Learning outcomes
Motor family map
AC induction and synchronous
3-phase cageSlip-ringSplit-phaseCapacitor-startPSCCSCRShaded-poleSynchronous1. Common motor principles
Force and torque
A current-carrying conductor in a magnetic field experiences a force. In a motor, forces on opposite rotor conductors form a turning moment called torque.
Back EMF
As a DC armature rotates, it generates an EMF opposing the supply. At start, back EMF is zero, so starting current must be limited by winding resistance or a controller.
Rotating magnetic field
Balanced three-phase currents produce a field that rotates at synchronous speed. In an induction motor, the rotor must lag behind this field so that current is induced.
Losses
Motor losses include stator and rotor copper loss, iron loss, friction, windage and stray-load loss. Efficiency is mechanical output power divided by electrical input power.
2. Main parts
Brushed DC motor cutaway
Presentation infographic image embedded for clearer classroom display.
Parts and functions
- Stator / field system
- Produces the stationary magnetic field using wound poles or permanent magnets.
- Armature / rotor
- Carries current and turns inside the field.
- Commutator
- Mechanical switching device that reverses current in armature coils.
- Brushes
- Stationary carbon contacts that carry current to the commutator.
- Shaft and bearings
- Transmit torque and keep the rotor centred with a small air gap.
- Frame, fan and enclosure
- Provide mechanical protection and remove heat.
3. Motor models: construction, behaviour and applications
DC Series Motor
Presentation infographic image embedded for clearer classroom display.
Construction
- Field winding has a few turns of thick conductor and is connected in series with the armature.
- The same current flows through the series field, brushes, commutator and armature.
- Laminated iron reduces eddy-current losses; carbon brushes transfer current to the rotating commutator.
Operating characteristics
- Starting torque: very high.
- Speed regulation: poor; speed rises sharply as load falls.
- Never run a large series motor uncoupled from its load.
Typical applications
- Electric traction and older cranes/hoists
- Automotive starter motors
- Heavy-duty winches
DC Shunt Motor
Presentation infographic image embedded for clearer classroom display.
Construction
- Shunt field has many turns of fine wire and is connected in parallel with the armature.
- Field current is relatively small and nearly constant.
- Brushes and commutator reverse armature current at the correct rotor position.
Operating characteristics
- Starting torque: moderate.
- Speed regulation: good over a normal load range.
- Speed can be adjusted by armature-voltage or field control.
Typical applications
- Machine tools and lathes
- Fans and centrifugal pumps
- Conveyors requiring steady speed
DC Compound Motor
Presentation infographic image embedded for clearer classroom display.
Construction
- Contains both a series field and a shunt field.
- In cumulative compound motors the two magnetic fields assist each other.
- In differential compound motors the fields oppose; this arrangement is uncommon because it can be unstable.
Operating characteristics
- Cumulative type gives strong starting torque.
- Speed regulation is better than a series motor.
- Can be short-shunt or long-shunt depending on the shunt-field connection.
Typical applications
- Presses and shears
- Elevators and rolling mills
- Conveyors with changing load
Separately Excited DC Motor
Shared comparison infographic embedded for the separately excited, PMDC and BLDC sections.
Construction
- Armature and field winding are supplied from separate DC sources or converter outputs.
- Independent field current allows direct control of magnetic flux.
- Usually includes a commutator and brushes unless implemented as an electronic drive system.
Operating characteristics
- Good speed regulation.
- Constant-torque region below base speed and field-weakening region above base speed.
- Requires more control equipment than a simple shunt motor.
Typical applications
- Industrial variable-speed drives
- Test rigs and dynamometers
- Older paper, steel and printing lines
Permanent-Magnet DC Motor (PMDC)
Shared comparison infographic embedded for the separately excited, PMDC and BLDC sections.
Construction
- Permanent magnets create the stator field, so no field winding or field copper loss is required.
- Rotor is a wound armature with commutator and brushes.
- Commonly built for compact, low-voltage operation.
