1. K. T. Chau The University of Hong Kong Hong Kong, China
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Electric Vehicle Machines and Drives – Design, Analysis and Application Part II Motor Drives for Electric Vehicles
K. T. Chau
The University of Hong Kong
Hong Kong, China

2. Motor Drives for Electric Vehicles – Content
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DC Motor Drives
Induction Motor Drives
Permanent Magnet (PM) Brushless Motor Drives
Switched Reluctance (SR) Motor Drives
Stator-PM Motor Drives
Magnetic-Geared (MG) Motor Drives
Vernier PM (VPM) Motor Drives
Advanced Magnetless Motor Drives

3. DC Motor Drives – System Configuration
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It consists of the DC-DC converter, DC machine, fixed gear (FG), differential, electronic controller and sensors.

4. DC Motor Drives – DC Machine
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The stator is the field circuit which incorporates the field winding or PMs to produce magnetic field excitation.
The rotor is the armature circuit which installs the armature winding where the armature current is bidirectional and switched by the commutator via carbon brushes.

5. DC Motor Drives – DC Machine Structures
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Different arrangements of the field circuit (F) and armature circuit (A) create different types of DC machines, hence providing different torque-speed characteristics.
It can be classified as:
Separately excited
Series
Shunt
Cumulative compound
Differential compound PM

6. DC Motor Drives – Torque-Speed Characteristics of DC Machines
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Without external control, the torque-speed characteristics at the rated armature and field voltages are depicted below:

7. DC Motor Drives – DC Motor Control
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In order to achieve a wide range of speed control, the armature voltage control and flux-weakening control should be independent, which can only be applied to the separately excited DC motor drive.
Below the base speed, the armature voltage is varied while the flux is kept at the rated value. Since the maximum allowable armature current is constant, the torque capability under armature voltage control can be kept constant, so-called the constant-torque region.
Above the base speed, the flux is weakened while the armature voltage is kept at the rated value. Since the armature voltage is fixed and the maximum allowable armature current is constant, the back EMF remains constant for all speeds. Hence, the power capability is constant, so-called the constant-power region.

8. DC Motor Drives – Operating Capabilities of Separately Excited DC Motor Drive
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Below the base speed, armature voltage control is applied to provide constant-torque operation.
Above the base speed, flux-weakening control is applied to provide constant-power operation. The torque capability becomes varying inversely with the motor speed.

9. DC Motor Drives – Torque-Speed Characteristics of Separately Excited DC Motor Drive
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Under armature voltage control, the slope of torque-speed characteristics (solid lines) does not change with the speed.
Under flux weakening control, the slope of torque-speed characteristics (dotted lines) is no longer kept constant, but affected by the flux.

10. Induction Motor Drives – System Configuration
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It consists of the 3-phase squirrel-cage induction motor, voltage-fed pulse-width modulated (PWM) inverter, electronic controller and sensors.

11. Induction Motor Drives – Induction Machine
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The most common type of induction machines is the squirrel-cage induction machine.
It consists of a stator incorporated with 3-phase armature windings, a rotor incorporated with cage bars that are short-circuited by two end-rings, two end bearings to support the rotor, and a frame with two end bells to house the machine.

12. Induction Motor Drives – Inverters
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Inverters for induction motors are generally classified into voltage-fed and current-fed types.
The 3-phase full-bridge voltage-fed inverter is almost exclusively used for induction motor drives.

13. Induction Motor Drives – Torque-Speed Characteristics of Induction Machines
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The torque-speed characteristic of the induction machine under rated voltage and frequency is depicted below:

14. Induction Motor Drives – Induction Motor Control
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There are three main types of control strategies for induction motor drives:
Variable-voltage variable-frequency (VVVF) control
Field-oriented control (FOC), also dubbed as vector control or decoupling control
Direct torque control (DTC)

15. Induction Motor Drives – VVVF Control Torque-Speed Characteristics
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It uses constant volts/hertz control for frequencies below the rated frequency, and variable-frequency control with constant rated voltage for frequencies beyond the rated frequency.
For very low frequencies, voltage boosting is applied.

16. Induction Motor Drives – VVVF Control Operating Capabilities
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Both the torque and air-gap flux under the VVVF control are functions of voltage and frequency. This coupling effect is actually responsible for the sluggish response.
The corresponding torque control is not fast and accurate enough for application to high-performance EVs.

17. Induction Motor Drives – FOC Control Principle
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By using FOC, the motor torque can be effectively controlled by adjusting the torque component of stator current along qe-axis while the field component of stator current along de-axis remains constant. Also, flux-weakening control can be easily realized by reducing the field component independently.
It can offer the desired fast transient response similar to that of the separately excited DC motor drive.
Torque control
Flux control

18. Induction Motor Drives – DTC Control Principle
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The DTC is a scalar control scheme that can offer comparable performance as the FOC for induction motor drives.
This scheme is to directly control the stator flux linkage and the torque by properly selecting the switching modes of the voltage-fed PWM inverter. The selection is made to restrict the torque and flux errors within the respective torque and flux hysteresis bands, hence to achieve fast torque response and flexible control.

