--- title: "Universal motor and why so called" subject: "Electrical Machine" chapter: 7 key: "semester_4/electrical_machine:universal-motor-construction-operation" source: https://www.bctengineer.com/q/semester_4/electrical_machine/universal-motor-construction-operation site: BCT Engineering --- # Universal motor and why so called Electrical Machine · Chapter 7: Fractional Kilowatt Motors - Asked in: 2075 Baisakh, 8 marks - Video: [universal motor | universal motor working principle | high efficiency motor for EVs & machines](https://www.youtube.com/watch?v=_tvMPIn59nQ) — Mind of Engineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/universal-motor-construction-operation - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-7 ## Question Explain construction, operation and characteristics of universal motor. Why is it called so? Give reason. ## Short answer A series-wound commutator motor with laminated stator and armature. Torque acts in the same direction on AC and DC because current and flux reverse together. Speed varies widely with load; on AC, reactive drop lowers speed and power factor. It is called universal because it works on both AC and DC supplies. ## Answer A universal motor is a series-wound commutator motor that can be operated on either a d.c. or a single-phase a.c. supply with nearly the same speed and torque. #### Construction ```text Laminated stator poles (field) +--------[ field coil ]--------+ Supply --+ +--- brushes +--- armature (commutator) ----+ Series connection: field in series with armature ``` - **Stator:** a laminated field core, either with salient poles and concentrated field coils (non-compensated type, small ratings) or with a distributed field winding and a compensating winding (compensated type, larger ratings). The field winding is in series with the armature. The core is laminated because the flux alternates on a.c. - **Armature:** a wound rotor on a laminated steel core, with a commutator and carbon brushes, as in a d.c. series motor. - A small air gap is used, and larger units have a compensating winding. #### Operation - On d.c., it works exactly like a d.c. series motor: T ∝ φ I_a ∝ I_a². - On a.c., the current and flux reverse together, so the torque (∝ φ I_a) always acts in the same direction. The torque pulsates at twice the supply frequency (average T ∝ I²), but because of the rotor inertia the speed remains steady. - On d.c., E_b = k φ N and V = E_b + I(R_a + R_se). On a.c. there is an additional reactive drop (IX), which lowers the power factor and the speed compared with d.c. #### Characteristics ```text N (speed) |\ | \ | \___ Torque - speed: | ---- very high speed at light load, +-----------> T speed falls sharply with load ``` - Very high starting torque (T ∝ I²). - The speed varies greatly with load (as in a series motor). At no load it can run up to 20 000 rpm or more, so it is always connected directly to the load or through gearing. - For the same voltage and load, the speed on a.c. is lower than on d.c., since part of the voltage is dropped across the reactance. - Speed is controlled by changing the voltage (using a thyristor or a tapped field). - Sparking at the brushes is greater on a.c. and radio interference may occur; a compensating winding reduces the reactance and improves commutation. #### Why it is called "universal" Because the same motor works on both a.c. and d.c. supplies. The torque direction does not depend on the polarity of the supply, because the field and armature currents reverse together. **Uses:** vacuum cleaners, mixer grinders, drills, sewing machines, hair dryers, portable tools. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is a universal motor wound? A. Field winding in series with the armature, with a commutator and brushes B. Field in shunt with the armature C. A squirrel-cage rotor with two stator windings D. Permanent-magnet field with a wound rotor and slip rings Answer: A. It is a series-wound commutator motor, laminated because the flux alternates on a.c. (Key point: Series connection) 2. Why does the torque keep its direction on a.c.? A. Field flux and armature current reverse together, so torque (proportional to phi I_a) keeps the same direction B. Only the armature current reverses C. A rectifier is built into the motor D. The brushes are removed on a.c. Answer: A. Since the same current flows in the field and armature, both reverse together and the product phi I_a stays of one sign. (Key point: Unidirectional torque on a.c.) 3. Which is true for the universal motor? A. Speed is nearly constant with load and is higher on a.c. than on d.c. B. Speed varies greatly with load, can run away at no load, and is lower on a.c. than on d.c. at the same voltage C. Speed is independent of voltage D. It has low starting torque Answer: B. Like a series motor its speed falls sharply with load and is dangerously high at no load. On a.c. part of the voltage is dropped across reactance, so speed is lower. (Key point: Speed behaviour and a.c. versus d.c.) 4. Why is it called a universal motor? A. It runs at any speed B. It works on both a.c. and d.c. supplies C. It works on three-phase supply only D. It can be built for any rating Answer: B. It is called universal because the same motor operates on a.c. and d.c. with similar performance. 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