What’s a Speedo Generator?

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A tachometer generator measures rotational speed using an internally generated electrical signal. It falls into three categories: DC, AC, and permanent magnet types. Accurate readings are crucial for proper machine functioning and speed control.

A tachometer generator is an instrument that measures the rotational speed of a shaft using an internally generated electrical signal. This reference signal or voltage is generated by providing the tachometer with a direct connection to the shaft which rotates the internal mechanism of the generator at the same speed as the shaft. The generated voltage is then read from the tach circuit and displayed on a readout or sent to an external device such as a speed controller. The instruments are precisely calibrated and the exact output voltage of the generator at any speed is a known factor which allows for accurate rotational speed readings. In terms of operation and construction, tachometer generators fall into three basic categories: direct current (DC), alternating current (AC), and permanent magnet types.

Accurate rotational speed readings are critical to the proper functioning of many machines, particularly when variable loads and power inputs are involved. Installation devices such as engine speed controllers often use the output voltage of a tachometer generator as a reference to control the speed of the motors they drive. These small generators are driven from the engine or the machines output shafts via a direct connection with a 1:1 rotation ratio or via a carefully calibrated auxiliary drive. As the car rotates, it drives the generator which produces a voltage that varies in amplitude and frequency according to the rotational speed. This voltage is then sent to a reading device or motor speed controller.

The basic construction of all types of tachometer generators closely follows that of conventional electric motors with a rotor rotating inside a stationary stator. DC tachometers have an array of permanent magnets around the inner surface of the stator and a rotor with several sets of windings connected to a commutator. When the rotor is rotated within the magnetic field of the stator, an electric current is generated in its windings. This current is transferred from the tachometer via a series of carbon brushes that run across the commutator.

AC types have two sets of wire coils in the stator, one of which is supplied with an AC voltage. This induces a secondary voltage in the other coil similar to the secondary coil in a transformer. The rotor of the AC tachometer provides a short circuit path for the AC voltage similar to the rotor in squirrel cage motors. As the rotor is rotated within the two stator windings, it affects the ratio between the primary and secondary windings which, in turn, affects the secondary voltage. This means that the secondary voltage depends on the rotational speed of the rotor, thus providing a known reference voltage to measure the rotational speed of the machine.

The permanent magnet tach generator is similar in operation to the DC types except for an inverted arrangement of a magnet set on the rotor and the coils in the stator. As the rotor rotates within the stator, the electrical reference current is induced in the stator windings. In all three cases, the voltage produced by the tachometer generator is directly proportional to the speed at which it rotates. This allows independent circuitry to display rotational speed on a graphical readout or to use the information as a reference to control machine speed.

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