Electrical System Design

Transcription

1 Electrical System Design UNIT 4 Stepper Motors What is Stepper Motor Stepper motor is a special type of electric motor that moves in precisely defined increments of rotor position(steps). A stepper motor moves in increments, or steps, rather than turning smoothly as a conventional motor. Thesizeoftheincrement is measured indegrees and can vary depending on the application. Due to precise control, stepper motors are commonly used in medical, satellites, robotic and control applications. There are several features common to all stepper motors that make them ideally suited for these types of applications. They are as under: - High accuracy: Operate under open loop - Reliability: Stepper motors are brushless. - Load independent: Stepper motors rotate at a set speed under different load, provided the rated torque is maintained. -Holdingtorque:Foreachandeverystep,themotorholdsitsposition without brakes. 1

2 What is Stepper Motor Stepper motor requires sequencers and driver to operate. Sequencer generates sequence for switching which determines the direction of rotation and mode of operation. Driver is required to change the flux direction in the phase windings. The block diagram of stepper motor system is shown in Figure Types of Stepper Motors Like most electric motors, a stepper motor consists of a stator (fixed external winding) and a rotor(rotating shaft with magnets). Step motors are normally of two types: (a) permanent magnet type. (b) variable reluctance type. (c)hybridtype. 2

3 Types of Stepper Motors Variable reluctance (stepper) motor (VR) The rotor consists of a magnetically soft material with different tooth pitch compared with the pole pitch of the stator. 3

4 Variable reluctance stepper motor (VR) The main principle of operation depends on aligning one set only of stator and rotor poles by energizing the stator windings. Therefore, the numberofpolesinthestatorandrotorwindingshastobedifferent.the stator windings are energized by a DC source in such a sequence to generate a resultant rotating air-gap field around the rotor in steps. The rotor is made of ferromagnetic material that provides a tendency to align the rotor axis along the direction of the resultant air-gap field. Therefore, the rotor tracks the motion of this stepped field. Principle of Operation 4

5 Principle of Operation Principle of Operation Reluctance of the magnetic circuit formed by the rotor and stator teeth varieswiththeangularpositionoftherotorhere,energizecoilsaanda (PhaseA)Rotor steps toalignrotorteeth1and4withstatorteeth1 and5 5

6 Principle of Operation EnergizecoilsBandB (PhaseB), Rotor steps forward, Rotorteeth3and6alignwithStatorteeth1and5 Energize Phase C Rotor steps forward another 15 Principle of Operation 6

7 Energize Phase D Rotor steps forward another 15 Principle of Operation Principle of Operation LetNs=#ofteethonthestator Nr=#ofteethontherotor β=stepangleinspace degrees Ns Nr β = 360 N N s r 7

8 Multi-Stack Variable Reluctance Stepper Motors Inthistype,themotorisdividedalongitsaxisintoanumberofstacks. Eachstackisenergizedbyaseparatewinding(phase)asshowninthe figure. These stacks are magnetically isolated from each other. The most common type is the three-stack, three-phase motors; however, number of stacks and phases up to seven are also available. Multi-Stack Variable Reluctance Stepper Motors Thesemotorsarecharacterizedbyhavethesamenumberofteethineach ofthestatorstacksasthoseineachoftherotorstacks 8

9 Permanent Magnet Stepper Motor (PM) A permanent magnet stepper motor consists of a stator with individually controllable windings and a rotor with a permanent magnet. The rotor may be configured as a cylindrical ferrite rod with multi-pole magnetization along its circumference. The permanent-magnetic rotor always aligns itself with the right polarity relative to the stator winding. Permanent magnet stepper motors have a holding torque to which a nonenergised motor can be subjected without causing a continuous rotation. Principle of Operation 9

10 Principle of Operation A permanent magnet stepper motor consists of a stator with individually controllable windings and a rotor with a permanent magnet. The rotor may be configured as a cylindrical ferrite rod with multi-pole magnetization along its circumference. The permanent-magnetic rotor always aligns itself with the right polarity relative to the stator winding. Permanent magnet stepper motors have a holding torque to which a nonenergised motor can be subjected without causing a continuous rotation. Principle of Operation 10

11 Principle of Operation Hybrid Stepper Motor This type of motor is frequently referred to as a permanent magnet motor. It uses a combination of permanent magnet and variable reluctance structure. Its construction is similar to that of an induction motor. The Figure shows a simplified type of hybrid motor to illustrate its construction. The rotor has two end pieces(yokes) with salient poles equally spaced but radially offset from each other by one-half tooth pitch. A circular permanent magnet separates them. The yokes have essentially uniform flux of opposite polarity. The stator is formed from laminatedsteel.themotorshowninthefigurehas4coilsarrangedin twogroupsof2coilsinseries.onecoilpairiscalledphaseaandthe otherphaseb.forthemotorillustrated,eachpolehasonetooth. 11

12 Hybrid Stepper Motor Principle of Operation 12

13 Hybrid Stepper Motor Hybrid Stepper Motor 13

14 Step Angle and Speed Step Angle and Speed 14

15 Drive Circuit There are two main drive circuits for stepper motors, namely; Uni-polar and Bi-polar drive circuits. Uni-polar Drive Circuit: Drive Circuit Uni-polar Drive Circuit: The figure presents a schematic diagram for a uni-polar drive circuit. This circuit is suitable for three phase variable reluctance stepper motors. Each phase winding of the motor is controlled by a separate drive circuit with a transistor as its controllable power switch. All drive circuits are energized by the same DC source. The transistor (power switch) of each winding has two modes of operation as follows: 15

16 Drive Circuit Drive Circuit 16

17 Drive Circuit Drive Circuit 17

18 Drive Circuit Drive Circuit Bi-polar Drive Circuit: The figure presents a schematic diagram for one phase of a bi-polar drive circuit. This circuit is suitable for permanent magnet or hybrid stepper motors. Each phase winding of themotoriscontrolledby aseparatedrivecircuit with atransistoras its controllable power switch. All drive circuits are energized by the same DC source. Each two transistors(power switches) of each phase winding are turned ON simultaneously. Two modes of operation occur as follows: 18

19 Drive Circuit Drive Circuit 19

20 Drive Circuit Higher efficiency than the uni-polar drive circuit as part of the stored energyinthephasewindingreturnsbacktothedcsourceduringthe power switches turn OFF mode. Fast decaying of the freewheeling current as the inductor discharge through the phase winding resistance, phase external resistance and the DC source. No freewheeling resistance is required. More power switches(devices) than the uni-polar drive circuit. More expensive than the uni-polar drive circuit. Mostofthelargesteppermotorstypes(>1kW)aredrivenbythebipolar drive circuit including variable reluctance types. Available Stepper Motors 20

21 Available Stepper Motors Switching Sequence Consider the 4-pole hybrid stepper motor shown in the figure 21

22 Switching Sequence The motor is derived by a Bi-polar drive circuit where the power switches are represented by contacts as illustrated Switching Sequence Wave Switching Sequence 22

23 Switching Sequence Wave Switching Sequence Switching Sequence Normal Switching Sequence 23

24 Switching Sequence Normal Switching Sequence Switching Sequence Half-Step Switching Sequence 24

25 Switching Sequence Half-Step Switching Sequence 25