Topic 15 of 18 • Chapter: Electromagnetic Induction • Physics

DC Motor – Complete Notes, Revision, Important Questions & Downloads

DC Motor is the chapter's electrical-to-mechanical conversion topic, and the local page organises it through Motor Principle and Construction, Back EMF, Motor Starter, and Mechanical Power and Efficiency. NEET tests this topic through the torque idea on a current-carrying coil, the role of the split-ring commutator in keeping rotation unidirectional, the back-emf relation e = E - iR, and the startup-current logic that makes a starter necessary. The topic is short but highly integrated: students must connect force on the armature, current reversal by commutation, speed-dependent back emf, and efficiency instead of studying each line as an isolated definition.

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Application TopicBack EMFStarter Logic

Expected QuestionsQ

0-1

usually asked as a direct concept check on commutator action, back emf, or why current is maximum at switching on

Time Required⏱

2 Hours

enough to lock the working diagram, the back-emf relation, and one or two starter or efficiency numericals

Difficulty⚡

Medium

the formulas are short, but students often mix motor and generator roles or forget how current changes from starting state to full-speed state

NRI USA Curriculum GapUS

Moderate

many students see motors in general physics or engineering examples, but NEET expects the exact textbook logic of commutation, back emf, starter resistance, and efficiency in one page

4Subtopics

28Practice Questions

4Free Downloads

2 hrsPrep Time

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NEET Weightage & Exam Pattern

Electromagnetic Induction

NEET Year	Questions from this Topic	Bar	Marks
2024	0	0 Q	0
2023	0	0 Q	0
2022	0	0 Q	0
2021	0	0 Q	0
2020	0	0 Q	0
Topic Weightage	0		0

DC Motor is more often used inside assertion-reason or diagram-based teaching questions than as a repeated high-frequency standalone NEET item.
The scoring hinge is the causal chain: torque rotates the coil, rotation creates back emf, back emf reduces current, and a starter protects the motor before speed builds up.
If a question looks purely numerical, the conceptual check is usually hidden in whether the student recognises that e is zero at switch-on and largest near full speed.

📊

0.0

Avg Questions / Year

🎯

Total Marks (6 yrs)

📈

Irregular

Pattern

⚠️

Medium

Difficulty

Preparation Strategy

Learn The Construction As A Working Chain Do not memorise armature, commutator, brushes, and poles as a list. Track how each part contributes to continuous rotation, because NEET-style questions often hide the answer in the function of one component.

Lock The Starting-State Logic At switch-on, omega = 0 so back emf is zero and current is maximum, i = E by R. This single condition explains why a starter is required and why later current falls as speed rises.

Remember What The Commutator Actually Does The split-ring commutator reverses current in the armature every half turn so the torque keeps the same sense of rotation. The trap is to think it reverses the direction of motion; actually it preserves the same rotational direction.

Use Efficiency Only After Back EMF Is Understood The textbook efficiency relation eta = e by E is compact, but it becomes meaningful only after you know that e is the back emf opposing the supply. Otherwise students substitute symbols mechanically and miss the physics.

Download Topic Notes

PDF · Cheat Sheet · MCQ Set · PYQ

📄

Full Topic Notes

Detailed notes on motor construction, commutator action, back emf, starter operation, and efficiency of a DC motor.

PDF5 Pages

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📝

Formula Sheet

One-page sheet for F = i l cross B, e = E - iR, i = (E - e) by R, and eta = e by E with startup and full-speed reminders.

PDF1 Page

Download Formulas

🎯

MCQ Practice

Practice set on torque direction, commutator role, current variation with speed, and starter resistance logic.

PDF28 Questions

Download MCQs

⏳

Previous Year Questions

Selected concept questions on back emf, starter function, and the difference between a motor and a generator.

PDF8 Questions

Download PYQs

Topic Coverage

2-Column Table

Column A	Column B
Motor Principle and Construction↗	Back EMF↗
Motor Starter↗	Mechanical Power and Efficiency↗

Quick Revision

Concept → Trap → Example

1) Motor Principle and Construction

Torque And Parts

A DC motor converts electrical energy into mechanical energy and works on the fact that a current carrying coil placed in the magnetic field experiences a torque. The working force on the armature follows F = i (l cross B).

