coil having a center
tap. You have two choices (use 1/2 coil (called unipolar), or
use the full coil (called bipolar series).
Eight wire motors have 2 pair wires per phase, in which case
one has 3 options (use just one coil of the two (unipolar), put both
coils in series (bipolar series), or put both coils in parallel
The sticker rating on these usually assumes to be rating each
half coil (for unipolar driver applications). Since we are
concerned with bipolar drivers, so we can choose to use only
half coil (of each of the 2 separate coils), in which case the
sticker rating would apply.
However, if we choose to utilize the entire coil (called a bipolar
series hookup, or bipolar parallel hookup) we have to apply some
(which are multiplied by the sticker ratings). I will give a table of
unipolar bipolar series bipolar
current factor * 1.0 * 0.7 * 1.4
phase resistance * 1.0 * 2.0 * 0.5
phase inductance * 1.0 * 4.0 * 1.0
For example, if the above 6.4A 0.5V steppers were 8 wire, then using
the table above, these would be ratings of the various hookups:
unipolar bipolar series bipolar
current factor 6.4 * 1.0 = 6.4 * 0.7 = 4.48 * 1.4 =
phase resistance .50 * 1.0 = 0.50 * 2.0 = 1.0 * 0.5 =
phase inductance 2.5mH* 1.0 = 2.5 * 4.0 = 10.0 * 1.0 = 2.5
The unipolar setup looks good at 6.4A and 80V with a gecko. However
only using 1 coil (of two) per phase will give us less torque than
the sticker torque. Note the small inductance which will give high
The bipolar parallel doesn't look good, we will only get 7.0/8.96 of the
rated torque (because of the 7 amp maximum/phase of the Gecko), but
note the 43.75V power supply limit (because 43.75V/(7A*.25ohm) = 25).
The bipolar series looks best (4.48A the 17.85 ratio given by
(17.85 = 80V/4.48V), but note the increased 10mH inductance limiting
the uppermost speed achievable.
You might read through the links below as well;
Last edited by CNCadmin; 03-02-2005 at 02:06 PM.