The bipolar motor produces more torque

AN235

8/23

Doc ID 1679 Rev 2

3

The bipolar motor produces more torque

The torque of the stepper motor is proportional to the magnetic field intensity of the stator

windings, which is proportional to the number of turns and the current in the winding, so

torque is proportional to N*I. A natural limit against any current increase is the danger of

saturating the iron core, though this typically isn't the major factor for stepper motors. Much

more important is the maximum temperature rise of the motor, due to the power loss in the

stator windings. The dissipation in the windings is equal to the square of the current times

the winding resistance. Since the resistance is proportional to the number of turns,

better uses the copper in the windings compared to a unipolar motor.

Consider the motor shown in Figure 1 B. When it is driven as a unipolar motor, current flows

from Vs to ground in one half of the center tapped winding through N turns. The power

dissipation and torque for the unipolar configuration are:

Equation 1

If the same motor is driven as a bipolar motor, from the ends with the center tap left

unconnected, the current flows through both halves or 2N turns and the dissipation and

torque are:

Equation 2

If Pd is set to be the same for both drive conditions, substituting one equation into the other

shows that

Equation 3

Since the bipolar driven motor better uses the copper in the windings it can deliver more

torque with the same dissipation in the windings than a unipolar motor built on the same

frame. Another way to look at it is that the bipolar motor has less dissipation to produce the

same torque.

Pd N

Iu

2

⋅

∼

and Tu N

Iu

⋅

∼

Pd 2N

Ib

2

⋅

∼

and Tb 2NIb

∼

Ib

Iu

1

2

-------

=

and Tb

2

Tu

⋅

=