DC Machines for SSC JE Electrical: DC Generators and Motors, EMF Equation, Types, Speed Control, Starters and Losses
Rahul Kumar
SSC Exam Expert & Content Editor
In this note
- 1.Construction of a DC Machine
- 2.Armature Windings
- 3.EMF Equation of a Generator
- 4.Types of DC Generators
- 5.DC Motor: Back EMF, Torque and Speed
- 6.Types of DC Motors
- 7.Speed Control of DC Motors
- 8.Starters
- 9.Armature Reaction and Commutation
- 10.Losses and Efficiency
- 11.Solved Examples
- 12.Quick Tips
- 13.Quick Revision Points
- 14.Practice Questions With Answers
- 15.How to Practise This Chapter
- 16.Frequently Asked Questions
PDF Notes: DC Machines for SSC JE Electrical: DC Generators and Motors, EMF Equation, Types, Speed Control, Starters and Losses
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DC machines are a core subject in SSC JE Electrical. The questions come from the construction, the EMF and torque equations, the characteristics of motors, the speed control and the starters. This guide on DC machines for SSC JE covers all of them, with tables, solved examples, practice questions and FAQs.
Construction of a DC Machine
| Part | Function |
|---|---|
| Yoke | The outer frame; gives mechanical support and carries the magnetic flux; made of cast iron or cast steel |
| Field poles and field winding | Produce the main magnetic field |
| Armature core and winding | The rotating part where EMF is induced; the core is laminated to reduce the eddy current loss |
| Commutator | Converts the AC induced in the armature into DC in a generator (and the reverse in a motor); made of hard-drawn copper segments |
| Brushes | Carbon or graphite; collect the current from the commutator |
| Interpoles | Small poles between the main poles to improve the commutation |
| Compensating winding | Placed in the pole faces to neutralise the armature reaction in large machines |
Armature Windings
| Winding | Number of Parallel Paths (A) | Use |
|---|---|---|
| Lap winding | A = P (number of poles) | Low voltage, high current machines |
| Wave winding | A = 2 | High voltage, low current machines |
EMF Equation of a Generator
Eg = P φ Z N / (60 A), where P is the number of poles, φ is the flux per pole, Z is the total number of armature conductors, N is the speed in rpm and A is the number of parallel paths.
Types of DC Generators
| Type | Field Winding | Features and Uses |
|---|---|---|
| Separately excited | Supplied from an external source | Stable output; used where wide voltage control is needed (testing, Ward-Leonard) |
| Shunt generator | Field in parallel with the armature | Voltage drops slightly with load; used for battery charging and general lighting |
| Series generator | Field in series with the armature | Voltage increases with load; used as boosters |
| Compound generator (cumulative) | Both shunt and series fields | Flat voltage; used for the supply to the motors and lighting; short-shunt and long-shunt connections |
- A self-excited generator builds up its voltage from the residual magnetism; the field resistance must be less than the critical resistance.
- The characteristics: open circuit (magnetisation) characteristic, internal and external characteristics.
- Voltage regulation of a generator = (no-load voltage − full-load voltage)/full-load voltage.
DC Motor: Back EMF, Torque and Speed
| Quantity | Formula |
|---|---|
| Back EMF | Eb = V − IaRa = PφZN/(60A) |
| Armature current | Ia = (V − Eb)/Ra |
| Torque | Ta = (P Z φ Ia)/(2πA) ∝ φ Ia |
| Speed | N ∝ Eb/φ |
| Mechanical power developed | Pm = Eb Ia = ω T |
The back EMF acts like a governor: when the load increases, the speed falls, the back EMF falls, the armature current rises and the torque rises to meet the load. At the start, Eb = 0, so the starting current is very high (V/Ra), and a starter is needed.
Types of DC Motors
| Motor | Torque and Speed | Uses |
|---|---|---|
| Shunt motor | T ∝ Ia; nearly constant speed (speed regulation 5–10%); medium starting torque | Lathes, fans, blowers, centrifugal pumps, machine tools |
| Series motor | T ∝ Ia²; very high starting torque; speed varies widely with load; never run at no load (dangerous over-speed) | Electric traction, cranes, hoists, trolley cars |
| Cumulative compound motor | High starting torque with a limited no-load speed | Elevators, rolling mills, presses, shears |
| Differential compound motor | Poor starting torque; nearly constant speed (rarely used) | – |
Speed Control of DC Motors
| Method | Details |
|---|---|
| Flux (field) control | A rheostat in the shunt field; speed rises as the flux decreases; gives speeds above the base (rated) speed |
| Armature (rheostatic) control | Resistance in series with the armature; speed below the base speed; losses are high |
| Voltage control | Ward-Leonard system: variable armature voltage from a motor–generator set; smooth control in both directions; used in rolling mills and lifts |
| Series–parallel control | For traction motors |
Starters
- The starter adds resistance in series with the armature at the start and reduces it as the motor speeds up.
