Electric Motor Diagram: How AC and DC Motors Work
Every motor -- from a tiny computer cooling fan to a 500 kW pump motor -- converts electrical energy into mechanical rotation using the same basic principle: a current-carrying conductor in a magnetic field experiences a force. What varies between motor types is how the magnetic field is produced and how the current is commutated to keep the rotation continuous. This guide covers the internal structure of both AC induction motors and DC motors, with labeled diagrams of the key components and how they produce torque.
The Common Principle: Lorentz Force
A conductor carrying current I in a magnetic field B experiences a force F:
F = BIL
where L is the length of the conductor. In a motor, the current-carrying conductors are wound onto a rotating assembly (the rotor). The fixed magnetic field is created by the stationary assembly (the stator). The force on the rotor conductors creates a torque that turns the shaft.
In practice, getting that torque to continue in one direction -- rather than just pulling the rotor to alignment and stopping -- requires either commutation (DC motors) or a rotating magnetic field (AC induction motors).
DC Motor Diagram
Main Components
Stator (field assembly) The stator of a wound-field DC motor consists of:
- Field poles: Electromagnets (or permanent magnets in small/PM motors) bolted to the inside of the cylindrical steel frame. Wound-field motors have field coils around each pole; permanent magnet motors (PMDC) use ceramic or rare-earth magnets instead.
- Yoke (frame): The outer steel cylinder that carries the magnetic flux between the poles.
Rotor (armature)
- Armature core: Laminated silicon steel drum, slotted around the circumference to hold the armature windings.
- Armature windings: Copper conductors wound through the slots and interconnected. Each winding is connected to one or more commutator segments.
- Commutator: A cylindrical assembly of copper segments separated by mica insulation, fixed to the shaft. The commutator is the key to DC motor operation -- it switches the current direction in the armature windings as the rotor turns, keeping the force always in the same rotational direction.
- Brushes: Carbon or graphite blocks held in brush holders, spring-loaded against the commutator. They carry current from the stationary external circuit to the rotating commutator. Brush wear is the primary maintenance issue in DC motors.
End shields / end bells Cast metal end plates that hold the rotor bearings and brush holders.
How a DC Motor Produces Rotation
- Current enters the motor through the positive brush, flows through the commutator segment in contact, and into the armature winding.
- The conductor in the magnetic field experiences the Lorentz force -- in this diagram, the force on the top conductor is toward the viewer (using the right-hand rule: field pointing left, current going away from viewer → force upward then curving into the page).
- This force creates a torque that begins to rotate the armature.
- As the armature turns approximately 90°, the commutator segments shift contact to the next brush. The current direction in the same physical conductors reverses -- but they are now on the other side of the poles, so the force direction in space remains the same.
- This commutation continues, maintaining continuous torque in one direction.
DC Motor Types and Their Wiring Diagrams
Permanent Magnet DC (PMDC): Two external terminals -- positive and negative. Reversing polarity reverses rotation. Used in toys, power tools, actuators.
Series DC motor: Field winding in series with the armature. Very high starting torque, speed varies dramatically with load. Used in traction, cranes, and historically in starter motors.
Shunt DC motor: Field winding in parallel with the armature (both connected across the same supply). Nearly constant speed with varying load. Used in machine tools and fans.
Compound DC motor: Both series and shunt field windings. Combines high starting torque with better speed regulation than a pure series motor.
AC Induction Motor Diagram
The three-phase squirrel cage induction motor is the most common motor in industrial use -- it has no brushes, no commutator, and no electrical connection to the rotor. It is also the motor covered in the 3-phase motor wiring diagram guide (terminal box connections, star/delta wiring).
Main Components
Stator
- Stator core: Laminated silicon steel ring, slotted on the inner circumference.
- Stator windings: Three sets of copper windings distributed in the slots, physically offset by 120° around the bore. When supplied with three-phase AC current (also offset by 120° in time phase), these windings create a rotating magnetic field -- the fundamental mechanism of the induction motor.
Rotor
- Squirrel cage: The most common rotor type. Short-circuit aluminum or copper bars cast in slots around a laminated steel core, connected at each end by aluminum end rings. It resembles a squirrel cage when the iron is removed, hence the name.
- No external electrical connections to the rotor -- that is the key difference from DC motors.
