6. Synchronous, Permanent-Magnet and Reluctance Drives

Introduction and context

AC drives based on synchronous and reluctance machines occupy an important position in modern electrical-drive engineering. They are used where high efficiency, precise speed control, high power density, rapid dynamic response, or operation over a wide speed range is required. Unlike induction motors, whose rotor currents are induced by slip, synchronous machines operate with the average rotor field locked to the rotating stator field. Reluctance machines produce torque because the rotor tends to move toward a position of minimum magnetic reluctance. Permanent-magnet machines obtain rotor excitation from magnets rather than from a wound field.

These distinctions influence nearly every aspect of drive design: inverter topology, commutation method, rotor-position sensing, starting, field weakening, fault behaviour, thermal design, controllability, and cost. The term “synchronous drive” therefore covers several technologically different systems rather than one single machine. A wound-field synchronous motor, a sinusoidal permanent-magnet synchronous motor (PMSM), a trapezoidal back-emf brushless DC motor (BLDC), a synchronous-reluctance motor (SynRM), and a switched-reluctance motor (SRM) may all be controlled electronically, yet their torque-production mechanisms and control requirements differ substantially.

This chapter develops a comparative understanding of these drives. The emphasis is not only on machine construction but also on the complete electromechanical system: converter, controller, position information, protection, and application constraints.

Learning objectives

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

1. Explain the operating principles of wound-field synchronous, PMSM, BLDC, synchronous-reluctance, and switched-reluctance drives.

2. Derive and interpret torque relationships using rotating-field, dq-axis, and reluctance-based models.

3. Distinguish sinusoidal and trapezoidal commutation and evaluate their effects on torque ripple and acoustic noise.

4. Explain why rotor position or rotor angle information is required in most electronically commutated drives.

5. Compare excitation, inverter, control, field-weakening, and fault characteristics.

6. Select a suitable drive for industrial, traction, servo, or high-speed applications.

7. Identify practical limitations, safety concerns, and emerging developments in these technologies.

The synchronous operating condition

For a machine with a stator having a balanced three-phase winding, the electrical angular speed of the rotating magnetic field is related to the supply frequency by

\[

n_s=\frac{120f}{P},

\]

where \(n_s\) is synchronous speed in revolutions per minute, \(f\) is electrical frequency, and \(P\) is the number of poles. In an electronically controlled drive, the inverter establishes the stator frequency and voltage rather than receiving them directly from a fixed-frequency grid.

A synchronous rotor must develop an average torque while maintaining an appropriate angular relationship with the stator field. If the rotor field falls too far behind the stator field, synchronism may be lost. The allowable angular displacement is often called the load angle or torque angle, represented by \(\delta\). In a wound-field machine, the rotor field is produced by a DC-excited winding. In a permanent-magnet machine it is produced by magnets. In a reluctance machine, torque results from anisotropy in magnetic reluctance rather than from a rotor excitation winding.

The drive controller consequently performs two linked tasks. It regulates electrical variables—usually phase currents or dq-axis currents—and it maintains the correct spatial relationship between those currents and the rotor magnetic structure. This is the central reason why rotor position is important.

Wound-field synchronous motor drives

A wound-field synchronous motor has a three-phase stator winding and a rotor carrying a DC field winding. The rotor field may be supplied through slip rings and brushes or by a brushless exciter mounted on the shaft. The stator is normally fed from a voltage-source inverter in variable-speed applications.

The rotor field can be adjusted by changing the field current. This provides a degree of control unavailable in a permanent-magnet motor. At constant mechanical load, increasing excitation can move the machine between underexcited, unity-power-factor, and overexcited operation. Overexcited synchronous motors can supply reactive power and may therefore support power-factor correction in large industrial installations.

For a simplified cylindrical-rotor machine, neglecting resistance, the electromagnetic power is approximately

\[

P_e\approx \frac{3VE}{X_s}\sin\delta,

\]

where \(V\) is the stator phase voltage, \(E\) is the internal excitation emf, and \(X_s\) is synchronous reactance. The corresponding torque is \(T_e=P_e/\omega_s\). This equation shows that torque depends on excitation, voltage, reactance, and load angle. It also shows why excessive load can cause loss of synchronism.

