Stepper Motors

Stepper motors operates on reluctance motor principle. The construction of these motors is quite simple. The stator has slotted structure equipped with two or more individual coils and rotor has salient projections but no coils. The rotor is designed with or without permanent magnet attached to its shaft and stepper motor is classified accordingly. If rotor has permanent magnet attached to its shaft, the motor is called permanent magnet stepper motor; otherwise it is called reluctance type stepper motor.

The input to the stator is in the form of pulses and on each pulse the rotor rotates in step by the desired angle. The angular shift per input pulse depends on the number of stator and rotor poles / projections. The train of pulses is generally fed through a logic driver so that the stator poles are excited in a preset mode.

Permanent Magnet Stepper Motors

This type of stepper motor behaves like a salient pole synchronous motor with rotor excitation provided by permanent magnets. Basic construction details of such motor for 18° step angle are given below—

Reluctance Type Stepper Motors

Typical constructional feature of a 18° step reluctance type stepper motor is given below—

The figure shows the rotor position with T1 switch powered. With the power transferred to T2, the rotor pole R2 shifts through 18° clockwise in order to align itself with stator pole S2 and so on.

Switching Modes

Various switching modes for stepper motor having 4-pole stator and 2-pole rotor are described below—

Switching mode 'A' (Full step)

Sequence of operations for full step (here 90°) are tabulated below and shown in figure—

StepT1T2T3T4
1+
2+
3+
4+

Switching mode 'B' (Full step)

Sequence of operations for full step (here 90°) are tabulated below and shown in figure—

StepT1T2T3T4
1++
2++
3++
4++

Switching mode 'C' (Half step)

It is a combination of switching modes 'A' and 'B'. Sequence of operations for half step (here 45°) are tabulated below—

StepT1T2T3T4
1+
2++
3+
4++
5+
6++
7+
8++

The design of the logic driver is much simplified if the stator windings are provided with centre taps as shown below—

The center taps are permanently connected to negative terminal and now stator only needs to be connected to the positive terminal of supply. Two switching arrangements with center taps are—

Switching mode 'D' (Full step)

This mode is center tap mode corresponding to switching mode 'A' and sequence of operations are—

StepT1T2T3T4
1+
2+
3+
4+

Switching mode 'E' (Half step)

This mode is center tap mode corresponding to switching mode 'C' and sequence of operations are—

StepT1T2T3T4
1+
2++
3+
4++
5+
6++
7+
8++

If the switching rate is increased, there comes a point where any further increase in the switching rate cannot accelerate the rotor from standstill to synchronous speed i.e. it cannot align with stator. The motor is then said to have reached pull-in rate. Between '0' rate and 'pull-in rate', the motor can start, stop, and reverse on command. This range is known as response range. The stepper motor can be successfully used as a positioning device in the control systems in this range.

Simplicity and economy are advantages of this motor and together with the fact that it is a digital device and can be harmonious with computer output. In the closed loop systems, stepper motor behaves like a conventional servo-motors. It has the advantage of higher response speed and lower power consumption.

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