Operating characteristics
- Good efficiency at small ratings.
- Simple speed control using pulse-width modulation (PWM).
- Brushes wear and magnets limit overload/temperature capability.
Typical applications
- Vehicle wipers and seat adjusters
- Battery tools and toys
- Small pumps, actuators and robotics
Brushless DC Motor (BLDC)
Shared comparison infographic embedded for the separately excited, PMDC and BLDC sections.
Construction
- Permanent-magnet rotor and multi-phase stator windings.
- An electronic controller switches the stator phases; Hall sensors or sensorless back-EMF detection may give rotor position.
- No mechanical commutator or carbon brushes.
Operating characteristics
- High efficiency and power density.
- Low maintenance and good speed control.
- Requires an electronic drive and correct commutation sequence.
Typical applications
- Computer and ventilation fans
- E-bikes, drones and EV auxiliaries
- Modern pumps, compressors and appliances
Three-Phase Squirrel-Cage Induction Motor
Presentation infographic image embedded for clearer classroom display.
Construction
- Laminated stator contains three windings spaced 120 electrical degrees apart.
- Rotor consists of conductive bars short-circuited by end rings, forming a cage.
- No electrical connection is made to the rotor; bearings support the shaft and a fan provides cooling.
Operating characteristics
- Self-starting from a three-phase supply.
- Rotor must run below synchronous speed to induce current and torque.
- Rugged, efficient and low maintenance.
Typical applications
- Pumps, fans and compressors
- Conveyors and machine tools
- Most fixed-speed industrial drives
Wound-Rotor / Slip-Ring Induction Motor
Presentation infographic image embedded for clearer classroom display.
Construction
- Stator is similar to a three-phase induction motor.
- Rotor has a three-phase winding connected to slip rings and brushes.
- External resistors can be connected to the rotor circuit during starting.
Operating characteristics
- High starting torque with limited starting current.
- External resistance can give staged starting and some speed control.
- More maintenance than a cage motor because of brushes and slip rings.
Typical applications
- Cranes, hoists and lifts
- Crushers and large conveyors
- High-inertia loads
Split-Phase Induction Motor
Shared single-phase AC motor reference chart embedded for this family of motors.
Construction
- Main winding and auxiliary start winding are displaced in the stator.
- Start winding has higher resistance to create a phase difference.
- A centrifugal switch or electronic relay disconnects the start winding at about 70–80% speed.
Operating characteristics
- Moderate starting torque.
- Simple and relatively inexpensive.
- Not suitable for very heavy starting loads.
Typical applications
- Small grinders and drills
- Light-duty fans and blowers
- Small workshop machines
Capacitor-Start Induction-Run Motor
Shared single-phase AC motor reference chart embedded for this family of motors.
Construction
- A start capacitor is connected in series with the auxiliary winding.
- Capacitor produces a larger phase displacement and stronger rotating field during starting.
- Centrifugal switch disconnects start winding and capacitor after acceleration.
Operating characteristics
- High starting torque.
- Runs on the main winding after the start circuit opens.
- Start capacitor is intermittent-duty and must not remain energised.
Typical applications
- Refrigeration compressors
- Pumps and air compressors
- Machines with difficult starting loads
Permanent Split Capacitor (Capacitor-Run) Motor
Shared single-phase AC motor reference chart embedded for this family of motors.
Construction
- Run capacitor and auxiliary winding remain connected continuously.
- Usually no centrifugal switch is required.
- Capacitor is continuous-duty, commonly metallised film construction.
Operating characteristics
- Lower starting torque than capacitor-start type.
- Smooth, quiet running and improved power factor.
- Speed control is possible with suitable electronic controllers.
Typical applications
- HVAC fans and blowers
- Room air conditioners
- Small pumps and office equipment
Capacitor-Start Capacitor-Run Motor
Shared single-phase AC motor reference chart embedded for this family of motors.
Construction
- Uses a large start capacitor and a smaller continuous-duty run capacitor.
- Start capacitor is disconnected after acceleration; run capacitor remains in circuit.
