Electric and Hybrid Vehicles Tutorial

1. Unit 3 – AC Motor

2. Contents
 Three phase induction motor – basics,
 Inverter Based Induction Motor Drive
 Modifying Torque-speed characteristics of 3 phase induction motor – Variable
voltage, variable frequency control, field oriented control, direct torque control.
Braking of the Induction Motor –
 Regenerative braking and dynamic braking.
 Losses in AC motors

3. Brushless DC motor
Advantages
 Wide speed range
 Excellent torque characteristics, good
medium and low speed torque
characteristics, large starting torque and
small starting current; strong overload
capacity;
 Soft start and soft stop, good braking
characteristics
 High reliability, good stability, strong
adaptability, simple maintenance
 Small size, light weight, high output;
Limitations
 Cost: Rare-earth permanent magnets are
much more expensive than other
permanent magnets.
 Limited constant power range: A large
constant power range is critical to
achieving high vehicle efficiency.
 High-speed performance: Surface-
mounted motors of permanent magnets
cannot achieve high speeds

4. Single Phase Induction Motor
 To uses single phases input supply single-phase induction motor is used, these
motors are less costly, easy to repair, and their reliability is higher.
 As the single-phase networks are less costly then the 3-phase system and most
appliances used in our homes, shops, offices are designed to operate on the
single-phase supply.
 A Single-ø Induction Motor comprises a single-ø winding that wound on the
stating portion of motor and its rotor consists of a cage winding.
 A revolving magnetic field is created when we provided single phase input at the
stating part of the induction motor.

6. • In a single-phase induction
motor, there are two winding
are used in stator.
• Out of these two windings,
one winding is the main
winding and the second is
auxiliary winding.

7. •The rotor of the induction motor is its
rotating part which rotates in the
magnetic field.
•There are two types of rotor of single
phase induction motor first one is wound
and the other is squirrel cage rotor.

8. •This type of rotor comprises of a sequence of conductor bars which are arranged in a cylindrical shape structure in the different slots.
•All these are connected with the slip ring on both sides. This assembly is said to be squirrel cage because its shape is like a squirrel.

9. Why single phase induction motor is not self
starting?
The Double-Revolving field
theory explains the entire
phenomenon. Single-phase
AC supply causes the
generation of two counter-
rotating magnetic fields in the
air gap. Both of these fields
are equal in magnitude but
opposite in direction. This
equal and opposite magnetic
field is generated due to the
AC voltage being in the
positive and negative half
cycle alternatively.

11. Split-Phase Induction Motor
•This type of induction motor is also named as a resistor
start motor.
•It comprises a cage rotor and its stating part is consists of
two windings. The first one is known as main winding and
the other is starting winding which is also named as auxiliary
winding.
•These windings are connected at an angle of ninety
degrees. The main winding has low resistance values.
•As the phase difference among its main and auxiliary
winding’s current is less so these motors provide less
starting torque.
•So, these motors used for such load that needs less
starting torque.

12. Capacitor-Start Induction Motor
•This type of induction motor produces a higher
starting torque about three to four-time than the full
load torque.
•For higher starting torque the capacitor values should
be higher, and the values of stating winding resistance
should be less.
•In this motor due to the capacitor, the current at
auxiliary windings leads the voltage.
•Due to this phase alteration among the main and
auxiliary windings current will increases and it will also
enhance the value of the initial torque.
•The starting torque of these motors is almost three
hundred percent of the torque value when a load is
connected. But its P.F is very less in case of the rated
load and speed.

13. Advantages of Induction Motor
• The most important advantage of an induction motor is that its
construction is quite simple in nature. When compared with a DC
Motor, the induction motor does not have Brushes and hence,
maintenance required is quite low. This leads to a simple
construction.
• The working of the motor is independent of the environmental
condition. This is because the induction motor is Robust and
mechanically strong.
• A Squirrel cage induction motor does not contain Brushes, Slip rings
and commutators. Due to this reason, the cost of the motor is quite
low.
• Due to the absence of Brushes, there are no sparks in the motor. It
can also be operated in hazardous conditions.
• An induction motor is a highly efficient machine with full load
efficiency varying from 85 to 97 percent.

14. Disadvantages of Induction Motor
1.A single phase induction motor, unlike a 3 phase induction motor, does not
have a self starting torque. Auxiliaries are required to start a single phase motor.
2. Speed control of an induction motor is very difficult to attain. This is because
a 3 phase induction motor is a constant speed motor and for the entire loading
range, the change in speed of the motor is very low.
3. Induction motors have high input surge currents, which are referred to
as Magnetising Inrush currents. This causes a reduction in voltage at the time of
starting the motor.
4. Due to poor starting torque, the motor cannot be used for applications which
require high starting torque.

