100 Power Electronics Questions and Answers
Q: Name some of the current controlled (current driven) devices…
SCR, GTO, GTR , BJT
Q: Name some of the voltage driven ( Voltage controlled) devices
IGBT, MCT, IGCT, SIT, MOSFET
Q: What is holding current in SCR?
It is the minimum current required to hold the SCR in forward conduction state.
When the forward current becomes less than holding current, SCR turns from forward conduction state to forward blocking state.
Q:What is latching current in SCR?
It is the minimum current required to latch(turn on) the SCR from forward blocking state to forward conduction state.
Q: What are the different turn on methods of SCR?
- Forward voltage triggering
- Gate Triggering
- dv/dt triggering
- Temperature triggering
- Light triggering
Q: What is snubber circuit?
The snubber circuit is used for the dv/dt protection of the SCR. It is a series combination of a resistor and a capacitor in parallel with the SCR.
Q: What is firing angle?
The angle between the zero crossing of the input voltage and the instant the SCR is fired is called as delay angle or firing angle.
Q:For High voltage applications will you prefer MOSFET or IGBT?
- For High voltage applications we have to use IGBT.
- Because MOSFETs are low voltage devices. ie, Their voltage rating is lesser than IGBT.
- General rule is MOSFETs are suitable for applications which has breakdown voltage less than 250V.
- The IGBTs are suitable for applications which has breakdown voltage upto 1000V.
Q: For High frequency applications will you prefer MOSFET or IGBT? Why?
- For High frequency applications, MOSFET is the right choice of the device.Because MOSFET has low switching losses compare to that of IGBT.General rule of thumb is for low-frequency applications having frequency range upto 20kHz, we have to use IGBT.
For high frequency applications having frequency range of more than 200kHz, we have to use MOSFET
- Q: What are the various parameters we have to consider, while selecting IGBT?
In IGBT datasheet, we have to focus on following parameters Collector to emitter voltage VCESCollector Current ICCollector Dissipation PCJunction Temperature Tj
- Q: For Selecting MOSFET, what are the major parameters we have to consider in the datasheet?
In MOSFET datasheet, we have to focus on following parameters:
- Drain to Source Voltage VDSSOn state Drain to Source Resistance RDS(ON)Reverse Recovery Current IRRReverse Recovery Charge QRRGate Charge QG
- Body Diode Characteristics
- ID at ambient temperature
- Channel power dissipation
- Channel temperature
Q:Why IGBT is very popular nowadays?
a. Lower hEAT requirements
b. Lower switching losses
c. Smaller snubber circuit requirements
Q: What are the different methods to turn on the thyristor?
a. Forward voltage triggering
b. Gate triggering
c. dv/dt triggering
d. Temperature triggering
e. Light triggering
Q: IGBT is a voltage controlled device. Why?
Because the controlling parameter is gate-emitter voltage.
Q: Power MOSFET is a voltage controlled device. Why?
Because the output (drain) current can be controlled by gate-source voltage.
Q: Power BJT is a current controlled device. Why?
Because the output (collector) current can be controlled by base current.
Q: What are the different types of power MOSFET?
a. N-channel MOSFET
b. P-channel MOSFET
Q: How can a thyristor turned off?
A thyristor can be turned off by making the current flowing through it to zero.
Q: Define latching current.
The latching current is defined as the minimum value of anode current which it
must attain during turn on process to maintain conduction when gate signal is removed.
Q: Define holding current.
The holding current is defined as the minimum value of anode current below
which it must fall to for turning off the thyristor.
12. What is a snubber circuit?
It consists of a series combination of a resistor and a capacitor in parallel with the
thyristors. It is mainly used for dv / dt protection.
Q: What losses occur in a thyristor during working conditions?
a. Forward conduction losses
b. Loss due to leakage current during forward and reverse blocking.
c. Switching losses at turn-on and turn-off.
d. Gate triggering loss.
Q: Define hard-driving or over-driving.
When gate current is several times higher than the minimum gate current
required, a thyristor is said to be hard-fired or over-driven. Hard-firing of a thyristor
reduces its turn-on time and enhances its di/dt capability.