Armature coil, split-ring commutators, carbon brushes, and field poles must be read as one working system rather than four separate labels.
Force on opposite arms of the coil is equal and opposite, producing torque instead of net translation.
Trap: using Fleming's right-hand rule here instead of remembering that the textbook explains motor motion through magnetic force on a current-carrying conductor.

Example (NEET-style)If current flows in one arm of the rectangular coil and the magnetic field is horizontal, F = i l B gives forces on opposite arms in opposite directions, so the pair forms a turning effect and rotates the armature instead of pushing the whole coil sideways.

2) Back EMF

Speed Dependent Opposition

Due to the rotation of armature coil in magnetic field a back emf is induced in the circuit. It is given by e = E - iR, and for constant magnetic field the page states e proportional to omega so that i = (E - e) by R = (E - k omega) by R.

Back emf is not an external supply; it is the motor's own induced emf opposing the applied voltage.
At switch-on omega = 0, so e = 0 and current becomes maximum, E by R.
Trap: claiming current rises with speed; in fact current falls as omega rises because back emf grows.

Example (NEET-style)Suppose E = 12 V and armature resistance is 2 ohm. At starting, e = 0 so current is 12/2 = 6 A. If at higher speed back emf becomes 8 V, current falls to (12 - 8)/2 = 2 A.

3) Motor Starter

Protection At Switch-On

At the time of start a large current flows through the motor which may burn it out, so a starter introduces suitable resistance in the circuit and this resistance decreases gradually to zero when the motor reaches safe speed.

Starter resistance is maximum at t = 0 because that is when back emf is absent.
The resistance is reduced only after the motor speed rises and the internal opposing emf becomes significant.
Trap: saying the starter is used to increase torque continuously; its immediate textbook purpose is to limit dangerous initial current.

Example (NEET-style)If a motor without starter would draw 10 A at startup, adding an external series resistance can reduce the initial current to a safe value until the armature begins rotating and develops its own back emf.

4) Mechanical Power and Efficiency

Input Versus Useful Output

The page writes efficiency of DC motor as eta = P mechanical by P supplied = e by E = back emf by supply voltage. That links useful mechanical conversion directly with how much of the supply is balanced by back emf at working speed.

Efficiency improves when a larger fraction of the supply is accounted for by back emf rather than resistive drop in the armature.
The relation eta = e by E is a compact exam formula, but it assumes the textbook motor model used on the page.
Trap: taking eta = iR by E; the resistive drop is the loss part, not the useful mechanical-output fraction.

Example (NEET-style)If supply voltage is 200 V and back emf at running speed is 160 V, the page formula gives eta = 160/200 = 0.8, so efficiency is 80 percent in that simplified model.

US Curriculum Gaps

Note for NRI/OCI students studying abroad.

AP Physics Often Mentions Motors But Not The Exact Split-Ring Story

Students may know that a motor turns because a current-carrying loop experiences torque, but NEET expects the specific textbook role of split-ring commutators and carbon brushes in keeping the rotation one-sided.

commutator keeps current reversal synchronized with rotation
brushes provide external connection while the armature rotates

Back EMF And Starter Logic Is Usually Less Exam-Driven

The page treats startup current, back emf, and starter resistance as one causal chain. Students used to broader engineering summaries often know the words but have not practised the exact current-versus-speed reasoning that NEET asks directly.

e is zero at switch-on
starter is required because i = E by R can be too large initially

Concept IQ Check

Exam-style checks

1In a DC motor, the split-ring commutator is used mainly to:Construction

increase the strength of the magnetic field

reverse the current in the armature every half turn so the torque keeps the same rotational sense

store electric energy temporarily

eliminate armature resistance

A DC motor rotates continuously only if the torque on the armature does not reverse after half a turn. The split-ring commutator solves that by reversing the current in the coil at the correct instant, so the force on the two arms remains arranged for the same sense of rotation. Option B captures that role exactly. Options A, C, and D assign unrelated functions to the commutator and ignore the motor's operating principle.

