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.
PART B
1. With necessary diagram explain the closed loop speed control of load commutated inverter synchronous motor drive. (16)
2. Draw the open loop volts/Hz speed control of multiple PM synchronous motors and volts/Hz speed control characteristics in torque –speed plane. (16)
3. Explain in detail the construction ,principle of operation and applications of Permanent Magnet Synchronous Motor. (16)
4. (a) Explain in detail about PMSM drives (8) (b)Explain power factor control of Synchronous Motor with relevant vector diagram. (8)
5. A 3 phase, 400 V , 50 Hz, 6 pole star connected round- rotor synchronous motor has Zs = 0+j2 . Load torque, proportional to speed squared, is 340 N-m at rated synchronous speed. The speed of the motor is lowered by keeping V/f constant and maintaining unity pf by field control of the motor. For the motor operation at 600 rpm, calculate a) supply voltage b) armature current c) excitation angle d) load angle e) the pull – out torque. Neglect rotational losses. (16)
6. (a) State and explain the role of damper winding in a synchronous motor? (8) (b) Write short note on cycloconverter fed synchronous motor. (8)
7. Describe the self control of synchronous motor fed from VSI. Discuss about the separately controlled synchronous motor fed from VSI. (16)
8. Write brief notes about (a)Constant margin angle control. (8) (b)Synchronous motor fed from cycloconverter. (8)
UNIT – V
PART – A
1. Give the four modes of operation of a Scherbius drive
The four modes of operation of static Scherbius drive are, Sub synchronous motoring. Sub synchronous regeneration Super synchronous motoring Super synchronous regeneration
2. Give the use of synchronous motors.
Synchronous 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 drives, etc.
3. How are the stator and rotor of the synchronous motor supplied?
The 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.
5. List out the commonly used synchronous motors.
Commonly used synchronous motors are, a. Wound field synchronous motors. b. Permanent magnet synchronous motors c. Synchronous reluctance synchronous motors. d. Hysterias 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.
The advantages of PMSM are, a. High efficiency b. High power factor c. Low sensitivity to supply voltage variations. The 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 synchronous motor.
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 small centrifuges.
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
Commutation of thyristors by induced voltages pf load is known as load commutation.
11. List out the advantages of load commutation over forced commutation.
Load commutation has a number of advantages over forced commutation It does not require commutation circuits Frequency of operation can be higher It can 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 self-controlled mode?
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 synchronous motor?
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 motor.
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 commutation?
The disadvantages of machine commutation are, a. Limitation on the speed range. b. The machine size is large c. Due to overexciting it is underutilized.
22. What is 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.
25. What are the characteristics of self controlled mode operated synchronous motor?
a) It operates at like dc motor also commutator less motor. b) These machines have better stability behavior. c) Do not have oscillatory behavior.
26. What are the characteristics of true synchronus mode operated synchronous motor?
The motor behaves like conventional synchronous motor i.e) hunting oscillations exists. The change in frequency is slow enough for rotor to truck the changes. Multi motor operation is possible here.
27. What is meant by sub synchronous speed operation?
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 operation?
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 system?
a) DC link static scherbius system b) Cyclo converter scherbius system
PART – B
1. Describe the operation of variable reluctance stepper motor. (16)
2. With a neat diagram describe the working of switched reluctance motor. (16)
3. (a)Compare the permanent magnet stepper motor with variable reluctance
stepper motor (8) (b)Discuss the advantages and disadvantages of using stepper motor (8)
4. Explain the operation of Brushless DC motor drives and its applications. (16)
5.Explain the operation of permanent magnet stepper motor drives. (16)
6.Explain about modern trends in industrial drive and its applications (16)
7. What is stepper motor? Explain in detail and its types. (16)