Q: Define circuit turn off time.
It is defined as the time during which a reverse voltage is applied across the thyristor during its commutation process.
Q:Why circuit turn off time should be greater than the thyristor turn-off time?
Circuit turn off time should be greater than the thyristor turn-off time for reliable
turn-off, otherwise the device may turn-on at an undesired instant, a process called
Q: What is the turn-off time for converter grade SCRs and inverter grade SCRs?
Turn-off time for converter grade SCRs is 50 – 100 ms turn-off time for converter
grade SCRs and inverter grade SCRs and for inverter grade SCRs is 3 – 50 ms.
Q: What are the advantages of GTO over SCR?
a. Elimination of commutation of commutating components in forced commutation,
resulting in reduction in cost, weight and volume.
b. Reduction in acoustic noise and electromagnetic noise due to elimination of
c. Faster turn-off, permitting high switching frequencies.
d. Improved efficiency of the converters.
Q: What are the advantages of free wheeling diode in rectifier circuit?
The input power factor is improved. It prevents the output voltage from becoming negative. Load current waveform is improved.
Q: What is meant by commutation?
The process of changing the direction of current flow in a particular path of the circuit. It is used to turn off the SCR.
Q. What is natural commutation?
The process of the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off is called as natural commutation.
Q What is forced commutation?
The process of the current flowing through the thyristor is forced to become zero by external circuitry is called as forced commutation.
Q What are the types of commutation with respect to commutation process?
- Voltage commutated chopper
- Current commutated chopper
- Load commutated chopper
Q:To design the buck converter, what are basic & essential information (parameters) we need to get from the Customer?
We need the following inputs from the customer,Output Voltage VOUT Input Voltage VIN Output Current (load current) IOUTMaximum Ripple voltage allowed at the output side Efficiency of the converter
Q. What is meant by cyclo-converter?
It is also known as frequency changer. It converts input power at one frequency to output power at another frequency with one stage conversion.
Q. What are the types of cyclo-converters?
- Step up cyclo-converter
- Step down cyclo-converter.
Q. What is step down cyclo-converter?
It is the converter whose output frequency is less than the input frequency.
Q. What is step up cyclo-converter ?
It is the converter whose output frequency is more than the input frequency.
Q. What does the Voltmeter in AC mode show? Is it RMS value or peak value?
Multimeter in AC mode shows RMS value of the voltage or current. Also when it is DC mode it will show the RMS value only.
Q. What are the control strategies of chopper?
The control strategies of chopper are
- Pulse width modulation PWM (Variable TON, Constant frequency)
- Frequency modulation (Constant TON or TOFF, Variable frequency)
- Current Limit Control (CLC)
Q. What is delay angle or what is firing angle of phase controlled rectifier?
The delay angle is the angle at which thyristors are triggered after zero crossing. After zero crossing of supply voltage, one pair of thyristors is forward biased. After delay angle(α) these SCRs are triggered.
Q. What is Universal Motor?
It is defined as a motor which can be operated either on DC or single-phase AC supply at approximately the same speed and output.The universal motor is built exactly like a series DC motor.
But a series DC motor cannot be run as a universal motor, even though both motors look the same internally and externally.We cannot use these motors in the industrial applications due to the low efficiency (25% -35%). It has high starting torque and a variable speed characteristic. It runs at dangerously high-speed on no load.
Q. Give some examples of power electronics applications in the day-to-day life?
We can list a huge number of power electronics applications. Few of the applications which we can see in our daily life are
- UPS – Uninterruptible Power Supply
- SMPS – Switch Mode Power Supply
- Speed Control of Motors
Q. What is meant by PMDC?
PMDC stands for Permanent Magnet DC Motor
A Permanent Magnet DC Motor is similar to an ordinary dc shunt motor except that its field is provided by permanent magnets instead of salient-pole wound field structure.
There are three types of permanent magnets used for such motors namely;
(i) Alnico Magnets
(ii) Ceramic magnets
(iii) Rare-earth magnets
The major advantages are low noise, small size, high-efficiency, low manufacturing cost.