Salient-pole wound-field machines have different direct- and quadrature-axis reactances. Their torque includes an excitation component and a reluctance component. A simplified expression is

\[

T_e=\frac{3}{\omega_s}\left[E_f I_q+(X_d-X_q)I_dI_q\right],

\]

with sign conventions depending on the adopted dq model. The first term is excitation torque and the second is reluctance torque.

A wound-field synchronous motor is attractive in high-power applications because excitation is controllable, magnets are not required, and reactive-power capability is available. Its disadvantages include the field-excitation system, increased mechanical and electrical complexity, slower excitation dynamics, and maintenance concerns when brushes and slip rings are used. Starting from a stationary condition also requires a suitable strategy, such as an inverter ramp, damper winding, or auxiliary starting motor.

Permanent-magnet synchronous motor drives

In a PMSM, rotor magnets establish a nearly constant air-gap flux. The stator inverter controls the current vector relative to the rotor magnetic axis. PMSMs are commonly classified as surface-mounted (SPM) or interior permanent-magnet (IPM) machines.

In an SPM motor, magnets are mounted on the rotor surface. The direct- and quadrature-axis inductances are often similar, so torque is primarily due to interaction between rotor magnet flux and stator current. In an IPM motor, magnets are embedded in the rotor. The rotor then has magnetic saliency, normally with different \(L_d\) and \(L_q\), enabling both magnet torque and reluctance torque.

Using a common dq convention, the electromagnetic torque is

\[

T_e=\frac{3}{2}p\left[\lambda_m i_q+(L_d-L_q)i_di_q\right],

\]

where \(p\) is the number of pole pairs, \(\lambda_m\) is the permanent-magnet flux linkage, and \(i_d,i_q\) are dq-axis currents. The first term is permanent-magnet torque; the second is reluctance torque. In many motor-oriented conventions, positive torque is produced mainly by positive \(i_q\), while \(i_d\) is used to adjust flux and exploit saliency.

For maximum torque per ampere, the controller selects an appropriate current angle. In an SPM machine, the preferred current is approximately quadrature to the magnet flux below the voltage limit. In an IPM machine, a deliberate d-axis current can increase torque per ampere by using reluctance torque. At high speed, negative d-axis current weakens the effective air-gap flux and permits operation beyond base speed, although this increases current demand and may reduce torque capability.

PMSMs offer high efficiency, excellent power density, low rotor losses, and strong servo performance. Their limitations include magnet cost, sensitivity to demagnetization, risk of uncontrolled generation during inverter faults, and dependence on accurate position information. High-temperature operation, short-circuit current, excessive negative d-axis current, and mechanical shock must all be considered in magnet selection and protection design.

BLDC drives and electronic commutation

A BLDC motor is a permanent-magnet machine generally designed with trapezoidal phase back emf and concentrated stator windings. Its inverter uses electronic commutation instead of brushes and a mechanical commutator. In conventional six-step operation, two phases conduct at a time while the third phase is unenergized or used for sensing. The conducting pair changes every 60 electrical degrees.

The controller uses rotor position to select the appropriate switching state. Hall sensors are commonly placed at 60- or 120-degree electrical intervals, although sensorless methods are also widely used. The ideal current waveform is rectangular and synchronized with the flat portion of the back emf. The average torque is then approximately proportional to the current magnitude and back-emf constant.

BLDC and PMSM terminology is partly application-dependent. Both use permanent magnets and electronic inverters. The principal distinction is the intended back-emf and current waveform: BLDC drives generally use trapezoidal commutation, whereas PMSM drives generally use sinusoidal currents and field-oriented control. A physical motor may operate under either strategy, but torque ripple and acoustic behaviour will depend on the compatibility between machine emf and applied current.

Six-step commutation is simple and computationally economical, making it suitable for fans, pumps, small traction systems, and appliances. Its disadvantages include commutation torque ripple, current discontinuity, electromagnetic interference, and increased acoustic noise. Higher-performance BLDC systems use advanced current shaping, overlapping commutation, or sinusoidal control to reduce these effects.

