Electrical Machines Notes FE Sem 1 BEE | Mumbai University

Module 4 covers the motors that convert electrical energy into mechanical energy in almost every industry and household. For each of the DC motor, the three-phase induction motor, the single-phase induction motor and the brushless DC (BLDC) motor, it explains the principle of operation, constructional details, classification and applications. The syllabus states that numericals are not expected, so the focus is on clear principles, labelled diagrams and comparisons. Understanding these machines helps in selecting the right motor for fans, pumps, lifts, tools and electric vehicles.

Summary

Principle of the DC motor: when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force F = B I L whose direction is given by Fleming's left-hand rule. In a DC motor the armature conductors carry current supplied through brushes and a commutator; the forces on conductors under the N and S poles form a torque that rotates the armature. The commutator reverses the current in each conductor as it passes from one pole to the next, so that the torque is unidirectional. As the armature rotates, its conductors cut the flux and a back EMF Eb = P phi Z N/(60 A) is induced, opposing the supply, with V = Eb + Ia Ra. Back EMF makes the motor self-regulating: armature current adjusts automatically to the load.

Construction of a DC machine: yoke (outer frame of cast iron or steel, providing mechanical support and the return path for flux), pole cores and pole shoes (laminated, carrying field windings and spreading the flux), field winding (produces the main flux), armature core (laminated silicon steel cylinder with slots), armature winding (lap or wave), commutator (copper segments insulated by mica, acting as a mechanical rectifier/inverter), brushes (carbon or graphite, collecting current), and bearings and shaft.

Classification and applications of DC motors: separately excited (field from a separate source); self-excited motors are shunt (field in parallel with armature; nearly constant speed; used in lathes, fans, blowers, machine tools), series (field in series; very high starting torque, speed varies widely with load and must never be started at no load; used in traction, cranes, hoists, electric trains) and compound (both series and shunt fields; cumulative compound gives high starting torque with reasonably constant speed; used in rolling mills, presses, shears, elevators).

Three-phase induction motor principle: a balanced three-phase supply to the three stator windings, displaced 120 degrees in space, produces a rotating magnetic field of constant magnitude rotating at synchronous speed Ns = 120 f/P. This field cuts the stationary rotor conductors and induces EMF and current in them (induction). By Lenz's law the rotor rotates in the same direction as the field to reduce the relative speed. The rotor can never reach Ns, because then there would be no relative motion, no induced current and no torque; the difference is expressed as slip s = (Ns - N)/Ns, typically 2 to 5 percent at full load. Rotor current frequency is s f.

Construction and classification of the three-phase induction motor: the stator is a laminated core with slots carrying a three-phase distributed winding, inside a cast-iron frame. Based on the rotor, motors are (1) squirrel-cage: rotor bars of copper or aluminium short-circuited by end rings, skewed slots; rugged, cheap, almost maintenance-free, moderate starting torque; used for pumps, fans, compressors, lathes, conveyors; and (2) slip-ring (phase-wound): three-phase rotor winding connected to slip rings and external resistances; high starting torque with lower starting current and some speed control; used for cranes, hoists, lifts and heavy loads.

Single-phase induction motor: a single-phase winding produces a pulsating (not rotating) field, which by double field revolving theory can be resolved into two equal fields rotating in opposite directions; at standstill their torques cancel, so the motor is not self-starting. Once rotated in either direction it continues to run. To start it, an auxiliary (starting) winding is added, displaced 90 degrees in space and with a current phase-shifted from the main winding current, to create a rotating field. Types: split-phase (resistance start; fans, washing machines, blowers), capacitor-start (high starting torque; compressors, refrigerators, pumps), capacitor-start capacitor-run (quiet, better pf; ceiling fans, air conditioners), permanent split capacitor (fans, blowers) and shaded-pole (very low starting torque; table fans, hair dryers, small toys).

