UNIT-3

Control of Induction motor & wound

rotor induction motor drive
ALPHA BREATHING
Evocation
 AC DRIVES
• AC motor Drives are used in many industrial and domestic
application, such as in conveyer, lift, mixer, escalator etc.
• Advantages :
 Increased power ratings
 High speed of operation
 Lightweight (20% to 40% lighter than equivalent DC motor)
 Inexpensive
 No commutator/ brush mechanism needed
 Low maintenance
• Disadvantages :
 The power control is complex
• There are two type of AC motor Drives :
1. Induction Motor Drives
2. Synchronous Motor Drives
 Induction Motor
• An induction motor operates on principle of induction.
• Rotor receives power from stator due to induction rather than direct conduction of
electric power.
• Induction Machines, the most widely used motor in industry, have been traditionally
used in open-loop control applications

• Applications induction machine variable-speed applications:

 Heating
 ventilation
  air conditioning systems
 waste water treatment plants   
 blowers
  fans 
 textile mills
  rolling mills, etc
• The stator winding are supplied with balanced three-phase AC voltage,
which produce induced voltage in the rotor windings.
• It is possible to arrange the distribution of stator winding so that there is
an effect of multiple poles, producing several cycle of magnetomotive
force (mmf) or field around the air gap.
• The speed of rotation of field is called the synchronous speed ws , which is
defined by :

ωs is syncronous speed [rad/sec]

Ns is syncronous speed [rpm]
p is numbers of poles
ω is the supply frequency [rad/sec]
f is the supply frequency [Hz]
Nm is motor speed
• The rotor speed or motor speed is :

m  s (1 S )
Where S is slip, as defined as : S  m NS  Nm
S Or S
S NS

3 Rr' Vs2
Td 
 Rr' 
2


S  s  Rs    X s  X r'
S 
 2

 
 Torque – speed Characteristic
• Three region operation :
1. Motoring : 0 S  1
2. Regenerating : S 0
3. Plugging : 1 S  2
 Torque Equation
• Starting speed of motor is m = 0 or S = 1,

Starting torque of motor is : 3 Rr' Vs2

Tst 
 ' 2
Rr  
 s  Rs     X s  X r  
' 2

 S  
So, the slip on maximum torque is :

Rr'
S max  
 R    X  X  
1
2 ' 2 2
s s r

Torque maximum is :
3 Vs2
Tmax 

2

2 s  Rs  Rs  X s  X r'  2 

Speed control induction motor
 Stator voltage control
 Stator frequency control
 Variable voltage and variable frequency control
VSI control
CSI control
 Rotor resistance control
 Slip power recovery scheme
Speed control induction motor :
• The speed of 3 phase induction motor can
be controlled by varying the stator voltage
and frequency.
• This method is suitable for speed control
of induction motor below the rated speed
• Variable voltage for speed control is
obtained by using ac power controller
 Ac Voltage controller circuits
(RMS voltage controllers)
An ac voltage controller is a type of thyristor power
converter which is used to convert a fixed
voltage, fixed frequency ac input supply to
obtain a variable voltage ac output
 AC power controller fed induction
motor drive
• Ac voltage controller is thyristor switch
connected between ac supply and motor
• Power flow can be controlled by varying
the rms value of ac voltage applied to the
motor
 Type Of Ac Voltage Controllers
• Single phase half wave ac voltage
controller (Uni-directional controller).
• Single phase full wave ac voltage
controller (Bi-directional controller).
• Three phase half wave ac voltage
controller (Uni-directional controller).
• Three phase full wave ac voltage
controller (Bi-directional Controller)
 Cont..
Types of control