- Auxiliary winding operates during starting and running.
Operating characteristics
- Very good starting torque.
- Good running power factor, efficiency and smoothness.
- Higher cost and more components than PSC or capacitor-start types.
Typical applications
- Larger refrigeration and air-conditioning compressors
- Pumps and woodworking machines
- Loads needing both strong starting and quiet running
Shaded-Pole AC Motor
Shared single-phase AC motor reference chart embedded for this family of motors.
Construction
- Each stator pole has a copper shading ring around part of the pole face.
- Current induced in the shading ring delays flux in the shaded section.
- The moving flux across the pole face produces weak starting torque in one fixed direction.
Operating characteristics
- Very low starting torque and low efficiency.
- Cheap, reliable and quiet at small sizes.
- Direction is usually fixed by the position of the shading rings.
Typical applications
- Small desk and extractor fans
- Microwave and refrigerator fans
- Small timing or display mechanisms
Synchronous Motor
Presentation infographic image embedded for clearer classroom display.
Construction
- Three-phase stator creates a rotating magnetic field.
- Rotor field is produced by DC excitation, permanent magnets or reluctance saliency.
- Large wound-field machines may use slip rings, a brushless exciter or damper bars.
Operating characteristics
- At steady state the rotor locks to the stator field and runs with zero slip.
- Standard wound-field types are not inherently self-starting without assistance.
- Can operate at leading power factor and support power-factor correction.
Typical applications
- Constant-speed large compressors and mills
- High-efficiency permanent-magnet drives
- Power-factor correction in large plants
Universal Motor
Presentation infographic image embedded for clearer classroom display.
Construction
- Series field and armature are connected in series, as in a DC series motor.
- Field core and armature are laminated to reduce AC iron losses.
- Uses brushes and a commutator.
Operating characteristics
- High starting torque and very high no-load speed.
- Can operate from single-phase AC or DC because field and armature current reverse together.
- Noisy, causes brush wear and may require interference suppression.
Typical applications
- Hand drills, vacuum cleaners and mixers
- Hair dryers and portable tools
- Small appliances requiring high speed
Stepper Motor
Presentation infographic image embedded for clearer classroom display.
Construction
- Multi-phase stator windings surround a toothed permanent-magnet, variable-reluctance or hybrid rotor.
- Electronic driver energises phases in a timed sequence.
- Often operated open-loop, but position feedback may be added.
Operating characteristics
- Accurate incremental positioning at low-to-medium speed.
- High holding torque when energised.
- Can lose steps if load torque exceeds available torque.
Typical applications
- 3D printers and CNC positioning
- Instrument drives and valves
- Robotics and office machines
4. Quick comparison
| Motor | Starting torque | Speed behaviour | Maintenance | Best suited to |
|---|---|---|---|---|
| DC series | Very high | Poor regulation; overspeed risk at no load | Brushes/commutator | Traction and heavy starting |
| DC shunt | Moderate | Nearly constant | Brushes/commutator | Steady-speed machinery |
| DC compound | High | Better than series | Brushes/commutator | Changing heavy loads |
| BLDC | High with controller | Wide controlled range | Low | Efficient variable-speed drives |
| 3-phase cage induction | Moderate to high by design | Below synchronous speed | Very low | General industrial duty |
| Slip-ring induction | High and controllable | Below synchronous speed | Higher | High-inertia starting |
| Capacitor-start | High | Near-constant under normal load | Switch/capacitor | Single-phase compressors and pumps |
| PSC | Low to moderate | Smooth and quiet | Low | Fans and HVAC |
| Shaded-pole | Very low | Fixed, load-dependent | Very low | Tiny low-cost fans |
| Synchronous | Needs starting method | Exactly synchronous | Depends on rotor | Constant speed / high efficiency |
| Universal | High | Very high, load-dependent | Brushes/commutator | Portable appliances |
5. Level 3 calculations
Synchronous speed
50 Hz, 4-pole motor:
Ns = 120 × 50 ÷ 4 = 1500 r/minA 2-pole motor would have 3000 r/min synchronous speed.
