Special machines:Hybrid Stepper Motor

Hybrid Stepper Motor

(a) Construction. It combines the features of the variable reluctance and permanent-magnet stepper motors. The rotor consists of a permanent- magnet that is magnetized axially to create a pair of poles marked N and S in Fig. 39.8 (b). Two end- caps are fitted at both ends of this axial magnet. These end-caps consist of equal number of teeth which are magnetized by the respective polarities of the axial magnet. The rotor teeth of one end-cap are offset by a half tooth pitch so that a tooth at one end-cap coincides with a slot at the other. The cross-sectional views perpendicular to the shaft along X –X ¢ and Y –Y ¢ axes are shown in Fig. 39.8 (a) and (c) respectively. As seen, the stator consists of four stator poles which are excited by two stator windings in pairs. The rotor has five N-poles at one end and five S-poles at the other end of the axial magnet. The step angle of such a motor is = (5 – 4) ´ 360º

(b) Working. In Fig.39.8 (a), phase A is shown excited such that the top stator pole is a S-pole so that it attracts the top N-pole of the rotor and brings it in line with the A –A ¢ axis. To turn the rotor,

phase A is denergized and phase B is excited positively. The rotor will turn in the CCW direction by a full step of 18º.

Next, phase A and B are energized negatively one after the other to produce further rotations of 18º each in the same direction. The truth table is shown in Fig. 4 39.9 (a). For producing clockwise rotation, the phase sequence should be Practical hybrid stepping motors are built with more rotor poles than shown in Fig. 39.9 in order to give higher angular resolution. Hence, the stator poles are often slotted or castleated to increase the number of stator teeth. As shown in Fig. 39.9 (b), each of the eight stator poles has been alloted or castleated into five smaller poles making Ns = 8 ´ 5 = 40º. If rotor has teeth, then step angle = (50 – 40) ´ 360º / 50 ´ 40 = 1.8º. Step angle can also be decreased (and hence resolution increased) by having more than two stacks on the rotor.

This motor achieves small step sizes easily and with a simpler magnet structure whereas a purely PM motor requires a multiple permanent-magnet. As compared to VR motor, hybrid motor requires less excitation to achieve a given torque. However, like a PM motor, this motor also develops good detent torque provided by the permanent-magnet flux. This torque holds the rotor stationary while the power is switched off. This fact is quite helpful because the motor can be left overnight without fear of its being accidentally moved to a new position.

Summary of Stepper Motors

1. A stepper motor can be looked upon as a digital electromagnetic device where each pulse input results in a discrete output i.e. a definite angle of shaft rotation. It is ideally-suited for open-loop operation because by keeping a count of the number of input pulses, it is possible to know the exact position of the rotor shaft.

2. In a VR motor, excitation of the stator phases gives rise to a torque in a direction which minimizes the magnetic circuit reluctance. The reluctance torque depends on the square of the phase current and its direction is independent of the polarity of the phase current. A VR motor can be a single-stack or multi-stack motor. The step angle b = 360º / mNr where Nr is the number of rotor teeth and m is the number of phases in the single-stack motor or the number of stacks in the multi-stack motor.

3. A permanent-magnet stepper motor has a permanently-magnetized cylindrical rotor. The direction of the torque produced depends on the polarity of the stator current.

4. A hybrid motor combines the features of VR and PM stepper motors. The direction of its torque also depends on the polarity of the stator current. Its step angle b = 360º / mNr.

5. In the 1-phase ON mode of excitation, the rotor moves by one full-step for each change of excitation. In the 2-phase-ON mode, the rotor moves in full steps although it comes to rest at a point midway between the two adjacent full-step positions.

6. Half-stepping can be achieved by alternating between the 1-phase-ON and 2-phase-ON modes. Step angle is reduced by half.

7. Microstepping is obtained by deliberately making two phase currents unequal in the 2-phase-ON mode.

Tutorial Problems

A stepper motor has a step angle of 1.8º. What number should be loaded into the encoder of its drive system if it is desired to turn the shaft ten complete revolutions ?

[2000]

2. Calculate the step angle of a single-stack, 4-phase, 8/6-pole VR stepper motor. What is its resolution ?

[15º; 24 steps/rev]

3. A stepper motor has a step angle of 1.8º and is driven at 4000 pps. Determine (a) resolution (b) motor speed (c) number of pulses required to rotate the shaft through 54º.

[(a) 200 steps/rev (b) 1200 rpm (c) 30]

4. Calculate the pulse rate required to obtain a rotor speed of 2400 rpm for a stepper motor having a resolution of 200 steps/rev.

[4000 pps]

5. A stepper motor has a resolution of 500 steps/rev in the 1-phase-ON mode of operation. If it is operated in half-step mode, determine (a) resolution (b) number of steps required to turn the rotor through 72º.

[(a) 1000 steps/rev (b) 200]

6. What is the required resolution for a stepper motor that is to operate at a pulse frequency of 6000 pps and a travel 180º in 0.025 s ?

[300 steps/rev]

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