3. Power-Electronic Converters for Drives

Introduction and Context

An electrical drive is an energy-conversion system in which electrical power is processed and delivered to a motor in a controlled manner. The converter is the interface between the electrical supply and the electromechanical system. It determines how rapidly the motor accelerates, how accurately speed and torque are controlled, whether regenerative braking is possible, and how much distortion is introduced into the supply and motor currents.

The central function of a power-electronic converter is switching. Semiconductor devices are operated between blocking and conducting states so that an available source of electrical energy is transformed into a voltage and current waveform suitable for the motor. Since the ideal switch dissipates no power, conversion can be highly efficient. Practical converters, however, experience conduction losses, switching losses, electromagnetic interference, harmonic distortion, thermal stress, and control constraints.

Converter selection is therefore not simply a matter of choosing a rated voltage and current. It requires consideration of motor type, speed range, torque requirements, braking energy, supply characteristics, power quality, switching frequency, thermal design, safety, reliability, cost, and applicable standards. This chapter develops the principal converter families used in drives and examines their operating principles, control methods, limitations, and industrial applications.

Learning Objectives

After studying this chapter, the reader should be able to:

1. Explain the operation and characteristics of controlled rectifiers and DC choppers.

2. Distinguish voltage-source and current-source inverters and identify suitable applications for each.

3. Analyse sinusoidal PWM, space-vector PWM, and selected advanced modulation methods.

4. Explain the structure and benefits of multilevel and matrix converters.

5. Relate switching frequency and device characteristics to losses, harmonics, thermal design, and electromagnetic interference.

6. Compare silicon devices with wide-bandgap devices such as SiC and GaN.

7. Select an appropriate converter topology for a drive application and justify the choice.

Converter Functions in an Electrical Drive

A converter may perform one or more of four basic functions: rectification, inversion, voltage regulation, and frequency conversion. A rectifier converts AC to DC, a chopper regulates DC voltage or current, and an inverter converts DC to variable-frequency AC. A bidirectional converter permits energy to flow from the source to the motor and, during regeneration, from the motor back to the source or an energy-storage element.

The power-processing chain for a conventional AC drive is often represented as

AC supply → rectifier → DC link → inverter → AC motor.

The DC link may contain a capacitor, an inductor, or both. A voltage-source inverter (VSI) uses a relatively stiff DC-link voltage, normally maintained by a capacitor. A current-source inverter (CSI) uses a large DC-link inductance to maintain approximately constant current. In a DC drive, a controlled rectifier or chopper directly regulates armature voltage and current.

Important performance variables include the average and instantaneous motor voltage, current ripple, torque ripple, input displacement factor, total harmonic distortion (THD), efficiency, regenerative capability, common-mode voltage, and semiconductor junction temperature. These variables are coupled. Increasing switching frequency may reduce low-order current ripple but usually increases switching loss and electromagnetic interference.

Controlled Rectifiers for DC Drives

A controlled rectifier uses thyristors or other controllable devices to obtain a variable average DC output from an AC source. For a single-phase fully controlled bridge with continuous load current, the ideal average output voltage is

V_d = (2V_m/π) cos α,

where V_m is the peak supply voltage and α is the firing angle. For a three-phase fully controlled bridge,

V_d = (3√2 V_{LL}/π) cos α,

where V_{LL} is the rms line-to-line voltage. These expressions assume ideal devices, negligible source impedance, and continuous current.

For 0° < α < 90°, the converter operates as a rectifier and supplies positive average voltage to the DC motor. If the motor emf and current conditions permit power reversal, operation with α greater than 90° produces negative average converter voltage and allows inverter operation. A dual-converter drive uses two bridges connected in inverse parallel. It can provide four-quadrant operation: forward motoring, forward braking, reverse motoring, and reverse braking.

Source inductance causes commutation overlap, during which two valve groups conduct simultaneously. The overlap reduces the average output voltage and increases reactive power demand. AC-side current is nonsinusoidal, containing characteristic harmonics. A six-pulse bridge principally produces harmonics of order 6k ± 1 on the AC side, although practical asymmetry and discontinuous current introduce additional components.

Thyristor converters remain useful in high-power DC drives and regenerative systems because of their high voltage and current capability. Their disadvantages include relatively slow switching, poor input power factor at large firing angles, line-current distortion, torque pulsation, and limited independent control of voltage and current. Modern installations frequently replace them with PWM active front ends when improved power quality and four-quadrant operation are required.

