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Electric motor

What Is a Universal Motor?

The universal motor is a type of electric motor that can work with both direct current and single-phase alternating current.

What is a universal motor?

Its constitution is similar to that of a direct current series motor, although with some modifications:

The pole cores, and the entire magnetic circuit, are constructed of insulated and stacked silicon iron sheets to reduce energy losses due to eddy currents. These currents are produced due to variations in magnetic flux when connected to an alternating current network.

It has a smaller number of turns in the inductor so as not to magnetically saturate the core and thus reduce losses due to eddy currents and hysteresis, increase the current intensity and, therefore, the motor torque and improve the power factor.

The universal motor has more turns in the armature to compensate for the decrease in flux due to the fewer number of turns in the inductor.

How Does a Universal Motor Work?

The universal motor is a type of single-phase motor that can operate in both direct current and alternating current. Its constitution is essentially that of a direct current series motor and its operating characteristics are analogous.

The following is a schematic representation of the universal motor diagram:

Scheme of a universal motor - alternating current and direct current

    The direct current series motor is characterized by having a strong starting torque and its speed is in inverse function to the load. This makes it easy to reach very high speeds when idling. At the moment in which the universal motor works in alternating current, this inconvenience is reduced because its application is usually in small power motors and the losses due to friction, bearings, etc., are high with respect to the total, so it is not They present the danger of accelerating so much, although they do reach speeds of up to 20,000 revolutions per minute (rpm). For this reason they are quite suitable for small electrical appliances.

    In order for an electric motor of this type to be able to work with alternating current, it is necessary that the stack of its inductor (the core of the electromagnets) is made of magnetic plate to avoid losses in the iron.

    The inductor winding of universal motors is usually bipolar, with two inductor coils. The universal motor works in direct current exactly the same as a series motor. If the motor is powered by alternating current, it starts on its own, since the current that flows through the inductor winding presents one hundred vibrations per second, the same happens to the current that flows through the induced winding, so that the moment of rotation and the direction of rotation remains constant.

    What Are the Characteristics of a Universal Motor?

    • In direct current it is a normal series motor with the same characteristics.
    • In alternating current it behaves in a similar way to a direct current series motor. As every time the direction of the current is reversed, it does so in both the inductor and the armature, so the motor torque retains its sense.
    • Lower power in alternating current in direct current, because in alternating current the pair is Pulsator.
    • Increased sparking in the brushes when operating in alternating current, as the armature coils are traversed by an alternating flux when shorted by the brushes, forcing a compensating winding to be put in the middle motors to counteract the induced electromotive force for this reason.

    Universal motors work with direct current or alternating current voltages. Such an electric motor, called universal, is used in electric saws, drills, cookware, fans, blowers, mixers and other applications where high rotational speed is required with weak loads or small resistant forces. These motors for alternating and direct current, including universal ones, are distinguished by their wound commutator and brushes.

    Stator of a universal electric motor

    The electrical circuit is very simple, it has only one path for the passage of current, because the circuit is connected in series. Its potential is greater because it has greater flexibility in overcoming inertia when it is at rest, that is, it has excellent starting torque, but it has a difficulty, and that is that it is not built for continuous or permanent use (for long periods of time ).

    Another difficulty of universal motors is electromagnetic emissions. Collector sparks ("crackles") together with its own magnetic field generate interference or noise in radio space. This can be reduced by means of step capacitors, 0.001 μF to 0.01 μF, connected from the brushes to the motor housing and connecting the motor housing to ground. These motors have the advantage that they reach high speeds, but with little force. There are also three-phase alternating current motors that operate at 380 V and other voltages.

    What Are the Components of the Universal Motor?

    The components of this engine are:

    • The fields ( stator)
    • Mass (rotor)
    • The brushes (the exciters) 
    • The covers (the engine side covers)

    The electrical circuit is very simple, it has only one path for the passage of current, because the circuit is connected in series. Its potential is greater because it has greater flexibility in overcoming inertia when it is at rest, that is, it has excellent starting torque, but it has a difficulty, and that is that it is not built for continuous or permanent use (for long periods of time ).

    What Are Universal Motors Used For?

      The use of these motors in alternating current is very widespread due to the higher starting torque compared to that of induction motors and due to their high rotation speed, which allows their size and price to be reduced. The universal motor is undoubtedly the most widely used electric motor in the household appliance industry.

      It is generally used in portable machine tools of all kinds, small appliances such as electric saws, drills, kitchen utensils, fans, blowers, mixers and other applications where high rotational speed is required with weak loads or small resistant forces. These motors for alternating and direct current, including universal ones, are distinguished by their wound commutator and brushes.

      What Are the Advantages and Disadvantages of the Universal Motor?

      Advantages of the Universal Motor

      The universal alternating current motor has the following advantages:

      • The universal motor has a high starting torque during cranking.
      • This type of electric motor can reach higher speeds (up to approximately 25,000 rpm) compared to the possible speeds than with asynchronous motors at the mains frequency.
      • As with a normal DC motor, the speed can be easily adjusted by varying the motor voltage.

      Disadvantages of the Universal Electric Motor

        On the other hand, this type of engine has the following drawbacks:

        • The commutator present in the electric motor is subject to wear and tear and often causes sparks under the influence of the resulting anchoring reaction. To limit EMC disturbances, the motor can be equipped with suppression capacitors, placed in parallel to the anchor and coupled series inductors on the supply lines to the commutator.
        • The universal motor features a low efficiency of around 80%, which results in a large amount of heat production. To avoid high losses due to eddy currents, not only the rotor but also the stator must be laminated.
        • The use of machines with a universal motor requires regular maintenance. With normal use, it would be good to blow the motor casing periodically with a compressor. This can extend the service life considerably.
        • Due to the torque of pulses, this type of electric motor produces a lot of vibrations and therefore a lot of noise.
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        Published: August 29, 2017
        Last review: April 9, 2019