 Phase angle control

 On-off control (Integral cycle control )
 Phase angle control

• The power flow to the load is controlled by

delaying the firing angle in each cycle.
• Less harmonics in to supply
• Torque α (Voltage)2
• To maintain torque, voltage is reduced then slip
will increase which decreases speed.
 ON-OFF Control (Integral cycle
control)
• The thyristors are employed as switches to
connect load to source for few cycles of
source voltage & disconnect after few
cycles.
• Thyristor acts as high speed switch
• Appropriate for small induction motors
• Poor efficiency for wide speed range
Principle of ON-OFF Control
Technique
V s n m

w t

V o
io

w t

ig 1 G a te p u ls e o f T 1

w t
ig 2 G a te p u ls e o f T 2

w t
 Stator voltage control

• Advantages:
 simple control circuit
 More compact
 Quick response time
 Economical method
• Disadvantages:
 Low input power factor
 Voltage & current waveforms are highly disorted due to harmonics
 Performance is poor
 Stator frequency control

• The torque and speed of induction motors can be controlled by

changing the supply frequency.
• The synchronous speed of induction motor is

• The two cases at constant voltage(frequency α 1/Flux)

 Low frequency operation. (flux increases)
 High frequency operation. (flux decreases)
• If the frequency is increased above its rated value, the flux and torque would decrease.
• If the synchronous speed corresponding to the rated frequency is called the base speed w b, the synchronous speed at any
other frequency becomes:

• Slip is given as:

• The motor torque:

 s   b
b  m 
S 1  m
 b  b

3 Rr' Vs2
Td 
 ' 2
R  

S  s  Rs  r   X s  X r'
S 
 2

 
 3 Rr' Vs2
Td 
 Rr' 
2


S b  Rs    X s  X r'
S
 2

 

If Rs is negligible, the maximum torque at the base speed as :

3 Vs2
Tmb 

2 S b X s  X r' 
The maximum torque at any other frequency is :
2
3 Vs
Tm 
2 S b  X s  X r'   2

At this maximum torque, slip S is :

Rr '
Sm 
  X s  X r' 
• Normalizing :
2
3 Vs
Tm 
2 S b  X s  X r'   2
Tm 1
 2
Tmb 
3 Vs2
Tmb 

2 S b X s  X r' 
Tm  2  Tmb

• During this operation of induction motor is similar to

working of dc series motor.
• It is known as field weakening mode because the air gap
flux gets reduced
 Speed torque curve
Td
Vs > Vs1 > Vs2
Tmax

Tst TL
Tst1
Tst2

2 1 
S=1 S=0
s
Nm =0 Ns
 Closed loop speed control
• Required to satisfy the steady state and transient
performance specifications of ac drives.
• For single quadrant operation 3 phase ac voltage
controllers are used.
• Plugging & reverse braking operations are possible with
3 phase ac voltage controllers .
• For four quadrant operation four quadrant ac voltage
controllers are used.
• Four quadrant operation with plugging is possible with
four quadrant ac voltage controllers
Closed loop control
Cont..
Speed torque characteristics
 Variable frequency Drives
• The speed of Induction Motor can be Calculated by given
equation                            
        Speed in RPM,  Ns =  120 f
                                                P
Where, f = frequency of AC supply, in Hz
  P = Number of Poles in Stator Winding.
And ,the Voltage Equation of Induction motor is given by    
V = 4.44 KfΦNT
Where V = e.m.f. induced in stator, in volt.
             Φ = air – gap flux in stator
         NT = number of turns in stator winding.
• As from above equations, It is clear that the  Synchronous speed
(Ns) is directly proportional to frequency (f)
           i.e.             Ns α f.
• Thus the speed of motor can be controlled by varying supply
frequency.
 Variable frequency Drives
• If a constant voltage is applied to the motor and the frequency is
reduced, the flux density in the motor would increase.
• This would drive the motor iron far into saturation causing excessive
excitation current and core loss.
• To prevent this, we must make the applied voltage directly
proportional to the applied frequency, making the flux density
constant.
• When the frequency is raised above the rated value, the voltage
must be held constant at its rated value.
• Thus, the flux in the motor decreases above rated frequency.
 Variable frequency operation of
induction motor drives
• Applicable to squirrel cage induction
motors.
• Speed controlled by varying supply
frequency.
• Variable frequency control allows good
running & transient performance.
• Used in special applications such as
underground &under water installations.
Applications :
 Fans
 Mills runner
 Blowers
 Spindle drives
 Conveyors
 Machine tools
 Pumps
 •The variable frequency application
can be obtained by using the following
devices or power circuits
 Voltage source inverter
 Current source inverter
 Cycloconverter
 Voltage source inverter
(VSI)
• Inverter which converts dc voltage into ac voltage
• Acts as voltage Source
• Capable of supplying variable frequency variable voltage
for speed control.
• Voltage α Frequency
Voltage source inverter
(VSI)
 Voltage source inverter
(VSI)
• Each thyristor is to be fired for an angle 180˚.
• So, to maintain the air-gap flux of Induction
Motor, the ratio of voltage and frequency must
be kept constant.
• The output voltage is varied by varying d.c.
input voltage.
• This adjustable d.c. voltage is obtained by using
a controlled rectifier and inverter.
 CSI fed induction motor drive
• In this ac drive, the induction motor is
controlled by using an current source
inverter. This type of a.c. drive is known as
Current source inverter drive .
CSI fed induction motor drive
 CSI fed induction motor drive
• Each thyristor are fired with a phase difference
of 60˚ in sequence of their numbers i.e. SCR1,
SCR2, SCR3, SCR4, SCR5, SCR6.
• A large inductance is connected in series to
maintain a constant current. The diodes (D1-
D6) and capacitors (C1-C6) provide
commutation of thyristor (SCR1-SCR6).
 CSI fed induction motor drive
• For a given speed, torque is controlled by varying dc link
current (Id) and by changing the dc link voltage (Vd).
• Thus, dc voltage can be varied also by two
arrangements.
• First, when the ac supply is used , a controlled rectifier is
connected between the supply and inverter and thus
speed of induction motor can be controlled.
• Another method is ,when dc supply is used, a chopper is
connected between dc supply and inverter. Hence, in
this way, the speed of induction motor is controlled.
Comparision of CSI & VSI fed induction
otor drives
 Types of Braking
 Dynamic Braking or rheostatic- electrical
energy is dissipated in braking resistors