DC Choppers

A DC chopper converts a fixed DC voltage into a controlled average voltage by high-frequency switching. For a step-down, or buck, chopper with duty ratio D,

V_o ≈ D V_{dc},

assuming continuous current and ideal components. The duty ratio is the fraction of each switching period for which the main device conducts. A step-up, or boost, chopper ideally gives V_o = V_{dc}/(1-D), although practical drive converters generally use more specialized buck–boost arrangements.

The motor armature inductance smooths current, while a freewheeling diode or synchronous switch provides a path when the main switch is off. Current control is commonly implemented through an inner current loop, with speed control forming an outer loop. This cascade improves torque response and limits armature current during acceleration.

Chopper classes are often described according to the permitted voltage and current polarities. A Class A chopper supports positive voltage and positive current and is suitable for forward motoring. Class B operation permits regenerative current. Class C combines two one-quadrant choppers to obtain two-quadrant operation, while Class E arrangements can provide four-quadrant operation. The practical topology must include appropriate freewheel paths, isolation, fault protection, and braking-energy management.

Compared with a phase-controlled rectifier, a chopper can operate from a DC link with faster response and lower low-frequency torque ripple. Its disadvantages are switching loss, conducted and radiated interference, and the need for careful layout and gate-drive design.

Voltage-Source Inverters

The VSI is the dominant converter for induction-machine, permanent-magnet synchronous-machine, and synchronous-reluctance-machine drives. A capacitor maintains a nearly constant DC-link voltage. Each inverter leg contains two complementary switches with antiparallel diodes or reverse-conduction capability. The upper and lower devices of one leg must never be turned on simultaneously; a short delay called dead time is inserted to prevent shoot-through.

A two-level, three-phase VSI has eight switching states. Six active states generate nonzero voltage vectors in the stationary α–β plane, while two zero states generate zero line-to-line voltage. By selecting switching states in a suitable sequence, the inverter synthesizes a rotating voltage vector. The motor inductance filters the high-frequency voltage components, producing an approximately sinusoidal current.

The inverter must accommodate freewheeling current when the motor back emf or inductance forces current to continue after a switch is turned off. In regenerative operation, energy flows from the motor to the DC link. If the front end is a diode rectifier, this energy cannot normally return to the AC supply. A braking resistor and chopper may dissipate it, or an active front end may return it to the grid.

Common VSI control strategies include volts-per-hertz control, field-oriented control (FOC), and direct torque control (DTC). Scalar control is simple and robust but has limited dynamic performance. FOC transforms measured currents into a rotating reference frame so that flux-producing and torque-producing components can be controlled approximately independently. DTC selects inverter states directly according to torque and flux errors, providing rapid response but potentially variable switching frequency and greater ripple unless refined modulation is used.

Current-Source Inverters

A CSI contains a large DC-link inductor that maintains a controlled current. The inverter switches direct this current among the motor phases. Because the DC-link current cannot change rapidly, the CSI has inherent current limitation and can be attractive for high-power applications. It is also naturally suited to motors with high short-circuit capability and to regenerative operation when the front-end converter is properly controlled.

The CSI requires a continuous path for the link current. Incorrect switching sequences can interrupt the current or impose excessive voltage on semiconductor devices. Motor-side capacitors are often used to support commutation and shape the output waveform. Compared with a VSI, a CSI generally has larger passive components, more complex commutation requirements, and a narrower range of application in modern medium-power drives. Nevertheless, current-source technology remains relevant in very high-power drives and where fault tolerance and regeneration are important.

The fundamental distinction is not that one inverter produces only voltage and the other only current. Both control motor voltage and current indirectly. Rather, the DC-link energy-storage element establishes the dominant controlled quantity: capacitor voltage in a VSI and inductor current in a CSI.

PWM and Space-Vector Modulation

Pulse-width modulation (PWM) controls the average value of inverter voltage by varying the duration of switching states. In sinusoidal PWM, a sinusoidal reference is compared with a triangular carrier. A reference greater than the carrier commands one switch state, and a reference lower than the carrier commands the complementary state. The modulation index determines the fundamental voltage magnitude, while the carrier frequency determines the principal switching-frequency region of the spectrum.

In the linear region of sinusoidal PWM, the fundamental phase voltage is approximately proportional to the modulation index. Overmodulation increases the fundamental voltage but introduces low-order harmonics and eventually produces six-step operation. Dead time also distorts the effective voltage, particularly at low speed where the commanded fundamental voltage is small.