- Q: What is meant by V/F control?
When the frequency is reduced, the input voltage must be reduced proportionally so as to maintain constant flux.
Otherwise the core will get saturated resulting in excessive iron loss and magnetizing current. This type of induction motor behaviour is similar to the working of dc series motors.
Q: What are the advantages of V/F control?
- Smooth speed control
- Small input current and improved power factor at low-frequency start
- Higher starting torque for low case resistance
Q: What is meant by stator current control?
The three-phase induction motor speed can be controlled by stator current control. The stator current can be varied by using current source inverter.
Q: What is meant by dynamic braking?
Dynamic braking of electric motors occurs when the energy stored in the rotating mass is dissipated in an electrical resistance. This requires the motor to operate as a generator to convert this stored energy into electrical.
Q: What is meant by plugging?
It is one method of braking of induction motor. When phase sequence of supply of the motor running at a speed is reversed, by interchanging connections of any two phases of stator with respect to supply terminals, operations shifts from motoring to plugging region.
Q: What is meant by rotor resistance control?
In a slip ring induction motor, a variable resistances are connected in the rotor circuit. By controlling the rotor resistance, the motor speed can be controlled as well as increase the starting torque and reduce the starting current. This method of speed control is called rotor resistance control method.
Q: What is meant by slip power?
The portion of air gap power, which is not converted into mechanical power, is called slip power.
Slip power is nothing but multiplication of slip(s) and airgap power (Pag)
Slip power = s. Pag
Q:What are the advantages of slip power recovery system?
In chopper method of speed control for SRM, the slip power is wasted in the external resistance and the efficiency also reduced. However, instead of wasting the slip power can be recovered by various schemes for the speed control of slip ring induction motor.
This system is called as slip power recovery system. The slip power can be recovered and fed back to the supply. So the overall efficiency also improved.
- Q: What is meant by Electrical Drives?
Answer: Systems employed for motion control are called as “Drives” and many employ any of the prime movers such as, diesel or petrol engines, gas or steam turbines, hydraulic motors and electric motors for supplying mechanical energy for motion control. Drives employing electric motors are known as ” electric drives”.
Here, We had given only the definition of electrical drive. But if you face this question in the face to face interview try to explain in detail. Draw the Block diagram of the electrical drive first. During the time, you recall about the basics and every part of the electrical drive system and explain as detail as possible.
Q: What are the different factors for the selection of electrical drives?
- Steady state operation requirements
- Transient operation requirements
- Requirements related to the source
- Capital and running cost, maintenance needs, life
- Space and weight restrictions
- Environment and location
Q: What are the parts of electrical drives?
- Electrical motors and load
- Power Modulator
- Control Unit
- Sensing unit
Q: What are modes of operation of electrical drives?
- Steady state
- Acceleration including starting
- Deceleration including stopping
Q: What is meant by four quadrant operation?
A motor operate in two modes-motoring and braking. In motoring, it converts electrical energy into mechanical energy, which supports its motion.
In braking, it works as a generator converting mechanical energy into electrical energy, and thus, opposes the motion. Motor can provide motoring and braking operations for both forward and reverse directions.
Q: What is meant by chopper?
Chopper converts fixed dc voltage into variable dc voltage
Q: What is brushless DC motor?
An inverter fed trapezoidal permanent magnet AC motor drive operating in self-controlled mode is called brushless dc motor.
Q: What is a PMSM?
PMSM = Permanent Magnet Synchronous Motor. In medium and small size Synchronous motors dc field can be produced by permanent magnets.
Q:Classification of PMSM?
According to construction
Surface mounted PMSM
Interior mounted PMSM
According to the nature of voltage induced
Q: What is meant by phase controlled rectifier?
It converts fixed ac voltage into variable dc voltage.
Q: Mention some of the applications of controlled rectifier.
a. Steel rolling mills, printing press, textile mills and paper mills employing dc motor drives.
b. DC traction
c. Electro chemical and electro-metallurgical process
d. Portable hand tool drives
e. Magnet power supplies
f. HVDC transmission system
Q: What is the function of freewheeling diodes in controlled rectifier?
