Power Electronics Interview Questions and Answers: Part-4

Power Electronics Interview Questions with Answers:

           Here, we had collected Various Power Electronics and Drives related Interview questions, Viva Questions. We are appending the new Q & A whenever we are getting from readers.
Some of the questions are asked to Experienced candidates based on their previous experience. If it is not related to your field, just ignore it.

[Q] What is AC voltage controller?
It is defined as the power electronics converter which converts the fixed AC voltage into a variable AC voltage without change in the frequency.

[Q] What is inverter? What are the types of Inverter?
A device which converts dc power into ac power at desired output voltage and frequency is called as Inverter.
Inverters are broadly classified into
Voltage Source Inverter (VSI)
Current Source Inverter (CSI)

[Q] What is chopper?
It is equivalent to a dc transformer in ac circuit. The Chopper is a static switch which is used to get the variable dc output voltage from a constant dc input voltage.
Chopper basics, types and principle of operation is discussed in a separate page. Click here to know more about Choppers

Step down chopper:
 In this circuit, the average output voltage [V_O = D . V_S] is less than the input supply voltage. It is also known as Buck converter.

Step up chopper:
It is also known as Boost converter. Here the average output voltage [V_O = V_S / 1 - D]  is more than the input supply voltage.

[Q] What is load commutation?
In load commutation, the load current flowing through the thyristor either becomes zero or is transferred to another device from the conducting SCR.

[Q] What is meant by pulse triggered devices?
As the name indicates, to trigger these devices just a pulse is sufficient. Continuous gate voltage of entire on time is not required. The advantage of pulse triggering is, it will avoid the hard triggering.
Ex. Thyristor, GTO

[Q] What is level-sensitive devices? give some examples
Some of the level sensitive devices are
MOSFET
IGBT
MCT
IGCT
In order to keep these devices in the ON state condition, we have to apply  gate current/voltage continuously to these devices.

[Q] What is meant by GTO?
It is a three terminal, four layer PNPN  Power Semiconductor device that can be turned on by a positive gate current and can be turned off by a reverse gate current.

[Q] What are the types of GTO?

Asymmetrical GTO
Symmetrical GTO

[Q] What does 10V AC mean? Is it the RMS voltage or Peak voltage or Average voltage?
In general,  AC voltages and currents are mentioned in RMS values only. It is sensible to compare with steady DC voltages and currents.
For example 230V AC supply means, 230Vrms AC Supply.

[Q] When we have to connect the SCRs in a parallel manner?
To meet the high current demand we will connect the SCRs in a parallel manner.
Similarly to meet the high voltage demand we have to connect the SCRs in series.

[Q] Define Gate Charge in MOSFET?
The gate charge is defined as the amount of charge required for the device during turn-on and turn-off time. It is the most important parameter we have to consider while selecting the MOSFET. The switching speed of the MOSFET depends on the speed at which the gate driver can charge or discharge the input gate charge.

More Power Electronics Interview Questions:
Power Electronics Interview Questions: Set-1
Power Electronics Technical Interview Questions

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Special motors (Permanent Magnet Synchronous Motor) Interview Questions

Special Machines (Permanent Magnet Synchronous Motor)  Interview Questions

[1] What is the PMSM?
In medium and small size synchronous motors, DC field can be produced by permanent magnets. Such kind of motors are called as PMSM

[2] What are the advantages and disadvantages of PMSM
Advantages

Elimination of field copper loss
Higher power density
Lower rotor inertia
Robust construction
Disadvantages

Power rating low
Motor power factor can not be controlled because of field excitation cannot be controlled

[3] What are the classification of PMSM
Based on construction
    Surface mounted PMSM
    Interior mounted PMSM
Based on the nature of voltage induced
    Sinusoidally excited
    Trapezoidally excited

[4] Give some applications of PMSM
Robots
Machine tools

[5] Define torque angle
The angle between excitation emf and supply voltage. This angle is called torque angle or load or power angle .

[6] What are the different classifications of synchronous motor?
(i) Wound field synchronous motor
      Cylindrical rotor construction
      Salient pole rotor construction
(ii) Permanent-Magnet Synchronous Motors
      Surface mounted (Projecting Type and Inset Type)
      Interior Buried Type
(iii) Synchronous Reluctance Motors
(iv) Hysteresis motors

[7] What are the different modes employed to achieve variable frequency control in synchronous motors?

Separate or true synchronous mode
Self controlled mode

[8] Why a Synchronous Motor without damper winding is used, in the drive system employing a Cycloconverter?
A Synchronous Motor without the damper winding is used, because the damper winding reduces the inductance of the machine, and therefore is ability to filter out harmonics in the output voltage of Cycloconverter. Since the drive operates in self-controlled mode, the damper winding is not needed for its converter.

Interview questions on more power electronics topics are covered in other posts... Please leave your comments below...

