Windings, Insulation and Dielectric Design

Introduction

The winding and insulation system jointly determine an electrical machine’s voltage, current capability, electromagnetic waveform, thermal life, reliability, manufacturability, and safety. A winding that produces the required fundamental magnetomotive force may still be unacceptable if it generates excessive harmonic loss, cannot fit within the slots, or exposes its insulation to damaging electric fields. Conversely, generous insulation dimensions can reduce copper area and increase loss or slot size.

Winding design is therefore an integrated electromagnetic, thermal, mechanical, and dielectric problem. The designer must select the winding layout, number of turns, conductor dimensions, slot geometry, insulation system, and terminal spacing while accounting for manufacturing tolerances and applicable standards.

Learning Objectives

After studying this chapter, students should be able to:

• distinguish concentrated, distributed, integral-slot, and fractional-slot windings;
• calculate coil pitch, slots per pole per phase, winding factors, turns, and conductor area;
• evaluate selected space-harmonic winding factors;
• estimate slot fill, current density, and specific electric loading;
• select practical slot and insulation arrangements;
• calculate electric stress in uniform and cylindrical insulation systems;
• explain partial discharge and inverter-related dielectric stress;
• perform a standards-based creepage and clearance selection; and
• integrate electromagnetic and dielectric requirements in a machine design.

1. Winding Architecture

1.1 Basic terminology

A coil consists of one or more turns, with each turn containing two active conductor sides connected by end windings. Conductors placed in slots generate magnetomotive force when carrying current and have voltage induced in them when linked by changing magnetic flux.

Let:

• S = number of stator slots;
• 2p = number of poles, where p is the number of pole pairs;
• m = number of phases;
• q = slots per pole per phase.

The fundamental winding classification parameter is

For an integral-slot winding, q is an integer. Fractional-slot windings have non-integer q. A concentrated tooth-coil winding commonly has q<1, although the term concentrated may also describe windings whose coil sides occupy only one slot per pole per phase.

1.2 Distributed windings

In a distributed winding, the coil sides of each phase belt are spread across several slots. Distribution improves the spatial approximation to a sinusoidal magnetomotive-force wave and can reduce selected harmonics. Its disadvantages include longer end windings, greater copper mass, and more complex insertion and connection.

Distributed windings are widely used in industrial induction motors, wound-field synchronous machines, and large permanent-magnet machines where low harmonic loss and smooth torque are important.

1.3 Concentrated and fractional-slot windings

A concentrated winding places each coil around a tooth or a small group of teeth. It offers short end turns, high copper utilization, simple modular manufacture, and good fault isolation. Fractional-slot concentrated windings are common in permanent-magnet machines.

Their limitations may include:

• larger space harmonics;
• additional rotor eddy-current loss;
• local tooth saturation;
• unbalanced magnetic forces for some slot–pole combinations; and
• increased acoustic noise or torque ripple.

A fractional-slot combination must therefore be evaluated using winding-function or phasor methods rather than assuming that integral-slot formulas apply.

2. Coil Pitch and Electrical Slot Angle

The electrical angle between adjacent slots is

where α is the electrical slot angle. For an integral-slot winding this is also 180°/(mq).

A full-pitch coil spans one pole pitch, or 180 electrical degrees. If a coil spans y slot pitches, its electrical coil span is

A short-pitched or chorded coil has β<180°. Its chording angle is

Short pitching reduces end-turn length and can suppress selected harmonics, but it also slightly reduces the fundamental induced voltage.

3. Winding Factors and Harmonics

3.1 Pitch factor

For harmonic order ν, the magnitude of the pitch factor is

Angles must be used consistently in degrees or radians. For the fundamental,

When β=180°−ε, the general harmonic relationship is

Thus, the commonly used identity k_pν=|cos(νε/2)| is valid only for odd harmonic orders. Many symmetric three-phase windings suppress even space harmonics, but this physical symmetry does not justify applying the odd-harmonic identity to an arbitrary even ν.

A harmonic is ideally eliminated by coil pitching when

for a nonzero integer n, provided that the chosen pitch remains physically realizable.

3.2 Distribution factor

For an integral-slot winding with q equal conductor groups separated by electrical angle α, the magnitude of the distribution factor is

This expression assumes equally spaced, equal-turn coil sides in a symmetrical phase belt. The limiting value is one when the denominator and numerator simultaneously approach zero.

For the fundamental,

Distribution reduces the fundamental slightly but often reduces higher harmonics much more strongly.

