Universal motor
A commutated series-wound motor operable on AC or DC power.
The universal motor is an electric motor that runs on either AC or DC power. It uses an electromagnet for its stator to generate the magnetic field. This is a commutated, series-wound design: the stator’s field coils connect in series with the rotor windings via a commutator, and it is also called an AC series motor. Its construction closely resembles that of a DC series motor, with slight modifications for proper AC operation. The motor works well on AC because the current in both the field coils and the armature (and their resulting magnetic fields) reverse polarity in sync with the power supply. This means the mechanical force always rotates in a consistent direction, determined by the commutator and field coil polarity, regardless of the applied voltage’s direction.
These motors offer high starting torque, high speed capability, and are lightweight and compact. They are common in portable power tools, household appliances, and similar equipment. Control is relatively simple, either electronically or electromechanically using tapped coils. However, the commutator’s brushes wear over time, making them less suitable for continuous use. Additionally, the commutator makes universal motors typically very noisy, both acoustically and electromagnetically.
Not every series-wound motor works well on AC. An ordinary DC series motor connected to AC runs poorly. Universal motors are modified for AC operation with a compensating winding and laminated pole pieces (instead of solid ones). Their armatures also have more coils and plates, with fewer windings per coil, which reduces inductance.
Even on AC, these motors can spin well above the mains frequency. Since most motor properties improve with speed, this allows them to be lightweight and powerful. Efficiency is generally low: around 30% for smaller motors and up to 70–75% for larger ones.
As series-wound motors, they respond to increased load by slowing down. Current rises, and torque increases with the square of the current because the same current flows through both armature and field windings. If stalled, current is limited only by total winding resistance, producing very high torque but risking overheating. Counter-electromotive force (EMF) helps limit armature current along with armature resistance. At startup, counter-EMF is zero, so armature current is very large due to low resistance.
- Efficiency range
- around 30% for smaller motors and up to 70–75% for larger ones
- Typical speed range
- 4000–16000 RPM
- Maximum speed
- over 20,000 RPM
- Power range common
- less than 1000 watts
Lore & Background
Universal motors are series wound. Shunt winding was used experimentally, in the late 19th century, but was impractical owing to problems with commutation. Various schemes of embedded resistance, inductance, and antiphase cross-coupling were attempted to reduce this. Universal motors, including shunt wound, were favoured as AC motors at this time as they were self-starting. When self-starting induction motors and automatic starters became available, these replaced the larger universal motors (above 1 hp) and the shunt wound.
In the past, repulsion-start wound-rotor motors provided high starting torque, but with added complexity. Their rotors were similar to those of universal motors, but their brushes were connected only to each other. Transformer action induced current into the rotor. Brush position relative to field poles meant that starting torque was developed by rotor repulsion from the field poles. A centrifugal mechanism, when close to running speed, connected all commutator bars together to create the equivalent of a squirrel-cage rotor. As well, when close to approximately 80 percent of its run speed, these motors can run as induction motors.
Universal motors have high starting torque, can run at high speed, and are lightweight and compact. They are commonly used in portable power tools and equipment, as well as many household appliances. They are relatively easy to control, either electronically or electromechanically using tapped coils. However, the commutator has brushes that wear, so they are less suitable for equipment that is in continuous use. In addition, partly because of the commutator, universal motors are typically very noisy, both acoustically and electromagnetically.
Reader's Guide
The universal motor's significance lies in its ability to combine high starting torque and compact, lightweight design with operation on either AC or DC power, making it uniquely suited for portable power tools and household appliances such as drills, vacuum cleaners, and blenders. Its series-wound construction allows very high speeds—up to over 20,000 RPM—far exceeding the synchronous speed limits of AC induction motors, which in 60 Hz countries is capped at 3600 RPM. This speed capability enables powerful yet small motors, though at the cost of relatively low efficiency (30% for small motors, up to 70–75% for larger ones) and significant noise from the commutator and brushes. The commutator also introduces maintenance issues and electromagnetic interference, limiting these motors to intermittent-duty applications. Speed control is straightforward via thyristor circuits or tapped field coils. While larger universal motors (above 1 hp) were largely replaced by self-starting induction motors and automatic starters, the universal motor remains prevalent in lower-power, high-speed applications where its unique characteristics are indispensable.
Did You Know?
- Universal motors can operate on both AC and DC power because the current in field coils and armature alternates synchronously, maintaining consistent rotation direction.
- Universal motors have no theoretical maximum speed for any particular applied voltage; they can exceed 20,000 RPM.
- The efficiency of universal motors ranges from around 30% for smaller motors to up to 70–75% for larger ones.
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