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What is a Tachometer Generator?

Paul Scott
Paul Scott

A tachometer generator is an instrument that measures the rotational rate 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 rotating the generator's internal mechanism at the same speed as the shaft. The generated voltage is then read by the tachometer circuitry and displayed on a readout or fed to an external device such as a speed controller. The instruments are precisely calibrated, and the exact output voltage of the generator at any given speed is a known factor making 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 correct operation of many machines, particularly where variable loads and power inputs are involved. Installation devices such as motor 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 by the motor or machines output shafts either by means of a direct connection giving a 1:1 rotational ratio or via a carefully calibrated auxiliary drive. As the machine turns, it drives the generator which produces a voltage differing in magnitude and frequency according to the rotational speed. This voltage is then sent to a readout device or motor speed controller.

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Man with a drill

The basic construction of all tachometer generator types closely follows that of conventional electric motors with a rotor revolving within a stationery stator. DC tachometers feature a set of permanent magnets around the inside surface of the stator and a rotor with several sets of windings connected to a commutator. When the rotor is turned within the stator's magnetic field, an electric current is generated in its windings. This current is transferred out of the tachometer via a set of carbon brushes which run on 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 generator offers a short circuit path for the AC voltage similar to the rotor in squirrel cage motors. When the rotor is turned within the two stator windings, it affects the relationship between the primary and secondary windings which, in turn, affects the secondary voltage. This means that the secondary voltage is dependent on the speed at which the rotor is spinning, thus supplying a known reference voltage to measure the rotational speed of the machine.

The permanent magnet tachometer generator is similar in operation to DC types with the exception of a reversed arrangement of a magnet set on the rotor and coils in the stator. When the rotor spins within the stator, the electric 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 turns. This allows independent circuitry to either display the rotational speed on a graphic readout or to use the information as a reference to control the speed of the machine.

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