# Universal motor operation

The current carrying excitation winding in the stator generates a magnetic field, which is amplified by the iron sheet package and grouped into defined poles. The same happens in the winding of the armature coil downstream. Depending on how the poles are established, this magnetic field created by the stator of the universal motor causes the rotor to drag behind or push it in front of it. In this case, the coils must be inverted in each half revolution (for machines with a pair of poles), for which a switch is necessary.

Considering a "single pole" version, both magnetic fields are perpendicular to each other. In accordance with the laws of physics, forces are created that seek to unite the two magnetic fields in a unipolar common field. By this force effect, the armature is rotated. However, in each of the induced rotation of the commutator, a change of electrical polarity occurs, provides the initial state of the magnetic field each time again, so that the armature rotates continuously while current flows through the armature and field winding.

The alternating-direct polarity caused by the alternating current does not influence the driving behavior of the universal motor, since both windings are always "reversed" at the same time. When working with sinusoidal alternating current, the torque follows a sinusoidal wave with dual network frequency. The minimums of this sinusoid are slightly negative. When operating with DC current the commutator motor generates a temporarily constant torque.

## Current tour

For the current application, the brushes move against the direction of rotation during the operation of the universal motor, the main flow of the pole induces a voltage of movement in the switching coil, which admits switching. In generator mode, the brushes must move in the direction of rotation, while the displacement of the brush depends on the operating state. Complete compensation is only possible at a specific operating point (nominal point). Changing a brush for improved commutation, but leads to a weakening of the excitation poles; Take into account the stability of the machine.

Since the switch does not adjust (ie it always switches perpendicular to the main field lines and not perpendicular to the "effective" field lines), sparking can be reduced by the brush holder that is mounted lightly twisted and then the operating state is switched perpendicular to the effective field lines. However, this requires adjustment in operation and is rarely done today for cost reasons. In contrast, universal motors for turning machines and compensating windings are used in large machines used, which "bend" the field lines as if they were in the ideal position. The investment poles are only used with larger single-phase universal motors, such as the traction motor. Small, single-phase motors do not have reverse poles or compensation windings.

## Universal motor problems

The inversion of the sinusoidal polarity of the stator field induces a voltage in the rotor, which falls on the brushes. This tension can not be compensated for by brush displacement. Therefore, it is not, unlike the direct current machine, an alternating current voltage induced to the carbon brushes at a constant temperature produce sparking wires and a high wear of the brushes (but remedy switching and winding compensators) . In addition to the associated pressure for the suppression of radio interference from this motor by capacitors in this way the service life of the motor is considerably reduced compared to induction machines.

## Operational behavior

In the single-phase series universal motor, the excitation current decreases with, which leads to an increase in speed with a decreasing torque. This behavior of an engine is known as a serial connection behavior. He has no fixed speed limit, the creation of larger units would run unhindered to the anchor beams. For this reason, a centrifugal switch is mounted on the motor shaft for the protection of some motors. This switch changes the ohmic resistance at critical engine speeds, or shuts down the engine completely.

• At rest when switched on, the highest current flows through the armature and the stator winding.
• The series motor has the highest starting torque of any electric motor.
• At idle or no load or low load, the series-wound motors pass through an increasing speed.
• Under load, the speed decreases, while the torque increases again due to the reduced speed, it stabilizes in a stable state.
• The speed depends a lot on the load.

## Principle

The current-carrying excitation winding in the stator generates a magnetic field, which is amplified by the iron sheet bundle and grouped into defined poles. The same happens in the winding of the armature coil downstream. Depending on how the poles are established, this magnetic field created by the stator of the universal motor causes the rotor to drag behind or push it in front of it. In this case, the coils must be inverted in each half revolution (for machines with a pair of poles), for which a switch is necessary.

Considering a "single pole" version, both magnetic fields are perpendicular to each other. In accordance with the laws of physics, forces are created that seek to unite the two magnetic fields in a unipolar common field. By this force effect, the armature is rotated. However, in each of the induced rotation of the commutator, a change of electrical polarity occurs, provides the initial state of the magnetic field each time again, so that the armature rotates continuously while current flows through the armature and field winding.

The alternating-direct polarity caused by the alternating current does not influence the running behavior of the universal motor, since both windings are always "reversed" at the same time. When working with sinusoidal alternating current, the torque follows a sinusoidal wave with dual network frequency. The minimums of this sinusoid are slightly negative. When operating with DC current the commutator motor generates a temporarily constant torque.

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Last review: May 23, 2018