1. What are
the components of the applied voltage to the induction motor?
The applied voltage to the induction motor has two
components at low frequencies. They are
a. Proportional to stator frequency.
b.To compensate for the resistance drop in the
stator.
The
second component deepens on the load on the motor and hence on rotor frequency.
2. What is
indirect flux control?
The method of maintaining the flux constant by
providing a voltage boost proportional to slip frequency is a kind of indirect
flux control. This method of flux control is not desirable if very good dynamic
behaviour is required.
3. What
is voltage source inverter?
Voltage
source inverter is a kind of D.C. link converter, which is a two stage
conversion device.
4. What
is the purpose of inductance and capacitance in the D.C. link circuit?
The
inductance in the D.C. link circuit provides smoothing whereas the capacitance
maintains the constancy of link voltage. The link voltage is a controlled
quality.
5. What
are the disadvantages of square wave inverter in induction motor drive?
Square wave
inverters have commutation problems at very low frequencies, as the D.C. link
voltage available at these frequencies cannot charge the commutating capacitors
sufficiently enough to commutate the thrusters. Those puts a limit on the lower
frequency of operation. To extend the frequency towards zero, special charging
circuits must be used.
6. What
is slip controlled drive?
When the
slip is used as a controlled quantity to maintain the flux constant in the
motor the drive is called slip enrolled drive. By making the slip negative
(i.e., decreasing the output frequency of the inverter) The machine may be made
to operate as a generator and the energy of the rotating parts fed back to the
mains by an additional line side converter or dissipated in a resistance for
dynamic barking.
By keeping
the slip frequency constant, braking at constant torque and current can be
achieved. Thus braking is also fast.
7. What
are the effects of harmonics in VSI fed induction motor drive?
The motor
receives square wave voltages. These voltage has harmonic components. The harmonics of the stator current cause
additional losses and heating. These harmonics are also responsible for torque
pulsations. The reaction of the fifth and seventh harmonics with the
fundamental gives rise to the seventh harmonic pulsations in the torque
developed. For a given induction motor fed from a square wave inverter the
harmonic content in the current tends to remain constant independent of input
frequency, with the rang of operating frequencies of the inverter.
8. What is a
current source inverter?
In a D.C. link converter, if the D.C. link current
is controlled, the inverter is called a current source inverter, The current in
the D.C. link is kept constant by a high inductance and he capacitance of the
filter is dispensed with . A current source inverter is suitable for loads
which present a low impedance to harmonic currents and have unity p.f.
9. Explain
about the commutation of the current source inverter.
The commutation of the inverter is load dependent.
The load parameters form a part of the commutation circuit. A matching is
therefore required between the inverter and the motor. Multimotor operation is
not possible. The inverter must necessarily be a force commutated one as the
induction motor cannot provide the reactive power for the inverter. The motor
voltage is almost sinusoidal with superimposed spikes.
10. Give the features from which a slip controlled
drive is developed.
The stator current of an induction motor operating
on a variable frequency, variable voltage supply is independent of stator
frequency if the air gap flux is maintained constant. However, it is a function
of the rotor frequency. The torque developed is also a function of rotor
frequency. The torque developed is also a function of rotor frequency only.
Using these features a slip controlled drive can be developed employing a
current source inverter to feed an induction motor.
11. How is the braking action produced in plugging?
In plugging, the barking torque is produced by
interchange any two supply terminals, so that the direction of rotation of the
rotating magnetic field is reversed with respect to the rotation of the motor.
The electromagnetic torque developed provides the braking action and brings the
rotor to a quick stop.
12. Where is rotor resistance control used?
Where the motors drive loads with intermittent type
duty, such as cranes, ore or coal unloaders, skip hoists, mine hoists, lifts,
etc. slip-ring induction motors with speed control by variation of resistance
in the rotor circuit are frequently used. This method of speed control is
employed for a motor generator set with a flywheel (Ilgner set) used as an
automatic slip regulator under shock loading conditions.
13. What are the advantages and disadvantages of
rotor resistance control?
Advantage of rotor resistance control is that motor
torque capability remains unaltered even at low speeds. Only other method which
has this advantage is variable frequency control. However, cost of rotor
resistance control is very low compared to variable frequency control.
Major disadvantage is low efficiency due to
additional losses in resistors connected in the rotor circuit.
14. Where is rotor resistance control used?
Where the motors drive loads with intermittent type
duty, such as cranes, ore or coal unloaders, skip hoists, mine hoists, lifts,
etc. slip-ring induction motors with speed control by variation of resistance
in the rotor circuit are frequently used. This method of speed control is
employed for a motor generator set with a flywheel (Ilgner set) used as an
automatic slip regulator under shock loading conditions.
15. What are the advantages and disadvantages of
rotor resistance control?
Advantage of rotor resistance control is that motor
torque capability remains unaltered even at low speeds. Only other method which
has this advantage is variable frequency control. However, cost of rotor
resistance control is very low compared to variable frequency control. Major
disadvantage is low efficiency due to additional losses in resistors connected
in the rotor circuit.
