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Electric motor

Universal motor

Universal motor

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

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

The polar nuclei, and the entire magnetic circuit, are built with silicon iron plates isolated and stacked to reduce energy losses by parasitic currents. These currents occur because of magnetic flux variations when connected to an alternating current network.

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

The universal motor has a greater number of turns in the armature to compensate for the decrease in flow due to the lower number of turns of the inductor.

Constitution and operating principle of the universal motor

The universal motor is a type of single-phase motor that can work both in direct current and in alternating current. Its constitution is essentially that of the direct current series motor and its operating characteristics are analogous. Next we represent this engine schematically:

Diagram 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. In this way it is easy to reach very high speeds when running in a vacuum. At the moment when the universal motor operates in alternating current, this disadvantage is reduced because its application is usually in small power motors and losses due to friction, bearings, etc., are high with respect to the total, so that they present the danger of accelerating so much, although if they reach speeds of up to 20,000 revolutions per minute (rpm). For this reason they are quite suitable for small appliances.

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

The inductor winding of the universal motors is usually bipolar, with two inductor coils. The universal motor operates in direct current exactly like a serial motor. If the motor is powered by alternating current, it starts on its own, since the current flowing through the inductor winding presents a hundred alternations per second, just as it happens to the current that the induced winding travels, so the moment of rotation and the direction of rotation remain constant.

Operating characteristics of universal motors

  • In direct current it is a normal series motor with its same characteristics.
  • In alternating current it behaves similarly to a direct current series motor. Like every time the direction of the current is reversed, it does so both in the inductor and in the armature, so that the torque retains its meaning.
  • Lower power in alternating current in DC, because in alternating torque is Pulsator.
  • Greater sizzles in the brushes when it works in alternating current, since the coils of the armature are crossed by an alternating flow when they are short-circuited by the brushes, which forces to put a compensating winding in the medium 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, blenders and other applications where high turning speed is required with weak loads or small resistant forces. These motors for alternating and direct current, including the universal ones, are distinguished by their winding commutator and brushes.

Stator of a universal electric motor

The electrical circuit is very simple, it has only one way for the passage of the 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 an excellent starting torque, but has a difficulty, and is that it is not built for continuous or permanent use (for long periods of time ).

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

Universal motor components

The components of this engine are:

  • The fields (stator)
  • The mass (rotor)
  • Brushes (exciters) 
  • Covers (lateral covers of the engine)

The electrical circuit is very simple, it has only one way for the passage of the 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 an excellent starting torque, but has a difficulty, and is that it is not built for continuous or permanent use (for long periods of time ).

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Use of universal motors

The use of these motors in alternating current is very widespread due to the higher starting torque with respect to that of the induction motors and because of its high rotation speed, which allows to reduce its size and its price. The universal motor is undoubtedly the most used electric motor in the appliance industry.

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

Advantages and disadvantages of the universal motor

The universal AC motor has the following advantages:

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

On the other hand, the universal motor has the following disadvantages:

  • The commutator present in the electric motor is subject to wear and often causes sparks under the influence of the resulting anchor reaction. To limit the EMC disturbances, the motor can be equipped with suppression capacitors, placed in parallel to the anchor and the series inductors coupled in the supply lines to the commutator.
  • The universal motor has 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 must be laminated, but also the stator.
  • The use of machines with universal motor requires regular maintenance. With normal use, it would be good to blow the motor housing periodically with a compressor. This can extend the useful life considerably.
  • Due to the pair of pulses, this type of electric motor produces many vibrations and, therefore, a lot of noise.
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References

Last review: April 9, 2019