Synchronous-reluctance drives

A synchronous-reluctance motor has no rotor winding and no permanent magnets. Its rotor is constructed with alternating high-permeability iron paths and flux barriers, creating magnetic saliency. The rotor aligns its low-reluctance axis with the rotating stator field. When controlled by an inverter, it runs synchronously.

The torque of a salient synchronous-reluctance motor is associated with the difference between the d- and q-axis inductances:

\[

T_e=\frac{3}{2}p(L_d-L_q)i_di_q.

\]

There is no permanent-magnet flux term. Torque therefore requires both appropriate d- and q-axis currents and sufficient saliency. Maximum torque per ampere is obtained by selecting the current vector so that the product of the two axis currents is optimized, subject to voltage, current, and thermal limits.

The absence of magnets and rotor copper reduces material cost and rotor losses. SynRMs can achieve high efficiency, particularly over a broad operating range, and are attractive for pumps, compressors, fans, and industrial motor replacements. Their disadvantages include lower torque density than many IPM motors, torque ripple caused by saliency and saturation, acoustic noise, and the need for careful rotor mechanical design. Flux barriers weaken the rotor mechanically, which is especially important in high-speed machines.

Switched-reluctance drives

An SRM has salient poles on both stator and rotor. The stator phases are independently energized by a power converter, while the rotor contains no windings, magnets, or electrical connections. When a phase is excited, the rotor tends to move toward the position of maximum inductance for that phase.

For a phase with flux linkage \(\lambda(i,\theta)\), the co-energy method gives torque as

\[

T_e=\left.\frac{\partial W'_c(i,\theta)}{\partial\theta}\right|_i.

\]

Under an unsaturated linear approximation, where phase inductance is \(L(\theta)\),

\[

T_e\approx \frac{1}{2}i^2\frac{dL(\theta)}{d\theta}.

\]

Positive torque is produced when the phase is energized over a region where inductance increases with rotor position. If current persists after the aligned position, \(dL/d\theta\) becomes negative and braking torque may result. Accurate turn-on and turn-off angles are therefore essential.

The asymmetric converter, commonly using two switches and two diodes per phase, permits independent phase control and regenerative operation. SRMs tolerate high temperatures because the rotor has no conductors or magnets, and they can continue operating with certain phase faults. They also have a very robust rotor and are suitable for harsh environments and high-speed operation.

The main challenges are torque ripple, radial force variation, acoustic noise, nonlinear magnetic behaviour, and the requirement for rotor-position information. Sensorless control is possible by estimating inductance or flux from phase voltage and current, but low-speed operation and parameter variation make it difficult. Advanced torque-sharing functions distribute torque between adjacent phases to reduce ripple.

Commutation, position information and control architecture

Commutation means coordinating inverter switching with rotor position so that the stator magnetic field produces torque in the desired direction. In a synchronous machine, an angular error changes the torque-producing current component and may cause instability or torque reversal. In an SRM, incorrect switching angles can directly produce negative torque.

Position may be measured using optical encoders, resolvers, Hall sensors, inductive sensors, or magnetic encoders. Encoders and resolvers support precise servo control but add cost and mechanical complexity. Hall sensors are economical and adequate for many BLDC drives, although their coarse resolution introduces commutation-angle errors. Resolvers are robust at high temperature and high speed but require excitation and signal-conditioning electronics.

Sensorless control estimates rotor position from electrical measurements. Back-emf methods are effective above a minimum speed but fail at standstill because back emf is negligible. High-frequency signal injection can exploit saliency and support low-speed or zero-speed estimation in suitable PMSM and SynRM designs. Observer-based methods, including flux observers, extended Kalman filters, and phase-locked loops, can improve performance but require computational resources and robust parameter adaptation.

Field-oriented control transforms three-phase currents into a rotating dq reference frame. The d-axis is aligned with rotor flux or a selected rotor reference, while the q-axis is associated primarily with torque. Inner current loops regulate \(i_d\) and \(i_q\), and outer speed or position loops generate current references. Space-vector pulse-width modulation makes efficient use of the DC bus and provides predictable switching.