3.3 Winding factor

If skew is absent, the harmonic winding factor magnitude is

For a winding skewed through electrical angle γ, an ideal continuous-skew factor may be included:

Then k_wν=k_pνk_dνk_sν. The skew expression assumes a uniformly distributed axial skew and neglects end effects.

For irregular or fractional-slot windings, a general phasor sum is safer:

Here N_j is the number of conductors represented by phasor j, θ_j is its electrical position, and s_j=+1 or −1 represents conductor direction. The phase sequence, harmonic sign, and connection must be retained when a signed result is needed.

3.4 Harmonic consequences

Low-order space harmonics can produce parasitic torque, radial force, noise, rotor heating, and additional core loss. In a balanced three-phase winding with appropriate symmetry, prominent rotating field harmonics commonly belong to orders 6r±1, where r is a positive integer. The fifth rotates oppositely to the fundamental, whereas the seventh rotates in the same direction but at a different synchronous speed.

Triplen harmonics are zero-sequence quantities. Their terminal-current behavior depends on the neutral and delta connections, while their local flux effects also depend on the magnetic circuit. Harmonic acceptance must therefore be based on the complete machine rather than on winding factor alone.

4. Turns and Induced Voltage

For approximately sinusoidal fundamental air-gap flux, the RMS induced phase voltage is

where:

• E_ph = RMS fundamental phase EMF, V;
• f = electrical frequency, Hz;
• Φ₁ = peak fundamental flux per pole, Wb;
• T = series turns per phase in each parallel path;
• k_w1 = fundamental winding factor.

The equation assumes sinusoidal time variation of flux. The factor 4.44 equals 4√2 and is not appropriate without correction for strongly nonsinusoidal flux waveforms.

The required turns are

For a star-connected three-phase winding, E_ph≈V_L/√3 when winding voltage drops are initially neglected. For a delta connection, E_ph≈V_L. Resistance and leakage-reactance drops must subsequently be included in performance calculations.

Worked Example 1: Distributed winding design

A three-phase, four-pole, 50 Hz, star-connected machine has 36 stator slots and a 400 V line voltage. The winding uses an eight-slot coil pitch. The design fundamental flux per pole is 0.0306 Wb.

The slots per pole per phase are

The electrical slot angle is

A full-pitch coil would span nine slots. The eight-slot coil has

The fundamental factors are

Therefore,

The phase voltage is 400/√3=230.94 V, so

Select 36 series turns per phase per parallel path.

For the fifth harmonic,

Because five is odd, the equivalent expression |cos(5×20°/2)| is valid. Also,

Thus k_w5=0.1399. For the seventh harmonic, k_p7=0.3420, k_d7=0.1774, and k_w7=0.0607. The short pitch and distribution therefore attenuate both harmonics substantially relative to the fundamental.

5. Conductor Area, Current Density, and Slot Fill

5.1 Current density

If a phase winding has a identical parallel paths, the conductor current is

The copper current density is

where J is in A/mm² when conductor copper area A_cu is in mm². Required area is therefore A_cu=I_c/J.

Allowable current density cannot be selected independently of cooling, duty, conductor geometry, frequency, altitude, ambient temperature, and insulation class. Typical low-voltage machine values often fall in the approximate range 3–7 A/mm², but this is a preliminary-design range, not a universal limit. High-frequency skin and proximity effects require AC-loss calculations and may favor strands, rectangular conductors, transposition, or litz wire.

5.2 Specific electric loading

The specific electric loading at the armature bore is

where Z is the total number of active conductors, I_c is conductor RMS current, and D is bore diameter in metres. The result is in ampere-conductors per metre. Definitions must be kept consistent when parallel paths are used; Z must count all physical conductors.

Higher electric loading increases torque capability but also increases copper loss, thermal stress, leakage field, and magnetic force.

5.3 Slot fill factor

The gross copper fill factor is

The numerator is bare copper area in one slot. The denominator is gross geometric slot area. Slot liner, turn insulation, conductor enamel, wedges, voids, tolerances, and insertion damage prevent unity fill.

Random-wound round-wire stators often achieve lower fill than carefully formed rectangular-conductor windings. A theoretically feasible fill may be impossible to manufacture because conductors must pass through the slot opening or bend around end turns.

Worked Example 2: Conductor and slot check

Continue Worked Example 1 with a phase current of 25 A, two parallel paths, and a target current density of 4 A/mm².