16. How is the resistance in the
output terminals of a chopper varied?
The resistance connected across the output
terminals of a chopper can be varied form O to R by varying the time ratio of
the chopper. When the chopper is always OFF, the supply is always connected to
the resistance R. The time ratio in this case is zero and the effective
resistance connected in R. Similarly when the chopper is always ON, the
resistance is short circuited. The time ratio in the case is unity and the
effective resistance connected is 0. Hence by varying the time ratio from 0 to
1, the value of resistance can be varied from R to O.
17. What is the function of
inductance L and resistance R in the chopper resistance circuit?
A smoothing inductance L is used in the circuit to
maintain the current at a constant value. Any short circuit in the chopper does
not become effective due to L.
The value of R connected across the chopper is
effective for all phases and its value can be related to the resistance to be
connected in each phase if the conventional method has been used. The speed
control range is limited by the resistance.
18. What are the disadvantages and advantages of
chopper controlled resistance in the rotor circuit method?
The method is very inefficient because of losses in
the resistance. It is suitable for intermittent loads such as elevators. At low
speeds, in particular the motor has very poor efficiency. The rotor current is
non-sinusoidal. They harmonics of the rotor current produce torque pulsations.
These have a frequency which is six times the slip frequency.
Because
of the increased rotor resistance, the power factor is better.
19. How is the range of speed control
increased?
The range
of speed control can be increased if a combination of stator voltage control
and rotor resistance control is employed. Instead of using a high resistance
rotor, a slip ring rotor with external rotor resistance can be used when stator
voltage control is used for controlling the speed.
20. Why the static scherbius drive has a poor
power factor?
Drive
input power is difference between motor input power and the power fed back.
Reactive input power is the sum of motor and inverter reactive power.
Therefore, drive has a poor power factor throughout the range of its options.
21. How is super synchronous speed achieved?
Super
synchronous speed can be achieved if the power is fed to the rotor from A.C.
mains. This can be made possible by replacing the converter cascade by a
cycloconverter. A cycloconverter allows power flow in either direction making
the static sherbets drive operate at both sub and supper synchronous speeds.
22. Give the features of static scherbius drive
The
torque pulsations and other reactions are minimal. The performance of the drive
improves with respect to additional losses and torque pulsations. A smooth
transition is possible from sub to super synchronous speeds without any
commutation problems. Speed reversal is not possible. A step up transformer may
be interposed between the lines and the converter, to reduce the voltage rating
of the converter.
23. Where is Kramer electrical drive system used?
Some
continuous rolling mills, large air blowers, mine ventilators, centrifugal
pumps and any other mechanisms including pumps drives of hydraulic dredgers
require speed adjustment in the range from 15 to 30% below or above normal. If
the induction motor is of comparatively big size (100 to 200 KW) it becomes
uneconomical to adjust speed by mean's pf external resistances due to copper
losses as slip power is wasted as heat in the retort circuit resistance. In
these case , the Kramer electrical drive system is used , where slip power
recovery takes places.
24. What is the use of sub synchronous converter
cascades?
Sub
synchronous converter cascades have been used, till now, in applications
requiring one quadrant operation. These can be employed for drives where at
least one electrical barking is required. A four quadrant operation can also be
made possible in these cascades, using suitable switching.
25. How is the speed control obtained in static
Kramer drive?
For speed
control below synchronous speed, the slip power is pumped back to the supply,
where as for the case of speed above synchronous speed, additional slip power
is injected into the rotor circuit.
26. What is static Kramer drive?
Instead
of wasting the slip power in the rotor circuit resistance, it can be converted
to 60 Hz A.C. and pumped back to the line. The slip power controlled drive that
permits only a sub synchronous range of speed control through a converter
cascade is know as static Kramer drive.
27. What is the use and functions of step down
transformer is static Kramer drive?
For a
restricted speed range closer to synchronous speed, the system power factor can
be further improved by using a step -down transformer.
The
step-down transformer has essentially two functions: besides improving the line
power factor, it also helps to reduce the converter power ratings.
28. What are the advantages of static Kramer drive?
The
static Kramer drive has been very popular in large power pump and fan-type
drives, where the range of speed control is limited near, but below the
synchronous speed. The drive system is very efficient and the converted power
rating is low because t has to handle only the slip power, In fact, the power
rating becomes lower with a more restricted range of speed control. The
additional advantages are that the drive system has D.C. machine like
characteristics and the control is very simple.
29. What are the causes of harmonic currents in
static Kramer drive?
The
rectification of slip power causes harmonic currents in the rotor, and these
harmonics are reflected to the stator by the transformer action of the machine.
The harmonic currents are also injected into the A.C. line by the inverter. As
a result, the machine losses are increased and some amount of harmonic torque
is produced. Each harmonic current in the rotor will create a reading magnetic
filed and its direction of rotation will depend on the order pf the harmonic.
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