1. What are the three modes of operation for electric drive?
- Deceleration and
- Steady State
2. What is meant by margin angle of commutation in the control of synchronous motors?
Margin angle control: When good power factor is required to minimize converter rating, the load side converter when working as an inverter is operated at a minimum γ, this control scheme is known as marginal angle control. Here γ is kept constant, hence this scheme is also know as constant margin angle control.
3. What is field weakening mode control?
In this mode of control, a constant armature voltage is maintained. The rotor speed is inversely proportional to the field current, by varying the field current, the rotor speed is changed. By weakening the field flux, the speed can be increased. This is known as field weakening mode control for drives.
4. Give the conditions under which the DC motor operates under continuous current mode.
- Armature current flow must be continuous
- Current through the motor is positive.
- E > Vs where VS → Source voltage
- E → Induced Emf.
5. Why is the speed control of a 3Φ inductor motor with constant supply voltage and reduced supply frequency not preferred?
Reduction in supply frequency and constant supply voltage causes as increase in the air gap flux, which leads to saturation of motor. Hence this method is not used for 3Φ induction motor speed control.
6. Why is a self-controlled synchronous motor free from hunting operation?
In self-control mode, the stator supply frequency is changed so that synchronous speed is the same as rotor speed. This ensures that rotor runs at synchronous speed for all operating points. Hence self controlled synchronous motor is free from hunting operation.
7. What are the advantages of closed loop speed control?
Advantages of closed loop speed control are;
- Enhancement in speed response
- Improvement in steady state frequency
8. Define steady state stability in drive characteristics.
- It is defined as the stability of an equilibrium point from the steady-state speed-torque curves of the motor and load, thus avoiding solution of differential equations valid for transient operation of the drive.
- Drive will operate in steady-state at the speed when motor torque equals the load torque, provided it is the speed of stable equilibrium.
9. What is meant by Regenerative braking?
In regenerative braking, generated energy is supplied to the source
For the above condition to happen
E > V and negative Ia
- E is supply voltage (induced voltage)
- V is armature voltage
- Ia is armature current.
10. What is the discontinuous conduction mode of a separately excited DC motor drive?
When current to armature does not flow continuously the motor is said to be operated indiscontinuous conduction.
This has two modes of operation:
- Duty interval (α ≤ ωt ≤ β) when motor is connected to the source and Va terminal voltage =VS, source value
- Zero current interval (β ≤ ωt ≤ π + α) when armature current ia = 0 and terminal voltage Va = E.
11. Define Time ratio control of a Chopper fed DC drives.
Time ratio control is obtained by changing the duty cycle of the chopper (i.e) Ton/T of the switched to change the average output voltage.
The types of time ratio control are
- Frequency modulation technique.
- Pulse width modulation technique.
12. What is slip power recovery scheme?
When rotor copper loss is neglected, Pr is equal to slip power, sPg where Pg is air gap power. Speed control below synchronous speed is obtained by controlling the slip power, without use of rotor resistance control. Therefore, these methods of speed control are classified as slip power recovery scheme. Two such schemes, static scherbius and Static Kramer Drives.
13. What are constant volt/frequency control?
Speed control in the inverter driven induction motor is achieved by means of variable frequency. Apart from frequency, the applied voltage needs to be varied to keep the air flus constant and not let it saturate,
Φμ α Vph/Fs (i.e) to maintain flux constant, the volt/frequency ratio has to be maintained constant. Such a speed control method is constant V/F control. Here, whenever stator frequency is changed, the stator input voltage is changed accordingly to keep the air gap flux constant.
14. What is self control of Synchronous motor?
In self-control of synchronous motor, the stator supply frequency is changed so that synchronous speed is the same as rotor speed. This ensures that rotor runs at synchronous speed for all operating points. Consequently, rotor cannot pull-out of step and hunting oscillations are eliminated.
15. Define Marginal angle control.
When good power factor is required to minimize converter rating, the load side converter when working as an inverter is operated at a minimum γ, this control scheme is know as marginal angle control. Here γ is kept constant, hence this scheme is also known as constant marginal angle control.