It serves two process.
a. It prevents the output voltage from becoming negative.
b. The load current is transferred from the main thyristors to the freewheeling diode,
thereby allowing all of its thyristors to regain their blocking states.
Q: What are the advantages of freewheeling diodes in a controlled in a controlled rectifier?
a. Input power factor is improved.
b. Load current waveform is improved and thus the load performance is better.
Q: What is meant by delay angle?
The delay angle is defined as the angle between the zero crossing of the input voltage and the instant the thyristor is fired.
Q: What are the advantages of single phase bridge converter over single phase mid-point converter?
a. SCRs are subjected to a peak-inverse voltage of 2Vm in a fully controlled bridge
rectifier. Hence for same voltage and currnt ratings of SCrs, power handled by
mid-point configuration is about
b. In mid-point converter, each secondary winding should be able to supply the load
power. As such, the transformer rating in mid-point converter is double the load
Q What is commutation angle or overlap angle?
The commutation period when outgoing and incoming thyristors are conducting is
known as overlap period. The angular period, when both devices share conduction is
known as the commutation angle or overlap angle.
Q: What are the different methods of firing circuits for line commutated converter?
a. UJT firing circuit.
b. The cosine wave crossing pulse timing control.
c. Digital firing schemes.
Q: Give an expression for average voltage of single phase semiconverters.
Average output voltage Vdc= (Vm / π) (1 + cos α).
Q: What are the advantages of six pulse converter?
a. Commutation is made simple.
b. Distortion on the ac side is reduced due to the reduction in lower order harmonics.
c. Inductance reduced in series is considerably reduced.
Q: What is meant by dc chopper?
A dc chopper is a high speed static switch used to obtain variable dc voltage from a constant dc voltage.
Q: What is duty cycle?
It is the ratio of the ON time of the chopper to total time period of the chopper.
D = Ton / [Ton + Toff]
Q:What is effect of having more duty cycle and less duty cycle?
If the duty cycle D is greater than 0.5, the core would not be completely demagnetized at the end of the off-period, and a DC magnetization of the core would build up, resulting in core saturation.
Q:What are the characteristics of ideal Opamp?
- Infinite open loop voltage gain
- Infinite input impedance
- Zero output impedance
- Infinite Bandwidth
- Zero offset voltage
Q: What are the applications of dc chopper?
a. Battery operated vehicles
b. Traction motor control in electric traction
c. Trolly cars
d. Marine hoists
e. Mine haulers
f. Electric braking.
Q: What are the applications of dc chopper?
a. High efficiency
b. Smooth acceleration
c. Fast dynamic response
Q: What is meant by step-up and step-down chopper?
In a step- down chopper or Buck converter, the average output voltage is less than
the input voltage. In a step- up chopper or Boost converter, the average output voltage is more than the input voltage.
Q: Write down the expression for average output voltage for step down chopper.
Average output voltage for step down chopper V0
= αVs, αis the duty cycle
Q: Write down the expression for average output voltage for step up chopper.
Average output voltage for step down chopper V0= Vs, αis the duty cycle
Q: What is meant by duty-cycle?
Duty cycle is defined as the ratio of the on time of the chopper to the total time
period of the chopper. It is denoted by α.
Q: What are the two types of control strategies?
a. Time Ratio Control (TRC)
b. Current Limit Control method (CLC)
Q: What is meant by TRC?
In TRC, the value of Ton/ T is varied in order to change the average output
Q: What are the two types of TRC?
a. Constant frequency control
b. Variable frequency control
Q: What is meant by FM control in a dc chopper?
In frequency modulation control, the chopping frequency f (or the chopping
period T) is varied. Here two controls are possible.
a. On-time Ton is kept constant
b. Off period Toff is kept constant.
Q: What is meant by PWM control in dc chopper?
In this control method, the on time Ton is varied but chopping frequency is kept
constant. The width of the pulse is varied and hence this type of control is known as PulseWidth Modulation (PWM).
Q: Write down the expression for the average output voltage for step down and step up
Average output voltage for step down chopper is VO= αVS.