15. Single Phase Induction Motor
 A single phase induction motor, does not have a self starting torque.
Auxiliaries are required to start a single phase motor.
 Speed control of an induction motor is very difficult to attain.
 Induction motors have high input surge currents, which are referred to
as Magnetising Inrush currents. This causes a reduction in voltage at the
time of starting the motor.
 Due to poor starting torque, the motor cannot be used for applications
which require high starting torque.

16. Three Phase Induction Motor
 The three-phase AC induction motor is a rotating electric machine that is designed to operate on a three-phase
supply.
 These three-phase motors consist of a stator and a rotor and between which no electrical connection exists.
 These stator and rotors are constructed with the use of high-magnetic core materials in order to reduce
hysteresis and eddy current losses.
 Stator frame can be constructed using cast iron, aluminium, or rolled steel.
 The stator frame provides necessary mechanical protection and support for stator laminated core, windings, and
other arrangements for ventilation.
 The stator is wounded with three-phase windings which are overlapped with one another at a 120-degree phase
shift fitted into slotted laminations.
 The six ends of the three windings are brought out and connected to the terminal box so that these windings are
excited by three-phase main supply.
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17. Three Phase Induction Motor
 These windings are of copper wire insulated with varnish fitted into insulated slotted laminations.
 At all working temperatures, this impregnated varnish remains rigid. These windings have high-
insulation resistance and high resistance to the saline atmosphere, moisture, alkaline fumes, oil,
and grease, etc.
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18. Stator Rotor
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19. Stator
Rotor
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20. Principle of Operation of 3-Phase
Induction Motor
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21. 3 Phase supply & Rotating Magnetic Field
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22. Advantages and Disadvantages of Three
Phase Induction Motor
 Three phase induction motors are widely used for various industrial applications because of their
following advantages -
1. They have very simple and rugged (almost unbreakable) construction
2. they are very reliable and having low cost
3. they have high efficiency and good power factor .
4. minimum maintenance required.
5. Three phase induction motor is self starting hence extra starting motor or any special starting arrangement
is not required.
 They also have some disadvantages are :
1. Speed decreases with increase in load, just like a DC shunt motor
2. If speed is to be varied, we have sacrifice some of its efficiency
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23. Three-Phase Induction Motor Torque-
Speed Characteristics
 Similar to other types of electric machines, a three-phase induction machine can work as a generator
and as a motor. For this machine, however, because the stator must be connected to the three-phase
circuit, the difference between being a motor or functioning as a generator lies in the speed of the
rotor.
 In general, if the rotor speed is higher than the synchronous speed, then it behaves as a generator,
and if the rotor speed is less than the synchronous speed, it becomes a motor.
 The synchronous speed is determined by the line frequency and the number of poles of the stator
winding.
 The developed rotating magnetic field, after the stator is electrically connected, revolves at the
synchronous speed. This causes the rotor to follow the rotating magnetic field and rotate (thus, a
motor), but, if the rotor shaft is given mechanical energy to rotate faster than the speed of the
magnetic field, then the machine behaves as a generator.
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24. Slip: The fact that the rotor of an induction machine does
not rotate with the same speed as the rotating magnetic
field (turning faster in a generator and slower in a motor).
Slip speed: Difference between the speeds of the rotor
and the rotating magnetic field in an AC induction
machine.
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25. Torque-slip Curve

26. Torque-speed Curve and Operating Region

27. Configuration of induction motor drive
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28. What do we really control?
 What is in our control?
 The voltage applied to the motor: amplitude, frequency, phase
 What do we want to control?
 The speed ( from Hall sensor, encoder, resolver….)
 The torque (inferred from the measured current)
 Flux ( inferred from speed and current)
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29. Conventional Closed Loop Control
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30. Speed Control of Induction Motor
 The Speed of Induction Motor is changed from Both Stator and Rotor Side. The speed control of
three phase induction motor from stator side are further classified as :
 V / f control or frequency control.
 Changing the number of stator poles.
 Controlling supply voltage.
 Adding rheostat in the stator circuit.
 The speed controls of three phase induction motor from rotor side are further classified as:
 Adding external resistance on rotor side.
 Cascade control method.
 Injecting slip frequency emf into rotor side.
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31. Speed Control by Changing the Slip
 The speed of an induction motor can be changed by changing its slip, and the slip
can be changed
 (1) by changing the rotor circuit resistance
 (2) by changing the supply voltage and
 (3) by injecting voltage in the rotor circuit.