16. What is field weakening mode control of drives?
- In this mode of control, a constant armature voltage is maintained.
- The rotor speed is inversely proportional to the field current, by varying the field current, the rotor speed is changed. By weakening the field flux, the speed can be increased. This is known as field weakening made control for drives.
17. What is speed feedback?
Here speed for the motor is given as signal from the speed sensor, which is compared with the reference speed and produces error signal based on which current is varied to provide control over the drive. A speed is taken as parameter for closed loop operation. The system is said to have speed feedback.
18. What are the types of load torques?
- Active load torque
- Passive load torque
Active Load torque: Load torques which have the potential to drive motor under equilibrium conditions.
Passive Load torque: Load torques which always oppose the motion and change their sign on reversal of motion.
19. List out the drawbacks of ac-dc converter fed dc drive.
- Poor power factor
- Presence of ripples in motor current
- Distortion of supply
20. What is TRC scheme?
In this scheme the value of duty cycle of a chopper is varied by constant frequency system (or) variable frequency system.
21. Write the real and reactive power equations of a balanced 3 phase ac system.
Real Power Equation
P = √3 VLIL cos Φ
- VL = line voltage (V)
- IL = line current (A)
- cos Φ = power factor
Reactive Power Equation
Q = √3 VLIL sin Φ (VAR)
22. What are the various applications of stator voltage control scheme?
- Pump drives.
- Suited for applications where torque demand reduces with speed.
23. Give the advantages of vector control method.
- It provides linearized control characteristics.
- It provides independent control of flux torque.
- It decouples the stator current into 2 components, one providing airgap flux and other producing the flux.
24. What is the necessity of delay unit in a open loop v/f control of synchronous motor?
The frequency command after passing through the delay circuit is applied to the voltage source inverter. This is done so that the rotor speed is able to track the changes in frequency.
25. Define self control of synchronous motor.
In self control mode of operation, the stator supply frequency is changed in proportion to rotor speed so that rotating field is produced by the stator always moves at same speed as rotor.
26. What is multiquadrant dynamics in drive characteristics?
A motor can operate in both motoring and breaking modes for forward and reverse directions.
- First Quadrant – Forward Motoring
- Second Quadrant – Forward Breaking
- Third Quadrant – Reverse Motoring
- Fourth Quadrant – Reverse Breaking
27. What are the different types of load?
- Constant torque type load
- Torque proportional to speed (Generator type load)
- Torque proportional to square of the speed (Fan type load)
- Torque inversely proportional to speed (Constant power type load.
28. What are the merits of four quadrant operation of a converter?
- Full forward and reverse speed can be achieved
- A wide range of speed control is possible
- Power is automatically regenerated during braking
- Continuous flow of current in load leads to continuous operation.
29. Define current limit control of a chopper fed DC drives.
In CLC (Current Limit Control), the chopper is switched ON and OFF, so that the current in the load is maintained between two limits (I0 min to I0 max).
30. How is speed feedback achieved in speed controller design?
Speed is usually sensed by tachometers, optical encoder, etc. This sensed speed in compared with reference speed and speed can be controlled.
31. What is field weakening mode control?
At constant power region, the stator voltage is kept constant and supply frequency is increased. Therefore the flux decrease or air gap flux is weakened. The control at this mode is called field weakening mode control.
32. Define vector control.
The process of controlling voltage and frequency to get the desired speed and torque is known as vector control.
33. Which type of control can be used for constant torque operation in synchronous motor? How?
V/f control can be used for constant torque operation in suprehension motor. This is because synchronous speed is directly proportional to frequency. Similar to induction motor constant flow operation below base speed is achieved by operating synchronous motor with constant (Yf).
34. What are different types of controls used in synchronous motor driver?
- True synchronous control or separate control.
- Self control.
