The document discusses rotor resistance control for induction motors, highlighting its method of speed control through external resistance connected via slip rings, applicable only to wound rotor motors. It explains the limitations of traditional variable rheostat methods and introduces a thyristor-based solution for varying rotor resistance in a more economical and effective manner. Additionally, it mentions the benefits of static rotor resistance control, including smooth operation and reduced maintenance.

1. ROTOR RESISTANCE CONTROL
PREET PATEL (151310109032)
POWER ELECTRONICS - II

2. CONTENTS
• Rotor resistance control of an Induction motor
• Closed loop Speed Control

3. ROTOR RESISTANCE CONTROL of an
INDUCTION MOTOR
• Rotor Resistance Control is also one of the methods by which we
can control the speed of the Induction motor.
• The speed of the wound induction motor can be controlled by
connecting an external resistance in the rotor circuit through slip
rings.
• This method is not applicable to cage rotor induction motor.

4. • As we know that,
𝑇 =
3𝑠𝑉2
𝜔 𝑠 𝑟
• So now if we consider that motor torque remains constant, then
resistance is directly proportional to slip, which in turn will vary the
speed.
• This method creates more losses, so this is only adopted for a narrow
range speed and used in lifting up of hoist, cranes and excavators.

5. Depending on the rotor resistance, the speed of the rotor falls or increases. The variation
of speed torque characteristics with respect to change in rotor resistance is shown in the
figures below.

6. Slip Power Control using Rotor Resistance
along with Chopper
• The conventional method for variation of rotor
resistance was done by inserting a variable
rheostat in series with the 3 phase winding of
motor.
• This method of speed control faced problems
and this was not economically viable.
• So in solution to this problem, thyristor is used
to which is connected across enables external
resistance and effective resistance is varied
simultaneously.

7. • Effective value of resistance across terminal A and B i.e., RAB is varied by varying duty cycle
of the transistor Tr, which in turn varies the resistance of the rotor circuit. Inductance is
added to the circuit to reduces the ripple and discontinuity in the dc link current Id.
• Rotor current waveform is shown in the when the ripple is neglected.

8. • Thus the rms value of the current will be:
• Resistance between A and B terminal will be zero when the transistor is on and it
will be R when it is off. Therefore average value of resistance between the
terminals is given by
Where, δ is the duty ratio of the transistor.
• Therefore the total rotor resistance is varied from Rr to 0.5(1-δ)R as δ is varied
from 1 to 0.
• Electrical power consumed by the RAB resistor per phase is
𝐼 𝑟 =
2
3
𝐼 𝑑
𝑅 𝐴𝐵 = (1 − 𝛿)𝑅
Power consumed per phase =
𝑃 𝐴𝐵
3
= 0.5 1 − 𝛿 𝑅𝐼 𝑟
2

9. Closed loop Speed Control
CURRENT
CONTROLLER
BASE
DRIVE
CURRENT
LIMITER
SPEED
CONTROLLER
δ
ωmωm
Id
B
A
Tr
R
Id
Ld
CHOPPER
IM
SPEED SENSOR
ωm*

10. • Closed loop speed control scheme with inner current control loop is shown in figure, which is
employed for successful performance achieved by using transistor chopper, which is of low cost
and Id has a constant value at the maximum torque point.

11. If the current limiter is made to saturate at this current, the drive will accelerate and decelerate at the
maximum torque giving very fast transient response. For plugging to occur, arrangement will have to be
made of reversal of phase sequence.
Compared to conventional rotor resistance control, static rotor resistance control has several advantages
such as smooth and stepless control, fast response, less maintenance compact size, simple closed-loop
control and rotor resistance remains balanced between the three phases for all operating points.

12. THANK YOU