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Qwestrum Engineering360 · Electrical & Electronics · Electrical Machines – II

Special Electrical Machines

Special machines — stepper, BLDC, switched-reluctance, universal and servo motors — trade the simplicity of the induction motor for precise position/speed control, and are analysed through their distinctive step angle, commutation and torque relations.
Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

  • SRM: doubly salient, no rotor winding
  • Linear induction motor for traction and material handling
  • Servo motors: high dynamic response with feedback

Topic details

Introduction

Stepper motors move in discrete steps of θ_s = 360/(mN_r); the number of pulses fixes the position, giving open-loop digital positioning used in printers and CNC. BLDC and PMSG use permanent magnets with electronic commutation, offering high efficiency and torque density.

Scope in B.Tech and GATE syllabus

Switched-reluctance motors have a doubly salient structure and no rotor winding or magnets, making them rugged and cheap; torque comes from the tendency of the rotor to align with the excited stator pole (minimum reluctance).

Key relations & formulas

Formulas (Indian textbook notation)

  • Universalmotor:serieswound−runsonACorDCUniversal motor: series wound - runs on AC or DC
Stepper:stepangleθs=360(m×Nr)Stepper: step angle \theta_{s} = \frac{360}{(m \times N_{r})}
(m phases, N_r rotor teeth)

Formulas (Indian textbook notation)

  • BLDC:trapezoidalback−EMF;electroniccommutationBLDC: trapezoidal back-EMF; electronic commutation

Notation and sign conventions

Relation 1 —
Universalmotor:serieswound−runsonACorDCUniversal motor: series wound - runs on AC or DC

Formulas (Indian textbook notation)

  • Universalmotor:serieswound−runsonACorDCUniversal motor: series wound - runs on AC or DC
Write this relation with symbols exactly as in Electrical Machines — Nagrath & Kothari before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 2 —
Stepper:stepangleθs=360/Stepper: step angle \theta_{s} = 360/
Stepper:stepangleθs=360(m×Nr)Stepper: step angle \theta_{s} = \frac{360}{(m \times N_{r})}
(m phases, N_r rotor teeth)
Write this relation with symbols exactly as in Electrical Machines — Nagrath & Kothari before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 3 —
BLDC:trapezoidalback−EMF;electroniccommutationBLDC: trapezoidal back-EMF; electronic commutation

Formulas (Indian textbook notation)

  • BLDC:trapezoidalback−EMF;electroniccommutationBLDC: trapezoidal back-EMF; electronic commutation
Write this relation with symbols exactly as in Electrical Machines — Nagrath & Kothari before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Fundamentals and definitions

For a stepper, total steps per revolution = 360/θ_s, and shaft speed (rpm) = (step rate in steps/s) × 60 / (steps per revolution). Microstepping subdivides each full step for finer resolution.

Governing relations in practice

Universal motors are series-wound and develop the same torque direction on AC or DC because both field and armature current reverse together; they achieve very high speed in mixers and power tools.

Design and analysis considerations

Servo systems close a feedback loop around a BLDC or DC motor with an encoder, achieving fast, accurate response; the machine is chosen for low inertia and high peak torque.

Assumptions and validity limits

State assumptions explicitly before using any relation for special electrical machines — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.
Wrong assumptions invalidate the entire solution even when the formula is correct. In Electrical Machines II viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in Electrical Machines II papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to special electrical machines.
4. Use equation 1:
Universalmotor:serieswound−runsonACorDCUniversal motor: series wound - runs on AC or DC
.
5. Use equation 2:
Stepper:stepangleθs=360/Stepper: step angle \theta_{s} = 360/
.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Special Electrical Machines appears in industrial motors and generators. In Indian electrical curricula this topic is tested because it connects theory to induction and synchronous machines.
GATE and semester exams often combine special electrical machines with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use special electrical machines?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

• Using electrical degrees where mechanical step angle is required
• Forgetting to divide by steps-per-revolution when converting pulse rate to rpm
• Assuming SRM needs rotor windings or magnets (it has neither)
• Thinking a universal motor reverses on AC (torque direction is preserved)

Quick revision checklist

Before attempting special electrical machines problems, confirm you can:
1. SRM: doubly salient, no rotor winding
2. Linear induction motor for traction and material handling
3. Servo motors: high dynamic response with feedback
Revise the solved examples in Electrical Machines — Nagrath & Kothari and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

Stepper motor speed from pulse rate

Problem

A stepper motor has a step angle of 1.8°. It is driven at 500 pulses per second. Find the steps per revolution and the shaft speed in rpm.

Solution

Steps per revolution = 360/1.8 = 200 steps.
At 500 steps/s, revolutions per second = 500/200 = 2.5 rev/s.
Speed = 2.5 × 60 = 150 rpm.

Conceptual check — Special Electrical Machines

Problem

In a Electrical Machines II semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of special electrical machines." What should a complete answer include?

Exams & GATE

Nagrath & Kothari — compare stepper, BLDC, SRM applications.

📖 Standard books (India)

  • Electrical Machines — Nagrath & Kothari

    Read: Syllabus unit

    Transformers, DC machines, and induction motors

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