Direct torque control instead regulates estimated torque and flux more directly, traditionally using hysteresis comparators and a switching table. It can provide rapid torque response, although conventional implementations may exhibit variable switching frequency and torque ripple. Modern predictive, deadbeat, and model-based controllers extend both field-oriented and direct-torque approaches.

Comparative selection of drive technologies

The correct selection depends on the complete duty cycle rather than peak efficiency alone. The following considerations are particularly important:

• Excitation: Wound-field motors offer adjustable excitation; PMSMs use permanent magnets; SynRMs and SRMs rely on reluctance.
• Power density: IPM and SPM motors generally provide high torque density. SynRMs and SRMs may require larger active volume for the same torque, although their simple rotors can be advantageous.
• Efficiency: PMSM and well-designed SynRM drives provide high efficiency. Wound-field machines can be optimized through excitation control. SRM efficiency is strongly affected by ripple, switching, and magnetic saturation.
• Control complexity: Six-step BLDC control is comparatively simple. PMSM, SynRM, and high-performance SRM drives require more accurate models, position information, and current regulation.
• Fault behaviour: SRMs are robust and can tolerate some phase faults. PMSMs can generate voltage when the rotor turns after an inverter failure, requiring short-circuit or isolation strategies.
• Materials and environment: PMSMs require magnets and may face supply-chain or temperature concerns. Wound-field, SynRM, and SRM technologies reduce or eliminate rare-earth magnet dependence.
• Noise and smoothness: Sinusoidal PMSM drives generally provide smooth torque. BLDC and SRM drives require additional measures to suppress commutation and radial-force ripple.
• Field weakening: IPM and wound-field machines offer useful flux-control mechanisms. SPM motors have more restricted high-speed operation because excessive negative d-axis current may demagnetize magnets or increase current stress.

A drive specification should therefore include speed range, continuous and intermittent torque, overload duration, duty cycle, ambient temperature, acoustic limits, regenerative requirements, safety integrity, maintenance access, and converter fault response.

Applications and professional practice

PMSM drives dominate many high-performance servo systems, robotics axes, electric-vehicle traction systems, compressors, and high-efficiency appliances. IPM motors are especially useful where both high torque density and extended constant-power operation are required. SPM motors are common in precision and moderate-speed applications where mechanical simplicity and smooth torque are important.

BLDC drives are widely used in computer cooling fans, pumps, power tools, small appliances, drones, and compact mobility systems. Their attractiveness comes from low maintenance, efficient electronic commutation, and relatively simple control.

Wound-field synchronous drives remain relevant in large compressors, pumps, mills, synchronous condensers, and high-power industrial installations. Their controllable excitation is valuable where power-factor management or wide-range field control is required.

SynRM drives are increasingly used for industrial pumps, fans, compressors, and general-purpose variable-speed drives where high efficiency and reduced dependence on permanent magnets are desired. SRM drives are appropriate for high-temperature systems, harsh environments, aircraft auxiliaries, starter-generators, washing machines, and applications where rotor simplicity and fault tolerance have priority.

Digital commissioning is now a major part of professional drive engineering. Motor identification routines estimate resistance, inductance, flux linkage, and inertia. Real-time data logging allows engineers to examine current ripple, thermal loading, vibration, speed error, and DC-link behaviour. Digital twins and finite-element models support electromagnetic and mechanical optimization before prototype construction, but model results must be validated experimentally because saturation, manufacturing tolerances, inverter dead time, and temperature significantly affect actual performance.

Illustrative case study: selecting a traction drive for an electric delivery vehicle

Background

A medium-duty electric delivery vehicle requires a traction motor rated for frequent acceleration, regenerative braking, operation from a variable DC battery voltage, and continuous operation in an enclosed underbody compartment. The design team is considering an IPM PMSM, a wound-field synchronous motor, and an SRM.

Situation and problem

The vehicle requires high launch torque, efficient operation during urban stop-start duty, a constant-power speed range above base speed, and limited acoustic noise. The system must also remain safe after an inverter fault and should avoid excessive dependence on scarce magnetic materials.

Analysis

The IPM PMSM provides high torque density and combines magnet torque with reluctance torque. Its negative d-axis control supports field weakening, making it well suited to the required speed range. However, the magnets may experience demagnetization under severe fault or thermal conditions, and the machine can produce back emf during freewheeling.