2A motor is just switched on. Which statement about back emf and armature current is correct at that instant?Back EMF

Back emf is maximum and current is minimum

Back emf is zero and current is maximum

Back emf equals supply voltage and current is zero

Back emf is independent of speed so current is constant

At the instant of switching on, the armature has not yet started rotating, so omega = 0. The page states that for constant field back emf is proportional to omega; therefore e = 0 at startup. Using i = (E - e) by R then gives i = E by R, which is the maximum possible armature current. Option B is correct. The other options contradict the direct speed dependence of back emf or the startup condition itself.

3Why is a starter needed in a DC motor?Starter

to increase the supply voltage suddenly

to reverse the magnetic field direction every second

to introduce resistance at startup and limit the large initial current

to make back emf zero throughout the motion

The page says that a large current flows through the motor at the time of start and may burn it out. This happens because back emf is absent initially, so the current would be very large. A starter introduces suitable resistance in the circuit at startup and then gradually reduces that resistance as the motor attains safe speed. Option C is therefore the only statement that matches the textbook mechanism and the reason for using a starter.

4In the simplified textbook relation for a DC motor, efficiency is written as:Efficiency

eta = iR by E

eta = e by E

eta = E by e

eta = E + e

The OCR page explicitly writes efficiency as mechanical power by supplied power and then simplifies that result to e by E, where e is the back emf and E is the supply voltage. Option B therefore reproduces the textbook relation directly. Option A uses the resistive drop, which represents loss rather than useful conversion. Option C inverts the ratio and would give values above unity for normal operation, which is not physical.

Practice Questions

Click "Reveal Answer" after attempting

1A DC motor has supply voltage 24 V and armature resistance 4 ohm. Find the armature current at the instant of switching on.

2 A

4 A

6 A

8 A

👁 Reveal Answer

Correct option: C. At switching on, omega = 0, so back emf is zero. Therefore i = E by R = 24/4 = 6 A. The whole point of the starter is to reduce this large initial current until speed rises and the motor develops back emf.

2At a certain speed a motor connected to a 220 V supply draws 5 A through an armature resistance of 4 ohm. What is the back emf?

20 V

180 V

200 V

240 V

👁 Reveal Answer

Correct option: C. Use e = E - iR. Here iR = 5 x 4 = 20 V, so e = 220 - 20 = 200 V. The sign matters: back emf opposes the supply, so the useful relation subtracts the resistive drop from the applied voltage.

3If the speed of a DC motor increases while the magnetic field remains constant, which statement is correct?

Back emf decreases and armature current increases

Back emf increases and armature current decreases

Both back emf and current increase

Neither changes

👁 Reveal Answer

Correct option: B. The page states e proportional to omega for constant magnetic field, so greater speed means larger back emf. Since i = (E - e) by R, the armature current must then decrease. Many students remember the proportionality but forget to pass that effect through the current equation.

4A motor has back emf 150 V when connected to a 200 V supply. According to the simplified page relation, what is the efficiency?

25%

50%

75%

133%

👁 Reveal Answer

Correct option: C. Using eta = e by E, efficiency = 150/200 = 0.75 or 75 percent. This result also shows physically that useful conversion is less than the supply input. Any option above 100 percent would immediately signal a wrong formula or inverted ratio.

Physics Revision Checklist

Check off chapters as you revise

Use this section for quick chapter tracking before mocks, part tests, and final NEET revision.

Motor Principle and Construction: label each part with its functionWrite one line each for armature, split-ring commutators, brushes, and field poles so you can explain how continuous rotation is maintained.

Back EMF: solve one startup and one running-state current questionUse e = E - iR and i = (E - e) by R once for omega = 0 and once for a finite back emf, then compare the two currents.

Motor Starter: explain why resistance is maximum at t = 0State the absence of back emf at startup and connect that directly to the need for extra series resistance.

Mechanical Power and Efficiency: compute eta = e by E in one exampleTake any supply voltage and back emf pair, calculate efficiency, and state what part of the supply is lost in the resistive drop.

