Universal motor + AC commutator motors

Reference notes

Universal motors are series-wound DC motors that also run on AC — the workhorse of nearly every corded hand-held power tool (drills, blenders, vacuums) and small kitchen appliance. Repulsion and other AC commutator motors are related historical machines. Use Next → to walk through how series construction enables AC operation, the droopy speed-torque characteristic, repulsion motors, and comparisons with induction / BLDC for small-motor applications.

Universal motor — why it runs on AC

• Series winding: field current = armature current (same I through both).
• Torque T = K · φ · I_a. Since φ ∝ I_field = I_a → T = K · I_a².
• When AC reverses I_a, I_a² remains positive → unidirectional torque despite alternating current.
• Both stator and rotor must be LAMINATED to manage eddy currents at 50/60 Hz.

Properties

• Very high RPM — 5000-20000 RPM possible. Not limited by line frequency like induction (3600 RPM at 60 Hz, 2-pole).
• High power density — 1 kW from a drill-sized motor. Induction equivalent would be 3-5× larger.
• Droopy T-speed curve — very high no-load speed (limited by friction/windage), speed drops sharply with load.
• Smooth speed control — via thyristor dimmer or PWM on the AC supply.
• Moderate efficiency — 40-65 %. Lower than induction (75-85 %) but acceptable for short-duty hand tools.
• Brush wear — typical brush life 500-1000 hours. Higher than DC due to commutation sparking on AC (small universal motors typically omit interpoles that DC machines use to ease commutation, so they accept some sparking).
• Noise — high RPM + commutation sparking → loud whine. The vacuum cleaner motor sound.

Applications

• Corded power tools — drills, circular saws, grinders, routers, jigsaws, sanders.
• Kitchen appliances — blenders, food processors, mixers.
• Household appliances — vacuum cleaners (canister and upright), hair dryers, electric knives.
• Sewing machines — small variable-speed universal motors with foot-pedal speed control.

Repulsion motor (historical)

• Single-phase stator + wound commutator rotor.
• Brushes SHORTED across each other at angle θ (10-30°) to stator field axis.
• Induced rotor currents produce a cross-axis MMF that interacts with the stator field → steady torque (similar mechanism to cross-field DC machines).
• Very high starting torque — 4-5× rated. Useful for hard-to-start loads.
• Variants: straight repulsion (operates continuously), repulsion-start induction-run (uses repulsion for starting, centrifugal switch then shorts the commutator to a complete short → runs as cage IM).
• Largely replaced by capacitor-start induction motors and modern VFD drives. Historical use in early-20th-century traction, refrigerator compressors, industrial hard-start loads.

Comparison with small-motor alternatives

Other historical AC commutator motors

• Compensated series motor — universal motor + compensating winding to cancel armature reaction. PF 0.8-0.9 vs uncompensated 0.4-0.6. Used in early AC traction.
• AC traction motors at 16⅔ Hz — Germany / Austria / Switzerland legacy railway power. Lower frequency was chosen to manage commutation issues with single-phase series traction motors.
• Schrage motor — variable-speed AC commutator with brush shifting. Covered in its own lesson.
• Atkinson, Desirabey motors — historical special-purpose AC commutator designs. Now syllabus-only.

Modern displacement by BLDC

Premium cordless tools (Milwaukee M18 Fuel, DeWalt FlexVolt, Makita LXT/XGT) have transitioned from brushed universal motors to BLDC + electronic control. Reasons: efficiency 85% vs 50%, no brush wear, better speed control, smaller size. Cost premium ~30-50% for the BLDC drive electronics. AC-corded tools remain dominantly universal-motor where battery runtime isn't a concern and cost matters more.