Asynchronous Induction Motors and Alternators: Principles and Applications

Posted on Jan 4, 2025 in Geology

Materials of the Elements and Rotor of a Single-Phase Asynchronous Induction Motor

In the first place, normally the rotor induces currents circulating in the rotor. Consequently, the appearance of a magnetic field and its interaction with the stator’s magnetic field occurs. Depending on the type of rotor, these machines are classified as: squirrel cage rotor (shorted) and wound rotor (slip-ring rotor). The winding consists of a set of stacked sheets forming a cylinder, which has slots in its exterior circumference where the winding is placed. It is made of ferromagnetic material to avoid losses. The sheets have silicon. The center has a circular hole to introduce the shaft. In the case of a squirrel cage type, it has a series of conductors shorted by two lateral rings. In the case of a wound rotor, it has a three-phase winding similar to the one located in the stator. The three phases are connected on one side in a star (Y) configuration, and the other ends are sent to slip rings. This arrangement makes it possible to introduce external resistances through the rings to limit the currents.

Operating Principle of a Three-Phase Synchronous Induction Machine

This is a machine whose rotor rotates at the same speed as the magnetic field (n = ns). Therefore, its speed depends on the frequency and the number of poles: n = ns = (60f) / p = constant. The stator can be the same as that of a single-phase machine. When powered by a sinusoidal signal, it generates a rotating magnetic field. It has two windings: the inductor, powered by DC, and the induced three-phase rotor. The rotor is formed by a permanent magnet created by an excitation, which can be a shunt-type dynamo mounted on the same shaft as the machine (alternator). It will follow the direction of the stator’s magnetic field while the resistant torque is less than the electromagnetic torque of the machine. In the rotor and stator of machines with power greater than 10 kVA, the inductor is located in the rotor (the case of alternators), and it can have salient or smooth poles. As the magnetic field of the stator reaches the synchronous speed instantaneously (from 0 to ns), and due to the inertia of the rotor, it needs to overcome its resistant torque. It is necessary to overcome the initial inertia at startup by means of an auxiliary system (excitation system). The rotor shaft reaches the synchronous speed. In modern synchronous machines, starting is achieved by means of a squirrel cage winding in the polar heads, allowing it to start as an asynchronous motor.

Applications:

  1. Salient pole rotors: Turbines in hydroelectric power plants and diesel generators.
  2. Smooth pole rotors: Steam turbines.

Operation of an Isolated Alternator: Voltage and Power Regulation

The behavior of a synchronous generator (alternator) varies if it is isolated (standalone grid) or connected to others (infinite grid). According to the operating principle of the machine, an isolated alternator rotates at n = constant. When connecting the first load, the first current induced in the stator will generate a reaction that opposes the inductor’s magnetomotive force (MMF), and the resultant MMF decreases, causing a decrease in voltage at the terminals. If it is desired that the voltage remains constant, it will be necessary to change the excitation current (iex) by means of a voltage regulator incorporated into the exciter. Also, as the load increases, the alternator reduces its speed. This is detected by the speed regulator, which acts to increase the steam or water flow of the turbine connected to the alternator, increasing the speed of the group. In this way, we achieve that the voltage at the terminals remains constant while increasing the load. In summary, in an alternator working isolated in the grid:

  1. The frequency depends on the speed of the prime mover that moves the synchronous machine.
  2. The power factor of the generator depends on the power factor of the load.
  3. The output voltage depends on “n” (rotation speed), iex, the induced current, and the power factor of the load.

Conditions for Coupling a Three-Phase Alternator to the Grid and How to Control its Operation

To couple a three-phase alternator to the grid, four conditions must be met:

  1. Equal Voltages: The alternator must have the same voltage at its terminals (U) as the grid. First, we start the prime mover (DC motor) and regulate the excitation current to achieve the desired speed of the alternator. Then, we excite the alternator. A voltmeter is placed at the terminals of the alternator and another at the grid, ensuring they indicate the same value.
  2. Equal Frequencies: The alternator must have the same frequency as the grid. To achieve this, we regulate the speed using frequency meters connected to the grid and the alternator, ensuring they indicate the same value.
  3. Phase Coincidence: The voltages of the grid and the alternator must be in phase. To verify this, we place a synchroscope between the alternator and the grid. When the lamps are off, there is no potential difference, and when they are on, there is the same potential, indicating they are in phase.
  4. Same Phase Sequence: The lamps must have the same phase sequence. If they turn on and off at the same time, they are in phase. If not, we interchange two phases either in the alternator or in the grid.

