A universal motor is a unique type of electric motor designed to operate efficiently on both alternating current (AC) and direct current (DC) power supplies. Its ability to run universally stems from its specific internal construction.
Understanding the Core Principle
Like most electric motors, the universal motor works based on the fundamental principle that a current-carrying conductor placed in a magnetic field experiences a force. This force, known as the Lorentz force, causes rotation, generating torque.
Series-Wound Design: The Key to Universality
The defining characteristic of a universal motor is its series-wound configuration. This means that the armature winding (the rotating part) and the field winding (which creates the magnetic field in the stationary part, called the stator) are connected in series.
Reference Information: According to byjus.com, "A universal motor is a special type of motor that runs on both AC and DC power supplies. Universal motors are series-wound (the armature and field windings are in series)."
Here's how this series connection allows operation on both AC and DC:
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DC Operation: When connected to a DC supply, current flows in one direction through both the field winding and the armature winding. The field winding creates a magnetic field of fixed polarity, and the current in the armature interacts with this field, producing a continuous torque that drives rotation. This operates much like a standard DC series motor.
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AC Operation: When connected to an AC supply, the voltage and current constantly reverse direction. Crucially, because the field winding and armature winding are in series, the current reverses simultaneously in both windings.
- When the current flows in one direction, the field winding creates a magnetic field of a certain polarity, and the armature current is in a corresponding direction, producing torque.
- When the AC voltage reverses, the current through both windings reverses. This means the magnetic field produced by the field winding also reverses its polarity, and the direction of the current in the armature winding also reverses.
- Since both the magnetic field direction and the armature current direction reverse at the same time, the direction of the force (and thus the torque) on the armature remains in the same direction. This results in continuous rotation, albeit with pulsating torque due to the varying magnitude of the AC current.
Simplified Comparison:
| Aspect | DC Operation | AC Operation |
|---|---|---|
| Field Polarity | Constant | Reverses with current direction |
| Armature Current | Constant direction | Reverses with current direction |
| Torque Direction | Constant | Constant (because field and current reverse together) |
| Torque Magnitude | Relatively smooth (for DC) | Pulsating (follows AC waveform magnitude) |
High Torque Capability
The series-wound design also contributes to the universal motor's ability to generate high torque, especially at low speeds. This characteristic makes them ideal for applications requiring a strong starting force.
Reference Information: The reference notes, "The series connection allows them to generate high torque".
Practical Application & Design
Because of their high torque and suitability for specific applications like portable power tools and appliances, universal motors are often built directly into the device they power.
Reference Information: "hence the universal motors are generally built into the device they are meant to drive."
Examples of devices using universal motors include:
- Vacuum cleaners
- Drills
- Blenders
- Sewing machines
- Hair dryers
In summary, a universal motor works by leveraging the interaction between a magnetic field and an armature current, enabled by its series-wound construction. This specific winding arrangement ensures that even when the power supply (and thus the current) reverses direction, as with AC, the resulting torque direction remains consistent, allowing the motor to run continuously.
