What is a universal motor?
Working principle
A universal motor is a series-wound commutator motor: the field winding and the armature are connected in series, so one current flows through both. Torque is the field flux times the armature current, and because that same current also makes the flux, torque grows roughly with the square of the current (T ∝ I²). Carbon brushes and a commutator feed the armature, exactly as in a DC series motor, which is why the same machine can run on direct current as well.
Why it runs on AC and DC
Reverse the supply and the field and the armature current reverse together, so T ∝ I² keeps its sign and the rotor keeps turning the same way. On AC the torque never goes negative: it pulsates between zero and twice its mean at 2f, 100 or 120 times a second. The reactance of the windings also takes part of the voltage, so at the same RMS voltage an AC-fed motor runs slower than on DC, with a power factor below 1.
Speed, load and runaway
The back-EMF is proportional to flux and speed. With little load the current drops, the field weakens, and nothing holds the rotor back, so it accelerates until friction and its own cooling fan limit it, often 10,000 to 35,000 rpm. Run unloaded, a small universal motor can destroy itself. The steep speed–torque curve, with the speed falling quickly as the load grows, also gives it a very high starting torque.
Speed control and applications
A triac delays the turn-on after every zero crossing of the supply. This phase-angle control lowers the RMS voltage to V·√(1 − α/π + sin 2α / 2π) and therefore the speed, so dimmer-style controllers regulate drills, mixers and vacuum cleaners. Universal motors are chosen for high speed, light weight and strong starting torque; brush sparking limits their life and causes radio interference.
Frequently asked questions
- Why can a universal motor run on both AC and DC?
- Its field and armature are in series, so when the supply polarity reverses both reverse at once. The torque depends on the square of the current (T ∝ I²), which does not change sign, so the rotor keeps turning the same way. On AC the torque simply pulsates at twice the supply frequency. Switch the supply type in the simulator and compare.
- Why does a universal motor run away without load?
- The speed is the back-EMF divided by the flux. With almost no load the current falls, the series field weakens, and the speed has to rise a long way before the back-EMF balances the supply. Only friction and the fan limit it, so speeds of tens of thousands of rpm are reached. Lower the load to zero and watch the runaway warning.
- How does a triac control the speed of a universal motor?
- The triac only starts conducting at the firing angle α after each zero crossing, so the motor sees a chopped sine wave with a lower RMS voltage, V·√(1 − α/π + sin 2α / 2π). Less voltage means less current at a given torque, so the speed falls. Raise the firing angle in the simulator to see the voltage and speed drop.