Universal motor

The single-phase series motor is an electric motor that runs on AC or DC, without having to make changes to the engine. It is identical to the DC series motor, with few details. The single-phase series motor belongs to the group of AC commutator motors, it is therefore also called single-phase commutator motor. Smaller single-phase series motors are also referred to as a universal motor or universal motor.

  • 4.1 Equivalent circuit diagram
  • 4.2 Torque 4.2.1 Torque equation derived from the internal power
  • 4.3.1 Transformer voltage
  • 4.3.2 reactance
  • 4.3.3 Movement voltage
  • 5.1 Special features compared to DC motors
  • 5.2 Differences between 16 2/3- and 50 -hertz motors
  • 7.1 advantages
  • 7.2 disadvantages
  • 8.1 Application Examples

Basics

The structure of the single-phase series motor is almost identical with that of the series-wound DC machines. It differs in its design only by the squat rack package that forms a unit with the pole pieces, the normal DC series motor. To reduce the eddy current losses caused by the alternating field, the entire magnetic circuit must be performed laminated ( dynamo sheet ). The principle of operation of single-phase series motor is the same as the DC machine. Due to the series connection of the armature and field winding of the exciter current ( excitation field ) and the armature current are in phase, that is, exciting current and armature current to change at the same time the sign of force and the direction of rotation remains the same.

Single-phase series motors are indeed designed for AC voltage, but can also be operated with DC voltage. However Conversely, a DC motor can not be operated with AC power, because most interference caused by eddy currents in the stator self-induction voltages would occur due to stray fields of the armature and field winding. Due to the inductive reactance of the output AC voltage of 15% is smaller than dc voltage. Small Einphasenreihenschlussmotoren today are designed as universal motors with a rated output of 3 kW and a rated speed of 3,000 to 30,000 revolutions per minute. Due to the high speed of this universal motors can be made smaller. Since in small motors, the current density is very large, results in a high power loss, the engine warms up. This problem is resolved but ( cooling) again due to the high speed.

Construction

The stator housing comprises the entire motor assembly, and for smaller motors, a cast part, for larger motors, a welded construction. Smaller motors have the stator frame cooling fins, bigger engines, however cavities for cooling air flow. The motor consists of a stator which has a DC motor such as that of the salient poles, but is performed in contrast to that of a laminated core. However, motors for high performance no more salient poles, with them, the stator consists punched dynamo sheet profiles. The magnetically active parts of stator and armature are composed of "packages" of one side against eddy currents isolated dynamo sheets. These are punched as the entire ring or as individual segments. The sheet package is pressed or shrunk into the stand. Both stator and armature laminations are usually shaped so that it takes only one mint a variety of pole pairs.

The electrically active parts consist mainly of the stator or field winding and the armature winding. Since the field winding is divided symmetrically to the anchor, which act part of the field winding coils as inductors and thus contribute to the suppression of radio interference at. The field windings and armature windings are in series, and connected " in series ". It is located in front of the armature winding one excitation winding (main winding). The series winding has only a few turns, but with a large cross section. The copper wire windings are wound in slots in the stator and the armature lamination packet or inserted as prefabricated windings. In the slots above and an insulation material layer is inserted. For electrical insulation of the wires to each other and to stabilize the windings can be impregnated with special paint and dried.

The rotor is identical with that of the direct current motor. The rotary armature consists of that applied to the axle shaft armature core and the armature coil winding. The armature shaft is supported in the stator frame, and transmits the torque of the electric drive is mechanically connected to the engine or the transmission. On one side of the laminated armature core, the commutator or collector is appropriate by the currents of the armature coil winding are guided via the carbon brushes mounted in the stator to the stator winding or at the terminals. The brush holder is used to hold the brushes in the required position.

According to the intended use, the stator frame fastening devices available. Have traction motors for electric railways special devices for resilient suspension between the support structure ( or bogie locomotive body ) and the pinion gear on the wheelset.