19. PM Brushless Motor Drives – PM Materials
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Ferrite
Alnico: iron-based aluminum-nickel-cobalt (Al-Ni-Co) alloy
Sm-Co: Samarium-cobalt alloy
Nd-Fe-B: Neodymium-iron-boron alloy

20. PM Brushless Motor Drives – System Configuration
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It consists of the PM brushless machine, voltage-fed inverter, electronic controller and sensors.
PM brushless machines have two main members: PM synchronous machine and PM brushless DC machine.

21. PM Brushless Motor Drives – PM Brushless Machine
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It mainly consists of a stator incorporated with the 3-phase armature winding and a rotor incorporated with PM poles.
Since the associated heat loss in the rotor is not significant, it generally does not require to mount fan blades on the rotor.

22. PM Brushless Motor Drives – PM Synchronous Machine Topologies
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surface-mounted
surface-inset
interior-radial
interior-circumferential

23. PM Brushless Motor Drives – PM Brushless DC Machine Topology
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In general, it adopts the surface-mounted PM rotor. Other types of PM rotors such as the surface-inset, interior-radial and interior-circumferential topologies can also be adopted, provided that the air-gap flux density distribution is close to trapezoidal.

24. PM Brushless Motor Drives – PM Synchronous Machine Principle
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The principle of PM synchronous machine is based on the interaction of sinusoidal back EMF waveforms and sinusoidal armature current waveforms.

25. PM Brushless Motor Drives – PM Brushless DC Machine Principle
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The principle of PM brushless DC machine is based on the interaction of trapezoidal back EMF waveforms and rectangular armature current waveforms.

26. PM Brushless Motor Drives – PM Brushless Motor Control
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The PM synchronous motor can adopt those control strategies that have been developed for the induction motor because both types of motors are based on sinusoidal waveforms.
The PM brushless DC motor needs to adopt dedicated control strategies because of its non-sinusoidal operating waveforms.
PM field excitation is inherently uncontrollable. In order to provide the constant-power operation for EV cruising, the flux-weakening control is desirable.
The FOC based flux-weakening control is applied to the PM synchronous motor to realize constant-power operation.
The phase-advance angle control is applied to the PM brushless DC motor to realize constant-power operation.

27. PM Brushless Motor Drives – FOC Based Flux-Weakening Control of PM Synchronous Motor
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The current and voltage vectors are controlled in such a way that the d-axis armature current is negative while the q-axis armature current is positive. Thus, the total flux linkage and hence the back EMF can be compensated.
The higher the d-axis inductance or lower the PM flux linkage, the better the flux-weakening capability can be provided.

28. PM Brushless Motor Drives – Phase-Advance Angle Control of PM Brushless DC Motor
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By purposely advancing the turn-on angle of the phase current, dubbed as the phase-advance angle control, the phase current can have sufficient time to rise up, and keep the current in phase with the back EMF under high-speed operation.
This phase-advance angle control can produce an equivalent flux-weakening effect to achieve the constant-power operation.

29. SR Motor Drives – System Configuration
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It consists of the SR machine, SR converter, sensor and controller.

30. SR Motor Drives – SR Machine
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It installs multiphase concentrated windings in the stator, but with no copper winding or PM piece in the rotor.
There are many possible topological structures for the SR machine, mainly depending on the number of phases as well as the numbers of stator and rotor poles.

31. SR Motor Drives – SR Machine Operation
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For motoring, the current is applied at the positive slope of phase inductance to create a positive torque.
For regeneration or braking, the current is applied at the negative slope of phase inductance to create a negative torque.

32. SR Motor Drives – SR Converters
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Based on the relationship between number of converter switches and number of machine phases m, there are 4 kinds of SR converters:
2m-switch
m-switch
(m+1)-switch
1.5m-switch

33. SR Motor Drives – SR Motor Control
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There are two main types of control strategies for SR motor drives:
Current chopping control (CCC) control is employed for speeds below the base speed to achieve constant-torque operation.
Advance angle control (AAC) control is employed for speeds above the base speed to achieve constant-power operation.
Beyond the critical speed, no more phase advancing is allowable. The phase advance angle is kept at its maximum value to offer the natural operation at which the torque drops significantly.

34. SR Motor Drives – CCC Operation
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Below the base speed, the back EMF is lower than the DC source voltage. By using a hysteresis current controller to control these switches, so-called the CCC, the phase current can be regulated at the rated value, hence offering the capability of constant-torque operation.

35. SR Motor Drives – AAC Operation
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Above the base speed, the back EMF is higher than the DC source voltage so that the phase current is limited by the back EMF. In order to feed the current into the phase winding, the switches of a particular phase need to be turned on well ahead of the unaligned position, so-called the AAC.

36. SR Motor Drives – Torque-Speed Capability
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The constant-torque operation, constant-power operation and natural operation are desirable for EV low-speed urban driving, medium-speed suburban driving and high-speed highway cruising, respectively.