32. Speed Control by Changing the Rotor
Circuit Resistance

33. Speed Control by Changing the Rotor
Circuit Resistance

34. Speed Control by Controlling the Supply
Voltage
Slip or speed of a motor can also be changed by controlling
the voltage fed to the motor.
The torque developed by the motor is directly proportional
to the square of the supply voltage. If the supply voltage is
decreased, the torque developed by the motor decreases
rapidly (T∝V2) and to pick-up the load slip increases or
speed decreases
• This method is never used for the speed control of three-
phase large induction motors because the voltage control
devices are very costly and bulky. However, this method
is usually employed with single-phase induction motors
e.g., ceiling fans, etc.

35. Speed Control by Injecting Voltage in the
Rotor Circuit
 The speed of an induction motor can also be controlled by
injecting a voltage at slip frequency directly into the rotor
circuit.
 First of all, this method was introduced by K.H. Schrage of
Sweden and the motor in which this method is employed is
called Schrage motor.
 If the injected emf has a component directly opposite to the
rotor induced emf, the motor speed decreases.
 On the other hand, if the injected emf has a component in
phase with the rotor induced emf the motor speed increases
and may rises beyond the synchronous speed.
 Now-a-days, Schrage motors are not preferred because of
their heavy cost and bulky construction but these are still

36. Speed Control by Changing the Supply
Frequency
 The frequency of the power supply is constant, therefore, to control the speed of an
induction motor by this method, the induction motor is connected to the alternator
operating independently.
 To control the speed, the frequency of the alternator is changed. This is a costly affair.
 Recent improvements in the capabilities of controlled rectifiers (SCR) and continued
decrease in the cost of their manufacturing, it has made it possible to control the speed of
induction motor by controlling the supply frequency fed to the motor.
 By this method 5 to 10% of rated speed of induction motors can be controlled.
 However, if the speed is to be controlled beyond this value, the motor design has to be
changed accordingly.
 Further, If the speed of the motor is changed by changing the frequency, to keep the flux
level constant in the stator core, supply voltage is also changed so that v/f is kept
constant.

37. Speed Control by Changing the Poles
 By means of suitable switch, the stator
winding connections can be changed in
such a manner that the number of stator
poles is changed.
 This changes the actual speed of motor
since actual speed of the motor is
approximately inversely proportional to the
number of poles.
 By suitable connections one winding can
give two different speeds.
 If more than two speeds are required. Two
separate winding are housed in same slots
and if each is arranged to give two speeds
then two windings can give four different

38. Necessity of a starter

39. Starting Methods of Squirrel Cage
Induction Motors
 The various starters which are employed to restrict the initial rush of current in
squirrel cage induction motors are given below:
 1. Direct On Line (D.O.L.) Starter;
 2. Primary resistance (or inductance) starter;
 3. Star/Delta Starter;
 4. Auto-transformer Starter.

40. Direct on Line (D.O.L.) Starter

41. Induction Motor Control – Vector
Control
 There are three main types of control strategies for induction motor drives: the
variable-voltage
 variable-frequency (VVVF) control,
 field-oriented control (FOC), and
 direct torque control (DTC).
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42. Variable-Voltage Variable-Frequency
Control
 VVVF control has been widely adopted for speed control of induction drives.
 It is based on 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 to compensate the difference between the
applied voltage and induced EMF.
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58. The first region is called the constant-torque region
in which the motor can deliver its rated torque for speeds
below the rated speed (normally called the
base speed 𝜔b).
In the second region, called the constant-power region,
the slip is increased gradually to the maximum value so
that the stator current remains constant and the motor can
maintain its rated power capability.
When the speed is above the critical speed 𝜔c, the slip
remains constant while the stator current decreases.
Thus, the torque capability declines with the square of
speed, the so-called reduced power region.
It should be noted that 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. That is, the
corresponding torque control is not fast and accurate
enough for application to high-performance EVs.
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60. Field Oriented Control

61. Introduction
 Scalar control such as the “V/Hz” strategy has its limitations in terms of performance.
 The scalar control method for induction motors generates oscillations on the produced torque.
 Hence to achieve better dynamic performance, a more superior control scheme is needed for
Induction Motor.
 With the mathematical processing capabilities offered by the micro-controllers, digital signal
processors and Filed Programmable Gate Array (FGPA), advanced control strategies can be
implemented to decouple the torque generation and the magnetization functions in an AC
induction motor.
 This decoupled torque and magnetization flux is commonly called rotor Flux Oriented
Control (FOC).