Slip
If the rotor runs at 1440 r/min:
s = (1500 − 1440) ÷ 1500 × 100 = 4%This slip is needed to induce rotor current.
Mechanical output
Torque 25 N·m at 1440 r/min:
P = 2πNT ÷ 60 ≈ 3.77 kWN is speed in r/min and T is torque in N·m.
6. Reading a motor nameplate
Teaching example only — always use the actual manufacturer’s nameplate and instructions.
- 230/400 V Δ/Y
- Each winding is designed for about 230 V. On a 400 V line-to-line supply, connect in star for direct-on-line operation unless the manufacturer specifies otherwise.
- 1440 min⁻¹
- Typical loaded speed of a 4-pole, 50 Hz induction motor; synchronous speed is 1500 r/min.
- S1 duty
- Continuous operation at a steady load until thermal equilibrium.
- IP55
- Protected against dust ingress sufficient to prevent harmful deposits and against water jets.
- Class F
- Insulation thermal class; actual permitted temperature rise depends on design and ambient conditions.
- IE3
- Efficiency class stated by the manufacturer for the rated operating point.
7. Motor-selection checklist
8. Flash-card revision
Use the cards to revise key motor terms, construction details, operating characteristics and applications. Select the card or press the Flip card button to reveal the answer.
Keyboard: Left/Right arrows change card; Space flips the card.
9. Self-marked knowledge check — 30 questions
Select one answer for each question. The first answer is recorded. The correct answer and a clear explanation appear immediately.
1 Question 1
Which part of a brushed DC motor mechanically reverses the current in the armature coils?
2 Question 2
Why must a large DC series motor never be operated with no mechanical load?
3 Question 3
What is the main operating advantage of a DC shunt motor?
4 Question 4
How is torque produced in a three-phase squirrel-cage induction motor?
5 Question 5
What is the synchronous speed of a 50 Hz, four-pole motor?
6 Question 6
Why does a loaded induction motor normally run below synchronous speed?
7 Question 7
Which single-phase motor disconnects its start capacitor after the motor accelerates?
8 Question 8
Which motor normally has very low starting torque and is commonly used for very small fans?
9 Question 9
Which component connects a wound rotor to external starting resistance?
10 Question 10
At normal steady-state operation, a synchronous motor runs:
11 Question 11
What replaces the brushes and commutator in a brushless DC motor?
12 Question 12
Why can a universal motor operate from either AC or DC?
13 Question 13
What field windings are fitted to a DC compound motor?
14 Question 14
What produces the main magnetic field in a permanent-magnet DC motor?
15 Question 15
What does “separately excited” mean in a DC motor?
16 Question 16
Why are motor stator and rotor cores laminated?
17 Question 17
What is the normal function of a centrifugal switch in a single-phase motor?
18 Question 18
What happens to the capacitor in a permanent split capacitor motor during normal running?
19 Question 19
What distinguishes a capacitor-start capacitor-run motor?
20 Question 20
In a balanced three-phase motor, the stator phase windings are displaced by:
21 Question 21
How are the rotor bars of a squirrel-cage rotor connected?
22 Question 22
What is the synchronous speed of a 50 Hz, six-pole motor?
23 Question 23
A four-pole motor has Nₛ = 1500 r/min and runs at 1440 r/min. What is its slip?
24 Question 24
A motor produces 10 N·m at 1440 r/min. Approximately what mechanical output power does it produce?
25 Question 25
Which motor is suitable when very high starting torque and adjustable lower starting current are required?
26 Question 26
Which motor is commonly used in portable electric drills and vacuum cleaners?
27 Question 27
How does a stepper motor move its shaft?
28 Question 28
What is the main function of motor bearings?
29 Question 29
Why is the starting current of a brushed DC motor normally high?
30 Question 30
Why does a squirrel-cage induction motor generally require less routine maintenance than a slip-ring motor?