Operation

Principle

The current flowing through the exciter winding in the stator generates a magnetic field that is enhanced by the iron core and bundled into defined Poland. The same happens in the downstream armature coil winding. This magnetic field built up by the stator causes, depending on how the poles are set so that it drags the rotor behind or pushed in front of. In this case, the coils must be reversed in polarity at each half revolution ( for machines with a pair of poles ) to which a commutator is required.

Looking at an execution only " single-pole ", both magnetic fields are transverse to each other. According to the laws of physics creates forces that seek to unite the two magnetic fields to a common unipolar field. This force effect of the armature is rotated. However, since rotation of each armature to the commutator, an electric polarity reversal occurs, the initial state of the magnetic field each time again sets, so that a continuous rotation is performed, as long as the current flows through the armature and field winding.

The constantly repeated by the alternating current induced reversal has no effect on the running performance, as always, both windings are " reversed " at the same time. When operating with a sinusoidal alternating current, the torque of a sinusoid at twice the mains frequency follows. The minima of the sine curve are slightly negative. For operation with the DC commutator motor generates a temporally constant torque.

Commutation

The brushes are moved against the direction of rotation of the commutation during motor operation, the Hauptpolfluss induced in the coil to move the commutating voltage, that supports the commutation. In generator mode, the brushes must be shifted in the direction of rotation, thereby shifting the brushes depends on the operating condition. Complete compensation is only in a certain operating point possible ( nominal point ). However, a brush shift to improve the commutation leads to a weakening of the field exciter poles ( has component opposite ); Observed stability of the machine.

Since the commutator is not adapting (that is, always switches perpendicular to the main field lines and not perpendicular to the "effective" field lines ), the sparking can be reduced by the brush carrier is slightly rotated assembled and in the operating state but switch perpendicular to the effective field lines. However, this requires an adjustment in operation and is rarely performed today for reasons of cost. Instead, in large machines commutating and compensating windings are used, which virtually " turn back " the field lines in the ideal location. Commutating poles are used only in larger single-phase series motors as the train engine. Small single-phase series motors have no commutating poles and no compensation winding.

Problems

Reversing the polarity of the stator, the sinusoidal voltage induced in the rotor which is dropped across the brushes. This voltage transformer can not be compensated by brushing shift. Therefore, there is, in contrast to the direct-current machine, an AC voltage induced at the carbon brushes, which leads to a constant brush fire and a high wear of the brushes (but remedy commutating and compensating windings). In addition to the associated constraint for radio interference suppression capacitors by this engine through the life of the engine is greatly reduced in comparison with induction machines.

Performance

When single-phase series motor the field current decreases with decreasing, which leads to a speed increase with decreasing torque. This behavior of an engine is referred to as a series circuit behavior. He has no fixed speed limit, which would be larger units unloaded run up to the bursting of the anchor. For this reason, a centrifugal switch is mounted on the motor shaft to hedge on some engines. This switch turns on critical engine speeds an ohmic resistance to, or shuts down the engine completely.

  • At standstill when turning on the highest current through the armature and stator winding.
  • The series motor has all electric motors, the largest start-up torque.
  • At idle, or with little or no stress -driven series motors go through with ever increasing speed.
  • Under load the speed decreases, while the torque rises again due to the reduced speed, it settles down, a stable state.
  • The speed is very load dependent.

Mathematical viewing

Equivalent circuit

Since the machine is operated with alternating current, except the ohmic resistances of the windings all inductive resistances are taken into account. The reactances are combined into a reactance:

  • Total reactance ...
  • ... Reactance of the exciter coil
  • ... Reactance of the armature winding
  • Commutating reactance of ... (if applicable)
  • ... Reactance of the compensation coil (if present)

Is the saturation of the magnetic circuit depends on (especially, the main field, the stray field ). therefore decreases with increasing load. is assumed to be constant for further drainage.

Since the axis of the armature coil is perpendicular to the excitation winding in the armature winding transformer voltage is not induced, that is, only the motion voltage is in phase with the field and current.

Torque

The torque is calculated analogous to the DC machine. There are to use the RMS values ​​for AC operation for current and voltage.

