Permanent Magnet Stepper Motor - Construction & Working

A stepper motor is an electro-mechanical device that actuates a train of step movements of a shaft in response to a train of input pulses. The step movement may be angular or linear. The step movement of the motor is according to the input pulse.

The control has two elements,

1. Number of Step Pulses (Which determines the angle of rotation).
2. Direction Data (Which determines the order in which the Phases are excited).

When a number of voltage signals are given to the motor, the shaft gets rotated through a known angle. The angle at which the motor rotates or moves for each signal is known as the step angle, expressed in degrees.

Construction :

The stator is a multi-polar (having a number of poles for different steps) type. The rotor may be a salient pole type. But generally, it is a smooth cylindrical type. It is made out of ferrite material which is permanently magnetized. Hence the motor is known as a permanent magnet stepper motor.

Let us consider a permanent magnet stepper motor with a stator having 4-poles. Around the poles, the exciting coils are wound. The rotor has 2 poles.

The voltage signals for different stator windings are given by using a driving circuit (used to control the motor). The driving circuit for a four-phase (four stator windings) permanent magnet stepper motor is shown in the below figure.

Operation :

When the input pulse is given to the motor with the help of a driving circuit, the motor starts rotating in step movements.

Case - 1

Here the switch SW₁ is closed and the supply is connected across the terminals of A₁ and A₂ due to its magnetic field a pole N is established on phase A. The rotor already consists of poles N and S. Due to torque developed, the rotor starts rotating to adjust its axis of poles as unlike poles attract and like poles repel each other i.e., the S pole of the rotor is aligned with A of the stator.

Case - 2

Now the supply is disconnected from phase A and it is connected across phase B by SW₂ i.e., to the terminals B₁ and B₂. Due to this, phase A is de-energized and phase B is energized. Here, again a magnetic pole N is established in phase B. Therefore the rotor further rotates with an angle of step movement according to the pole axis of phase B.

Similarly, in cases 3 and 4 when the phases C and D are connected to the supply sequentially. The rotor further changes its motion with an angle according to its respective pole axis. We can see that the motion of this rotor is in step movement.

The major limitation of this type of motor is small step movements are impossible because the number of poles that can be made on the stator is limited. This is overcome by the utilization of a variable reluctance type stepper motor.

Advantages :

• Its main advantage is the presence of a detent torque. Detent Torque is defined as the maximum static torque that can be applied to the shaft of an unexcited motor without causing continuous rotation. Under this torque, the rotor comes back to the normal rest position even if excitation ceases. Such positions of the rotor are referred to as the detent positions.
• The motor has full torque at standstill (if the windings are energized).
• Excellent response to starting/stopping/reversing.
• Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependent on the life of the bearing.

Disadvantages :

• Low torque to Inertia ratio.
• Acceleration is slow.
• Very slow dynamic response.
• The step angles are of high range i.e., 30° to 90°.