Select a standard conductor having 3.30 mm² bare copper area. The actual current density is

The number of active conductors per slot is

Thus the bare copper area per slot is 12×3.30=39.6 mm². If the gross slot area is 100 mm²,

A 39.6% gross copper fill may be manufacturable for an appropriate winding process, but the final design must be checked using the insulated conductor dimensions, liner thickness, wedge space, tolerances, and insertion trials.

6. Slot Geometry Selection

Slots must accommodate conductors and insulation while controlling leakage reactance, tooth flux density, mechanical retention, cooling, noise, and manufacture.

A preliminary average tooth flux-density estimate is

where Φ_t is the flux assigned to one tooth at the considered instant, k_Fe is lamination stacking factor, b_t is tooth width, and l_e is effective stack length. Determining Φ_t requires the actual air-gap flux distribution; simply dividing pole flux equally among teeth is only a preliminary approximation.

Slot leakage, local saturation, and harmonic fields are normally refined using finite-element analysis. Nevertheless, analytical calculations remain essential for checking model inputs and identifying implausible simulation results.

7. Insulation Systems

7.1 Insulation hierarchy

Machine insulation is a coordinated system rather than a single material. It commonly includes:

• strand or wire enamel;
• turn-to-turn insulation;
• coil-to-coil and phase separators;
• slot liners and ground-wall insulation;
• sleeving over leads and joints;
• wedges, bindings, tapes, and impregnation resin;
• terminal supports and barriers.

The weakest interface may determine the life of the entire winding. Compatibility must be established among enamel, resin, tape, coolant, cleaning chemicals, and impregnation process.

7.2 Thermal classification

IEC 60085 identifies thermal classes by numerical temperature indices, including 105, 120, 130, 155, 180, 200, 220, and 250 °C. Historical letter designations such as A, E, B, F, and H remain common. A class rating characterizes an evaluated insulation system; it is not automatically conferred by using one high-temperature material.

The permissible temperature rise must account for ambient temperature, hotspot allowance, measurement method, duty, and the applicable machine standard. Long insulation life generally requires operating margin below the thermal-class limit.

7.3 Impregnation and mechanical integrity

Dip-and-bake, trickle impregnation, vacuum-pressure impregnation, and resin-rich processes fill voids and bond conductors. Effective impregnation improves heat transfer, mechanical stiffness, contamination resistance, and partial-discharge performance. Poor curing or trapped voids can create weak dielectric regions.

Electromagnetic forces act approximately with the square of current during severe faults. End windings and slot conductors must therefore withstand short-circuit forces without abrasion, displacement, or delamination.

8. Dielectric Stress

8.1 Uniform-field approximation

For two large parallel electrodes separated by homogeneous insulation thickness d, the nominal field is

where E is in V/m, V is potential difference, and d is in metres. This approximation neglects edge effects, voids, material interfaces, and electrode curvature.

Dielectric strength obtained from material coupons must not be used directly as a continuous design stress. Ageing, temperature, waveform, defects, manufacturing variability, and safety margin require substantially lower operational stress.

8.2 Cylindrical insulation

For a long coaxial arrangement with conductor radius a, outer grounded radius b, and homogeneous permittivity, the radial electric field is

The maximum occurs at the conductor surface:

The field is therefore nonuniform. Sharp corners, strand crossovers, and poorly terminated semiconductive layers can similarly intensify local stress.

8.3 Interfaces and voids

Normal electric flux density is continuous across an interface without free surface charge:

A gas void has much lower permittivity than surrounding solid insulation, so its electric field can be substantially higher. Because gases also have lower breakdown strength, voids are common partial-discharge sites.

9. Partial Discharge and Converter-Fed Stress

Partial discharge is a localized electrical discharge that bridges only part of an insulation system. It may occur in internal voids, along surfaces, or near sharp electrodes. The measured apparent charge, often reported in picocoulombs, is a terminal-equivalent quantity and is not necessarily equal to the actual charge at the discharge site.

Important quantities include:

• partial-discharge inception voltage, at which repetitive discharge begins;
• partial-discharge extinction voltage, below which it ceases;
• apparent charge and pulse repetition rate;
• phase-resolved discharge pattern under AC excitation.

Random-wound low-voltage windings are commonly designed and qualified to avoid repetitive partial discharge under specified service impulses. Form-wound medium- and high-voltage systems may instead use controlled, discharge-resistant insulation with stress grading and qualification tests.

