Special Purpose Motors

Stepper motor:

The stepper motor is an electromechanical incremental actuator that can convert digital pulses input into analog output shaft motion in steps.It rotates by a specific number of degrees in response to an electrical pulse.It is used in the digital control system.It is generally applicable to the printer,X-Y recorder,robotics,etc.

There are two types of stepper motor:

1.Variable Reluctance type motor

a.Single stepper motor

b.Multi-stack stepper motor

2.Permanent magnet type motor

The construction and working principle of single stack stepper motor are described below:

The schematic diagram in fig 3.6.1 reflects the constructional view of the single stack stepper motor.When the stator phases are excited with dc current in proper sequence,the resultant air gap field steps around through 45⁰ and the rotor follows the axis of air gap field by virtue of reluctance torque.

At any position 1,when the switch S_A is closed,only coil A is excited,with current I_A ,and the direction of air gap field will be vertically upward and the rotor will align in the vertical direction.Similarly,when the switch S_A and S_B are closed,then coil A and coil B are excited,with current I_A and I_B, and the direction of air gap field will shift in clockwise direction by an angle of 45⁰.This process continues and after 360⁰ rotation in 8 steps,the sequence is repeated.Thus,In this way, the stepper motor operates in steps.

Schrage motor:

Schrage motor is essentially a combination of wound rotor induction motor and frequency converter. Schrage motor can be treated as an inverted polyphase induction motor. Unlike induction motors the primary winding of Schrage motor is on the rotor. Three phase supply is given to the primary with the help of 3 slip rings. The secondary winding is on the stator. Apart from primary and secondary, there is a third type of winding called as tertiary winding which is connected to the commutator. The primary and tertiary are housed in the same rotor slots and are mutually coupled. The secondary winding terminals are connected to the commutator via three sets of movable brushes A₁A₂ , B₁B₂ and C₁C₂. The brush position can be changed by a wheel provided at the back of the motor. The angular displacement between the brushes determines the injected emf into the secondary winding which is required for speed and power factor control.The schematic diagram of Schrage motor is given in the fig3.6.2 below.

At standstill conditions due to three phase currents flowing in the primary winding a rotating field is produced. This rotating field cuts the secondary with a synchronous speed n_s. Therefore, according to Lenz’s law, the rotor will rotate in a direction so as to oppose the cause i.e. to induce slip frequency emfs into secondary. Therefore, the rotor rotates opposite to the direction of rotation of synchronously rotating field. Now air gap field is rotating at slip speed n_s - n_r with respect to secondary. Therefore, the emf collected by the stationary brushes is at slip frequency and hence suitable for injection into secondary.

The Schrage motor is applicable in following fields:

Used in drives requiring variable speed such as cranes , fan , centrifugal pumps , conveyors etc.

Servo motor:

Servomotors are those motor which is used for the feedback control system as output actuators. Power ratings of such motors are very low in the range of a fraction of a watt to few hundred watts. They have a high-speed response which requires low rotor inertia. Generally, their rotors are smaller in diameter and longer in length and are operated at low speed. They are used in the control system of robots, machine tools, tracking system, etc. They are available in both ac and dc types.

DC servo motor:

The figure below shows the schematic diagram for an armature controlled DC servo motor. Here the armature is energized by amplified error signal and field is excited by a constant current source.

The field is operated at well beyond the knee point of magnetizing saturation curve. In this portion of the curve, for the huge change in magnetizing current, there is a very small change in mmf in the motor field. This makes the servo motor is less sensitive to change in field current. Actually for armature controlled DC servo motor, the motor should response to any change of field current.

Again, at saturation the field flux is maximum.The general torque equation of dc motor is, torque T ∝ φI_a. Now if φ is large enough, for every little change in armature current (I_a) there will be prominent changes in motor torque.

The torque speed characteristics of dc servomotor is shown below:

AC servomotor:

The a.c. the servomotor is basically two-phase induction motor. The output power of a.c. servo motor varies from a fraction of watts to few hundred of watts. The operating frequency is 50 Hz to 400 Hz. Most of the servomotors used in the low power servomechanism are a.c. servomotors.

The a.c. the servomotor basically consists of a stator and a rotor as shown in the figure below. The stator carries two windings, uniformly distributed and displaced by 90^oin space, from each other.One winding is called as main winding or fixed winding or reference winding. The reference winding is excited by a constant voltage a.c. supply.

The other winding is called as control winding. It is excited by the variable control voltage, which is obtained from a servo amplifier. The winding are90^oaway from each other and control voltage is90^oout of phase with respect to the voltage applied to the reference winding. This is necessary to obtain rotating magnetic field.To reduce the loading on the amplifier, the input impedance i.e. the impedance of the control winding in increased by using a tuning capacitor in parallel with the control winding.

The torque speed characteristics of ac servomotor are also shown in figure below and its torque equation is given by the following an expression.

T= k_mV_a - F_mω_m

Where,

K_m=motor torque constant

F_m=friction coefficient

AC servo motor can be applied in following fields.

1. Instrument server
2. Process controllers
3. Robotics
4. Self-balancing recorders
5. Machine tools