AN235

Step sequence

Doc ID 1679 Rev 2

7/23

Figure 4.

Half-step sequence for a two-phase bipolar motor

The main advantage of half-step is the increased resolution. The main disadvantage of half-

step operation is that in the half-step state the motor has only about 70% of the torque as

when driven to the full-step state. This is the direct result of lower flux density in the stator.

In the full-step state the magnetic vector generated by the stator is the vector sum of the

magnetic vectors of the two coils. When both coils are excited evenly, the vector sum of the

two is at a 45° angle and has a magnitude of

vector. When only one coil is driven, as in the half-step states, the total magnetic vector is

only the vector for one coil. This results in a 30% reduction in torque for the half-steps. The

reduction in torque can be compensated for by increasing the current in the one driven coil

for the half-steps. If the current in increased by

Figure 5, the torque is essentially equal for both full-step and half-step states.

Figure 5.

Current waveform to reduce half-step torque ripple

I1

I2

I1

I1

I1

I1

I2

I2

I2

I2

I1

I2

I1=0

I2=0

I1=0

I2=0

12

3

4

56

7

8

AM16502v1

1.4

1

0

-1

-1.4

1.4

1

0

-1

I Coil 1

I Coil 2

-1.4

AM16503v1