35. What are the speed torque conventions used in multi-quadrant operation?
- I quadrant – Forward motoring – ω, T is positive
- II quadrant – Forward braking – ω is positive T is negative
- III quadrant – Reverse motoring – ω, T is negative
- IV quadrant – Reverse braking – ω is negative T is positive.
36. Give any one example of active load torque.
- Torque due to the force of gravity.
- Torque due to tension, compression and torsion.
37. What is the difference between continuous and discontinuous conduction modes?
- In continuous conduction, the speed torque characteristics are parallel straight line while in discontinuous conduction, torque is less than rated and mainly used for low power drives.
- In discontinuous conduction mode, α > 90° while in continuous conduction mode α > 90°.
38. What are the applications of chopper fed dc drives?
- Battery operated vehicles.
- Traction motors control in electric traction.
- Trolley cars.
- Electric braking.
39. Write any two advantages of closed loop control system.
- Greater accuracy.
- Improved dynamic response.
- Reduced effects of disturbances.
40. What do you mean by field weakening mode control?
With constant supply voltage and increased supply frequency operation, air gap flux get reduced. This mode is said to be field weakening mode. Now the control of induction motor is similar to that of d.c. series motor control.
41. What are the drawbacks of stator voltage control method?
- The voltage and current waveforms are highly distorted due to harmonies.
- Operating efficiency is low.
- At low speeds, motor currents are exercise.
- Maximum torque available from the motor decreases with reduction in stator voltage.
42. Write any two advantages of vector control techniques.
- Accuracy is high.
- Fast response.
- Less Deviations.
43. Define self-control technique of synchronous motor.
The stator supply frequency is changed proportion to the rotor speed so that the rotating field produced by the stator always moves at the same speed as the rotor.
44. Write any two advantages or permanent magnet synchronous motor.
- Elimination of field cooper loss.
- Higher power density.
- Lower rotor inertia.
- Robust construction.
45. What are the disadvantages of DC motors as compared to AC motors?
- Higher cost.
- Higher rotor inertia.
- Maintenance problems with commutators and brushes.
- EMI problems.
46. What are the advantages of chopper fed DC drive?
- High efficiency.
- Flexibility in control.
- Quick response.
- Regeneration down to very low speed.
47. What is mechanical time constant of D.C. machine?
Mechanical Time Constant : τm = J/B.
48. What is the role of current limiter in the closed loop control of D.C. drives?
Current limiter saturates even for a small speed error. It sets current reference for inner current control loop at a value corresponding to the maximum allowable current.
49. Mentioned the merits and demerits of stator voltage control.
- The control circuit is simple and compact
- Quick response time.
- Voltage and current waveforms are highly disorted.
- Performance is poor under running condition at low speed.
- Operating efficiency is low.
50. What are components of load torque?
- Friction torque (TF)
- Windage torque (Tw)
- Torque required to the useful mechanical work (Tm).
51. Why does the armature voltage control is not preferred for the speeds above the rated value in dc motors?
The armature voltage control is not preferred for the speed above the rated speed in dc motors because, by increasing the controller resistance to the potential drop across the armature us decreased which decreases the speed.
52. Name any four simulation software packages that can be used for electrical drives.
The simulation tools are:
53. What is meant by super synchronous operation?
The SRIM speed can be controlled above synchronous speed. The supply is fedback to the rotor side.
54. What are the advantages of PMSM?
- Elimination of field copper loss.
- Higher power density.
- Lower rotor inertia.
- Robust construction.
55. What are the drawbacks of stepped wave inverter fed drives?
- Due to low frequency harmonics, the motor losses are increased.
- Motor develops pulsating torque due to fifth, seventh and thirteenth harmonics cause jerky motion.
- Harmonic content in motor current increases at low speeds.
56. What are the advantages of static rotor resistance control over conventional rotor resistance control?
Static rotor resistance control (vs) Conventional rotor resistance control.
- Power wastage in resistance is reduced.
- Smooth and wide range of speed control.
- High Line Power factor.
57. When can a synchronous motor be load commutated?
When the synchronous motor operated at a leading power factor thyristor of the 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.