Description of a tachogenerator

A tachogenerator is an electric machine that converts the speed of the motor shaft into a signal of electric current. A tachogenerator is a part of an electric motor that is mounted or attached to a shaft. Than the output of the tachogenerator is connecting to the visual display or to the auto-tuning device that controls a speed of the motor shaft.

Main application fields of tachogenerators:

  • measuring the rotation speed of machines and mechanisms (asynchronic electric motors and DC motors);
  • oscilloscope curves of rotation speed change of shafts in function of time, for example: acceleration, braking, reverse of the electric motor;
  • for feedback in tracking systems.

Types of tahogenerators

Tachogenerators are divided into:

  • DC (induction);
  • AC (asynchronous, synchronous).

Description of dc-tahogenerators

DC-tachogenerators (Induction) - are generators of low power with independent electromagnetic excitation or excitation from permanent magnets.

Depending on the installation, DC-tachogenerators are available with a hollow shaft or with their own bearing system.

Components of the tachogenerator with shaft:

  • rotor (directly mounted on the motor shaft);
  • brush holder with brushes;
  • collector (in the form of an iron housing in which magnets are mounted or an iron ring with magnets placed in the middle);
  • locking screws.

A tachogenerator with its own bearing system is like a low-power DC-motor from the outside and is mounted on an electric machine by means of a clutch or adapter.

When operating the DC-tachogenerator it is important to follow the manufacturer's advice regarding the material of the brushes. In the case of incorrectly selected brush material, the tachogenerator will work with an error or increase the degree of wear and tear of the rotor, which will lead to breakdown.

Advantages of DC-tachogenerator:

  • weight and size is relatively small;
  • lack of phase error (at higher output power);
  • excitation by permanent magnets (allows to work without a power source for the excitation circuit).

Disadvantages of a DC-tachogenerator:

  • the presence of sliding contacts between the brushes and the collector (leads to the decreasing in the reliability of the tachogenerator and to the instability of the original characteristic);
  • presence of a zone of insensitivity;
  • ripple of the output voltage;
  • radio interference.

Description of ac-tachogenerators

AC-tachogenerators are divided into:

  • synchronous tachogenerators;
  • asynchronous tachogenerators.

Synchronous tachogenerators are non-collector machines of alternating current, which are manufactured as single-phase with a star-shaped permanent magnet rotor. This type of tachogenerators works with a variable frequency which complicates its use in simple circuits. It has a low sensitivity to the possibility of changing the direction of the motor shaft torque, so it is used for actuators of mechanisms with a low rotation speed.

Asynchronous tachogenerators are a two-phase executive induction motors with a short-circuited rotors (the rotor is made with a cavity in the middle). Excitation is carried out through the AC-network.

As well as asynchronous motors with a short-circuited rotor they have two phase windings on the stator with offset axes in space with relative to each other by 90°. One of them connects to the network and is called excitation winding, and the other winding removes the output voltage of the tachogenerator.

Advantages of AC-tachogenerators:

  • contactlessness (no sliding contacts);
  • low inertia (due to the low moment of inertia of the rotor);
  • the presence of a small moment of resistance (due to a lack of radial and axial forces acting on the rotor);
  • high reliability;
  • stability of characteristics.

Disadvantages of AC tachogenerators:

  • non-linearity of the original characteristic;
  • presence of phase error;
  • the presence of zero (residual) voltage;
  • low output power;
  • large dimensions and weight.

Thus, any type of tachogenerator has its disadvantages and advantages. Because of this when choosing a tachogenerator it is necessary to proceed from the specific conditions of its operation and requirements to the tachogenerator from the automatic device for which it is intended.