SESSION:29

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1)       Why PMSM operating in self-controlled mode is known as Commutatorless DC Motor?

Load side controller performs somewhat similar function as commutator in a DC machine. The load side converter and synchronous motor combination functions similar to a DC machine.

First it is fed from a DC supply and secondly like a DC machine. The stator and rotor field remain stationary with respect to each other at all speeds. Consequently, the drive consisting of load side converter and synchronous motor is known as commutator less DC motor.

2)       What is Pulse Mode?

For speeds below 10% of base speed, the commutation of load side converter SCRs is done by forcing the current through the conducting SCRs to zero.

This is realised by making source side converter to work as inverter each time load side converter SCRs are to be turned off.

Since the frequency of operation of load side converter is very low compared to the source frequency, such kind of operation can be realised.

The operation of inverter is termed as Pulsed Mode.

3)       What is meant by self-control?

·         As the rotor speed changes the armature supply frequency is also changes proportionally so that the armature field always moves (rotates) at the same speed as the motor.

·         The armature and rotor field move in synchronism for all operating points.

·         Here accurate tracking of speed by frequency is realised with the help of rotor position sensor.

4)       What is full converter? What is dual converter?

Full converter: 

When positive as well as negative half cycles of the supply are rectified, it is called full converter.

Dual converter: 

When the polarities of output voltage as well as current can be reversed then it is said to be dual converter. It operates in all the four quadrants of the V_O – I_O plain.

5)       What are the advantages and disadvantages of buck/boost regulator?

Advantages

1.    Provides inverted output

2.    Both buck/boost operations achieved simultaneously

3.    High efficiency

4.    Short circuit protection can be easily implemented

Disadvantages

1.    Output voltage polarity opposite to that of input

2.    Voltage stress on switching device in the chopper is high

6)       List the merits and demerits of AC voltage controller.

Merits

1.    They use line commutation, hence no extra commutation circuits are required

2.    They have high efficiencies, since device losses are reduced.

Demerits

1.    large ripple and harmonics are present in the output

2.    Output waveforms are not sinusoidal.

7)       What the two types of AC voltage controllers? which one these is preferred and why?

Types of AC voltage controllers:

1.    ON-OFF type AC voltage Controller

2.    Phase angle type AC voltage controller

ON-OFF control:

In this type of controller, the full input voltage is applied to output for some number of cycles and no voltage is applied to output for some number of cycles. In this control, output voltage waveform is discontinuous and contains large ripple.

Phase angle control:

In phase angle control, each half cycle of the source voltage is applied partially or fully to the load. Firing angle of the SCRs is charged to control output voltage. Output voltage waveform is continuous but phase controlled.

The type of AC controller which is highly preferred is Phase angle control, because it contains less ripple and discontinuities in output.

8)       What are the effects of load inductance on the performance of an AC voltage controller?

·         The load inductance makes the output current to flow continuously.

·         With load inductance, the commutation becomes difficult if TRIAC switches are used

·         The control fails sometimes due to high inductive load.

9)       Define string efficiency of SCRs connected in series.

String efficiency is given as

η = [VS / nVDI ] x 100

It indicates the amount by which the sting is utilized.

10)    What is PWM? List its various techniques.

In PWM the output voltage can be varied by changing the ON pulse width of the switching devices.
Various PWM techniques are

·         Single pulse modulation

·         Multiple pulse modulation

·         Sinusoidal pulse modulation

·         Modified sinusoidal pulse modulation

11)    Define modulation index of PWM. What is its use? 

The ratio of amplitude of reference signal to amplitude of carrier signal is given as modulation index of PWM.

m = Ar / Ac

Modulation index determines the RMS value of the output voltage. As modulation index increase, RMS value of output also increases.

12)    Mention the disadvantages of Frequency Modulation (FM) scheme used in choppers.

Disadvantage:

·         The frequency of the chopper is not fixed. Hence it is difficult to design exact filters

·         The control circuit becomes complex, since frequency is not fixed

13)    What is fast recovery diode? Mention its uses.

·         Fast recovery diodes have the reverse recovery time of the order of 25-100nS.

·         Fast recovery diode consists of a highly doped P type and n+ type layers sandwiched by a lightly doped n– layer.

·         The thickness of n– layer is increased to increase the voltage blocking capacity.

·         The reverse recovery time is reduced by reducing the stored charge in the neutral and minimizing carrier life time. This is achieved by forming recombination centers in neutral region.

·         For very high frequency applications, PIN diodes are used.

·         Fast recovery diodes have ratings from 1A/50V to 1kA/3kV with reverse recovery time less than 5μS.

14)    Why is pulse triggering preferred over RC triggering?

Pulse triggering is preferred higher than RC triggering because of its following advantages:

·         The SCR has small turn ON time ie, upto 5uS. Hence a pulse of gate drive is sufficient to turn on the SCRs.

·         Once the SCR turns ON, there is no need of gate drive. Hence gate drive in the form of pulses is suitable.

·         The DC gate voltage and current increases losses in the SCR. Pulsed gate drive has reduced losses.

·         The pulsed gate drive can be easily passed through isolation transformers to isolate SCR and the trigger circuit.