- Three-point starter (for shunt and compound motors) has the terminals L, A and F; it has a no-volt release coil and an overload release; the drawback is that the no-volt coil is in series with the field.
- Four-point starter has a separate connection for the no-volt coil, so the speed can be raised by the field control without releasing the coil.
- Series motors are started with a series rheostat starter or directly under load.
Armature Reaction and Commutation
- Armature reaction is the effect of the armature flux on the main field flux; it distorts the flux (cross-magnetising effect) and weakens it (demagnetising), and shifts the magnetic neutral axis.
- Its effects are reduced by interpoles, compensating windings and the shifting of the brushes.
- Commutation is the process of the reversal of the current in a coil when it passes the brush; poor commutation causes sparking.
- Improvements: interpoles, resistance commutation (carbon brushes) and the correct brush shift.
Losses and Efficiency
| Loss | Details |
|---|---|
| Copper losses | I²R in the armature, field and brush contact; variable losses |
| Iron (core) losses | Hysteresis loss and eddy current loss in the armature core; nearly constant |
| Mechanical losses | Friction and windage; constant |
| Stray losses | Other small losses |
The efficiency is maximum when the variable losses equal the constant losses. The efficiency of large DC machines is about 85–95%.
Solved Examples
Example 1. A 4-pole wave-wound generator has 500 conductors, a flux of 0.02 Wb per pole and runs at 1,200 rpm. Find the EMF.
E = (4 × 0.02 × 500 × 1200)/(60 × 2) = 400 V.
Example 2. A motor on a 220 V supply has an armature resistance of 0.5 Ω and takes 20 A. Find the back EMF.
Eb = 220 − 20 × 0.5 = 210 V.
Example 3. Find the mechanical power developed in Example 2.
Eb Ia = 210 × 20 = 4,200 W.
Example 4. Which DC motor is used in electric traction?
The series motor (high starting torque).
Quick Tips
- Memorise the EMF equation and the torque relation T ∝ φ Ia.
- Remember that a series motor must never run without a load.
- Learn the speed control methods and their ranges.
- Remember that lap winding has A = P and wave winding has A = 2.
Quick Revision Points
- Eg = PφZN/60A.
- Eb = V − IaRa.
- A series motor has the highest starting torque.
- A shunt motor runs at nearly constant speed.
- Interpoles improve commutation.
Practice Questions With Answers
- Which DC motor has the highest starting torque?
Answer: Series motor - What is the back EMF formula?
Answer: Eb = V − IaRa - What is the number of parallel paths in a lap winding?
Answer: A = P (equal to the number of poles) - What is the number of parallel paths in a wave winding?
Answer: 2 - Which motor is used in cranes and traction?
Answer: Series motor - Which part of a DC machine converts AC to DC?
Answer: Commutator - Which method gives speed control above the base speed?
Answer: Field (flux) control - Which system gives smooth speed control in both directions?
Answer: Ward-Leonard system - What is the effect of armature reaction?
Answer: It distorts and weakens the main flux - When is the efficiency of a DC machine maximum?
Answer: When the variable losses equal the constant losses
How to Practise This Chapter
Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the Electrical Engineering notes page, read the previous topic Basic Electrical Circuits and AC Fundamentals and the next topic Synchronous Machines.
Frequently Asked Questions
What is a DC generator?
A machine that converts mechanical energy into direct current electrical energy.
What is back EMF?
The EMF induced in the armature of a motor that opposes the supply voltage.
Why is a starter needed for a DC motor?
Because the starting current is very high as the back EMF is zero at start.
Why should a series motor never run at no load?
Its speed rises dangerously high because the flux is very small at a light load.
What is a commutator?
A device made of copper segments that rectifies the AC in the armature to DC.
What is armature reaction?
The effect of the armature magnetic field on the main field.
What are interpoles?
Small poles between the main poles to improve the commutation.
What is a three-point starter?
A starter used for shunt and compound motors, with a no-volt release and an overload release.
What is the Ward-Leonard system?
A system of speed control by varying the armature voltage with a motor generator set.
Is this topic asked in SSC JE?
Yes. DC machines are asked in every SSC JE Electrical paper.
What is a self-excited generator?
A generator whose field is supplied by its own output.
What is the function of brushes?
They collect the current from the commutator and deliver it to the external circuit.
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Basic Electrical Circuits and AC Fundamentals for SSC JE Electrical: RMS, Impedance, Power Factor, Resonance and Three-Phase
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Synchronous Machines for SSC JE Electrical: Alternator, EMF Equation, Voltage Regulation, Parallel Operation and Synchronous Motor
Rahul Kumar
SSC Exam Expert & Content Editor
Rahul is a senior SSC exam strategist with 8+ years of experience helping aspirants crack CGL, CHSL and MTS exams. He writes in-depth notification breakdowns, exam pattern guides and preparation strategies.