 Regenerative Braking – Electrical energy

is fed back to the supply

 Plugging– Reverse voltage braking

 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.
• The braking operation can be understood easily. When
the motor is running on 1-phase supply, the motor is fed
by positive and negative sequence, net torque produced
by the machine at that point of time is sum of torques due
to positive and negative sequence voltage.
• At high resistance the net torque is found to be negative
and braking occurs.
Dynamic braking
 Regenerative Braking
• We know the power (input) of an induction motor is given
as.
Pin = 3VIscosφs

• Here, φs the phase angle between stator phase voltage

V and the stator phase current Is.
• Now, for motoring operation φs < 90° and for braking
operation φs > 90°. When the speed of the motor is more
than the synchronous speed, relativespeed between the
motor conductors and air gap rotating field reverses, as
a result the phase angle because greater than 90° and
the power flow reverse and thus regenerative braking
takes place.
speed torque curves
 Cont..
• 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.
• with a variable frequency source
regenerative braking of induction motor
can occur for speeds lower than
synchronous speed.
 Torque-Speed Curve of Induction
Motor
Three regions in torque-speed curve:
1) Plugging (braking) region (1<s<2)
Rotor rotates opposite to direction of air gap flux.
Can happen, for example, if stator supply phase
sequence reversed while rotor is moving.
2) Motoring region (0<s<1)
Te=0 at s=0. As s increases (speed decreases), Te
increases until max. torque (breakdown torque) is
reached. Beyond this point, Te decreases with
increasing s.
 Torque-Speed Curve of Induction
Motor
3) Regenerating Region (s<0)
Here the induction machine acts as a
generator. Rotor moves faster than air
gap flux resulting in negative slip
Torque-Speed Curve of Induction
Motor
Speed torque characteristics
 Speed control of induction motor
• The speed and torque of induction motors
can be controlled by

– Stator voltage control

– Rotor resistance control
Stator voltage control
 Stator voltage control

The stator voltage can be varied by three-phase

– ac voltage controllers,
– voltage-fed variable de-link inverters, or
– pulse-width modulation (PWM) inverters.

