EEE Dept./Mahalakshmi Engineering college,Trichy-621213 Page 1 MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI – 621213 QUESTION BANK WITH ANSWER --------------------------------------------------------------------------------------------------------------- Sub. Code : EE2352 Semester : VI Subject : SOLID STATE DRIVES Unit : IV ---------------------------------------------------------------------------------------------------------------- INDUCTION MOTOR DRIVES 1. What are the different methods of braking applied to the induction motor? Regenerative braking Plugging Dynamic braking. 2. What are the different methods of speed control of IM? Stator voltage control Supply freq. control Rotor resistance control Slip power recovery control. 3. What is meant by stator voltage control? The speed of the IM can be changed by changing the stator voltage, because the torque is proportional to the square of the voltage. 4. Mention the application of stator voltage control. This method is suitable for applications where torque demand reduced with speed, which points towards its suitability for fan and pump drives. 5. Mention the applications of ac drives. AC drives are used in a no. of applications such as fans, blowers, mill run- out tables, cranes, conveyors, traction etc. 6. What are the three regions in the speed-torque characteristics in the IM? Motoring region(0<=s<=1) Generating region(s<0) Plugging region (1<=s<=2) where s is the slip. 7. What is the advantage of stator voltage control method?
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MAHALAKSHMImahalakshmiengineeringcollege.com/pdf/eee/VIsem/EE2352/UNIT 4.pdfGive the four modes of operation of a Scherbius drive. EEE Dept./Mahalakshmi Engineering college,Trichy-621213
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The four modes of operation of static Scherbius drive are,
Sub synchronous motoring.
Sub synchronous regeneration
Super synchronous motoring
Super synchronous regeneration
27. How is the static Scherbius drive operated in super synchronous motoring mode?
In super synchronous motoring mode, the shaft speed increases beyond
the synchronous speed, the slip becomes negative and the slip power is absorbed by the rotor. The slip power supplements the air gap power for the total mechanical power output. The line therefore supplies slip power in addition to
stator input power. At this condition, the phase sequence of slip frequency is reversed so that the slip current – induced rotating magnetic filed is opposite to
that of the stator
PART-B
1)Explain construction and working principle of three phase induction
motor.(AU/2012/11/)
Construction:
The induction motor consists of two main parts,
Stator
Rotor
Stator:
the stator made up of a stamping with alternate slot and tooth. Stamping are insulated
from each other. Each stamping is 0.4 to 0.5 mm thick. The slot house the three phase winding
just like a three phase alternator. The three phase winding is called stator winding. It may be
connected either in star or delta. The stator winding is made for a fixed number of poles.
Rotor:
There are two type of rotors used in induction motor. They are,
2. External resistance cannot be connected to rotor circuit
Applications of squirrel cage induction motor
Squirrel cage induction motors are used in lathes, fans, blowers water pumps, grinder
etc..,.
Advantages of slip ring induction motor
1. The starting torque can be controlled by varying the rotor circuit resistance.
2. The speed of the motor can be controlled by varying the rotor circuit resistance.
Disadvantages of slip ring induction motor
1. Wound – rotor machine is heavier.
2. High cost
3. High rotor inertia
2)Analysis and performance of three phase induction motor:
Per phase equivalent circuit of a three phase induction motor is shown in the fig. and
are the stator referred values of rotor resistance Rr rotor reactance Xr. Slip is defined by
ms
mmss
_____________ (1)
p
4ππms Rad/sec _______________ (2)
Since, stator impedance drop is generally negligible compared to terminal voltage V, the equivalent circuit can be simplified to that shown in the fig:
3)Operation with unbalanced source voltage and single phasing
As supply voltage some time become unbalanced, it is useful to know the effect of unbalanced voltages on motor performance.
