CHAPTER 11 SPECIAL MACHINES Electrical Machines 1 Introduction

CHAPTER 11 SPECIAL MACHINES Electrical Machines 1

Introduction Conventional electrical machines are generally used for bulk energy conversion. However, there are some electrical machines that are used for specific applications. These machines are increasingly used in position control systems, robotics and mechatronics, electric vehicles and high-speed transportation. Electrical Machines 2

Stepper Motor This motor rotates at fixed angular steps in response to input current pulse. When a series of pulses are supplied, the motor rotates through a definite known angle. The angle through which the motor shaft rotates for each pulse input is called a step angle. Electrical Machines 3

Stepper Motor The step angle is given by the expression Electrical Machines 4

Classification of Stepper Motor Stepper motors are classified as follows: i) Variable reluctance stepper motor ii) Permanent magnet stepper motor iii) Hybrid stepper motor Electrical Machines 5

Variable Reluctance Stepper Motor It has a salient pole stator with concentric winding, which forms the different phases of the stator. Its rotor has no winding nor is it a permanent magnet. It is simply a salient pole structure with teeth in it. Electrical Machines 6

Variable Reluctance Stepper Motor Variable reluctance stepper motor is of two types i) Single-stack stepper motor ii) Multiple-stack stepper motor Electrical Machines 7

Single-stack stepper motor • The stator have an even number of poles and the windings of opposite poles are connected in series to form one phase. • Rotor is made up of laminated silicon steel or solid silicon steel. • The number of stator poles and the rotor teeth should not be equal. • This will make the motor self starting. Electrical Machines 8

Single-stack stepper motor Six-pole stator with four rotor teeth Switch connection Electrical Machines 9

Single-stack stepper motor The operating sequence Electrical Machines 10

Modes of Operation of Stepper Motor Two-phase ON mode of operation Electrical Machines 11

Modes of Operation of Stepper Motor Half Step mode of operation Electrical Machines 12

Multi-stack Variable Reluctance Stepper Motor • A multi-step variable reluctance stepper motor has same number of poles on stator and rotor. • In this type, the motor is divided along its axis into a number of stacks. Electrical Machines 13

Multi-stack Variable Reluctance Stepper Motor Three-stack variable reluctance stepper motor Electrical Machines 14

Permanent Magnet Stepper Motor In a permanent magnet stepper motor, the rotor is made up of permanent magnet and the stator is same as that of variable reluctance stepper motor. It operates on the reaction between a permanent magnet rotor and the electromagnetic field. Electrical Machines 15

Permanent Magnet Stepper Motor • It has higher inertia and lower acceleration/deceleration rates. • Step pulse rate up to 300 pulses per second can be obtained as compared to 1200 pulses per second in variable reluctance stepper motors. Electrical Machines 16

Permanent Magnet Stepper Motor • In this motor, larger step sizes ranging from 300 to 900 can be obtained. • It has a higher torque per ampere of stator currents. Electrical Machines 17

Permanent Magnet Stepper Motor Four-pole two-tooth rotor permanent magnet stepper motor Electrical Machines 18

Permanent Magnet Stepper Motor Electrical Machines 19

Permanent Magnet Stepper Motor Electrical Machines 20

Permanent Magnet Stepper Motor Electrical Machines 21

Hybrid Stepper Motor Hybrid stepper motors have improved step resolution with increment angles ranging from 3. 6 to 1 degree for 100 to 400 steps per revolution respectively. Electrical Machines 22

Hybrid Stepper Motor The system incorporates the advantages of both permanent magnet and variable reluctance types and utilises a special arrangement of permanent magnet poles over the entire concentric multi-toothed rotor/shaft assembly. Electrical Machines 23

Hybrid Stepper Motor • The multi-toothed rotor assembly exhibits excellent coordination with magnetic fluxes generated by the stator windings, which guide the rotor movement in a well-controlled manner across the intermediate air gaps. Electrical Machines 24

Switched Reluctance Motor • Switched reluctance motor is a brushless AC motor. • Only the stator of the motor has windings. The rotor of the motor has no winding nor is it a permanent magnet. • It is made up of laminated steel. Electrical Machines 25

Switched Reluctance Motor • Switched reluctance motor cannot run directly from DC supply or AC supply. It must always be electronically commutated. • When the stator of the motor is excited by passing current, a torque is produced in the rotor due to the tendency of the rotor to align with the excited stator poles. Electrical Machines 26

Switched Reluctance Motor Electrical Machines 27

Permanent Magnet DC motor • Permanent magnet DC motor (PMDC) is almost similar to DC shunt motor. • Its field is made up of permanent magnet. • The armature construction is same as that of a DC motor and has armature slots for the windings, commutator segments and brushes. Electrical Machines 28