Space-vector PWM treats the three-phase inverter as a vector synthesizer. During each sampling period, the reference vector is approximated by two adjacent active vectors and one or more zero vectors. If the reference lies in a sector, the dwell times of the adjacent vectors are calculated so that their time-weighted average equals the reference. Space-vector PWM makes better use of the DC-link voltage than conventional sinusoidal PWM and provides a systematic framework for switching-sequence optimization.

Alternative methods include discontinuous PWM, which clamps one phase for part of the fundamental cycle and can reduce switching losses; selective harmonic elimination, which chooses switching angles to cancel specified harmonics; and model-predictive control, which evaluates possible switching states using a cost function. The choice depends on whether priority is given to dynamic response, common-mode-voltage reduction, acoustic noise, switching loss, computational burden, or harmonic performance.

Multilevel Converters

Multilevel converters synthesize an output waveform from several discrete voltage levels. The principal families are neutral-point-clamped, flying-capacitor, and cascaded H-bridge converters. A three-level neutral-point-clamped inverter, for example, can apply positive, zero, or negative half-DC-link voltage to each phase. With more levels, the voltage steps become smaller and the output waveform more closely resembles a sine wave.

The main benefits are reduced dv/dt, lower motor insulation stress, lower filter requirements, improved voltage quality, and the ability to operate at high voltage using devices with lower individual voltage ratings. The principal challenges are capacitor-voltage balancing, unequal device losses, increased gate-drive complexity, more complicated fault management, and a larger number of possible switching states.

Cascaded H-bridge converters are attractive where isolated DC sources are available, such as photovoltaic or battery systems. Modular multilevel converters (MMCs) use many submodules and are prominent in high-voltage transmission and large motor-drive research. Their modular construction supports high voltage and redundancy, but circulating-current control, capacitor balancing, and protection are significant design issues.

Matrix Converters

A matrix converter directly converts fixed-frequency AC to variable-frequency AC without a large DC-link energy-storage capacitor. It consists of bidirectional switches connecting each input phase to each output phase. Appropriate switching must prevent input-phase short circuits and avoid interruption of inductive output current.

Matrix converters offer sinusoidal input currents, controllable input displacement factor, bidirectional power flow, and compact construction. They are useful where regeneration and reduced low-frequency energy storage are important. Their disadvantages include complex commutation, sensitivity to supply disturbances, limited maximum voltage transfer ratio, and demanding protection and control. A conventional matrix converter has a practical maximum voltage transfer ratio below unity, commonly around 0.866 under ideal modulation conditions.

The indirect matrix converter separates the function into a virtual rectifier and inverter, allowing established modulation concepts to be applied while retaining the absence of a bulky electrolytic DC capacitor. Reliability can improve because film capacitors may replace electrolytic capacitors, but switching and control complexity remain high.

Switching Losses, Thermal Design, and Harmonics

Semiconductor losses consist primarily of conduction and switching losses. Conduction loss depends on the device voltage drop or on-state resistance and the current waveform. Switching loss occurs during turn-on and turn-off while voltage and current overlap. A first approximation is

P_sw ≈ f_s(E_on + E_off),

where f_s is switching frequency and E_on and E_off are the energy losses per transition. Gate-drive loss, diode reverse-recovery loss, capacitor loss, and magnetic-component loss must also be included in a complete design.

Higher switching frequency generally reduces motor-current ripple and may shift dominant voltage harmonics above the audible range. It also increases switching loss, thermal stress, and electromagnetic interference. The optimum frequency is therefore a system-level compromise rather than a universal value.

Harmonics can be considered at the input, in the DC link, and at the motor terminals. They increase motor heating, torque pulsation, acoustic noise, bearing currents, and supply distortion. Mitigation methods include multipulse transformers, passive filters, active front ends, optimized PWM, sine-wave filters, common-mode chokes, shielded cables, proper grounding, and bearing-current protection. The design should distinguish differential-mode noise from common-mode noise because the suppression methods differ.

Thermal design begins with semiconductor loss estimation and proceeds through junction-to-case, case-to-heatsink, and heatsink-to-ambient thermal resistances. The junction temperature must remain below its rated limit under both steady-state and transient conditions. Real drives require derating for altitude, ambient temperature, blocked ventilation, overload duty, and enclosure conditions.

