Three-phase collector motor
The three-phase collector motor belongs to the group of electric motors with infinitely variable speed and lossless control.
Basics
One differentiates essentially:
- Three-phase collector motors with main circuit behavior
- Three-phase collector motors with shunt behavior
In terms of their basic mechanical and electrical structure, three-phase collector motors differ from three- phase asynchronous motors only in that a normal DC rotor is used instead of the squirrel cage rotor . In three-phase collector motors, the stator windings are regularly equipped with a three-phase winding.
In times when variable-speed drives with electronics were still very expensive, the motor was often used as a variable-speed drive in the textile and paper industry. In the meantime, however, they have been almost completely replaced by three-phase asynchronous motors with frequency converters .
The three-phase shunt motor in a rotor-fed version is the only type that can still be found sporadically today.
The motors have a very high power factor , but are expensive because of the enormous construction effort and are poorly efficient .
Three-phase main circuit collector motors
These motors are similar in their operational behavior to a DC main circuit motor, i.e. H. with a fixed brush position, its speed decreases with increasing torque and increases with decreasing torque. The stator of these motors has a normal three-phase winding, which is usually connected in series with the rotor via a series transformer. The terminals U, V, W of the primary winding of the series transformer are connected to the mains. Terminals X, Y, Z of the primary winding are connected to terminals U, V, W of the motor stator winding. The secondary winding of the series transformer is connected in star. The beginnings u, v, w are connected in series with the rotor.
The purpose of the series transformer is to reduce the respective mains voltage to a voltage value that is favorable for commutation (approx. 100 to 150 volts when starting up, approx. 70 volts when idling). It transfers only a fraction of the motor power to the rotor circuit, which is smaller the closer the speed approaches the synchronous one. The series transformer is therefore only designed for part of the motor output.
If the mains voltage is very high, the entire energy supplied to the motor is reduced to the operating voltage in a transformer connected in front of the stator and rotor .
Starting and regulating the speed
Starting, regulating the speed and reversing the direction of rotation takes place in the three-phase main circuit collector motor only by moving the movable brushes arranged on the collector . When the motor is switched on, the brushes must be in the so-called zero position. The windings of the motor then only consume a small amount of current (around 10 to 20% of the rated current). The tightening torque is almost zero in this brush position, i.e. H. the runner does not start.
If the movable brushes are moved out of the zero position, the rotor starts up. The direction of rotation of the rotor is always opposite to the direction in which the brushes are moved. If you move the brushes closer to the zero position, the speed drops, if you remove them, the speed increases. In this way, within the control range of the motor, any desired speed can be set for any required power.
The power factor of the three-phase main circuit collector motor increases to 1 at the highest rated speed and rated power; but it sinks below the rated speed and rated power, d. H. when the brush adjustment angle is small. This disadvantage can be eliminated to a certain extent by using the star / delta switching if the star connection is used at low speed and power output.
If the brush displacement is small (low speed), the motor tends to behave unstably, i.e. H. it becomes difficult to set it to a certain speed. Because of the main circuit behavior of the machine, it is not suitable for all other drives that come into question. They were mainly used for drives where the main circuit behavior was desired or at least not disruptive, e.g. B. for fans , blowers, pumps , conveyors, roller lines, etc.
Three-phase shunt collector motors
A distinction is made between rotor-fed and stator-fed three-phase current shunt collector motors. In the case of the rotor-fed types, the mains voltage is fed to the rotor; in the case of the stator-fed types, it is fed to the stator.
Rotor-fed three-phase current shunt collector motors
The rotor has a normal three-phase winding that is connected to the network via three slip rings. In addition, a normal DC armature winding is arranged in the rotor, which, as can be seen in the circuit diagram, is connected to the collector. The stator winding is not connected to the network, but only to the rotor winding. The speed is controlled by two sets of brushes that can be moved against each other by taking a controllable multiphase voltage of the appropriate frequency from the collector and feeding it to the stator winding. Depending on the frequency of the voltage fed to the stator, the rotor can be brought to over or under synchronous speed. The control range of such motors is usually 1: 2 or 1: 3.
The three-phase shunt collector motors are similar in their operational behavior to the direct current shunt motor , i.e. H. with a fixed brush position, the speed is only slightly dependent on the load. The efficiency and power factor of these motors are also favorable in the lower control range. At nominal load the power factor = 1.
Stator-fed three-phase current shunt collector motors
In the stator-fed collector motor, the mains voltage is fed to the stator winding (designed as a normal three-phase winding). The rotor has a normal DC armature winding. The brushes arranged on the collector are connected to the secondary winding of a three-phase transformer fed from the mains. The secondary winding of the transformer is provided with taps, which enable certain voltage values to be impressed on the rotor. The stator-fed three-phase current shunt collector motors of the type described above were already outdated by the rotor-fed motors around 1950.
Sources and further information
literature
- Fritz Raskop: The electrical engineering trade . 3rd edition (1949), Technischer Verlag Herbert Cram, Berlin
- Hans Günter Boy, Horst Flachmann, Otto Mai: The master's degree in electrical machines and control technology. 4th edition, Vogel Buchverlag, Würzburg, ISBN 3-8023-0725-9