Power Electronics Technical Interview Questions [Ametek Aerospace]

Power Electronics Technical Interview Questions asked in Ametek Aerospace

               One of our friend shared his interview experience. The position is for Senior Power Electronics Engineer with 5 year experience. We believe that it will be usefull for your interview preparation...

The following questins were asked in the interview...

[1]  How to choose a micro controller?

[2]  What is the difference between a LDO and a Linear regulator?

[3]  What is the difference between I2C and SPI?

[4]  What is the difference between logic analyzer and a oscilloscope?

[5]  On what crises do we select components/Selection of a capacitor?

[6] Opamps/ADC/DAC how do you interface them externally /which ADC/DAC you have been used  and why do you selected them/resolution ?

[7]  What are usages and issues with J-Tag debugger?

[8]  Any experience in Automation?

[9]  Emi/Emc testing and how do you conduct them?

[10]  What is the ESD protection how do you select any component to be ESD protected?

[11]  Experience with other protocols Explain?

[12]  Selection of capacitor, inductor?

[13]  Frequency required and interfacing with other peripherals?

[14]  Mixed signal interfacing circuits?

[15] Emc/Emi testing for immunity and radiation testing process and how do you conduct them and  where it has been done in your product..

[16]  FMEA analysis?

[17]  Power supply design for single phase for Dc-Dc converter?

[18]  DFT,DFM Analysis,reliability,RPN number preference

[19]  Production manufacturing issues?

[20]  Design process in HW/SW?

[21]  How do you conduct design reviews?

[22]  PCB manufacturing And placement analysis?

[23] How do you come to know that a micro controller is programmable and what are the issues that occurred while debugging?

Apart from the technical questions, our friend was faced with the usual interview questions like:

 [1] Tell me about yourself?

 [2] Carrier experience and Roles?

 [3] Past and present experience?

 [4] Kind of projects handled?

 [5] Preferred area of working

 [6] Why are you looking for a change?

 [7] Willing to relocate to Bangalore?

 [8] Roles and kind of job to be handled

 About work experience and clients working for in the present organization

 Roles and reporting personalities

 Projects description.

Please share your interview experience with our readers.... 

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Please don't just read and go... Please leave your comments...Your comments are highly appreciated...

Power Electronics Graduate Trainee Interview [BGR Energy]

These questions were asked in Power Electronics Graduate Trainee Interview conducted by BGR Energy Systems Company at MIT Chennai.

[1] Draw and Explain the SCR V-I characteristic?

Vak = +ve & Vg = 0
(i) When a positive voltage is applied to anode with respect to cathode, the junctions J1 and J3 are         forward biased, but the junction J2 is reverse biased. 

(ii) The SCR is in its forward blocking state. At this time the Gate signal is not applied. 

(iii) As shown in figure a small amount of forward leakage current is flows through the device.

Vak = + & Vg = +ve
(i) When the small amount of positive voltage is applied, while positive voltage is applied to anode with respect to cathode, the junction J3 becomes forward biased. 

(ii) Thus the SCR conducts a large value of forward current with small voltage drop. With the application of gate signal the SCR changed from forward blocking state to forward conducting state. It is called as latching.Without gate signal it happens at forward breakdown voltage (Vfbd).

(iii) When the gate signal value is increased, the latching happens for low Vak voltages as mentioned in the figure. In the presence of forward current (i.e. after the thyristor is turned on by a suitable gate voltage) it will not turn off even after the gate voltage has been removed.  

(iv) The thyristor will only turn off when the forward current drops below holding current. The holding current is defined as the minimum current required to hold the SCR in the forward conduction state.

Vak = -ve

(i) When a negative voltage is applied to anode with respect to cathode, the junctions J1 and J3 are reversed biased, but the junction J2 is forward biased. The SCR is in its reverse blocking state. 

(ii) As shown in figure a small amount of reverse leakage current flows through the device. 

If the applied voltage is more than reverse breakdown voltage, the device will collapse and the large amount of current flows through it.

[2] Explain Buck topology

(i) When the switch is ON condition (ie, T_ON), the input provides energy to inductor and output. 
(ii) The difference between the input and output voltage is applied to inductor as shown in the below waveform.
(iii) When the switch is turned off, the inductor current will flow in the same direction to load. It is inductor property to maintain the current flow in the same direction.
(iv) The freewheeling diode D1 is forward biased at this time and completes the inductor current path.

The output of the buck converter with continuous output current is 

Vout = D*V_IN

D = Duty cycle = T_ON / [T_ON + T_OFF]

[3] Compare MOSFET and BJT

             BJT                                                                     MOSFET

More Power handling Capability                             Less Power handling capability

Low switching speed                                             Fast switching speed

High On state resistance                                        Low on state resistance

Has second breakdown voltage problem                 No second breakdown voltage problem