Shaft, bearings, and frame: Same general construction as a DC motor.
How an AC Induction Motor Produces Rotation
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The three-phase stator windings produce a rotating magnetic field that spins at the synchronous speed: Ns = 120f / P, where f is supply frequency (60 Hz in US) and P is the number of poles.
- For a 2-pole motor at 60 Hz: Ns = 120 × 60 / 2 = 3,600 RPM
- For a 4-pole motor at 60 Hz: Ns = 1,800 RPM
- For a 6-pole motor at 60 Hz: Ns = 1,200 RPM
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The rotating magnetic field sweeps past the stationary rotor bars. By Faraday's law, the changing flux induces a voltage in the short-circuited rotor bars, causing a rotor current to flow.
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The rotor current is in a magnetic field (the stator's rotating field). Lorentz force acts on the rotor bars, creating torque. The rotor accelerates in the direction of the rotating field.
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The rotor never reaches synchronous speed. If it did, there would be no relative motion between the field and the rotor bars, no induced EMF, no rotor current, and no torque. The rotor always lags the rotating field by the slip: s = (Ns - Nr) / Ns. At full load, slip is typically 2 to 5%.
Single-Phase Induction Motor
Single-phase AC does not create a rotating field -- it creates a pulsating field. A single-phase motor cannot start on its own because the pulsating field produces zero net torque at standstill. A starting method is required:
- Capacitor-start: A start capacitor in series with an auxiliary winding creates a phase-shifted current, simulating a rotating field for starting. A centrifugal switch disconnects the capacitor at about 75% of rated speed.
- Capacitor-run / PSC: The run capacitor stays in circuit continuously, improving power factor and efficiency.
- Shaded-pole: A copper shading ring on each pole creates a lagging flux in part of the pole face, providing weak starting torque. Used in small fans, clocks, and appliances where starting torque is not critical.
Comparing AC Induction and DC Motor Diagrams
| Feature | DC Motor | AC Induction Motor |
|---|---|---|
| Rotor connection | External (via brushes and commutator) | None (inductively coupled) |
| Speed control | Easy -- vary armature voltage | Needs VFD for smooth speed control |
| Starting torque | Very high (especially series) | Moderate (6--8x FLC inrush) |
| Maintenance | Brushes and commutator wear | Bearings only |
| Efficiency | Good, but brush losses | High, especially larger sizes |
| Cost | Higher (more complex construction) | Lower |
Torque-Speed Characteristic
For a DC shunt motor, torque is nearly proportional to armature current, and speed drops slightly with load -- a relatively flat speed curve.
For an AC induction motor, the torque-speed curve has a characteristic hump at the breakdown torque point (typically 200 to 300% of rated torque) before the motor accelerates through to the running region near synchronous speed. Understanding this curve is important when selecting a motor for high-inertia loads.
Create Your Own Electric Motor Diagram
Whether you are teaching motor theory or documenting a specific motor installation, clear diagrams of internal components and winding connections are invaluable. With CircuitDiagramMaker, you can:
- Draw labeled cross-sections of DC and AC motor internal components
- Diagram the field winding and armature connections for series, shunt, and compound DC motors
- Show three-phase stator winding layout and the terminal box connections
- Draw the torque-speed characteristic with labeled operating points
- Export your motor diagrams for presentations or maintenance documentation
Create your own electric motor diagram -- free
Three-Phase Motor Terminal Markings: IEC vs NEMA
The terminal box wiring diagram uses different lead labels depending on which standard applies. IEC 60034-8 labels the six winding leads U1, V1, W1 (winding starts) and U2, V2, W2 (winding ends), with U1, V1, W1 typically connected to the incoming supply. NEMA MG 1, used in North America, labels the same six leads T1 through T6, with T1, T2, T3 connected to the supply and T4, T5, T6 as the ends of the same three windings (T7, T8, T9 appear on some part-winding-start motors with extra winding taps). The two systems map directly onto each other -- U1/T1, V1/T2, W1/T3, U2/T4, V2/T5, W2/T6 -- but a terminal box wired to one standard will not match a diagram drawn to the other unless you convert the labels first.