The wound-field motor allows excitation to be reduced at high speed and eliminates permanent magnets. It can offer strong field-weakening control, but the field system adds losses, mass, and response complexity. For a vehicle with rapid torque transients, the excitation dynamics may be less attractive unless a sophisticated brushless exciter and controller are used.

The SRM has a robust rotor, strong high-temperature tolerance, and good fault isolation. Its principal disadvantages are torque ripple, acoustic noise, and the additional effort required in converter and control design. These concerns are significant in a vehicle intended for operation in residential areas.

Approach and solution

The team selects an IPM PMSM for the primary design, with resolver feedback, field-oriented control, active thermal monitoring, and a defined inverter-fault strategy. The controller uses maximum-torque-per-ampere control below base speed and controlled negative d-axis current above base speed. A mechanical containment design and demagnetization analysis are included. An SRM is retained as a technology alternative for a future variant intended for high-temperature or low-cost operation.

Outcome

The selected drive meets the traction requirements with a compact motor and smooth torque. The design review identifies that efficiency maps, not rated-point efficiency, must govern the final decision because the urban duty cycle contains long periods of partial load and regeneration.

Learning points

1. Drive selection must be based on the complete duty cycle.

2. High torque density does not remove the need for fault and thermal analysis.

3. Field weakening, regenerative braking, and uncontrolled generation must be considered together.

4. Acoustic performance can be as important as electromagnetic performance in vehicle applications.

5. An alternative technology may be preferable when material availability or environmental severity changes.

Limitations, risks and common design errors

A frequent error is to treat a PMSM, BLDC motor, and SRM as a motor alone rather than as a motor-converter-controller system. The same machine can exhibit very different performance under different commutation and current-control strategies.

Incorrect encoder alignment is another common problem. A small electrical-angle error can reduce torque per ampere, increase current, and produce heating. In a BLDC drive, incorrect Hall-sequence interpretation may cause severe vibration or reverse operation. In an SRM, incorrect turn-off timing can create negative torque and large ripple.

Ignoring saturation leads to inaccurate predictions of torque, inductance, field weakening, and fault current. Linear equations are useful for first analysis, but final design generally requires nonlinear magnetic models and measured data. In addition, inverter dead time, voltage drops, sampling delay, current-sensor offset, and DC-link ripple can materially affect low-speed performance.

Permanent-magnet machines require analysis of demagnetization, rotor retention, overspeed, and short-circuit current. Wound-field machines require safe excitation isolation and attention to brush or exciter reliability. SRMs require acoustic and vibration assessment, not merely efficiency measurement. All drives require protection against overcurrent, overvoltage, overspeed, overheating, and loss of position feedback.

Safety, standards and governance considerations

Drive systems are energy-conversion systems capable of producing hazardous torque and high DC-link voltage even when commanded speed is zero. Safe design requires electrical isolation, controlled discharge of capacitors, emergency-stop behaviour, safe torque off where applicable, and protection against unintended restart. Functional-safety analysis should address sensor failure, inverter short circuit, processor malfunction, loss of communication, and invalid position signals.

Compliance must be assessed against the standards and regulations applicable to the target industry and jurisdiction. Important engineering practices include documented requirements, traceable software changes, validated protection thresholds, electromagnetic-compatibility testing, thermal qualification, overspeed testing, and clear maintenance procedures. Safety claims should be supported by verification evidence rather than by reliance on nominal controller behaviour.

Current developments and emerging directions

Current research and industrial development focus on rare-earth-reduced or rare-earth-free traction motors, improved SynRM rotors, ferrite-assisted machines, advanced SRM torque-sharing control, and integrated motor-inverter units. Wide-bandgap semiconductor devices can reduce switching losses and increase switching frequency, but they introduce faster voltage transitions, insulation stress, bearing-current concerns, and electromagnetic-compatibility challenges.

Artificial-intelligence methods are being investigated for condition monitoring, parameter estimation, fault diagnosis, and adaptive sensorless control. Their value is greatest when combined with physically meaningful models and verified fallback logic. A data-driven classifier should not be allowed to obscure a known safety limit or replace deterministic protection.

