11.3 DC Motors — Series, Shunt, Compound & Starter-Generators
Key Takeaways
- Motor torque depends on flux and armature current (T ∝ Φ Ia); speed relates to back EMF and flux; reverse field or armature current to reverse rotation
- Series motors: high starting torque, speed rises as load falls — never run unloaded; used for starters and actuators needing strong breakaway torque
- Shunt motors: roughly constant speed with load; moderate starting torque — good for fans, pumps, and steady-speed drives
- Compound motors combine series and shunt fields for tailored torque–speed behaviour (cumulative vs differential)
- Aircraft starter-generators crank as series-like DC motors, then switch to generate mode to supply the DC bus after light-off
11.3 DC Motors — Series, Shunt, Compound & Starter-Generators
Quick Answer: Torque T ∝ Φ Ia. Back EMF Eb grows with speed and flux; Ia = (V − Eb)/Ra. Reverse field or armature current to reverse rotation. Series = high starting torque, runaway risk off-load. Shunt ≈ constant speed. Compound mixes both. Starter-generators motor to start the engine, then generate ~28 V DC.
This section finishes topic 3.12 with motor performance factors, the classic winding types, comparison tables you should memorise, and the aircraft starter-generator that links motor and generator theory in one LRUs.
Back EMF — The Motor’s Self-Regulation
As soon as a DC motor rotates, armature conductors cut field flux and induce a back EMF (counter EMF) Eb that opposes supply voltage (Lenz). Armature current becomes:
Ia = (V − Eb) / Ra
| Condition | Eb | Ia behaviour |
|---|---|---|
| Just starting (n = 0) | Eb ≈ 0 | Ia ≈ V/Ra — very large; needs limiting |
| Accelerating | Eb rises | Ia falls |
| Steady speed | Eb just below V | Ia matches torque demand |
| Heavy mechanical load | Speed dips → Eb falls | Ia rises → more torque |
Worked example 1. V = 28 V, Ra = 0.05 Ω, at speed Eb = 26 V. Ia = (28 − 26)/0.05 = 40 A. At standstill with no starter resistor, Ia = 28/0.05 = 560 A — why large motors need starting resistance or current-limited supplies.
Factors Affecting Power, Torque, Speed, and Direction
Torque and power
| Quantity | Dependence (Module 3 form) | Notes |
|---|---|---|
| Torque T | T ∝ Φ Ia | More flux or more armature current → more torque |
| Electrical input | ≈ V Itotal | Supply loading |
| Mechanical power | ≈ Eb Ia (idealised developed power) | Or T × ω on the shaft |
Worked example 2. If Φ is constant and Ia doubles under load, torque roughly doubles (before saturation / armature-reaction limits).
Speed
From V = Eb + Ia Ra and Eb ∝ Φ n:
n ∝ Eb / Φ ≈ (V − Ia Ra) / Φ
| Increase… | Typical speed effect (qualitative) |
|---|---|
| Supply voltage V | Speed tends to rise |
| Field flux Φ | Speed tends to fall (for given V) — field weakening raises speed |
| Load torque | Speed drops slightly or more, depending on motor type |
Direction of rotation
From Fleming’s left-hand rule, reverse either field polarity or armature current → reverse force → reverse rotation. Reverse both → same direction.
| Method | How |
|---|---|
| Reverse armature leads | Swap A-circuit polarity |
| Reverse field leads | Swap F-circuit polarity |
| Series motor caveat | Field and armature are in series — reversing supply alone does not reverse rotation; you must reverse one winding relative to the other |
Series, Shunt, and Compound — Construction of Connections
| Type | Field connection | Field current vs armature |
|---|---|---|
| Series | Field in series with armature | Same current; few turns of heavy wire |
| Shunt | Field in parallel with armature | Field current ≈ V/Rf; many turns of fine wire |
| Compound | Both series and shunt fields on poles | Combines characteristics |
Series motor characteristics
| Feature | Behaviour |
|---|---|
| Starting torque | Very high — at start Ia (and series field current) are large → Φ and Ia both large → T ∝ Φ Ia huge |
| Speed vs load | Speed falls as load rises; speed rises dangerously if load is removed (Φ collapses as current falls) |
| Off-load rule | Do not run series motors unloaded (belt failure / unloaded starter) — runaway risk |
| Typical uses | Engine starters, some actuators, traction-type loads needing breakaway torque |
Worked scenario. A series starter cranks a high-compression engine: locked-rotor current builds massive torque. Once running unloaded on the bench without a load, speed can climb until centrifugal damage — hence the prohibition.