Motor Principle and Construction: compare motor action with generator actionWrite two lines distinguishing electrical-to-mechanical conversion in a motor from mechanical-to-electrical conversion in a generator so the chapter applications do not get mixed.

DC Motor FAQs

Notes · Downloads · Revision · Important Questions

Why does a current-carrying coil rotate in a DC motor?

A current-carrying conductor placed in a magnetic field experiences force, and opposite sides of the armature coil experience equal and opposite forces that form a couple. That couple produces torque and rotates the armature. The motor therefore converts electrical input into mechanical rotation through magnetic force on the current-carrying coil.

What exactly does the split-ring commutator do?

It reverses the current in the armature after each half turn. Without that reversal, the torque on the coil would reverse and the armature would tend to oscillate instead of rotating continuously in one direction. The commutator therefore preserves the same sense of rotation by changing current direction at the correct instant.

Why is back emf called opposing emf?

It is induced because the armature rotates in the magnetic field, and its direction opposes the applied supply according to electromagnetic induction. That is why the current relation is written as i = (E - e) by R rather than (E + e) by R. The faster the motor runs, the stronger this opposing effect becomes.

Why is the starting current maximum in a DC motor?

At the instant of switching on, the armature is not yet rotating, so omega = 0 and the back emf is zero. The only opposition to current is then the armature resistance, which is usually small. Hence i = E by R becomes very large initially, making the starting current the maximum current drawn by the motor.

Why is a starter used only at startup and not throughout operation?

A starter is needed only when the armature has not yet developed back emf. Once the motor gains speed, its own induced back emf limits the current naturally, so the external starter resistance is gradually reduced to zero. Keeping that extra resistance permanently would only waste power and reduce useful performance.

Does back emf mean the motor is wasting energy?

No. Back emf is a sign that the rotating armature is behaving as an induced-emf system inside the motor. In the page formula for efficiency, a larger back emf at running speed corresponds to better conversion of supply into mechanical output relative to pure resistive loss. It opposes the source, but it is also part of how the motor regulates current during useful operation.

How does the page define efficiency of a DC motor?

The OCR page writes efficiency as mechanical power by supplied power and simplifies it to eta = e by E in the textbook model. That means the useful-output fraction is directly related to the ratio of back emf to supply voltage. The formula is compact, but it should be used only with the same assumptions and symbols used on the page.

What is the fastest way to revise DC Motor for NEET?

Start with the construction diagram and the function of the commutator, then revise the two current relations e = E - iR and i = (E - e) by R, and finish with the startup-state statement omega = 0 implies e = 0. That three-step revision covers almost every direct concept question the page can generate.

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DC Motor

DC Motor – Complete Notes, Revision, Important Questions & Downloads

1) Motor Principle and Construction

2) Back EMF

3) Motor Starter

4) Mechanical Power and Efficiency

AP Physics Often Mentions Motors But Not The Exact Split-Ring Story

Back EMF And Starter Logic Is Usually Less Exam-Driven

Physics Revision Checklist

NEET NRI Counseling & Admission eBook Download

Schedule Trial Session For NEET Prep

Motor Principle and Construction

Back EMF

Motor Starter

Mechanical Power and Efficiency

Subtopics

Motor Principle and Construction

Back EMF

Motor Starter

Mechanical Power and Efficiency

NEET > Physics > Electromagnetic Induction and Alternating Currents Chapters

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DC Motor

Overview content

DC Motor – Complete Notes, Revision, Important Questions & Downloads

1) Motor Principle and Construction

2) Back EMF

3) Motor Starter

4) Mechanical Power and Efficiency

AP Physics Often Mentions Motors But Not The Exact Split-Ring Story

Back EMF And Starter Logic Is Usually Less Exam-Driven

Physics Revision Checklist

NEET NRI Counseling & Admission eBook Download

Schedule Trial Session For NEET Prep

Motor Principle and Construction

Back EMF

Motor Starter

Mechanical Power and Efficiency

Subtopics

Motor Principle and Construction

Back EMF

Motor Starter

Mechanical Power and Efficiency

NEET > Physics > Electromagnetic Induction and Alternating Currents Chapters

Comments

Leave a comment

Comments (0)