More-electric aircraft, industrial decarbonization, autonomous machines, and high-efficiency refrigeration are increasing demand for compact, controllable, and fault-tolerant drives. The future is therefore unlikely to be dominated by one machine type. Instead, application-specific optimization will determine whether magnets, field windings, or reluctance structures provide the best overall solution.

Best-practice design workflow

A disciplined design process begins by defining the mechanical duty cycle and operating envelope. The engineer then selects a candidate machine family and establishes its electromagnetic, thermal, mechanical, and converter models. Position-sensing and sensorless requirements should be decided early, not added after the motor design is complete.

The controller should be designed around explicit objectives: torque smoothness, efficiency, dynamic response, field weakening, regeneration, or fault tolerance. Parameter identification and calibration must follow prototype testing. Finally, the integrated system should be evaluated under nominal, overload, thermal, sensor-fault, inverter-fault, and communication-fault conditions. This workflow prevents a drive from being judged solely by a laboratory efficiency point that does not represent its real application.

Chapter Summary

Synchronous, permanent-magnet, and reluctance drives differ fundamentally in how rotor excitation and torque are produced. Wound-field synchronous motors provide controllable excitation and reactive-power capability, while PMSMs provide high efficiency and power density through permanent-magnet flux. BLDC drives use electronic, commonly six-step, commutation and are economical where some torque ripple is acceptable. SynRMs produce torque from rotor saliency without magnets or rotor windings. SRMs use independently excited stator phases and the tendency of the rotor to move toward increasing inductance.

The torque equations clarify these distinctions: PMSM torque contains magnet and, in IPM machines, reluctance components; SynRM torque depends on inductance difference and the product of dq currents; SRM torque depends on the derivative of magnetic co-energy with respect to rotor position. Commutation and rotor-angle information are central to all electronically controlled systems. Field-oriented control, sensorless estimation, torque-sharing control, and predictive methods provide increasingly sophisticated performance.

Selection must consider the complete duty cycle, efficiency map, torque density, field weakening, noise, materials, fault behaviour, thermal environment, safety, and cost. No drive is universally superior. Professional design treats the motor, converter, controller, sensors, protection, and application as one integrated system.

Key Terms

Synchronous speed: Speed of the rotating stator field, determined by supply frequency and pole number.

Load angle: Electrical angular displacement between the rotor field and the stator rotating field.

Wound-field synchronous motor: Synchronous motor whose rotor flux is produced by a DC field winding.

Permanent-magnet synchronous motor: Synchronous motor whose rotor excitation is supplied by permanent magnets.

Surface-mounted PMSM: PMSM with magnets mounted on the rotor surface and usually limited saliency.

Interior PMSM: PMSM with embedded magnets and significant rotor saliency.

BLDC motor: Permanent-magnet motor designed commonly for trapezoidal back emf and electronic commutation.

Electronic commutation: Inverter switching synchronized with rotor position to produce directed torque.

Saliency: Difference between magnetic properties, particularly inductance, along rotor d and q axes.

Reluctance torque: Torque produced by the tendency of a magnetic system to move toward minimum reluctance.

Synchronous-reluctance motor: Synchronous motor with a salient, magnet-free, unexcited rotor.

Switched-reluctance motor: Reluctance motor whose stator phases are independently switched according to rotor position.

Field-oriented control: Control method that regulates current components in a reference frame aligned with rotor flux or a selected rotor axis.

Field weakening: Operation using reduced effective air-gap flux to extend speed beyond base speed.

Torque per ampere: Control objective that maximizes torque for a specified stator-current magnitude.

Back emf: Voltage induced by rotor flux motion relative to the stator winding.

Sensorless control: Estimation of rotor position or speed from electrical measurements without a direct position sensor.

Torque ripple: Periodic variation of electromagnetic torque about its average value.

Magnetic co-energy: Energy-based quantity whose rotor-position derivative gives electromagnetic torque at constant current.