BLDC motor: a brushless DC motor has a permanent-magnet rotor and a stator carrying a (usually three-phase) winding. The mechanical commutator and brushes are replaced by electronic commutation: Hall-effect sensors (or back-EMF sensing) detect the rotor position and an electronic controller (inverter) switches the stator phase currents in sequence, producing a stepwise rotating field that the magnet rotor follows. Types: inner-rotor and outer-rotor, sensored and sensorless. Advantages: high efficiency, no brush wear or sparking, low noise, high speed, good speed-torque control, long life. Applications: ceiling fans (energy-efficient fans), computer cooling fans, hard disk and CD drives, drones, electric vehicles and e-bikes, medical equipment, HVAC and appliances.

Mind map

Working of a three-phase induction motor

Important definitions

Formulas

Key points to remember

• Numericals are not expected in this module; principle, construction, classification and applications are asked.
• A DC series motor must never be started without load, because with very small current the flux is small and speed rises to dangerous values.
• Back EMF limits armature current; at start Eb = 0, so a starter is needed to limit the starting current of a DC motor.
• The commutator in a DC motor acts as a mechanical inverter, converting DC from the supply into alternating current in the armature conductors.
• The rotor of an induction motor can never run at synchronous speed; slip is never zero for a running induction motor.
• Rotor and stator of induction motors are laminated to reduce eddy current loss.
• Reversing any two supply leads of a three-phase induction motor reverses the direction of rotation.
• A single-phase induction motor is not self-starting; a capacitor or split-phase arrangement gives the starting torque.
• In capacitor-start motors a centrifugal switch disconnects the auxiliary winding at about 70 to 80 percent of synchronous speed.
• BLDC motors replace brushes with electronic commutation, so they need a controller and rotor position information.
• BLDC ceiling fans consume much less power than conventional capacitor-run induction fans, which is why they are promoted for energy saving.

Solved examples

Example 1. (Illustrative, numericals not asked in the MU paper.) A 4-pole, 50 Hz three-phase induction motor runs at 1440 rpm. Find the synchronous speed, slip and rotor frequency.

Ns = 120 f/P = 120 x 50/4 = 1500 rpm.

s = (Ns - N)/Ns = (1500 - 1440)/1500 = 0.04 = 4 percent.

Rotor frequency fr = s f = 0.04 x 50 = 2 Hz.

Example 2. (Illustrative.) A 230 V DC shunt motor has an armature resistance of 0.5 ohm and draws an armature current of 20 A. Find the back EMF. What would the armature current be at the instant of starting if no starter were used?

Eb = V - Ia Ra = 230 - 20 x 0.5 = 220 V.

At the instant of starting N = 0, so Eb = 0 and Ia = V/Ra = 230/0.5 = 460 A, which is 23 times the running current. This shows why a starter (series resistance) is required to limit starting current.

Example 3. (Illustrative.) List the synchronous speeds of 2, 4, 6 and 8 pole induction motors on a 50 Hz supply and explain why a ceiling fan motor has many poles.

Ns = 120 x 50/P: 2 poles = 3000 rpm, 4 poles = 1500 rpm, 6 poles = 1000 rpm, 8 poles = 750 rpm.

A ceiling fan must run slowly (a few hundred rpm) without gears, so its motor is built with a large number of poles; for example a 16-pole motor has Ns = 120 x 50/16 = 375 rpm.

Example 4. (Conceptual.) Explain with reasoning why an induction motor cannot run at synchronous speed.

Torque in an induction motor is produced only when rotor current flows.

Rotor current flows only if EMF is induced in the rotor, which requires relative motion between the rotating field and the rotor conductors.

If the rotor reached Ns, the relative speed would be zero, no EMF and no current would be induced, and the torque would become zero.

The rotor would then slow down due to friction and load, relative motion would reappear and torque would be produced again. Hence the rotor always runs slightly below Ns, with slip of 2 to 5 percent at full load.

Example 5. (Conceptual.) Select a suitable motor for each application with a reason: (a) electric train, (b) lathe, (c) water pump, (d) refrigerator compressor, (e) energy-efficient ceiling fan, (f) crane with heavy starting load.

