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Rehfuss motors are named for the armature winding and field winding working in series. Single-phase series-wound motors are AC/DC universal motors; they can operate on either AC or DC power supplies. The single-phase series-wound motor is widely used. Its advantages include high speed, high starting torque, compact size, light weight, resistance to stalling, and a wide applicable voltage range. Speed control can be achieved simply and easily by adjusting the voltage.

Rehfuss motor operating mode

Motor excitation methods: Generators with this excitation method use a dedicated DC generator, known as a DC exciter. The exciter is generally coaxial with the generator. The generator's field winding receives DC current from the exciter via slip rings mounted on the main shaft and stationary brushes. This excitation method offers advantages such as independent excitation current, reliable operation, and reduced auxiliary power consumption. It was the primary excitation method for generators in past decades and benefits from mature operational experience. Disadvantages include slower excitation regulation speed and high maintenance workload, so it is rarely used in units above 10MW. Excitation without an exciter: In this method, no dedicated exciter is installed. Instead, the excitation power is drawn from the generator itself, rectified, and then supplied to the generator's field winding. This is called self-excited static excitation. Self-excited static excitation can be divided into self-shunt excitation and self-compound excitation. In self-shunt excitation, the excitation current is obtained from a rectifier transformer connected to the generator output, rectified, and then supplied to the generator field. This method features simple structure, fewer components, lower investment, and reduced maintenance. In addition to the rectifier transformer, self-compound excitation includes a high-power current transformer connected in series with the generator stator circuit. The function of this transformer is to provide a larger excitation current to the generator during short circuits, compensating for insufficient output from the rectifier transformer. This method utilizes two excitation sources: a voltage source obtained through the rectifier transformer and a current source obtained through the series transformer. Excitation via AC exciter: Some modern large-capacity generators use an AC exciter to provide excitation current. The AC exciter is also mounted on the generator main shaft. Its AC output is rectified and supplied to the generator rotor field. In this case, the generator's excitation method is separately excited. Since a static rectifier is used, it is also called separate static excitation. An AC pilot exciter provides the excitation current. The AC pilot exciter can be a permanent magnet generator or an AC generator with a self-excited constant voltage device. To improve excitation regulation speed, the AC exciter typically uses a medium-frequency generator of 100–200HZ, while the AC pilot exciter uses a medium-frequency generator of 400–500HZ. In these generators, both the DC field winding and the three-phase AC winding are wound in the stator slots. The rotor has only teeth and slots, with no windings, resembling a gear. Therefore, it has no rotating contact parts such as brushes or slip rings, offering advantages like reliable operation, simple structure, and convenient manufacturing. Disadvantages include higher noise and larger harmonic components in the AC electromotive force.

Rehfuss motor structure

The basic structure of a motor consists of a stator, a rotor, and other accessories. Function of the stator core: It is part of the motor magnetic circuit and holds the stator windings. Function of the stator windings: They form the electrical circuit of the motor. When three-phase AC is applied, they generate a rotating magnetic field. Function of the frame: It fixes the stator core and front/rear end caps to support the rotor, providing protection and heat dissipation. Function of the rotor: It is part of the motor magnetic circuit and holds the rotor windings in the core slots. Function of the rotor windings: They cut the stator's rotating magnetic field to generate induced electromotive force and current, forming electromagnetic torque that causes the motor to rotate. Characteristics: The structure is relatively complex, so wound-rotor motors are less widely used than squirrel-cage motors. However, by inserting additional resistors into the rotor winding circuit via collector rings and brushes, the starting, braking, and speed control performance of asynchronous motors can be improved. Therefore, they are used in equipment requiring smooth speed control within a certain range, such as cranes, elevators, and air compressors. Other accessories of three-phase asynchronous motors: 1, End caps: Support function. 2, Bearings: Connect rotating and stationary parts. 3, Bearing end caps: Protect bearings. 4, Fan: Cools the motor.

Rehfuss motor structure

The motor is divided into two parts: stator and rotor. Note: Do not confuse commutating poles with commutators. The stator includes: main magnetic poles, frame, commutating poles, brush assembly, etc. The rotor includes: armature core, armature (shu) winding, commutator, shaft, and fan. The rotor part of a DC motor consists of the armature core, armature, commutator, and other components. The following details each component of the construction. 1. Armature core: Its function is to house the armature winding and conduct magnetic flux, reducing eddy current and hysteresis losses in the armature core during motor operation. 2. Armature: Its function is to generate electromagnetic torque and induced electromotive force for energy conversion. The armature winding consists of many coils made of glass-fiber-covered flat copper wire or high-strength enamel-coated wire. 3. Commutator, also known as a rectifier: In a DC motor, it converts the DC current from the brushes into AC current in the armature winding, keeping the direction of electromagnetic torque stable. In a DC generator, it converts the AC electromotive force in the armature winding into DC electromotive force at the brush terminals. The commutator is a cylinder composed of many segments insulated from each other by mica. Each end of every armature coil is connected to two different commutator segments. In a DC generator, the commutator converts the alternating electromotive force in the armature winding into DC electromotive force between the brushes. Current flows through the load, and the DC generator outputs electrical power to the load. Simultaneously, current flows through the armature coils. Interaction with the magnetic field generates electromagnetic torque, the direction of which is opposite to that of the generator. The prime mover must overcome this magnetic torque to drive the armature rotation. Thus, while the generator outputs electrical power to the load, it consumes mechanical power from the prime mover, achieving the conversion of mechanical energy into electrical energy.

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