Silicon and Wide-Bandgap Devices

Silicon IGBTs remain widely used because they provide a mature manufacturing base, good controllability, robust short-circuit capability, and economical performance over a broad voltage and power range. Silicon MOSFETs are effective at lower voltages and high switching frequencies, while thyristors remain important at very high power.

Silicon-carbide (SiC) MOSFETs and diodes offer higher critical electric field, lower switching loss, high-temperature capability, and fast reverse recovery. They can enable higher switching frequency, smaller filters, improved efficiency, and reduced cooling-system size. Gallium-nitride (GaN) devices offer very fast switching and low charge at lower and medium voltage ratings, making them attractive for compact high-frequency converters.

Wide-bandgap devices do not automatically produce a superior drive. Their fast transitions can increase voltage overshoot, common-mode current, electromagnetic interference, and sensitivity to parasitic inductance. Gate-loop design, insulation coordination, measurement bandwidth, layout, and protection must therefore be upgraded. The economic decision should compare total system cost, including cooling, filters, reliability, control hardware, and maintenance, rather than semiconductor purchase price alone.

Selecting a Converter for a Drive Application

A rational selection process begins with the motor and operating cycle. Required speed range, overload torque, braking duty, supply type, motor voltage, and power level establish the first constraints. The designer then evaluates whether the system needs four-quadrant operation, low input distortion, isolation, compactness, high reliability, or compatibility with regenerative energy.

A diode-bridge VSI is often appropriate for economical variable-speed induction-motor drives when regeneration is not required. An active-front-end VSI is preferable when power regeneration, near-unity power factor, and low input current distortion justify additional cost. A controlled rectifier or dual converter remains appropriate for some high-power DC motor applications. A multilevel converter is attractive for medium-voltage motors, while a matrix converter is considered where compactness and bidirectional AC–AC conversion outweigh control complexity.

The selection should also consider standards for electromagnetic compatibility, electrical safety, harmonic current, insulation, and functional protection. A converter that performs well electrically may be unsuitable if it cannot meet grid-code, thermal, acoustic, maintainability, or cybersecurity requirements in the intended installation.

Applied Case: Regenerative Pump Drive in a Water-Transfer Station

Background

An illustrative water-transfer station uses a medium-voltage induction motor to drive a pump. The pump normally operates at variable speed to match changing demand. During rapid reduction of flow, the rotating hydraulic system returns energy to the motor shaft, causing regenerative operation.

Situation and Analysis

The existing drive consists of a diode rectifier, a capacitor DC link, and a two-level VSI. It provides good motoring control but cannot return energy to the utility. A braking resistor can absorb the energy, but frequent braking would increase thermal stress and operating cost. The plant also has a requirement to reduce input-current distortion.

Three alternatives are considered: a braking resistor with the existing front end, a regenerative active front end, and a medium-voltage multilevel active converter. The resistor is the least expensive but wastes regenerated energy. The active front end permits controlled bidirectional power flow and can shape the input current close to sinusoidal. The multilevel option reduces output dv/dt and device voltage stress but has greater capital cost and balancing complexity.

Approach and Solution

For the stated duty, an active-front-end VSI is selected. The front end uses a current-control loop synchronized to the grid voltage. The d-axis current controls active power and the q-axis current controls reactive power. During motoring, power flows from the grid to the DC link; during braking, the d-axis current reverses and energy flows from the DC link to the grid. The motor-side inverter uses field-oriented control with speed and torque limits.

A precharge circuit limits capacitor inrush current. Semiconductor temperature is monitored, and protection is provided for overcurrent, DC-link overvoltage, loss of synchronization, and cooling failure. Harmonic performance is verified at the point of common coupling rather than only at the converter terminals.

Outcome

The design avoids routine dissipation of braking energy and improves control of input power factor. Its additional switching devices, sensing, filtering, and control software increase cost and require trained maintenance personnel. The case illustrates that topology selection must be based on the complete duty cycle and grid interface, not merely on motor rated power.

Learning Points

The principal lessons are that regenerative capability is a system property, input power quality must be evaluated at the point of common coupling, and advanced converter topologies create corresponding requirements for protection, control, thermal design, and maintenance.

Challenges, Professional Practice, and Emerging Directions

Common design mistakes include ignoring dead-time distortion, selecting a switching frequency without calculating thermal losses, treating the DC link as an ideal voltage source, omitting regeneration analysis, and specifying a wide-bandgap device without redesigning the commutation loop. Inadequate grounding and cable termination can produce bearing currents and premature motor failure.