(a) Electric train: DC series motor, because it gives very high starting torque and is always coupled to the load.

(b) Lathe: DC shunt motor (or squirrel-cage induction motor), because speed remains nearly constant with load.

(c) Water pump: three-phase squirrel-cage induction motor (single-phase capacitor motor for domestic pumps), because it is rugged, cheap and maintenance-free.

(d) Refrigerator compressor: single-phase capacitor-start induction motor, because it needs high starting torque on a single-phase supply.

(e) Energy-efficient ceiling fan: BLDC motor, because of high efficiency and smooth electronic speed control.

(f) Crane: slip-ring induction motor or DC series/compound motor, because it provides high starting torque with limited starting current.

Important questions and answers

Q1. Explain the principle of operation of a DC motor. What is the significance of back EMF? (5 marks)

Principle: a current-carrying conductor placed in a magnetic field experiences a force F = B I L, whose direction is given by Fleming's left-hand rule.

When the armature of a DC motor is supplied through brushes and commutator, the conductors under the N pole carry current in one direction and those under the S pole in the opposite direction. The forces on all conductors act in the same rotational sense and produce torque, so the armature rotates. The commutator reverses the current in each conductor as it moves from one pole to the next, keeping the torque unidirectional.

Back EMF: the rotating conductors cut the flux and an EMF Eb = P phi Z N/(60A) is induced, which opposes the applied voltage (Lenz's law). V = Eb + Ia Ra, so Ia = (V - Eb)/Ra.

Significance: (1) it limits the armature current; (2) it makes the motor self-regulating: when load increases, speed falls slightly, Eb falls, Ia rises and develops more torque, and vice versa; (3) Eb Ia is the electrical power converted to mechanical power.

Q2. Explain the construction of a DC machine with a neat sketch. (6 marks)

1. Yoke: outer frame of cast iron (small machines) or rolled steel; gives mechanical support and carries the magnetic flux between poles.

2. Poles: pole core and pole shoe bolted to the yoke; laminated to reduce eddy losses; the pole shoe spreads the flux and supports the field coils.

3. Field winding: copper coils on the pole cores; when excited, they produce the main flux, with alternate N and S poles.

4. Armature core: cylindrical, laminated silicon steel with slots on the periphery; keyed to the shaft; houses the armature conductors.

5. Armature winding: insulated copper conductors in the slots, connected as lap or wave winding.

6. Commutator: cylinder of hard-drawn copper segments insulated by mica, connected to armature coils; converts the DC supply into alternating current in the conductors so that torque is unidirectional.

7. Brushes: carbon or graphite blocks pressed on the commutator by springs, held in brush holders; connect the external circuit to the rotating armature.

8. Shaft and bearings: support the rotating armature. A labelled cross-sectional sketch should be drawn.

Q3. Classify DC motors and give the characteristics and applications of each. (6 marks)

Separately excited motor: field supplied from an independent source; good speed control; used in rolling mills and drives with wide speed range.

Shunt motor: field connected in parallel with armature; flux nearly constant, so speed is almost constant from no load to full load; medium starting torque. Applications: lathes, drilling machines, fans, blowers, centrifugal pumps, machine tools.

Series motor: field in series with armature; torque proportional to Ia^2 before saturation, so very high starting torque; speed falls sharply with load and becomes dangerously high at no load. Applications: electric traction, cranes, hoists, trolleys, vacuum cleaners (universal motor version).

Compound motor: has both shunt and series fields. Cumulative compound combines high starting torque with fairly constant speed. Applications: presses, shears, rolling mills, elevators, conveyors. Circuit diagrams of each connection should be drawn.

Q4. Explain the working principle of a three-phase induction motor. Why is it called an asynchronous motor? (6 marks)

When a balanced three-phase supply is given to the stator winding (three windings displaced 120 degrees in space), it produces a rotating magnetic field of constant magnitude rotating at synchronous speed Ns = 120 f/P.