Converter-fed machines face fast voltage rise, cable reflection, unequal turn-voltage distribution, and repetitive bipolar or unipolar impulses. At the instant of a steep-front pulse, winding capacitances dominate, so the first turns of a coil may receive a disproportionate share of the voltage. Mitigation includes:

• inverter-duty wire and enhanced turn insulation;
• controlled cable length and grounding;
• output reactors, dv/dt filters, or sine-wave filters;
• optimized coil and lead placement;
• reduced terminal overshoot; and
• qualification under representative repetitive impulses.

IEC 60034-18-41 and IEC 60034-18-42 provide qualification frameworks for converter-fed insulation systems, respectively addressing partial-discharge-free Type I systems and partial-discharge-tolerant Type II systems within their stated scopes.

10. Creepage and Clearance

10.1 Distinction

Clearance is the shortest distance through air between conductive parts. It is principally selected from impulse withstand voltage, electric-field conditions, altitude, and the applicable standard.

Creepage is the shortest distance along an insulating surface. It is principally selected from working voltage, pollution degree, material tracking resistance, and insulation type.

These distances must be evaluated separately. A long surface path does not compensate for inadequate air clearance, and a large clearance does not guarantee adequate creepage.

IEC 60664-1 provides a widely used insulation-coordination framework for low-voltage equipment. A motor’s product standard, installation standard, or national adoption may impose additional or different requirements and takes precedence where applicable.

10.2 Design parameters

The selection process requires:

1. working RMS or DC voltage across the insulation;
2. nominal supply system and overvoltage category;
3. required impulse withstand voltage;
4. basic, supplementary, reinforced, or functional insulation classification;
5. pollution degree;
6. insulating material group based on comparative tracking index, CTI;
7. field condition and altitude;
8. manufacturing tolerances and contamination controls.

Typical material groups are based on CTI ranges. Group IIIa covers 175≤CTI<400, while groups with higher CTI have better tracking resistance. CTI data must apply to the actual molded or processed material.

10.3 Standards-based worked selection

Consider an illustrative 400 V three-phase motor terminal board installed at 3000 m altitude. It is connected to a 230/400 V low-voltage distribution system under overvoltage category III. The environment is pollution degree 2, and the terminal support is material group IIIa. Basic insulation is required between phases. The field is treated as inhomogeneous, and the phase-to-phase working voltage is 400 V RMS.

Using the IEC 60664-1 insulation-coordination procedure:

• a 230/400 V supply in overvoltage category III gives a rated impulse voltage of 4 kV;
• the tabulated basic clearance for a 4 kV impulse in an inhomogeneous field at altitudes up to 2000 m is 3.0 mm;
• the altitude correction factor at 3000 m is 1.14.

Therefore,

Select at least 4.0 mm physical clearance after accounting for tolerance and assembly displacement.

For 400 V working voltage, pollution degree 2, and material group IIIa, the corresponding basic creepage requirement is 4.0 mm. Select at least 4.0 mm creepage, again measured along the shortest credible contaminated surface path.

The design therefore provides:

A barrier may increase creepage only if its geometry satisfies the applicable measurement rules. Unqualified coating must not simply be assumed to reduce pollution degree. Reinforced-insulation values must be selected from the relevant provisions rather than obtained by automatically doubling every basic-insulation distance.

Standards are revised, and national or product standards may alter the result. A professional design record should identify the standard edition, table entries, voltage basis, altitude, pollution degree, CTI evidence, and measured production tolerance.

11. Integrated Illustrative Case: Inverter-Duty Stator

Background

A manufacturer is developing a four-pole, 36-slot, 400 V industrial motor for variable-speed operation. The motor must provide smooth torque, fit an existing stator frame, and operate at an installation altitude of 3000 m. This is an illustrative case.

Problem

The preliminary winding must satisfy induced-voltage, thermal, slot-fill, harmonic, and terminal-insulation requirements. The inverter produces fast-front pulses, making turn insulation and terminal overshoot important.

Analysis and approach

The designer selects the three-phase distributed winding calculated in Worked Example 1: q=3, eight-slot coil pitch, k_w1=0.9452, and 36 turns per phase per parallel path. The fifth- and seventh-harmonic winding factors are only 0.1399 and 0.0607, respectively.

With two parallel paths and 25 A phase current, 3.30 mm² conductors operate at 3.79 A/mm². Twelve conductors per slot produce 39.6% gross copper fill in a 100 mm² slot. A manufacturing trial is still required because gross fill does not represent insertion force or insulated dimensions.

