Chapter 14

Chapter 14 - 14-1 Chapter 14 Reluctance Drives:...

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Unformatted text preview: 14-1 Chapter 14 Reluctance Drives: Stepper-Motor and SwitchedReluctance Drives Exit 2001 by N. Mohan Print TOC " ! 14-2 Reluctance Drives Stepper – Motor and Switched – Reluctance Drives ! Reluctance Drives • Stepper – Motor drives - Accurate position control without feedback - Electrical pulse input gives discrete angle change - Types: Variable Reluctance Permanent Magnet Hybrid • Switched – Reluctance drives - Variable-reluctance stepper-motor designed to go into saturation - Feedback necessary Exit 2001 by N. Mohan TOC " ! 14-3 Stepper-Motor Positioning θm step angle Electrical pulse input l 0 0 t t ! Each pulse moves motor a discrete angle – step angle ! Counting pulses tells how far motor has turned without actually measuring (no feedback) Exit 2001 by N. Mohan TOC " ! 14-4 Reluctance Motors – Operating Principles λ i (t ) + θ fixed λ2 ∆λ R S T λ1 e (t ) − 2 1 at θ + ∆θ at θ 0 i1 i ! Salient rotor aligns to salient stator Exit 2001 by N. Mohan TOC " ! 14-5 Stepper - Motors ! Variable – reluctance motors - relies on rotor saliency ! Permanent magnet motors - relies on rotor magnets ! Hybrid motors - relies on both saliency and rotor magnets Exit 2001 by N. Mohan TOC " ! 14-6 Variable – Reluctance Stepper Motors 1 θ = rotor 3 tooth pitch θ =0 ia z z ib 2 θ = rotor 3 tooth pitch ia θ =1 rotor tooth pitch z z ic Exit 2001 by N. Mohan TOC " ! 14-7 Variable - Reluctance Stepper Motors (cont…) ! Rotor and stator saliency ! Unequal number of poles ! Stator current effectively pulls rotor pole in line with stator pole 3600 step-angle = qN r Exit 2001 by N. Mohan q = number of phases N r = number of rotor poles TOC " ! 14-8 Permanent Magnet Stepper Motor θ = 0o θ = 45o A S N A B N S z S A S N N S S N B N B B N S S z S N A A N S N S A S N z N B θ =135o θ = 90o z N S B N S B N S B S A A N A N S A S N N S A θ =180o zS N A ! Permanent magnets replace salient poles of variablereluctance motor Exit 2001 by N. Mohan TOC " ! 14-9 Hybrid Stepper Motor L R stator S windings N shaft S end cap N end cap air gap stator outer casing L' R' ! Uses both rotor saliency and permanent magnets on rotor ! Saliency on ends of rotor – ends not lined up ! Permanent magnet makes one of the rotor ends a south pole and the other a north pole Exit 2001 by N. Mohan TOC " ! 14-10 Hybrid Stepper Motor (cont…) 8 1 N S S 1 N 2 N 8 S 2 N N S N S 7S S3 S S 7S N N S3 N S N N 6 S S S N 5 4 N N 6 1 8 N 1 S rotor 4 tooth pitch S S 7 1 2 S S S S S N 6 S S S 5 Exit 2001 by N. Mohan 4 N 5 S4 8 S N N N N 2 N 3 7 N N N 3 N N 6 N N N S4 5 TOC " ! 14-11 Stepper-Motor Equivalent Circuit i ph + Rph L ph v ph − Exit 2001 by N. Mohan + e ph − TOC " ! 14-12 Half–Stepping and Micro-Stepping ia ib ! Possible to move rotor by half steps by exciting two stator windings equally ! Finer steps possible by exciting two windings unequally Exit 2001 by N. Mohan TOC " ! 14-13 PPU for Variable-Reluctance Stepper-Motor + v ph + − T1 i ph L ph + e ph D1 − Vd Phase winding D2 T2 Rsense − i ph Iref t T1 on T1 on T1 off T1 ,T2 off T2 on Exit 2001 by N. Mohan TOC " ! 14-14 PPU for Variable-Reluctance Stepper-Motor (cont…) ! Currents do not need to reverse ! Circuit uses “incomplete” switch poles that can pass current only one direction through the motor phase Exit 2001 by N. Mohan TOC " ! 14-15 PPU for PM and Hybrid StepperMotor + T1 D1 Phase winding T3 D3 Vd D4 + v ph − T4 T2 D2 − ! Phase currents must be reversible ! Complete switch poles used Exit 2001 by N. Mohan TOC " ! 14-16 Switched-Reluctance Motor Drives ia θ = -30o θ = 0o ia λa θ = 0o θ = -30o ia ! ! ! ! Exit 2001 by N. Mohan Variable-reluctance stepper-motor Closed-loop, uses feedback Motor goes into saturation Rugged, inexpensive TOC " ! 14-17 Summary/Review ! What are the three broad categories of reluctance drives? ! How is the principle on which reluctance drives operate different than that seen earlier with other drives? ! Write down the reluctance torque expression. What does the direction of torque depend on? ! Describe the operating principle of a variable-reluctance stepper-motor. ! Describe the operating principle of a permanent-magnet steppermotor. ! Describe the operating principle of a hybrid stepper-motor. ! What is the equivalent-circuit representation of a stepper-motor? ! How is half-stepping and micro-stepping achieved in steppermotors? Exit 2001 by N. Mohan TOC " ! 14-18 Summary/Review ! What is the nature of power-processing units in stepper-motor drives? ! Describe the operating principles of switched-reluctance drives. ! What are the application areas of switched-reluctance drives? Exit 2001 by N. Mohan TOC " ! ...
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