• However, due to limited speed range requirements, the ac voltage

controllers are normally used to provide the voltage control.
• The ac voltage controllers are very simple.
• However, the harmonic contents are high and the input PF of the
controllers is low.
• They are used mainly in low-power applications, such as fans,
blowers, and centrifugal pumps, where the starting torque is low.
 Advantages
• Simple Control circuit
• Compact size
• Quick response time
• Economical method of speed control
 Disadvantages

• Performance is poor under running

conditions
• Operating efficiency is low
• Maximum torue decreases as stator
voltage reduces
• Voltage &current waveforms are distorted
due to harmonics
 Rotor resistance Control
• This method increases the starting torque while limiting the starting
current.
• However, this is an inefficient method and there would be
imbalances in voltages and currents if the resistances in the rotor
circuit are not equal.
• A wound-rotor induction motor is designed to have a low-rotor
resistance so that the running efficiency is high and the full-load slip
is low.
• The increase in the rotor resistance does not affect the value of
maximum torque but increases the slip at maximum torque.
• The wound-rotor motors are widely used in applications requiring
frequent starting and braking with large motor torques (e.g., crane
hoists).
• Because of the availability of rotor windings for changing the rotor
resistance, the wound rotor offers greater flexibility for control.
 Rotor resistance Control

• However, it increases the cost and needs maintenance

due to slip rings and brushes.
• The wound-rotor motor is less widely used as compared
with the squirrel-case motor.
• In a wound-rotor motor, an external three-phase resistor
may be connected to its slip rings
• The developed torque may be varied by varying the
resistance Rx. If Rx is referred to the stator winding and
added to Rr, to the developed torque can be determined.
• Effective resistance,
Re = R (l - k)
Stator voltage control
 Advantages
& Disadvantages

Advantages:
• Absence of in rush starting current
• High line power factor
• Absence of line current harmonics
• Wide range of speed control
Disadvantages:
• Slip energy is wasted in rotor circuit resistance
• Reduced efficiency
• Speed changes widely with load variation
• Unbalance in voltage & currentsof rotor circuit
 STATIC KRAMER DRIVE
• Instead of wasting the slip power in the
rotor circuit resistance it can be pumped
back to line.
• This system is applicable only for sub
synchronous speed operation
• The voltage at the slip rings is forced to be
in phase with the rotor currents by the
diode rectifier.
 STATIC KRAMER DRIVE
• The magnitude of the slip ring voltage is
set by the DC link voltage, which is in turn
set by the inverter connected back to the
AC supply.
• In the diagram above and the analysis
presented, the inverter used is a thyristor
converter.
 Cont..
• This simple analysis of the static kramer drive
illustrates the operation of the drive. It neglects
the voltage drops in the drive and any possible
commutation overlap in the diode rectifier.
• The voltage at the input to the diode rectifier is
given by
 Cont..
• the dc link voltage can be found from the diode input
line-line voltage as

• Considering the thyristor converter, this circuit can be

thought of as a thyristor rectifier connected in reverse,
and the DC link voltage is related to the line-line inverter
voltage as
 Cont..
• Substituting the above expressions, the voltage injected into the
rotor can be calculated as

• In the case that the inverter line-line voltage is connected to the

supply through a transformer, as shown in the diagram above, the
injected voltage can be related to the supply voltage as
 Cont..
• Using this simplified analysis together with the slip
energy recovery torque equations, the thyristor firing
angle required for a particular torque at a particular
speed can be found.
• Slip ,s= -1/b(cos α)
• If b=1,slip= - (cos α)
• So, α=90˚---- s=0(synchronous speed)
• α=180˚---- s=1(speed is 0)
Cont..
 Advantages
• Slip power is saved
• Very efficient
• Converter rating is low
• Control is simple
 Disadvantage
• Power factor is low
• Harmonic current
• Losses increases & derating
sub synchronous & super synchronous speed
operation

• sub synchronous speed operation:

• Speed control below synchronous speed.
• Sip power fed back to the supply.

• super synchronous speed operation

• Speed control above synchronous speed.
• Supply fed back to rotor side