Converter software and firmware are safety-critical in many industrial applications. Access control, authenticated updates, event logging, and separation of control networks from general-purpose networks reduce the risk of unauthorized changes. Functional safety requires clearly defined fault responses, including safe torque off where applicable. Engineers must also account for hazardous stored energy in DC-link capacitors and ensure that maintenance procedures include verified discharge and isolation.

Current developments include silicon-carbide medium-voltage drives, modular converters with condition monitoring, digital twins for thermal and lifetime prediction, sensorless control supported by real-time estimation, and data-driven maintenance based on switching-waveform and temperature information. Artificial intelligence can assist fault diagnosis and remaining-useful-life estimation, but it should supplement, not replace, deterministic protection and validated control laws.

Best practice is to design the converter, motor, cable, filter, grounding system, controller, and cooling arrangement as one electromagnetic and thermal system. Simulation should be followed by hardware-in-the-loop testing, controlled laboratory tests, and verification under abnormal operating conditions. Measurements must use probes and bandwidths suitable for fast switching edges; otherwise apparently acceptable waveforms may conceal dangerous overshoot or common-mode currents.

Chapter Summary

Power-electronic converters regulate the energy exchanged between the electrical supply and a motor. Controlled rectifiers provide variable DC voltage and can support regenerative DC drives, although firing-angle control introduces reactive power and harmonics. DC choppers regulate DC motor voltage rapidly through duty-ratio control and can be arranged for one- to four-quadrant operation.

Voltage-source inverters dominate modern AC drives because capacitor-supported DC links and PWM permit flexible control of induction, synchronous, permanent-magnet, and reluctance machines. Current-source inverters use an inductive DC link and remain useful in selected high-power applications. Sinusoidal PWM, space-vector PWM, discontinuous PWM, selective harmonic elimination, and predictive methods represent different compromises among voltage utilization, switching loss, harmonic quality, and control complexity.

Multilevel converters reduce voltage steps and device stress but introduce balancing and protection challenges. Matrix converters provide direct bidirectional AC–AC conversion with controllable input power factor but require sophisticated commutation. Switching losses, harmonics, thermal limits, electromagnetic compatibility, and regeneration must be analysed together. Silicon devices remain economical and mature, while SiC and GaN can improve efficiency and power density when their fast-switching requirements are properly managed.

Key Terms

Controlled rectifier: A rectifier whose average DC output is regulated by controlling semiconductor firing or switching.

Firing angle: The delay angle, measured from the natural commutation point, at which a thyristor is triggered.

DC chopper: A switching converter that regulates DC voltage or current.

Duty ratio: The fraction of a switching period during which a switch is commanded to conduct.

Voltage-source inverter: An inverter supplied by a stiff DC-link voltage, usually supported by a capacitor.

Current-source inverter: An inverter supplied by a stiff DC-link current, usually maintained by an inductor.

DC link: The intermediate energy-storage and filtering section between converter stages.

PWM: Pulse-width modulation used to control the average value of a switched waveform.

Space-vector PWM: A modulation method that represents three-phase inverter states as vectors in a stationary reference plane.

Dead time: The intentional interval between turning off one device and turning on its complementary device.

Four-quadrant operation: Operation allowing both directions of rotation and motoring and regenerative braking in each direction.

Regeneration: Transfer of mechanical or electrical energy from the motor back to the source or an energy-storage element.

Multilevel converter: A converter that synthesizes its output from three or more discrete voltage levels.

Matrix converter: A direct AC–AC converter using an array of bidirectional switches.

Switching loss: Power dissipated during semiconductor transitions when voltage and current overlap.

THD: Total harmonic distortion, a measure of the rms harmonic content relative to the fundamental component.

Active front end: A controlled rectifier that regulates power flow and input current waveform, including regeneration.

Wide-bandgap device: A semiconductor device based on a material such as SiC or GaN with a larger bandgap than silicon.

Common-mode voltage: The voltage component shared by all motor terminals relative to a reference, often associated with bearing currents and insulation stress.

Review Questions

Short Answer Questions

1. What is the principal difference between a voltage-source inverter and a current-source inverter?

2. Define duty ratio and state its ideal relationship with output voltage in a buck chopper.

3. Why is dead time inserted in an inverter leg?

4. What is meant by four-quadrant operation?

5. Identify two causes of harmonic distortion in a motor-drive converter.

Long Answer Questions

1. Derive and explain the average-output-voltage relationship of a three-phase fully controlled rectifier and discuss the effects of firing-angle control.