Review Questions

Short Answer Questions

1. What is meant by load angle in a synchronous motor?

2. State the principal difference between PMSM and BLDC commutation.

3. Why does an IPM motor produce reluctance torque?

4. What is the role of turn-off angle in an SRM?

5. Give two reasons why rotor-position information is important in synchronous drives.

Long Answer Questions

1. Explain the construction, torque production, and control of a wound-field synchronous motor drive.

2. Derive and interpret the dq-axis torque equation of a PMSM.

3. Compare six-step BLDC control with sinusoidal field-oriented control.

4. Explain the operating principles and practical limitations of synchronous-reluctance and switched-reluctance drives.

5. Discuss the factors that should guide selection of a drive for an industrial variable-speed application.

Analytical / Application Questions

1. An IPM motor operates below base speed with a specified current limit. Explain how the controller would select d- and q-axis currents for maximum torque per ampere, and how this strategy changes above base speed.

2. An SRM exhibits high acoustic noise and torque ripple during commutation. Identify likely electromagnetic causes and propose control and design measures.

3. A traction application requires high power density, regenerative braking, low noise, and safe operation during inverter faults. Compare an IPM PMSM, wound-field synchronous motor, and SRM, and justify a selection.

22. Multiple Choice Questions with Answers

Question 1

Which torque component is unique to an IPM PMSM when compared with an SPM PMSM?

A. Induction torque caused by rotor slip

B. Reluctance torque caused by saliency

C. Torque caused by rotor copper loss

D. Torque caused by mechanical commutation

Correct Answer: B. Reluctance torque caused by saliency

Question 2

In an SRM, positive motoring torque is normally produced when a phase is energized while its inductance is:

A. Decreasing with rotor position

B. Constant with rotor position

C. Increasing with rotor position

D. Zero at all rotor positions

Correct Answer: C. Increasing with rotor position

Question 3

The principal purpose of rotor-position information in a synchronous drive is to:

A. Increase the DC-link voltage

B. Synchronize inverter switching with the rotor magnetic structure

C. Eliminate all stator resistance

D. Determine the mechanical load independently of speed

Correct Answer: B. Synchronize inverter switching with the rotor magnetic structure

Question 4

Which drive can directly control rotor excitation to support reactive-power management?

A. Wound-field synchronous drive

B. Surface-mounted PMSM only

C. Switched-reluctance drive

D. Six-step BLDC drive only

Correct Answer: A. Wound-field synchronous drive

Question 5

A conventional BLDC drive most commonly uses:

A. Six-step electronic commutation with trapezoidal current or back-emf characteristics

B. Mechanical brushes and a commutator

C. Slip-frequency control with rotor bars

D. DC excitation of the rotor winding

Correct Answer: A. Six-step electronic commutation with trapezoidal current or back-emf characteristics

Question 6

Why is negative d-axis current commonly used in a PMSM above base speed?

A. To increase rotor resistance

B. To weaken effective air-gap flux and reduce the voltage requirement

C. To stop all electromagnetic torque

D. To create mechanical commutation

Correct Answer: B. To weaken effective air-gap flux and reduce the voltage requirement

Question 7

Which feature is a major advantage of an SRM rotor?

A. It contains a high-current rotor winding

B. It requires rare-earth magnets

C. It has no rotor windings or permanent magnets

D. It operates only from a fixed-frequency supply

Correct Answer: C. It has no rotor windings or permanent magnets

Question 8

A position-sensor alignment error in a field-oriented PMSM drive will most directly cause:

A. Improved torque per ampere under all loads

B. Incorrect dq current orientation and possible torque reduction

C. Removal of inverter switching losses

D. Constant mechanical speed independent of load

Correct Answer: B. Incorrect dq current orientation and possible torque reduction

Question 9

Which application characteristic generally favours a wound-field synchronous motor over a PMSM?

A. A need for adjustable rotor excitation and reactive-power control

B. A requirement for no excitation system under any condition

C. A requirement for trapezoidal back emf

D. A requirement for a rotor with flux barriers only

Correct Answer: A. A need for adjustable rotor excitation and reactive-power control

Question 10

Sensorless back-emf position estimation is difficult at standstill primarily because:

A. The stator has no inductance

B. Back emf is negligible when rotor speed is zero

C. The inverter cannot switch at zero speed

D. Permanent magnets produce no flux at standstill

Correct Answer: B. Back emf is negligible when rotor speed is zero