Star (Wye) vs Delta Connection: Reading the Terminal Box
Dual-voltage three-phase motors are commonly wired to run in either star (wye) or delta configuration, depending on the supply voltage available. In the star connection, the three winding ends (U2, V2, W2 or T4, T5, T6) are bonded together at a single point to form the star point, and the supply connects to the three starts (U1, V1, W1 or T1, T2, T3). In the delta connection, each winding end connects to the start of the next winding (U1-W2, V1-U2, W1-V2), forming a closed triangle, and the supply connects at the three corners. Star connection applies the phase voltage across each winding; delta applies the full line voltage across each winding. The terminal box typically has small brass link bars used to bond the terminals into the correct pattern -- the diagram printed on the motor nameplate or terminal box cover tells you which links to use for the supply voltage you have.
Common Mistakes When Reading Motor Diagrams
Reversing rotation with only one lead swapped. To reverse the rotation direction of a three-phase induction motor, you must swap any two of the three supply leads. Swapping only one lead, or all three, does not reverse rotation -- it either does nothing or reproduces the same rotating field sequence.
Confusing series and shunt field connections. In a DC motor wiring diagram, the field winding symbol looks the same whether it is wired in series or shunt -- what distinguishes them is where the winding connects relative to the armature, not the symbol itself. Trace the connection path rather than assuming the configuration from the symbol alone.
Misreading slip as a fixed value. Slip changes with load. A nameplate or diagram note showing rated slip at full load does not mean the motor runs at that slip continuously -- at light load, slip and rotor current both drop close to zero.
Assuming PMDC lead polarity always matches wire color. Unlike line-voltage AC wiring, small PMDC motor leads are not standardized by color across manufacturers. Confirm polarity from the diagram or with a meter rather than assuming red is always positive.
Key Takeaways
- Both AC and DC motors use the Lorentz force (F = BIL) to convert electrical energy into mechanical torque.
- DC motors use a commutator and brushes to reverse current direction in the rotor windings, maintaining continuous torque in one direction.
- The three main DC motor types -- series, shunt, and compound -- differ in how the field windings are connected relative to the armature.
- AC induction motors produce a rotating magnetic field from three-phase windings; this field induces rotor currents that create torque without any electrical connection to the rotor.
- Squirrel cage induction motor speed at no load: Ns = 120f / P. At full load, the rotor runs at 2 to 5% below synchronous speed (slip).
- Single-phase induction motors require an auxiliary starting method -- capacitor-start, capacitor-run, or shaded-pole -- because a single-phase field alone produces no net starting torque.
- AC induction motors have lower maintenance requirements than DC motors because there are no brushes or commutator to wear.
Frequently asked questions
What happens if you swap the field winding leads on a wound-field DC motor?
Swapping only the field leads (not the armature leads) reverses the direction of the magnetic field relative to the armature current, which reverses the motor's rotation direction. Swapping both field and armature leads at the same time cancels out, and the motor spins the same direction as before.
How do you reverse rotation on a capacitor-start single-phase motor?
Swap the two leads of the start (auxiliary) winding relative to the run (main) winding, not the main supply leads. This changes the phase relationship between the two windings' magnetic fields, reversing the direction the rotor accelerates in when the motor starts.
Does NEMA frame size affect a motor's wiring diagram?
NEMA frame size mainly defines mounting dimensions, shaft height, and shaft diameter, not the internal winding connections. It can affect terminal box size and lead length, but the winding diagram itself -- star, delta, series, or shunt -- depends on the motor's electrical design, not its frame size.
Can you control an AC induction motor's speed without a VFD?
Yes, though it is less common today. Pole-changing motors have windings that can be reconnected for a different number of poles, giving two or more fixed speeds without electronic control. Eddy-current clutches are another older alternative, but a VFD gives continuously variable speed and is now the standard approach.
Why does a motor hum but not turn?
On a single-phase motor, humming without rotation usually means the start capacitor or centrifugal switch has failed, leaving no starting torque. On a three-phase motor, it often means the motor is single-phasing -- one of the three supply legs has lost power, typically from a blown fuse or loose connection.
What is the difference between a squirrel-cage and a wound-rotor induction motor?
A squirrel-cage rotor has short-circuited bars cast into the rotor with no external connections. A wound-rotor motor instead has rotor windings brought out to slip rings, letting external resistance be added to the rotor circuit to increase starting torque and control speed, at the cost of extra maintenance.