SESSION:27

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1)       What is Minimum Oil Circuit Breaker (MOCB)?

These types of circuit breakers only use oil as the interrupting medium and not as insulating material in all chambers. These types of breakers are widely used in transmission and distribution networks, but have a known sensitivity to transient recovery voltages (TRV) and are prone to re-strikes during the switching of capacitor banks. The operation of a minimum oil circuit breaker is similar to that of the bulk oil circuit breaker, as the arc is contained within the arc chamber. This results in a pressurized bubble of vaporized hydrogen around the arc. This pressure is released by an orifice when contact separation distance is reached. This results in the rapid cooling of the area between arcing contacts and fast deionisation at current zero. This in turn gives rise to transient recovery voltages over the contacts aiming to reinstate the current flow. This can result in a restrike and an unsuccessful interruption. This effect can further be complicated with the switching of capacitor loads and out-of-phase currents, but can be addressed by pressurizing the device with dry nitrogen.

2)       Mention some disadvantages of electromechanical sensors.

Following are the disadvantages:

·         The position sensors have a tendency to be unreliable because of environmental factors such as dust, high temperature, humidity and vibration

·         The cost of the sensors rises with the position resolution. Hence, if high-performance control is required, and expensive high-resolution encoder needs to be employed

·         There is an additional manufacturing expense and inconvenience due to the sensor installation on the motor shaft. In addition, consideration must be given to maintenance of the motor because of the mechanical mounting of the sensors, which also adds to the design time and cost.

·         Mechanical position sensors entail extra electrical connections to the motor. This increases the quantity of electrical wiring between the motor and the motor drive. This wire normally needs to be shielded from electromagnetic noise and thus further adds to the expense of the drive system.

·         The allocation of space for the mounting of the position sensor may be a problem for small applications.

3)       What are the advantages of constant margin angle control of synchronous motor drive fed by an inverter?

Following are the advantages:

·         Eliminates the hunting and stability problems

·         It does not require commutation circuits

4)       What is self-control mode of synchronous motors?

In self-control mode, the stator supply frequency is changed in proportion to the rotor speed so that the rotating field produced by the stator always moves at the same speed as the rotor.

5)       What is constant margin angle control of synchronous motor drive?

In self-control synchronous motor drive, the load side converter is operated as an inverter and maximizes motor power factor. This control is called constant margin angle control.

6)       Why a synchronous motor without damper winding is used, in the drive system employing a cycloconverter?

A synchronous motor without damper winding is used because the damper winding reduces the inductance of the machine, and therefore, its ability to filter out harmonics in the output voltage of cycloconverter. Since the drive operates in self-control mode, the damper winding is not needed for its conversion purpose.

7)       Why the load commutated inverter fed synchronous motor drive found suitable for high-speed and high power applications?

The load commutated inverter fed synchronous motor drive used for high-speed and high power applications because it has high-efficiency and four quadradent operation with regenerative breaking.

8)       Give the advantages of using cycloconverter for synchronous motor speed control.

Following are the advantages:

·         Smooth low-speed operations

·         Four quadrant operation with regenerative braking

·         Good dynamic response

9)       Define four quadrant operations?

A motor operate in two modes namely motoring and braking.
In motoring, it converts electrical energy into mechanical energy, which supports its motion.

In braking it works as a generator converting mathematical energy into electrical energy and thus, opposes the motion.
Motor can be provided motoring and braking operations for both forward and reverse directions.

10)    What are the types of braking?

Braking is of three types:

·         Regenerative’ braking

·         Dynamic braking

·         Plugging

11)    Define braking. What are the different types of braking in dc motor?

In braking, the motor works as a generator developing a negative torque which opposes the motion. Types of braking are

·         Regenerative braking

·         Dynamic OR Rheostat braking and

·         Plugging or Reverse voltage braking.

12)    What is Switched Reluctance Motor (SRM)?

The switched reluctance motor is a double salient, singly excited motor. This means that it has salient pole on the rotor and the stator, but only one member carries windings. The rotor has no windings, magnets or cage windings. It works on variable reluctance principle.

13)    Why is an open circuit test, in the case of transformers, done with the high voltage side open and the low voltage side connected to the source?

In open circuit test secondary of Transformer remain open and we apply Rated Voltage to primary side. Now suppose a transformer has rating like 220/33kV and if you apply the rated voltage in HV side then we will need 220 kV which may not be feasible and economical while if you apply rated voltage in secondary then we will only need 33 kV which is very less than 220 kV. That is why we connect low voltage side to the source. 

14)    What are the types of induced emf?

There are mainly two types of induced emf:

·         Statically Induced EMF

·         Dynamically Induced EMF

The statically induced emf involves no physical movement of conductor or magnet. Further there are two types of static induction:

Self Induction: EMF induction in coil or conductor due to change in flux associated with itself. E.g. Electromagnetic Ballast or choke

Mutual Induction: EMF induction takes place in coil or conductor due to change in current & hence flux in other conductor. E.g. Transformer

Dynamically emf is induced in generator.

15)    What are the two types of control techniques in SRM?

Two types of control techniques in SRM are:

·         Hysteresis type control

·         PWM type control