2. Explain the structure, switching states, and control principles of a two-level voltage-source inverter.

3. Compare sinusoidal PWM and space-vector PWM with respect to voltage utilization, switching sequence, and harmonic performance.

4. Discuss the operating principles, advantages, and limitations of neutral-point-clamped and cascaded H-bridge multilevel converters.

5. Explain the effects of switching frequency on losses, harmonics, thermal design, and electromagnetic interference.

Analytical / Application Questions

1. A regenerative drive uses a diode rectifier and a capacitor DC link. Explain why energy cannot normally be returned to the utility and recommend two engineering solutions.

2. A medium-voltage motor drive requires low dv/dt, reduced insulation stress, and moderate regenerative capability. Compare a two-level VSI, a multilevel active converter, and a matrix converter, then recommend one topology with justification.

3. A SiC-based inverter exhibits excessive voltage overshoot and common-mode current after its switching frequency is increased. Analyse likely causes and propose changes to layout, gate drive, filtering, and control.

22. Multiple Choice Questions with Answers

Question 1

For a three-phase fully controlled bridge operating with continuous current, increasing the firing angle α from 30° to 60° will primarily

A. increase the average DC output voltage

B. decrease the average DC output voltage

C. eliminate source harmonics

D. force the converter into six-step inverter operation

Correct Answer: B. decrease the average DC output voltage

Question 2

An ideal buck chopper supplied by 400 V operates with a duty ratio of 0.65. Its approximate average output voltage is

A. 140 V

B. 260 V

C. 400 V

D. 615 V

Correct Answer: B. 260 V

Question 3

The main energy-storage element that makes a voltage-source inverter a voltage-stiff converter is the

A. large DC-link capacitor

B. large DC-link inductor

C. commutation capacitor on the motor side

D. gate-drive transformer

Correct Answer: A. large DC-link capacitor

Question 4

The purpose of dead time in an inverter leg is to

A. increase the motor fundamental frequency

B. prevent simultaneous conduction of complementary switches

C. provide regenerative power to the grid

D. eliminate all switching losses

Correct Answer: B. prevent simultaneous conduction of complementary switches

Question 5

In space-vector PWM, the reference voltage vector during one sampling interval is synthesized primarily by

A. one random switching state only

B. two adjacent active vectors and zero vectors

C. the DC-link capacitor and no switches

D. three independent sinusoidal transformers

Correct Answer: B. two adjacent active vectors and zero vectors

Question 6

Which feature is most characteristic of a multilevel converter compared with a conventional two-level inverter?

A. It cannot operate with PWM

B. It produces larger voltage steps at the motor terminals

C. It can reduce dv/dt and individual device voltage stress

D. It requires no capacitor-voltage management

Correct Answer: C. It can reduce dv/dt and individual device voltage stress

Question 7

A matrix converter is distinguished from a conventional VSI because it

A. converts AC to variable-frequency AC without a large DC-link energy-storage capacitor

B. can operate only with a fixed output frequency

C. uses only unidirectional switches

D. cannot provide bidirectional power flow

Correct Answer: A. converts AC to variable-frequency AC without a large DC-link energy-storage capacitor

Question 8

If switching frequency is increased while the converter topology and current remain unchanged, which trend is normally expected?

A. Switching loss decreases and thermal stress disappears

B. Switching loss increases, although current ripple may decrease

C. Input displacement factor becomes exactly unity

D. All common-mode voltage is eliminated

Correct Answer: B. Switching loss increases, although current ripple may decrease

Question 9

A drive with a diode front end is frequently braking and its DC-link voltage rises excessively. The most direct remedy is to

A. remove the motor inductance

B. add a braking chopper and resistor or install a regenerative front end

C. increase inverter dead time indefinitely

D. replace the DC-link capacitor with an open circuit

Correct Answer: B. add a braking chopper and resistor or install a regenerative front end

Question 10

A principal design implication of replacing a silicon IGBT with a fast SiC MOSFET is the need to pay greater attention to

A. slow mechanical commutation only

B. parasitic inductance, voltage overshoot, and electromagnetic interference

C. eliminating all gate-drive protection

D. reducing insulation coordination requirements

Correct Answer: B. parasitic inductance, voltage overshoot, and electromagnetic interference