Drive Circuits for Stepper Motor:

A Drive Circuits for Stepper Motor is usually driven from a low voltage dc source. When a phase is to be energised, the dc source is connected to the phase by a semiconductor switch S (Fig. 8.9).

The phase current builds up at the rate decided by the phase winding’s electrical time constant. When the phase is to be de-energised, switch is turned off, which transfers the current to freewheeling diode D_F. The current drops to zero, again at the rate decided by the time constant of the phase winding. Motor torque, which is a function of i_ph, builds up and decays in the same manner. In order to maximize torque capability of a step motor, Drive Circuits for Stepper Motor should be such that the current builds up and decay as fast as possible, ideally as shown by dotted lines in Fig. 8.9(b). This is specially important when high stepping rates are required, as demonstrated in Fig. 8.9(c). The Drive Circuits for Stepper Motor are designed to incorporate this requirement.

Unipolar Drive for Variable Reluctance Motors:

In case of variable reluctance motors, phase currents need only be switched on or off and the current polarity does not matter. Unipolar drive, which is capable of supplying current only in one direction, is sufficient. A simple unipolar drive circuit suitable for low power two phase variable reluctance motor is shown in Fig. 8.10.

When switch S₁ is closed, phase A winding is connected to the dc source V_d and the phase current builds up and when it is opened the phase current decays in the freewheeling path consisting of phase A, D_F and R_F. The external resistor R_E reduces the electrical time constant, thereby speeding up the current build-up. Value of external resistor R_E is chosen to fix the value of the electrical time constant and then the source voltage V_d is chosen to produce the rated current I_R in the phase winding. Thus

where R_P is the phase winding resistance.

During on period of the switch, phase current also flows through the external resistor R_E, causing major proportion of the energy drawn from the source to be dissipated in R_E. Further, energy stored in the phase winding inductance during the on period of the switch is all dissipated in free-wheeling circuit resistances when the switch is turned off. Because of these energy losses, the unipolar circuit of Fig. 8.10 is highly inefficient, and therefore, is suitable only for low power Drive Circuits for Stepper Motor.

An efficient unipolar drive circuit capable of providing fast current build-up and decay is shown in Fig. 8.11. It uses chopper principle. Circuit shown is only for one phase. Each other phase will employ a similar circuit. The dc source voltage can now be much larger than in the Drive Circuits for Stepper Motor of Fig. 8.10.

Figures 8.11(a) (b) and (c) show drive circuit, phase voltage and current waveforms respectively. To energise the phase, semiconductor switches S₁ and S₂ are closed at t = t_o. This connects phase winding to the dc source voltage V_d and phase current i_ph builds up fast. When it crosses the rated current I_R by a prescribed amount, S₁ is switched off. The phase current freewheels through S₂ and D₂ and decreases below I_R. After a fixed interval, S₁ is turned on. Phase current i_ph increases. When it exceeds the rated current I_R by the prescribed amount, again S₁ is turned off. Thus, alternately turning the switch S₁ on an off, the phase current value is maintained to be nearly I_R. At t = t₀ + T, the phase is de-energised by turning off both S₁ and S₂. Phase current now flows through the path consisting of D₁, source V_d and D₂ and major proportion of energy stored in the phase winding inductance is fed back to the source V_d. Since, phase current has to flow against a large voltage V_d, it decays fast to zero.

Bipolar Drive for Permanent Magnet and Hybrid Motors:

A simple bipolar drive circuit for one phase is shown in Fig. 8.12. Each other phase will employ a similar circuit. The phase winding carries a positive current when semiconductor switches S₁ and S₂ conduct and it carries a negative current when S₃ and S₄ conduct.

The phase winding is energised with a positive current when S₁ and S₂ are turned on. The external resistance R_E reduces the electrical time constant allowing rapid build-up of phase current. V_d and R_E are chosen to satisfy Eq. (8.1). The phase is de-energised by turning off S₁ and S₂. Winding current now flows through the path consisting of D₃, source V_d and D₄. The major proportion of energy stored in phase winding inductance is fed back to the source and phase current decays rapidly to zero. Due to the presence of external resistance R_E, the drive circuit is inefficient, although it is more efficient than the unipolar drive circuit of Fig. 8.10.

By eliminating R_E, and using the chopper principle, an efficient bipolar drive circuit is obtained. The circuit is then operated as follows.

To energise the phase winding with a positive current, S₁ and S₂ are turned on. The phase current builds up rapidly. When it exceeds the rated current by a prescribed amount (Figs. 8.11 (b) and (c)), S₁ is turned off. The phase current freewheels through S₂ and D₄. After a fixed interval, S₁ is turned on again. Thus, phase current fluctuates around the rated value I_R. When phase is to be de-energised, both S₁ and S₂ are turned off. Winding current flows through the path consisting of D₃, source V_d and D₄. Phase current decays rapidly and energy stored in the winding inductance is recovered by the supply.