Shunt motor characteristics
| Feature | Behaviour |
|---|---|
| Starting torque | Moderate (field already at roughly full Φ from supply; Ia limited by starter if fitted) |
| Speed vs load | Nearly constant speed; small drop as load increases |
| Field loss | If shunt field opens while running, Φ collapses → speed may race — protection matters |
| Typical uses | Fans, pumps, blowers, machine tools, steady-speed accessories |
Compound motor characteristics
| Subtype | Series + shunt sense | Effect |
|---|---|---|
| Cumulative compound | Series field aids shunt field | Good starting torque + reasonably flat speed curve |
| Differential compound | Series field opposes shunt | Rare for drives; speed can rise with load — usually avoided |
Aircraft and industrial training emphasise cumulative compound when a blend of series starting punch and shunt speed stability is needed.
Comparison Table — Motor Types (Memorise)
| Property | Series | Shunt | Cumulative compound |
|---|---|---|---|
| Starting torque | Highest | Moderate | High (between series and shunt, design-dependent) |
| Speed regulation | Poor (varies widely with load) | Good (fairly constant) | Fair to good |
| No-load speed | Dangerously high | Safe finite speed | Safe if shunt field present |
| Field wire | Few turns, thick | Many turns, thin | Both |
| Reversing supply only | Does not reverse rotation | Reverses rotation | Depends — reverse armature or both fields per procedure |
| Best for | Starters, high breakaway torque | Constant-speed loads | Mixed torque + speed needs |
Starter-Generators (Aircraft)
A starter-generator combines a DC motor and DC generator in one machine on the engine accessory gearbox.
Construction (typical)
| Element | Role in starter-generator |
|---|---|
| Armature & commutator | Shared rotating member |
| Series field windings | Heavy-current path for start torque |
| Shunt / control fields | Excitation and voltage control in generate mode |
| Brushes | Carry hundreds of amps during start; continuous generate current later |
| Clutch / drive / spline | Couples to engine; may include overrunning or drive features per type |
| Start contactor / generate contactor / GCU | Switches windings and connects to battery vs bus; regulates voltage |
Dual-mode operation
| Mode | When | Energy flow | Electrical behaviour |
|---|---|---|---|
| Start (motor) | Engine cranking | Battery/APU electrical → mechanical cranking | Series (or series-dominant) connection; very high current; limited duty time |
| Generate | After self-sustaining speed / light-off | Mechanical drive → electrical to bus/battery | Field controlled by Generator Control Unit (GCU); output ~28 V DC typical |
Worked sequence.
- Start switch → battery power through series field + armature → high torque cranks turbine.
- Engine lights and accelerates past starter cut-out speed.
- Start circuit drops out; generate contactors/GCU enable generation.
- Same machine now driven by the engine supplies DC bus and recharges battery.
Maintenance angles tied to theory. Brush wear from high start current, commutator condition, series field integrity, GCU regulation, and correct residual magnetism/polarity for generate build-up all map back to §§11.1–11.2.
Power, Torque, Speed — Quick Decision Table
| Goal | Typical action / type choice |
|---|---|
| Maximum breakaway torque | Series or cumulative compound; ensure start current path intact |
| Steady RPM under varying load | Shunt or cumulative compound |
| Raise running speed (field control) | Weaken field (Φ ↓) carefully — watch commutation and overspeed |
| Reverse rotation | Reverse armature or field (not both); follow series-motor relative reverse rule |
| One unit for start + DC power | Starter-generator with proper switching/GCU |
Exam Focus Checklist
- Write Ia = (V − Eb)/Ra and explain high starting current.
- State T ∝ Φ Ia and n ∝ (V − IaRa)/Φ.
- Compare series / shunt / compound with the table above.
- Explain why series motors must not run unloaded.
- Describe starter-generator motor-then-generate sequence on aircraft.
Topic 3.12 is complete when you can move fluently from Fleming and commutation, through generator EMF and armature reaction, to motor types and starter-generators — the DC rotating-machine core of Module 3 before AC theory begins.
At the instant a DC motor is energised from rest, why is armature current often very large?
Which DC motor type develops the highest starting torque and must not be run without load?
Compared with a series motor, a shunt DC motor’s speed–load behaviour is best described as:
In a typical aircraft starter-generator, what happens after the engine reaches self-sustaining speed?