This field cuts the stationary rotor conductors and induces EMF in them by Faraday's law. Since the rotor conductors are short-circuited (end rings or external resistance), currents flow in them.

These current-carrying conductors lie in the magnetic field of the stator and experience force, producing torque. By Lenz's law the rotor rotates in the direction of the rotating field so as to reduce the relative speed which causes the induced current.

The rotor speed N is always less than Ns, since at Ns there would be no relative motion and no torque. The difference is the slip s = (Ns - N)/Ns.

Because the rotor never runs at synchronous speed, it is called an asynchronous motor; because rotor current is produced by induction (no electrical connection), it is called an induction motor.

Q5. Compare squirrel-cage and slip-ring induction motors. (5 marks)

Rotor: squirrel-cage has bars short-circuited by end rings; slip-ring has a three-phase insulated winding connected to slip rings.

Construction: squirrel-cage is simple, robust and cheap; slip-ring is complicated and costly.

External resistance: cannot be added to a squirrel-cage rotor; can be added to a slip-ring rotor through brushes.

Starting torque: moderate for squirrel-cage; high for slip-ring (with rotor resistance) with lower starting current.

Maintenance: squirrel-cage almost none; slip-ring needs maintenance of brushes and slip rings.

Efficiency: squirrel-cage higher; slip-ring slightly lower due to rotor resistance and brush losses.

Applications: squirrel-cage for pumps, fans, blowers, lathes, compressors; slip-ring for cranes, hoists, lifts, elevators and loads needing high starting torque.

Q6. Why is a single-phase induction motor not self-starting? Explain how it is made self-starting. (6 marks)

A single-phase winding produces a pulsating field that alternates along one axis but does not rotate. By the double field revolving theory, this pulsating field of maximum value phi_m can be resolved into two fields of phi_m/2 rotating in opposite directions at synchronous speed. At standstill each induces equal rotor currents and develops equal and opposite torques, so the net starting torque is zero and the motor does not start. If the rotor is given a push in either direction, the torque due to the field rotating in that direction becomes larger and the motor continues to run.

To make it self-starting, a rotating field must be created at start. An auxiliary (starting) winding is placed 90 electrical degrees apart from the main winding, and its current is made to differ in phase from the main winding current by using higher resistance (split-phase), a capacitor in series (capacitor-start or capacitor-run), or shading coils on the poles (shaded-pole). The two phase-displaced currents in space-displaced windings produce a rotating field which gives starting torque. In split-phase and capacitor-start motors a centrifugal switch disconnects the auxiliary winding at about 75 percent of synchronous speed.

Q7. Explain the types of single-phase induction motors with their applications. (5 marks)

1. Split-phase (resistance-start): auxiliary winding of high resistance and low reactance, phase difference about 20 to 30 degrees; low to moderate starting torque; used in washing machines, fans, blowers, small pumps, grinders.

2. Capacitor-start: capacitor in series with the auxiliary winding gives nearly 90 degrees phase difference and high starting torque; auxiliary winding cut out by centrifugal switch; used in refrigerators, compressors, air conditioners, pumps.

3. Capacitor-start capacitor-run (two-value capacitor): large capacitor for starting, smaller one remains in circuit; high starting torque, quiet running, better power factor; used in ceiling fans, air conditioners, compressors.

4. Permanent split capacitor: one capacitor permanently in series with auxiliary winding; moderate starting torque, smooth running; used in fans, blowers, room coolers.

5. Shaded-pole: part of each pole is surrounded by a copper shading ring causing a lagging flux; very low starting torque, low efficiency, cheap; used in table fans, exhaust fans, hair dryers, record players and toys.

Q8. Explain the construction, working and applications of a BLDC motor. (6 marks)

Construction: the stator is a laminated core with a three-phase (star-connected) winding, similar to an induction motor stator. The rotor carries permanent magnets (ferrite or rare-earth such as neodymium) forming alternate N and S poles. Hall-effect sensors mounted on the stator sense rotor position. An electronic controller (three-phase inverter of MOSFETs or IGBTs) supplies the windings from a DC source. Rotor may be inside (inner rotor) or outside the stator (outer rotor, as in ceiling fans).

