Industrial automation lab.

Viva questions and answers.

1)What is the functional difference between a Diode and a SCR.?

  • Diode is used as un controlled rectifier.
  • SCR can be used as a controlled rectifier.

2)How do you turn on and turn off an SCR?

  • To turn ON an SCR, it should be forward biased and a positive gate pulse should be applied. Once the device is turned on, it remains in on condition even if the gate pulse is removed.
  • To turn off the SCR, Anode current must be reduced below the value of holding current.

3)Define latching current and holding current of SCR.

  • Latching current is that value of minimum anode current at which the device switches from off state to ON state and latches on to ON state even upon removal of gate signal, once the device is turned on.
  • Holding current is that value of minimum anode current, below which the anode current should be reduced to turn off the device.

4)What is a controlled rectifier?

  • Controlled rectifier is an electronic circuit that converts a constant AC at its input into variable DC at its output.

5)What is the principle of operation used to obtain controlled rectification?

  • Principle of phase control is used in controlled rectifiers.

6)What is natural and forced commutation of SCR?

  • Natural commutation occurs in SCR circuits with AC input supply, in which the SCR is reverse biased during negative half cycles and turns off.
  • Forced commutation is a method that is used to turn off SCR in SCR circuits with DC input supply, using external circuit employing R, L, C and auxiliary SCRs that applies reverse current / Voltage to turn off SCR at appropriate instants.

7)What is forward break over voltage w.r.t SCR? How does it depend on gate current?

  • Forward break over of SCR(VBO) is that value of forward bias, at which the device switches from forward blocking state to forward conduction state , due to break down of the reverse biased inner junction.
  • Magnitude of the forward break over voltage (VBO) reduces with increase in gate current (IG).

8)Define firing angle.

  • Firing angle is that value of phase angle w.r.t input Voltage, at which the SCR is triggered.

9)Mention the range of firing angle control in R and RC gate triggering circuits.

  • Firing angle control range in R triggering circuit is 0 to π/2 andin RC gate triggering circuit it is 0to π.

10)What is the functional difference between SCR and TRIAC?

  • SCR is a uni directional device that conducts when forward biased and can be used to obtain controlled rectification.
  • TRIAC is a bi directional device that conducts in both half cycles of AC supply and can be used as AC Voltage controller.

11)What is a snubber circuit?

  • Snubber circuit is a series Resistance, Capacitor circuit that is connected across a thyristor to provide dv/dt protection.

12)What is a DC chopper?

  • DC chopper is a device that converts a constant DC at its input to variable DC at its input.

13) What is duty cycle of a Chopper?

  • Duty cycle (α) of a chopper is the ratio of ON time (TON) of the Chopper to the total time (T)of the chopper.
  • i.e α = TON/T=TON/(TON+TOFF).

14)How do you calculate output Voltage in a chopper?

  • The output voltage in a chopper can be calculated using the formula, E0= αEiVolts.

Where, E0= Output of the chopper,

α= Duty cycle. (TON/T=TON/(TON+TOFF)).

Ei= Input DC Voltage.

15)What are the concepts of constant frequency and variable frequency control strategies of chopper?

  • In constant frequency control strategy, the duty cycle is varied by varying the ON time TON of chopper, keeping the total time T=TON+TOFF constant.
  • In variable frequency control strategy, the duty cycle of chopper is varied by varying ON time TON of chopper, keeping off time TOFF constant or by varying off time TOFF of chopper, keeping ON time TON constant.

16)What is an inverter?

  • Inverter is a power electronic circuit that converts DC power at its input to AC power at its output.

17)What is a cyclo converter?

  • Cyclo converter is a power electronic circuit which is a direct frequency changer which provides AC of different frequency at its output.

18)What is a Universal motor?

  • Universal motor is an electric motor that can be run using DC power supply or AC power supply.

19)What are single phase semi converters and single phase fully controlled bridge converters?

  • Single phase semi converters are single phase half controlled full wave bridge converter circuits in which two SCRs and two diodes are used.
  • Single phase fully controlled bridge converters are single phase fully controlled full wave bridge converter circuits in which four SCRs are used.

20)What are the different control schemes used for speed control of a DC motor?

21)Different control schemes used for speed control of a DC motor are i) Armature Voltage control method ii) Field control method and iii) Combined Armature Voltage control method and Field control method.

22)How speed and armature Voltage is are related in a DC motor?

  • Speed of a DC motor is directly proportional to Armature Voltage.

23)What is a ladder program?

  • Ladder program is a programming methodology used to write PLC programs.

24)What are the different timer functions used in a PLC?

  • Different timer functions used in a PLC are i) On delay timer ii) Off delay timer iii) Retentive On delay timer.

25)Name a few instructions used in PLC programming.

  • Examine if close, Examine if open, Output enable, Output Latch, on delay timer, off delay timer, UP/DN counter functions etc.