Operating Principle of a Universal Single-Phase Motor

It is a series-wound commutator motor, similar to a DC motor with series excitation. It is called “universal” because it can operate on DC. Its operation is based on the fact that a sinusoidal and pulsating MMF produced by a single-phase AC can be decomposed into two rotating MMFs in opposite directions. It is impossible to reverse the direction of rotation because inverting the polarity of the voltage reverses the direction of the current, both in the inductor and in the induced (since they are in series), so the direction of the torque and rotation remains constant.

Advantages: Very high starting torque, proportional to i2 (Tarr = ki2). It has no speed limitation problems; it can be very high.

Disadvantages: It has poor commutation. To reverse the direction of rotation, it is necessary to change the connection between the inductor and the induced.

Applications: Machines that require high starting torque and speeds above 3000 rpm, such as grinders, drills, etc.

Operating Principle of a Squirrel Cage Induction Motor and its Laws

It consists of a three-phase stator winding that, when powered by a three-phase system, generates a rotating magnetic field of constant value and speed: ns = (60f) / p (synchronous speed). Preferably, we take: f(t, θ) = 3/2 * Φ * cos(ωt – θ) (or its equivalent, a magnetic flux Φ permanently rotating at a constant speed ns). In the bars of the rotor, an electromotive force (EMF) will be generated: e = (-dΦ) / dt (Faraday’s law). As they are shorted at their ends by rings, currents will result: i = e / Z (Ohm’s law). Inside the rotating magnetic field of the stator, a torque is produced (Laplace’s law): T = F * d = B * i * l * Φ * d = i. Therefore, it rotates at a speed n < ns. The relative difference in speeds is called slip: s = (ns – n) / ns.

Operating Principle of a Wound Rotor Asynchronous Induction Motor

This motor consists of a three-phase stator identical to all three-phase rotating machines (three groups of windings) that, when connected in star or delta and powered by a three-phase system of frequency f, creates a rotating magnetic field (B) of constant value and speed ns = (60f) / p. The rotor is formed by three groups of coils, with the same number of poles as the stator, connected in a star configuration, and their ends are connected to slip rings. Through these rings and brushes, it is possible to introduce additional variable resistances R2‘, allowing for progressive variation of torque and speed, which is not possible with a squirrel cage.

Advantages: We can regulate the speed and start at maximum torque.

Disadvantages: Greater constructive complexity, more expensive maintenance.

Applications: Phase shifters, frequency generators, variable speed motors.

Comparison: The wound rotor motor has the same stator as the squirrel cage rotor. Instead of shorted bars, its rotor winding is three-phase with the same number of poles as the stator, connected in a star configuration. The center of the star is internal, and the other ends are connected to phosphor bronze slip rings. Through brushes, we can access the terminals (u, v, w) from the outside. By accessing these terminals, we can modify the resistance by adding external resistances rx and thus vary the impedance. Consequently, we can vary the current, torque, speed, starting conditions, etc. This is not possible with a squirrel cage.

Direct and Stator Resistance Starting of Squirrel Cage Induction Motors

Direct Starting: When applying the nominal voltage directly to the terminals of the machine, it absorbs a current I and produces a starting torque Tarr. It is used only in low-power motors (UN < 5kW), and its advantage is the high starting torque.

Stator Resistance Starting: It consists of inserting a rheostat in series with the three-phase stator, which reduces the voltage in the motor, so that the voltage that reaches it is not the nominal voltage but a lower one. As the motor increases its speed, the resistance value decreases. The advantage is that it is inexpensive. The disadvantages are high power losses and a significant reduction in starting torque due to the high dispersion of energy (heat).