Working: depending on the rotor position signalled by the Hall sensors, the controller energises two stator phases at a time in a fixed sequence of six steps. The energised winding produces a field that attracts the rotor magnet; just before alignment, the controller switches to the next phase pair, so the stator field keeps moving ahead of the rotor and produces continuous torque. Speed is controlled by varying the applied voltage or PWM duty cycle. This electronic commutation replaces the brushes and commutator of a conventional DC motor.

Advantages: high efficiency, no brush wear or sparking, low maintenance, low noise, high power-to-weight ratio, precise speed control, long life. Disadvantages: needs an electronic controller, higher cost.

Applications: energy-efficient ceiling fans, computer fans, hard disk drives, drones, electric vehicles, e-bikes, washing machines, air conditioners, medical and aerospace equipment.

Q9. Compare a conventional DC motor with a BLDC motor. (4 marks)

Commutation: mechanical (brushes and commutator) in DC motor; electronic in BLDC.

Rotor: wound armature in DC motor; permanent magnet in BLDC.

Field: produced by stator field winding or magnets in DC motor; by stator winding in BLDC (magnets on rotor).

Maintenance: brushes wear and need replacement in DC motor; practically none in BLDC.

Sparking and noise: present in DC motor; absent in BLDC.

Efficiency and life: lower and shorter for DC motor; higher and longer for BLDC.

Control: simple voltage control for DC motor; BLDC needs a controller with rotor position sensing.

Cost: DC motor cheaper initially; BLDC costlier but saves energy.

Q10. Define slip and explain its significance in an induction motor. (3 marks)

Slip is the difference between synchronous speed and rotor speed expressed as a fraction (or percentage) of synchronous speed: s = (Ns - N)/Ns.

Significance: slip decides the relative speed between the rotating field and the rotor, and hence the rotor EMF, rotor current, rotor frequency (fr = s f) and torque. At standstill s = 1; at no load s is very small (about 1 percent); at full load s is about 2 to 5 percent. Slip can never be zero for a motor because then no torque would be developed.

Exam tips

• For each motor, structure your answer as principle, construction (with sketch), classification, and applications; this matches the syllabus wording and the marking scheme.
• Always mention Fleming's left-hand rule for DC motors and the rotating magnetic field with Ns = 120 f/P for induction motors.
• Prepare comparison tables: shunt vs series vs compound, squirrel-cage vs slip-ring, DC motor vs BLDC.
• Learn two or three applications for every motor type; examiners frequently ask 'give applications'.
• BLDC is a newer topic; be ready with construction, electronic commutation and advantages.
• Draw circuit diagrams of shunt, series and compound connections and of capacitor-start motors with the centrifugal switch.

FAQs

Are numericals on motors asked in FE BEE?

No. The MU syllabus states that numericals are not expected for this module. Questions focus on principle of operation, construction, classification and applications.

Why does a DC series motor need to be started on load?

At no load its armature current and therefore flux are very small, and speed is inversely proportional to flux, so the speed rises to dangerously high values that can damage the motor.

What is the difference between an induction motor and a BLDC motor?

An induction motor's rotor current is induced by the rotating field and the rotor runs with slip; it works directly from AC mains. A BLDC motor has a permanent-magnet rotor that runs in step with an electronically switched stator field, needs a controller and DC supply, and has no slip and higher efficiency.

Why are ceiling fans now made with BLDC motors?

BLDC fans consume much less power than conventional capacitor-run induction-motor fans of the same size (commonly about half or less), offer smooth electronic speed control and run quietly, so they save electricity.

These notes follow the University of Mumbai first-year engineering syllabus (NEP 2020, Basic Electrical & Electronics Engineering) and were prepared with AI help and checked.