Average output voltage for step up chopper is VO= αVSx [1/ ( 1- α)].
Q: What are the different types of chopper with respect to commutation process?
a. Voltage commutated chopper.
b. Current commutated chopper.
c. Load commutated chopper.
Q: What is meant by voltage commutation?
In this process, a charged capacitor momentarily reverse biases the conducting
thyristor and turn it off.
Q: What is meant by current commutation?
In this process, a current pulse is made to flow in the reverse direction through
the conducting thyristor and when the net thyristor current becomes zero, it is turned off.
Q: What is meant by load commutation?
In this process, the load current flowing through the thyristor either becomes zero
or is transferred to another device from the conducting thyristor.
Q: What are the advantages of current commutated chopper?
a. The capacitor always remains charged with the correct polarity.
b. Commutation is reliable as load current is less than the peak commutation current ICP.
c. The auxiliary thyristor TA is naturally commutated as its current passes through zero value.
Q: What are the advantages of load commutated chopper?
a. Commutating inductor is not required.
b. It is capable of commutating any amount of load current.
c. It can work at high frequencies in the order of kHz.
d. Filtering requirements are minimal.
Q: What are the disadvantages of load commutated chopper?
a. For high power applications, efficiency becomes very low because of high
switching losses at high operating frequencies.
b. Freewheeling diode is subjected to twice the supply voltage.
c. Peak load voltage is equal to twice the supply voltage.
d. The commutating capacitor has to carry full load current at a frequency of half
e. One thyristor pair should be turned-on only when the other pair is commutated.
This can be realized by sensing the capacitor current that is alternating.
Q: What is meant by inverter?
A device that converts dc power into ac power at desired output voltage and
frequency is called an inverter.
Q: What are the applications of an inverter?
a. Adjustable speed drives
b. Induction heating
c. Stand-by aircraft power supplies
e. HVDC transmission
Q: What are the main classification of inverter?
a. Voltage Source Inverter
b. Current Source Inverter
Q: Why thyristors are not preferred for inverters?
Thyristors require extra commutation circuits for turn off which resulting
Increased complexity of the circuit. For these reasons thyristors are not preferred for
Q: How output frequency is varied in case of a thyristor?
The output frequency is varied by varying the turn off time of the thyristors in the
inverter circuit, i.e. the delay angle of the thyristors is varied.
Q: Give two advantages of CSI.
a. CSI does not require any feedback diodes.
b. Commutation circuit is simple as it involves only thyristors.
Q: What is the main drawback of a single phase half bridge inverter?
It require a 3-wire dc supply.
Q: Why diodes should be connected in antiparallel with the thyristors in inverter circuits?
For RL loads, load current will not be in phase with load voltage and the diodes
connected in antiparallel will allow the current to flow when the main thyristors are
turned off. These diodes are called feedback diodes.
Q: What types of inverters require feedback diodes?
VSI with RL load.
Q: What is meant a series inverter?
An inverter in which the commutating elements are connected in series with the
load is called a series inverter.
Q: What is the condition to be satisfied in the selection of L and C in a series inverter?
Q: What is meant a parallel inverter?
An inverter in which the commutating elements are connected in parallel with the
load is called a parallel inverter.
Q: What are the applications of a series inverter?
The thyristorised series inverter produces an approximately sinusoidal waveform
at a high output frequency, ranging from 200 Hz to 100kHz. It is commonly used for
fixed output applications such as
a. Ultrasonic generator.
b. Induction heating.
c. Sonar Transmitter
d. Fluorescent lighting.
Q: How is the inverter circuit classified based on commutation circuitry?
a. Line commutated inverters.
b. Load commutated inverters.
c. Self commutated inverters.
d. Forced commutated inverters.
Q: What is meant by McMurray inverter?
It is an impulse commutated inverter which relies on LC circuit and an auxiliary
thyristor for commutation in the load circuit.
Q: What are the applications of a CSI?
a. Induction heating
b. Lagging VAR compensation
c. Speed control of ac motors
d. Synchronous motor starting.
Q: What is meant by PWM control?