The slot opening is kept narrow enough to limit permeance ripple but wide enough for the selected insulated conductor. Rounded slot corners, a robust liner, phase separators, and a mechanically secure wedge are specified. Thermal analysis checks the end-winding hotspot and reduced air density at altitude.

For the terminal board, the IEC 60664-1 procedure gives a corrected minimum clearance of 3.42 mm and a creepage requirement of 4.0 mm. The drawing specifies 4.0 mm minimum after tolerance. Production gauges verify the shortest air and surface paths.

Because of inverter supply, the team also specifies inverter-duty turn insulation, limits cable-related overvoltage, and performs representative impulse qualification. If measured terminal overshoot exceeds the insulation-system capability, a dv/dt filter is added rather than relying on nominal 400 V operation.

Outcome

The integrated design meets the preliminary electromagnetic and insulation constraints without enlarging the stator. It proceeds to finite-element loss analysis, thermal testing, surge comparison testing, dielectric qualification, and prototype endurance testing.

Learning points

• Fundamental winding performance alone does not establish machine reliability.
• Harmonic reduction, conductor sizing, and slot manufacturability must be evaluated together.
• Inverter peak voltage and rise time can be more important to turn insulation than RMS voltage.
• Creepage and clearance require different inputs and must be documented against a specified standard edition.

12. Design Challenges and Contemporary Practice

Analytical equations use idealized sinusoidal fields, uniform materials, and exact geometry. Real machines contain skew, saturation, eccentricity, end effects, random conductor placement, resin-rich and resin-poor regions, thermal gradients, and production variation. Prototype validation therefore remains necessary.

Modern workflows combine parameterized winding software, electromagnetic finite-element analysis, thermal networks, electric-field simulation, and optimization algorithms. AI-assisted optimization can explore slot–pole combinations or insulation geometries, but it cannot establish compliance without traceable constraints, verified data, and engineering review. Designs generated by software should be checked for connection feasibility, symmetry, fault behavior, manufacturability, and standards compliance.

Ethical engineering practice requires conservative reporting of insulation capability, accurate use of thermal-class claims, traceable test evidence, and disclosure of service limitations. Safety margins must not be silently reduced to meet cost or power-density targets.

Chapter Summary

Electrical-machine winding design begins with slot, pole, phase, and coil-pitch selection. Distributed windings generally provide good waveform quality, while concentrated and fractional-slot windings offer short end turns and manufacturing advantages at the cost of potentially stronger harmonics.

Pitch, distribution, skew, and total winding factors quantify the contribution of winding geometry to fundamental and harmonic EMF. The harmonic pitch-factor identity involving cosine applies only to odd harmonic orders; even orders require a sine expression. Required turns follow from the sinusoidal EMF equation, while conductor area follows from phase current, parallel paths, and permissible current density.

Slot design must balance copper area, tooth saturation, leakage, cooling, mechanical retention, and manufacturability. Insulation must be treated as a coordinated system whose thermal, mechanical, chemical, and dielectric properties are jointly qualified.

Dielectric design requires attention to nonuniform electric fields, material interfaces, voids, partial discharge, and converter impulses. Clearance is selected principally from impulse voltage and altitude, whereas creepage depends on working voltage, pollution degree, and material tracking resistance. Compliance requires use of the applicable standard edition, end-product requirements, tolerances, and verification testing.

Key Terms

Coil pitch: Electrical or mechanical separation between the two active sides of a coil.

Clearance: Shortest distance through air between two conductive parts.

Comparative tracking index: Material test index used to classify resistance to surface tracking.

Concentrated winding: Winding whose coils occupy a small number of slots, often around individual teeth.

Creepage distance: Shortest path between conductive parts along an insulating surface.

Current density: Conductor current divided by bare conductor cross-sectional area.

Distribution factor: Ratio describing phasor reduction caused by distributing conductors among slots.

Distributed winding: Winding whose phase conductors are spread over several slots per pole.

Fractional-slot winding: Winding for which slots per pole per phase is non-integer.

Ground-wall insulation: Main insulation separating a winding from the grounded magnetic core or frame.

Partial discharge: Localized discharge that bridges only part of an insulation system.

Pitch factor: Ratio describing the effect of coil span on induced EMF or MMF harmonic magnitude.

Pollution degree: Standardized classification of environmental contamination affecting insulation surfaces.