37. Stator-PM Motor Drives – Stator-PM Machines
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Focusing on those stator-PM machines which are viable for EV propulsion, there are 3 major types:
Doubly-salient PM (DSPM) machine
Flux-reversal PM (FRPM) machine
Flux-switching PM (FSPM) machine
As these 3 types of stator-PM machines are all based on PM excitation, they are classified into the same group – flux uncontrollable. Additionally, with the inclusion of independent field winding or magnetizing winding in the stator for flux control, the stator-PM machines become flux controllable which can be further classified as:
Hybrid-excited PM (HEPM)
Flux-mnemonic PM (FMPM)

38. Stator-PM Motor Drives – Stator-PM Machines Types
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39. Stator-PM Motor Drives – Potentiality
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The stator-PM motor drives have high potentiality for EV application because they can solve 2 fundamental problems of the existing PM brushless motor drives, including the PM synchronous motor drive and PM brushless DC motor drive: 
There are no PMs in the rotor, thus avoiding the problem on how to firmly mount them on the high-speed rotor and hence to withstand the high centrifugal force.
All PMs are located in the stator, thus facilitating the cooling arrangement and hence solving the thermal instability problem of PMs.

40. Stator-PM Motor Drives – Evaluation for EVs
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DSPM
FRPM
FSPM
HEPM
FMPM
Power density
Medium
Good
High
Torque density
Efficiency
Controllability
Superb
PM immunity
Weak
Robustness
Strong
Manufacture
Easy
Hard
Maturity
Low

41. Magnetic-Geared Motor Drives – Rationale
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The in-wheel motor is either a low-speed gearless outer-rotor one or a high-speed planetary-geared inner-rotor one.
The low-speed gearless outer-rotor one takes the advantage of gearless operation, but its low-speed design causes bulky size and heavy weight.
The high-speed planetary-geared inner-rotor one takes the merits of reduced overall size and weight, but suffers from transmission loss, acoustic noise and need of regular lubrication.
By incorporating the magnetic gear into the electric motor, the magnetic-geared motor can simultaneously possess the advantages of high-speed motor design and low-speed output motion as well as pseudo-gearless transmission.

42. Magnetic-Geared Motor Drives – Magnetic-Planetary-Geared Machine
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The outer rotor of the machine shares its rotating body with the sun gear of the magnetic planetary gear.

43. Magnetic-Geared Motor Drives – Magnetic-Coaxial-Geared Machine
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The outer rotor of the machine shares its rotating body with the inner gear of the magnetic coaxial gear.

44. Vernier PM (VPM) Motor Drives – Rationale
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Direct drive takes the definite advantages of high torque, fast response, gearless operation, and zero transmission loss. Among the available direct-drive machines, the vernier PM machine inherently offers high torque at low speeds, thus eliminating the use of mechanical gear or even magnetic gear to amplify the torque for low-speed operation.
The PMs create a multipole magnetomotive force (MMF) field which is modulated by the permeance variation of teeth. The number of PM pole-pairs and the number of iron teeth are similar so that the armature flux has a relatively low pole number. The behavior of this magnetic circuit resembles the action of the vernier gauge, where there are patterns of alignment and misalignment.

45. Vernier PM Motor Drives – Rotor-PM VPM Machine
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When the rotor rotates one PM pole pitch, the flux linkage reverses the polarity. The speed of rotation of the alignment pattern is thus much higher than that of the rotor itself. This speed reduction from the armature field speed to the rotor speed is so-called the magnetic gearing effect, which works as a mechanical gear to reduce the speed or amplify the torque. Therefore, the VPM machine has the inherent feature of low-speed high-torque operation.

46. Vernier PM Motor Drives – Stator-PM VPM Machine
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The stator-PM VPM machine operates in a similar manner to the rotor-PM VPM machine –both of them employ the magnetic gearing effect to achieve low-speed high-torque operation. However, as reflected by the name, this machine installs its PM poles in the stator, rather than in the rotor. Actually, it can be considered as a special case of the flux-reversal PM (FRPM) machine.

47. Advanced Magnetless Motor Drives – Rationale
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The supply of PM materials especially those rare-earth elements is so limited and fluctuating that the corresponding market prices are soaring and volatile.

48. Advanced Magnetless Motor Drives – Machine Types
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The family of magnetless machines is coined, aiming to distinguish from the family of PM machines. Conceptually, the induction machine and switched reluctance machine are a kind of magnetless machines.
There are 5 major types of advanced magnetless machines:
Synchronous reluctance (SynR) machine
Doubly-salient DC (DSDC) machine
Flux-switching DC (FSDC) machine
Vernier reluctance (VR) machine
Doubly-fed vernier reluctance (DFVR) machine
The axial-flux machine (AFM) morphology of the above magnetless machines is particularly attractive for in-wheel motor drive application.

49. Advanced Magnetless Motor Drives – Evaluation for EVs
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SynR
DSDC
FSDC VR
DFVR
AFM
Power density
Fair
Good
High
Low
Superb
Torque density
Efficiency
Power factor
Controllability
Robustness
Manufacture
Hard
Easy
Material cost
Maturity