DC electric motor

Where it fits. This topic continues directly from "force on a current-carrying conductor" inside the Electro-Magnetism section of the ICSE Class 10 Electricity and Magnetism chapter. You are expected to describe the principle, construction, and working of a simple DC motor, and explain why the split-ring commutator is essential. There is no numeric torque or motor-efficiency treatment at this level — everything below is qualitative and diagram-based, exactly as examined.

Principle. A DC motor converts electrical energy into mechanical (rotational) energy. The underlying fact is simple: a current-carrying conductor placed in a magnetic field experiences a force, and Fleming's Left-Hand Rule fixes that force's direction from the field direction and the current direction. A motor's whole design trick is to arrange a rectangular coil so that its two sides parallel to the axis of rotation always feel forces pointing in opposite directions — one side pushed up while the other is pushed down. That pair of opposite forces, acting on opposite sides of the same coil, forms a couple, and a couple produces rotation.

Construction. Four parts, each with one job:

• Armature coil — a rectangular loop of insulated copper wire, free to rotate about a fixed axis.
• Magnetic field — usually two permanent magnets with opposite poles facing each other, so the coil sits between a north pole and a south pole.
• Split-ring commutator — a ring split into two insulated halves, each attached to one end of the coil, rotating together with it.
• Carbon brushes — fixed contacts that press against the rotating commutator, connecting the coil to the fixed external DC supply.

Working. Current entering the coil splits into two sides running parallel to the axis, carrying current in opposite directions (since the current goes around the loop). With the field pointing the same way across both sides but the currents opposite, Fleming's Left-Hand Rule gives forces in opposite directions on the two sides — a couple that starts the coil turning. As the coil sweeps past the position where its plane is perpendicular to the field (the vertical position), the two halves of the commutator swap which brush they touch. That swap reverses the current direction in the coil at exactly the moment needed, so the side that has just moved to where the other side used to be still feels a force in the same overall sense of rotation. The coil never gets a chance to be pulled backward — it just keeps turning the same way, half-turn after half-turn.

This section answers the question every exam paper asks in some form: why does the coil spin continuously instead of just rocking back and forth once and stopping? A convincing answer has to track both the force on the coil and the current direction through more than a single half-turn, because a single push does not by itself prove continuous rotation — it only shows the coil starts moving.

Step 1. Start with the coil at some angle to the field, current flowing so that side AB feels an upward force and side CD feels a downward force (Fleming's Left-Hand Rule, opposite currents in the two sides, same field direction). This couple begins turning the coil, say clockwise as viewed from one end.

Step 2. Track what happens as the coil approaches the vertical position, where its plane is perpendicular to the field. At this exact instant the forces on AB and CD, though still opposite, act along the same line as the coil's plane — they no longer produce a turning effect, only a moment of zero torque. The coil does not stop here because it carries rotational momentum from Step 1 and coasts through this position.

Step 3. As the coil's momentum carries it past vertical, side AB moves into the space where CD used to be, and CD moves into AB's old space. If nothing else changed, the current in the coil would still be flowing the same way relative to the external circuit, which would now put a downward force where an upward one used to be — the couple would reverse and start pulling the coil backward, causing it to oscillate rather than spin.

Step 4. This is exactly the instant the split-ring commutator earns its name. As the coil crosses vertical, the two commutator halves — rigidly attached to the coil — swap which brush each one touches, and that swap reverses the current direction flowing around the coil.

Step 5. With the current now reversed, the side that has just moved into AB's old position still gets pushed upward, and the side in CD's old position is still pushed downward, in the same overall sense as before. The couple keeps acting in the same rotational direction, so the coil sails through vertical and keeps spinning the same way rather than snapping back — and this reversal repeats every half-turn, sustaining continuous rotation for as long as current flows.

A rectangular coil ABCD, with AB uppermost and CD lowest, sits between a north pole on the left and a south pole on the right, so the field points left to right. Current flows around the loop in the order A to B to C to D to A. Find the direction the coil starts rotating, and describe what the commutator does a half-turn later.

Step 1. Identify which sides matter: AB and CD run parallel to the rotation axis and carry current in opposite directions (current goes around the loop A to B to C to D to A), so only these two sides feel the turning forces.

Step 2. Apply Fleming's Left-Hand Rule to side AB: forefinger along the field (left to right), middle finger along the current in AB, thumb gives the force — say, upward.

Step 3. Apply the rule to side CD: its current runs opposite to AB's (same loop, opposite side), so with the same field the force on CD comes out downward, opposite to AB's force.

Step 4. Upward force on AB plus downward force on CD form a couple, so the coil begins rotating in one sense — call it clockwise as viewed from one end.

Step 5. A half-turn later, AB and CD have swapped positions, and at that exact moment the commutator halves also swap which brush they touch, reversing the coil current. The side now sitting where AB started still gets pushed upward, and the side where CD started still gets pushed downward — so the coil keeps rotating clockwise instead of reversing.

Competitive stretch

Suppose the commutator is replaced by two continuous, unbroken slip rings, each permanently connected to the same brush throughout the rotation (as used in an AC generator). Trace the same coil through one half-turn.

Step 1. With slip rings, current direction in the coil relative to the external circuit never reverses — there is no swap at vertical.

Step 2. As the coil's momentum carries it past vertical (same physical situation as Step 3 of Why it works above), the unchanged current direction now produces a couple acting the opposite way relative to the coil's motion.

Step 3. The coil decelerates, stops, and swings back — it oscillates rather than completing full rotations, and cannot run as a motor. This is precisely why slip rings belong to AC generators (which want an alternating output) while a split-ring commutator belongs to a DC motor (which wants one-directional spin from a fixed supply).

• Believing the magnetic field reverses to keep the motor spinning — it is the current direction in the coil that reverses (via the commutator), while the external field from the magnets stays fixed throughout.
• Confusing a split-ring commutator (DC motors) with continuous slip rings (AC generators) — mixing these up describes the wrong machine entirely, since one sustains rotation and the other would only oscillate.
• Assuming the turning force is constant throughout the rotation — it is momentarily zero when the coil's plane is perpendicular to the field, and the coil coasts through that instant on its own rotational momentum.
• Forgetting that the brushes are fixed and only the commutator (attached to the coil) rotates — reversing this in a diagram or explanation is a common labelling mistake.

Ask the tutor: Want a slower, part-by-part walkthrough of how the commutator times its current reversal, or help sketching the motor's construction diagram yourself?