Permanent Magnet DC Motor • The working principle of PMDC motor is similar to the general working principle of a DC motor. • Permanent magnet dc motors are extensively used where smaller power ratings are required, such as in toys, small robots, computer disc drives etc. Electrical Machines 29

Brushless DC Motor (BLDC) • BLDC motors have many similarities with AC induction motors and brushed DC motors in terms of construction and working principles respectively. • Motor operation is based on the attraction or repulsion between magnetic poles. Electrical Machines 30

Linear Induction Motor • A Linear Induction motor (LIM) is a special type of induction motor which gives linear motion instead of rotational motion as in the case of conventional induction motor. • It operates on the principle as that of a conventional induction motor. Electrical Machines 31

Linear Induction Motor • The LIM operates on the same principle as that of a rotary squirrelcage induction motor. • Application of linear induction motor are in automatic sliding doors in electric trains, mechanical handling equipment, such as in the propulsion of a train or tram along a certain route, metallic conveyor belts etc. Electrical Machines 32

Hysteresis Motor A hysteresis motor is a single-phase synchronous motor with no salient poles. Neither does it have DC excitation. It starts by virtue of the hysteresis loss induced in its hardened steel secondary member by the revolving field of the primary and continues to operate normally at synchronous speed. Electrical Machines 33

Hysteresis Motor • The stator of the hysteresis produces a synchronously revolving field from a single-phase supply. • The rotor of the hysteresis motor does not have any winding. It is made with magnetic material of high hysteresis losses. Electrical Machines 34

Hysteresis Motor • It is used in sound recording instruments, sound producing equipments, high-quality record players, electric clocks, printers, timing devices etc. Electrical Machines 35

Repulsion–Induction Motor • It is a combination of a repulsion motor and a squirrel-cage induction motor. • The stator of a repulsion motor is of non-salient pole type provided with slots. • The rotor is connected to the commutator as in the case of the armature of a DC machine. Electrical Machines 36

Repulsion–Induction Motor • The difference between AC series motor and repulsion motor lies the way in which power is supplied to the armature. • In repulsion motors, the armature is supplied by induction from the stator windings. Electrical Machines 37

Repulsion–Induction Motor • This motor is ideally suited for applications where low voltage is a problem or high starting torque is required. • The main disadvantages of repulsion motor are its power factor is very poor, it has very high noload speed, its commutator and brushes wear our quickly Electrical Machines 38

Universal Motor • It is a single-phase motor which can run on either DC or singlephase AC supply. • The armature and field winding of this motor are connected in series and hence, it has a high starting torque. Electrical Machines 39

Universal Motor • The armature of the universal motor is similar to that of a small DC motor with laminated core having either straight slots or skewed slots and a commutator to which the leads of the armature winding are connected. Both the core and commutator are pressed on the shaft. Electrical Machines 40

Universal Motor • When the universal motor is fed with DC supply, it works as a DC series motor. • When fed with AC supply, it still produces unidirectional torque. Since armature winding and field winding are connected in series, they are in same phase. Electrical Machines 41

Servo Motor • Servo motors, also called servos, are self-contained electric devices that rotate or push parts of a machine with great precision. It is used in toys, home electronics, aeroplanes and in many other places. Electrical Machines 42

Servo Motor • The servo motor is a small DC motor that runs on a battery supply and rotates at high speed but giving very low torque. • There are basically three basic types of servo motors, namely, servos with positional rotation, continuous rotation and linear servo. Electrical Machines 43

Servo Motor • Servos take commands from a series of pulses sent from the computer or radio. In the battery device used in servos, “low” is considered to be ground or 0 volts and “high” is the battery voltage. Servos tend to work in a range of 4. 5 to 6 volts. Electrical Machines 44

Schrage Motor • Schrage motor is a variable speed motor with wound rotor construction. • The three-phase windings of the wound rotor are delta-connected. • The end points of the deltaconnected rotor are brought out and connected on to slip-rings. Electrical Machines 45

Schrage Motor • The three phases of the stator windings are connected to the commutator segment by two sets of brushes. • The two sets of brushes can be moved to connect to the same point on the commutator, or moved apart in either direction. Electrical Machines 46

Single-phase AC Series Motor • In a DC motor, if the direction of both field and armature current is reversed, the direction of torque does not change. When a DC series motor is connected to an AC supply, both field and armature current get reversed, thereby producing unidirectional torque Electrical Machines 47

Single-phase AC Series Motor • The characteristics of such motor are similar to that of the DC series motor. The torque varies as square of the armature current and speed varies inversely as the armature current. The speed of such motor can be dangerously high at no-load condition and hence, it is always started with some load. Electrical Machines 48