Anna University Previous Year Question Papers for EEE 7th sem
Anna University Previous Year Question Papers for EEE 7th Sem Power Electronics and Drives Paper
 What is PWM? List its various techniques [Apr-2005]
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
 Define modulation index of PWM. What is its use? (Nov-2005)
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 increase.
 What are the control strategies of chopper? (Nov-2003)
What are the methods of controlling the output voltage of a chopper (Apr-2005)
- Pulse Width Modulation(variable Ton, Constant frequency)
- Frequency Modulation (Constant Ton or Toff , Variable frequency)
- Current Limit Control (CLC)
 Mention the disadvantages of Frequency Modulation (FM) scheme used in choppers (Nov -2004)
- 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
 List the applications of CSI [Nov - 2004]
- Speed control of AC motors
- Induction heating
- VAR compensation
- Synchronous motor starting
 Draw the VI characteristics of SCR and mark the holding current and latching current in the characteristics (Nov-2004)
 What is fast recovery diode? mention its uses (Nov-2004)
- 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.
 Distinguish between holding current and latching current of SCR (Nov -2005)
- Latching current is effective at the time of turning ON whereas as holding current is effective at the time of turning OFF the SCR.
- Latching current is the minimum current that should flow at the time of triggering to turn ON the SCR whereas once the SCR is already in ON state its current should not reduce below holding current otherwise it turns OFF.
- Latching current is greater than holding current even though their magnitudes are much related.
 Why is pulse triggering preferred over RC triggering? (Nov-2005)
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.
 Distinguish between VSI and CSI [ Nov- 2002]
What are the main difference between current driven inverters and voltage driven inverters [ Apr-2005]
|Input is constant voltage||Input is constant current|
|Short circuit can damage the circuit||Short circuit cannot damage the circuit|
|Peak current of power device depends
|Peak current of power device is limited|
|Current waveform depends on load||Voltage waveform depends on load|
|Freewheeling diodes are required in case of
|Freewheeling diodes are not required|
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