As three – phase set of unbalanced voltages (Va,Vb and Vc) can be resolved in to three phase balanced positive sequence, negative sequence and zero sequence voltages, using
symmetrical components relations:
Vp= 1/3(Va+αVb+2 Vc)
Vn= 1/3(Va+2 Vb+αVc)
V0= 1/3(Va+Vb+Vc) ______________ (1)
Where α= 120sinsin120cos120 je j ___________ (2)
For positive sequence voltages, equivalent circuits are same, except that V is replaced by Vp. The positive sequence rotor current and torque are obtained by replacing V by Vp. Thus
2'
rS
2'
rs
p'
rp
)Xj(XS
RR
VI
__________________(3)
2'
rS
2'
rs
2
r
2
P
ms
P
)X(XS
RR
/SRV3T
_______________(4)
The slip is
ms
mmsnS
_____________ (5)
Sn= (2-S) ______________(6)
When S is replaced by (2-S), and V is replaced by Vn, Then the following expressions are
Torque has a negative sign because for negative sequence voltages the synchronous speed is the
synchronous speed is (- ms )
The rms rotor current and torque are given by
2'
rS
2'
rs
2
r
2
n
2'
rS
2'
rs
2
r
2
P
ms
nP
1/2'2
rn
'2
rp
'
r
)X(XS
RR
S)-/(2RV
)X(XS
RR
/SRV3
TTT
)I(II
______________ (9)
Positive sequence, negative sequence and the resultant speed – torque characteristics are shown
in fig:
Operation with unbalanced rotor impedance
Earlier unbalanced rotor impedance was employed in starting and speed control. It is useful to examine the effect of unbalance on motor performance. Unbalalance rotor impedance
causes the unbalance in rotor currents. The unbalanced rotor currents can be resolved in to positive and negative sequence components.
For (0≤S≤0.5), the speed of negative sequence rotor field is positive, therefore interaction between this field and stator current induced by it, produce a positive on the stator, and the fig
shows the nature of the speed torque characteristics.
Speed – torque curves with unbalanced rotor resistance
4)Explain Various method of Starting in three phase Induction Motor.
Starting arrangement is chosen based on the load requirements and nature of supply.
The following methods are employed for starting of induction motor.
Star-Delta Starter
In this method motor designed run in delta connection and connected in star during
starting, this allows reduction in stator current and voltage by 1/√3.
Circuit breakers CBr and CBs are closed to start the motor in star connection and when
the motor started to run in steady state speed, the circuit breakers CBs and CBr are closed run the motor in Delta connection
If stator copper loss, core loss and friction loss are ignored, then the motor efficiency is
given by S1P
Pη
g
m
Stator voltage control
7) Explain the closed loop speed control of VSI fed & CSI fed induction motor
drive.
VSI fed Induction Motor Drives
Voltage source inverter allows a variable frequency supply to be obtained from a dc supply.
VSI can be operated as a stepped wave inverter or PWM inverter. When operated as stepped wave inverter, transistors are switched in sequence with time difference.
Frequency of the inverter operation varied by varying the T and output of the inverter
also varied accordingly.
Inverter output line and phase voltage are given by the following Fourier series:
Rotor resistance can also vary steplessly using the above circuit. The ac output voltage of the rotor can be converted with Diode Bridge and it is given to parallel combination of
the resister and transistor switch.
The resistance value can be varied with help of the duty ratio of the transistor.
The RMS rotor current will be
dr I3
2I
Therefore average value of resistance between the terminals is given by
RAB= Rδ1
Power consumed by RAB is
PAB= δ)R(1IRI 2
dAB
2
d
Power consumed per phase= 2
rAB δ)I0.5R(1/3P
Thus total rotor circuit resistance per phase will be δ)0.5R(1RR TrT
8)Explain the voltage\frequency control of Induction motor drives.
Open Loop Volts/Hz Control
Open loop volts/Hz control of an induction motor is very familiar in the industry.
For adjustable speed applications, frequency control is natural.
The phase voltage command Vs* is generated from frequency command and the flux is