Slot fill factor: Ratio of bare copper area to gross slot area under a stated convention.

Specific electric loading: Total armature ampere-conductors per unit bore circumference.

Thermal class: Temperature classification assigned to an evaluated electrical insulation system.

Turn insulation: Insulation between adjacent turns of a winding.

Winding factor: Product or phasor-derived factor representing the effectiveness of winding distribution, pitch, and sometimes skew.

Review Questions

Short Answer Questions

1. Distinguish an integral-slot winding from a fractional-slot winding.
2. Why does short pitching reduce selected winding harmonics?
3. Define gross slot fill factor.
4. Distinguish partial discharge from complete dielectric breakdown.
5. Why is altitude relevant to clearance but not normally used as a direct creepage multiplier?

Long Answer Questions

1. Derive the distribution factor for q equal conductor groups separated by electrical angle α.
2. Compare distributed and fractional-slot concentrated windings in terms of harmonics, copper utilization, manufacturing, and noise.
3. Explain how conductor current density, slot geometry, cooling, and insulation class interact in stator design.
4. Discuss the mechanisms and mitigation of dielectric stress in converter-fed machine windings.
5. Describe a standards-based procedure for determining creepage and clearance in a low-voltage machine terminal assembly.

Analytical / Application Questions

1. A three-phase, six-pole, 54-slot winding has a coil span of eight slots. Calculate q, electrical slot angle, coil-span angle, chording angle, and fundamental pitch and distribution factors.
2. Design the conductor area for a 60 A phase winding with three parallel paths and a target current density of 5 A/mm². If there are 15 conductors per slot and the gross slot area is 125 mm², determine the copper fill factor using the minimum conductor area.
3. An insulation layer forms a coaxial system with conductor radius 4 mm, outer radius 9 mm, and applied voltage 6 kV. Calculate the maximum electric field and explain where partial discharge is most likely if an air void exists near the conductor.

Multiple Choice Questions with Answers

Question 1

A three-phase, four-pole winding has 36 slots. What is the number of slots per pole per phase?

A. 1

B. 2

C. 3

D. 6

Correct Answer: C. 3

Question 2

For a 36-slot, four-pole stator, what is the electrical angle between adjacent slots?

A. 10°

B. 20°

C. 30°

D. 40°

Correct Answer: B. 20°

Question 3

A coil has an electrical span of 160°. What is its fundamental pitch factor?

A. 0.6428

B. 0.8660

C. 0.9598

D. 0.9848

Correct Answer: D. 0.9848

Question 4

When β=180°−ε, which expression correctly gives the magnitude of the pitch factor for an even harmonic order ν?

A. |cos(νε/2)|

B. |sin(νε/2)|

C. |tan(νε/2)|

D. 1/ν

Correct Answer: B. |sin(νε/2)|

Question 5

Which quantity is primarily used with impulse withstand voltage and altitude to select insulation spacing?

A. Clearance

B. Creepage

C. Copper fill factor

D. Current density

Correct Answer: A. Clearance

Question 6

A phase current is 30 A and the winding has two identical parallel paths. If the bare conductor area is 3 mm², what is the conductor current density?

A. 2.5 A/mm²

B. 5 A/mm²

C. 10 A/mm²

D. 20 A/mm²

Correct Answer: B. 5 A/mm²

Question 7

A slot contains 40 mm² of bare copper and has a gross area of 100 mm². What is its gross copper fill factor?

A. 0.25

B. 0.40

C. 0.60

D. 2.50

Correct Answer: B. 0.40

Question 8

Which feature is a principal advantage of a fractional-slot concentrated winding?

A. It always eliminates all space harmonics

B. It requires no turn insulation

C. It can provide short end windings

D. It guarantees zero cogging torque

Correct Answer: C. It can provide short end windings

Question 9

In a homogeneous coaxial insulation system, where is the electric-field magnitude greatest?

A. At the midpoint of the insulation

B. At the outer conductor

C. At the inner conductor surface

D. It is uniform everywhere

Correct Answer: C. At the inner conductor surface

Question 10

Why can an internal gas void experience greater electric stress than surrounding solid insulation?

A. The gas necessarily has greater permittivity

B. Normal electric flux continuity produces a larger field in the lower-permittivity region

C. The solid insulation has zero conductivity

D. The void eliminates the applied voltage

Correct Answer: B. Normal electric flux continuity produces a larger field in the lower-permittivity region