62. Field Oriented Control
 Field Oriented Control describes the way in which the control of torque and speed are directly
based on the electromagnetic state of the motor, similar to a DC motor.
 FOC is the first technology to control the “real” motor control variables of torque and flux.
 With decoupling between the stator current components (magnetizing flux and torque), the
torque producing component of the stator flux can be controlled independently.
 Decoupled control, at low speeds, the magnetization state of motor can be maintained at the
appropriate level, and the torque can be controlled to regulate the speed.
 FOC has been solely developed for high-performance motor applications which can operate
smoothly over the wide speed range, can produce full torque at zero speed, and is capable of
quick acceleration and deceleration

63. Working Principle of Field Oriented
Control
 The field oriented control consists of controlling the stator currents represented by a vector.
 This control is based on projections that transform a three phase time and speed dependent
system into a two coordinate (d and q frame) time invariant system.
 These transformations and projections lead to a structure similar to that of a DC machine
control.
 FOC machines need two constants as input references: the torque component (aligned with the
q coordinate) and the flux component (aligned with d coordinate).

73. Basic module of Field Oriented Control

75. Field Oriented Control
 Advantages of Field Oriented Control
1. Improved torque response.
2. Torque control at low frequencies and low speed.
3. Dynamic speed accuracy.
4. Reduction in size of motor, cost and power consumption.
5. Four quadrant operation.
6. Short-term overload capability.
 On the other hand, FOC has some drawbacks including the fact that a sensor is needed for
determining the exact rotor position. This information is required to generate the magnetic
field resulting in maximum torque. In most cases, an electro-mechanical sensor or a
complex software algorithm (observer) is used.

77. Direct Torque Control

78. Introduction
 Direct torque control (DTC) is an emerging technique for controlling PWM inverter-fed induction
motor (IM) drives.
 It allows the precise and quick control of the IM flux and torque without calling for complex
control algorithms.
 In principle, moreover, it requires only the knowledge of the stator resistance.

79. Space vectors

80. Switching State Vectors

81. Principle of DTC

82. Principle of DTC

83. Invertor Voltage vectors and corresponding stator
flux variation in the time Δt

84. Conventional DTC Controller

85. Estimation block of Conventional DTC
Controller

86. Look up Table for DTC

87. Trajectory of stator flux vector in DTC

88. Effect of selected space vector

89. Features of DTC

90. Comparison of Vector control and DTC

91. Braking of Induction Motor
 Whenever controlling of motors is done, braking is the most important term, so as with
induction motors. Induction motor braking can be done by different methods, which
are-
1. Regenerative braking of induction motor
2. Plugging Braking of induction motor
3. Dynamic braking of induction motor is further categorized as
• AC dynamic breaking
• Self excited braking using capacitors
• DC dynamic braking
• Zero Sequence braking

92. Regenerative Braking of Induction Motor
 For motoring operation φs < 90o and for
braking operation φs > 90o.
 If the source frequency is fixed then the
regenerative braking of induction motor can
only take place if the speed of the motor is
greater than synchronous speed, but with a
variable frequency source regenerative
braking of induction motor can occur for
speeds lower than synchronous speed.
 The main advantage of this kind of braking
can be said that the generated power is use
fully employed and the main disadvantage
of this type of braking is that for fixed
frequency sources, braking cannot happen
below synchronous speeds.

93. Plugging Braking of Induction Motor
 Plugging induction motor braking is done by reversing the phase
sequence of the motor.
 Plugging braking of induction motor is done by interchanging
connections of any two phases of stator with respect of supply
terminals and with that the operation of motoring shifts to
plugging braking.
 During plugging the slip is (2 – s), if the original slip of the running
motor is s, then it can be shown in the following way.

94. Dynamic Braking of Induction Motor
 AC Dynamic Braking:
 This type of induction motor braking is
obtained when the motor is made to run on a
single phase supply by disconnecting any one
of the three phase from the source, and it is
either left open or it is connected with another
phase.
 When the disconnected phase is left open, it is
called two lead connection and when the
disconnected phase is connected to another
machine phase it is known as three load
connection.

95. Dynamic Braking of Induction Motor
 Self excited braking using capacitors:
 In this method there capacitors are kept
permanently connected across the source
terminals of the motor.
 The value of the capacitors are chosen
depending upon their capability to deliver
enough reactive current to excite the motor
and make it work as a generator.
 So, that when the motor terminals are
disconnected from the source the motor
works as a self excited generator and the
produced torque and field is in the opposite
direction and the induction motor braking
operation occurs.

96. Dynamic Braking of Induction Motor
 DC Dynamic Braking:
 To obtain this type of braking the stator of a
running induction motor is connected to a DC
supply. Two and three load connections are the
two common type of connections for star and
delta connected stators.
 Method of operation:
 The moment when AC supply is disconnected
and DC supply is introduced across the terminals
of the induction motor, there is a stationery
magnetic field generated due to the DC current
flow and as the rotor of the motor rotates in that
field, there is a field induces in the rotor winding,
and as a result the machine works as a generator
and the generated energy dissipates in the rotor
circuit resistance and dynamic braking of
induction motor occurs.