With   - Inner torque   - Rms current   - Excitation field strength ( proportional to the current I)   - Anchor motor constant

The excitation field caused by the current flowing through the armature and the field winding. Therefore, the torque is proportional to the square of the operating current. Under load, the device shall:

  • Load moment ...
  • Torque loss ...

Due to the inertia of the machine and the load resulting in a mean speed is established. As a result of the pulsating torque of average speed but is superimposed on an oscillating speed.

Torque equation derived from the internal power

The moment pulsates at twice the mains frequency.

In commutating coil induced voltages

Transformer voltage

The coil axis of the coil coincides with a commutation of the excitation field axis, that is, the commutating voltage of the transformer coil, a variable excitation field induced.

  • Transformer voltage ...
  • Magnetic flux ...
  • Mains frequency ...
  • Number of turns per coil ...

The transformer voltage is proportional to the line frequency, but independent of the speed ( it also occurs at a standstill on ) and lags the current 90 ° before.

Reactance

The reactance opposes the change in current. Commutation = tK. The current change depends on the commutation from → reactance depends on the commutation.

  • Reactance ...

Movement voltage

Since the turn of the field varies sinusoidally in time, the voltage induced movement of commutation depends.

  • Voltage ... the turning field
  • Peripheral speed of the armature ...
  • ... Magnetic flux density of the field reversal
  • With commutating
  • - Without commutating

With the turn of Poland the transformer voltage can not be compensated.

The spark voltage can be fully compensated only for a specific speed. At standstill, the transformer voltage is not compensated by the commutating poles, that is, must be kept small.

  • Small → D -A -CH- rail power supply with 16 2/ 3 Hz,
  • Flux per pole small → large number of pole pairs

Current locus

With the current locus to obtain a relationship for the current vector between the impedance and the speed n as a real variable. As with the asynchronous machine is created by inverting a circle. Each stream pointer can be assigned to a selected resistance scale, a fixed speed. The lower half can not be used as in the asynchronous design to the flow arrows in the generator mode.

The maximum current is obtained for:

  • Power ...
  • Voltage ...
  • Impedance ...
  • ... Fictitious synchronous speed (which is a synchronous motor having the same construction could )
  • Frequency ...
  • Number of pole pairs ...
  • Erregerwindungszahl ...
  • ... Reactance of the exciter coil
  • Phase shift angle ...
  • Reactance ...

Special

Unique features compared to DC motors

The magnetic field of an AC motor track pulsates with the frequency and induced in the short-circuited under the brush rotor windings is harmful to the commutation voltage. This voltage is called voltage transformer (); it is proportional to the frequency, the number of turns and magnetic flux. The other stresses occurring in the shorted turn are:

  • The reactance (), which is proportional, depending on the rotor current and the rotational speed ( the reactance is formed by reversing the polarity of the leakage flux of the commutating coil)
  • Originating from the non-compensated residual rotor field voltage (), which is also of the armature current and the speed dependent

These voltages are compensated by the turn of the field voltage, which is also of the rotor current and the rotational speed -dependent. ', And together form the so-called spark voltage. The transformer voltage can not be compensated for the entire speed range with simple means. If you were to move the turn of field voltage in phase, one could compensate the spark voltage completely. This is made possible by the parallel connection of an ohmic resistor to the commutating for a particular operating point.

The transformer voltage during start-up at its greatest and must not exceed certain values. In order to keep small, the following measures can be taken. These are usually only for large motors Application:

  • Out of the excitation flux per pole (which leads to large numbers of poles ) to get great benefits
  • And number of turns equal to 1 and loop winding. This in turn requires a high number of armature coils and therefore a large number of segments, which is limited for mechanical reasons.

Thus, the driven by the transformer voltage current is reduced using partial Spreizkohlebürsten that increase the resistance between the shorted coil to the collector.

