1. Give the four modes of operation of a Scherbius
modes of operation of static Scherbius drive are,
2. Give the use of synchronous motors.
motors were mainly used in constant speed applications. The development of
semiconductor variable frequency sources, such as inverters and
cycloconverters, has allowed their use in draft fane, main line traction, servo
3. How are the stator and rotor of the
synchronous motor supplied?
stator of the synchronous motor is supplied from a thyristor power converter
capable of providing a variable frequency supply. The rotor, depending upon the
situation, may be constructed with slip rings, where it conforms to a
conventional rotor. It is supplied with D.C. through slip rings. Sometimes
rotor may also be free from sliding contacts (slip rings), in which case the
rotor is fed from a rectifier rotating with rotor.
4. What is the difference between
an induction motor and synchronous motor?
An induction motor operates at lagging power factor
and hence the converter supplying the same must invariable is a force
commutated one. A synchronous motor, on the other hand, can be operated at any
power factor by controlling the field current.
the commonly used synchronous motors. Commonly used synchronous motors are,
field synchronous motors.
magnet synchronous motors
reluctance synchronous motors.
6. Mention the main difference between the wound field
and permanent magnet motors.
When a wound filed motor is started as an induction
motor, D.C. field is kept off. In case of a permanent magnet motor, the field
cannot be 'turned off’.
7. Give the advantages and
applications of PMSM.
advantages of PMSM are,
a. High efficiency
b. High power factor
sensitivity to supply voltage variations.
application of PMSM is that it is preferred of industrial applications with
large duty cycle such as pumps, fans and compressors.
8. Give the uses of a hysteresis
Small hysteresis motors are extensively used in
tape recorders, office equipment and fans. Because of the low starting current,
it finds application in high inertia application such as gyrocompasses and
9. Mention the two modes employed
in variable frequency control
Variable frequency control may employ and of the
two modes. a. True synchronous mode b. Self-controlled mode
10. Define load commutation
of thyristors by induced voltages pf load is known as load commutation.
11. List out the advantages of
load commutation over forced commutation.
commutation has a number of advantages over forced commutation
not require commutation circuits
of operation can be higher
operate at power levels beyond the capability of forced commutation.
12. Give some application of load commutated
inverter fed synchronous motor drive.
Some prominent applications of load commutated
inverter fed synchronous motor drive are high speed and high power drives for
compressors, blowers, conveyers, steel rolling mills, main-line traction and
aircraft test facilities.
13. How the machine operation is performed in
For machine operation in the self -controlled mode,
rotating filed speed should be the same as rotor speed. This condition is
relaised by making frequency of voltage induced in the armature. Firning pulses
are therefore generated either by comparison of motor terminal voltages or by
rotor position sensors.
14. What is meant by margin angle of commutation?
The difference between the lead angle of firing and
the overlap angle is called the margin angle of commutation. If this angle of
the thyristor, commutation failure occurs. Safe commutation is assured if this
angle has a minimum value equal to the turn off angle f the thyristor.
15. What are the disadvantages of VSI fed
synchronous motor drive?
VSI synchronous motor drives might impose fewer
problems both on machine as well as on the system design. A normal VSI with
180° conduction of thyristors required forced commutation and load commutation
is not possible.
16. How is PNM inverter supplied in VSI fed
When a PWM inverter is used, two cases may arise
the inverter may be fed from a constant D.C. source in which case regeneration
is straight forward. The D.C. supply to the inverter may be obtained form a
diode rectifier. In this case an additional phase controlled converter is
required on the line side.
17. What is D.C. link converter and cycloconverter?
D.C. link converter is a two stage conversion
device which provides a variable voltage, variable frequency supply.
Cycloconverter is a single stage conversion device
which provides a Variable voltage, variable frequency supply.
18. What are the disadvantages of cycloconverter?
A cycloconverter requires large number of
thyristors and ts control circuitry is complex. Converter grade thyristors are
sufficient but the cost of the converter is high.
19. What are the applications of cycloconverter?
A cycloconverter drive is attractive for law speed
operation and is frequently employed in large, low speed reversing mils
requiring rapid acceleration and deceleration. Typical applications are large
gearless drives, e.g. drives for reversing mills, mine heists, etc.
20. Give the application of CSI fed synchronous
Application of this type of drive is in gas turbine
starting pumped hydroturbine starting, pump and blower drives, etc.
21. What are the disadvantages of machine
disadvantages of machine commutation are,
a.Limitation on the speed range.
b. The machine size is large
c. Due to
overexciting it is underutilized.
the use of an auxiliary motor?
Sometimes when the power is small an auxiliary
motor can be used to run up the synchronous motor to the desired speed.
23. What are the advantages of
brushless D.C. motor?
The brushless D.C. motor is in fact an inverter-fed
self controlled permanent synchronous motor drive. The advantages of brushless
D.C. motor are low cost, simplicity reliability and good performance.
24. When can the synchronous
motor be load commutated?
When the synchronous motor operates at a leading
power factor thyristors of load side converter can be commutated by the motor
induced voltages same way as the thyristors of a line commutated converter are
commutated by line voltages.
the characteristics of self controlled mode operated synchronous motor?
operates at like dc motor also commutator less motor.
machines have better stability behavior.
c) Do not have
the characteristics of true synchronus mode operated synchronous motor?
behaves like conventional synchronous motor i.e) hunting oscillations exists.
change in frequency is slow enough for rotor to truck the changes.
motor operation is possible here.
27. What is meant by sub synchronous speed
The sub synchronous speed operation means the
SRIM speed can be controlled below the synchronous speed. i.e) the slip power
is fed back to the supply.
28. What is meant by super synchronous speed
The super synchronous speed operation means
the SRIM speed can be controlled above the synchronous speed. i.e) the supply
is fed back to the rotor side.
29. What are the two types of static scherbius
a) DC link static scherbius system
b) Cyclo converter scherbius system