In this method, a fixed dc input voltage is given to the inverter and a controlled
ac output voltage is obtained by adjusting the on and off periods of the inverter
components. This is the most popular method of controlling the output voltage and this
method is termed as PWM control.
Q: What are the advantages of PWM control?
a. The output voltage can be obtained without any additional components.
b. Lower order harmonics can be eliminated or minimized along with its output
voltage control. As the higher order harmonics can be filtered easily, the filtering
requirements are minimized.
Q: What are the disadvantages of the harmonics present in the inverter system?
a. Harmonic currents will lead to excessive heating in the induction motors. This
will reduce the load carrying capacity of the motor.
b. If the control and the regulating circuits are not properly shielded, harmonics from
power ride can affect their operation and malfunctioning can result.
c. Harmonic currents cause losses in the ac system and can even some time produce
resonance in the system. Under resonant conditions, the instrumentation and
metering can be affected.
d. On critical loads, torque pulsation produced by the harmonic current can be
Q: What are the methods of reduction of harmonic content?
a. Transformer connections
b. Sinusoidal PWM
c. Multiple commutation in each cycle
d. Stepped wave inverters
Q: What are the disadvantages of PWM control?
SCRs are expensive as they must possess low turn-on and turn-off times.
Q: What does ac voltage controller mean?
It is device which converts fixed alternating voltage into a variable voltage without change in frequency.
Q: What are the applications of ac voltage controllers?
a. Domestic and industrial heating
b. Lighting control
c. Speed control of single phase and three phase ac motors
d. Transformer tap changing
Q: What are the advantages of ac voltage controllers?
a. High efficiency
b. Flexibility in control
c. Less maintenance
Q: What are the disadvantages of ac voltage controllers?
The main draw back is the introduction of harmonics in the supply current and the
load voltage waveforms particularly at low output voltages.
Q: What are the two methods of control in ac voltage controllers?
a. ON-OFF control
b. Phase control
Q: What is the difference between ON-OFF control and phase control?
ON-OFF control: In this method, the thyristors are employed as
switches to connect the load circuit to the source for a few cycles of the load voltageand
disconnect it for another few cycles. Phase control: In this method, thyristor switches
connect the load to the ac source for a portion of each half cycle of input voltage.
Q: What is the control range of firing angle inac voltage controller with RL load?
The control range is Φ< α<180°, where Φ= load power factor angle.
Q. What type of gating signal is used in single phase ac voltage controller with RL load?
High frequency carrier gating signal is used for single phase ac voltage controller
with RL load.
Q. What are the disadvantages of continuous gating signal?
a. More heating of the SCR gate.
b. Increases the size of pulse transformer.
Q. What is meant by high frequency carrier gating?
Thyristor is turned on by using a train of pulses from αto π. This type of signal
is called as high frequency carrier gating.
Q. What is meant by sequence control of ac voltage regulators?
It means that the stages of voltage controllers in parallel triggered in a proper
sequence one after the other so as to obtain a variable output with low harmonic content.
Q. What are the advantages of sequence control of ac voltage regulators?
a. System power factor is improved.
b. Harmonics are reduced in the source current and the load voltage.
Q: What is meant by cyclo-converter?
It converts input power at one frequency to output power at another frequency
with one-stage conversion. Cycloconverter is also known as frequency changer.
Q: What are the two types of cyclo-converters?
a. Step-up cyclo-converters
b. Step-down cyclo-converters
Q: What is meant by step-up cyclo-converters?
In these converters, the output frequency is less than the supply frequency.
Q: What is meant by step-down cyclo-converters?
In these converters, the output frequency is more than the supply frequency.
Q: What are the applications of cyclo-converter?
a. Induction heating
b. Speed control of high power ac drives
c. Static VAR generation
d. Power supply in aircraft or ship boards
Q: What is meant by positive converter group in a cycloconverter?
The part of the cycloconverter circuit that permits the flow of current during
positive half cycle of output current is called positive converter group.
Q:What is meant by negative converter group in a cycloconverter?
The part of the cycloconverter circuit that permits the flow of current during negative half cycle of output current is called negative converter group.