So that the power factor as close as possible to 1, the rotational voltage has to be large. This is possible by a large number of sipes, a low frequency and a high speed. The power is proportional to the number of pole pairs, the magnetic flux, the rotor current pad and the rotor peripheral speed. The number of pole pairs is limited due to the constructive potential brush maintenance grant. Because of the voltage transformer is the magnetic flux limited for thermal reasons, the rotor current density and mechanical reasons the rotor peripheral speed. The motor voltage is determined by the number of plates and the number of pole pairs (due to the allowable voltage lamellae ). Caused by inductive resistors, a phase shift between the current and the voltage. The pulsating torque of twice the mains frequency is replaced by a negative percentage as a function of the phase shift. The reversal of the direction of the torque leads to large mechanical stresses and a shaking of the engine, especially at start-up.

Running a DC shunt motor with AC power, would thus be absurd, because a phase difference between stator and rotor current would be produced by 90 ° by the inductance of the stator which would make the average torque generated to zero.

Differences between 16 2/3- and 50 -hertz motors

For 50 -hertz motors the same laws as for 16 2/3-Hertz-Motoren apply, only in this case the transformer voltage plays an even greater role. Because of the three times this frequency would be three times as large. Therefore, one must take special measures to prevent this. With the same design principles can, at the same transformative power, reach only 1/3 the power of 16 2/3-Hertz-Motoren. To prevent this, you have over 16 2/3-Hz-Motoren reduce in which either the iron length is reduced to 1/ 3 or triples the number of pole pairs of the magnetic flux to 1/3. However, the number of pole pairs can not be increased arbitrarily ( brush maintenance grant ). The low frequency of 16 2/3 Hz facilitate commutation.

If the arm length is performed, however, reduced to 1/3 and the number of pole pairs is maintained, as can be two partial motors stored on a common shaft, these engines then called tandem engines. These motors can then be in the same installation conditions 2/3 of the power of 16 2/3-Hertz-Motoren reach. Such tandem engines, however, are extremely complicated and expensive. Ultimately, they have, as well as 16 2/3-Hertz-Motoren lost significance because Drehstromasynchronantriebe offer some advantages.

Speed ​​setting

Pros and Cons

Benefits

  • Good speed setting options
  • High torque at low speed
  • Large tightening torque ( Beneficial for mixers, switches, drilling machines )
  • Lower mass of iron and copper as the same in an asynchronous motor performance ( particularly beneficial for portable devices with high engine power, eg vacuum cleaner, electric drill )

Disadvantages

  • Higher production costs than asynchronous
  • Speed ​​change regulation is necessary in load ( hyperbolic curve ) → constant speed
  • Or brushes, commutator needs maintenance
  • Brush fire
  • Radio interference suppression is necessary

Applications

As traction motors single-phase series motors were ( with small turning Poland and compensation coils ) used earlier. For this purpose, the operating voltage of 15,000 volts was transformed down to about 20 volts to about 600 volts by means of traction transformer and associated tap selector. To avoid unbalanced load large single-phase series motors can not be operated on the public network.

Today, single-phase series motors are universal motors as the major small engines. In addition to the drive for power tools, this engine is used in almost every electrical appliance. Often these devices are equipped with a tap changer, which for various load cases between taps of the field winding can be switched. But your short life prohibits its continuous use.

Also in washing machines they are often to be found - but for this they need to be reversed in polarity and equipped with a tachometer generator, in order to control the direction of rotation and speed can. The use of advantage here is washed out with a simple motor without change gear as well as being able to hurl.

Application Examples

  • Vacuum cleaner
  • Mixing, kneading and cutting machines
  • Mixer
  • Washing machines
  • Model Railways
  • Hand drills
  • Angle
  • Circular saws

Standards and regulations

  • EN 60 034 Part 1 General requirements for rotating electrical machines
  • EN 60 034 Part 8 Terminal markings and direction of rotation for electrical machines
  • DIN IEC 34 Part 7 types rotating electrical machines
  • EN 60034-5 protection of rotating electrical machines
  • EN 60034-6 types of cooling for rotating electrical machines
Magnetic core Electrical reactance Induction motor Hyperbola Repulsion motor
50509
de