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Revolution Counter

A revolution counter is a device that tracks revolutions of a disc, shaft, or similar object. It can return

data about the speed of rotation, as well as simply recording the number of rotations. Some are

analog, requiring a mechanical connection with the device they measure, while others may use

techniques like laser sighting to collect information. A well-known example of a revolution counter isthe tachometer in a car, with a display providing information about the engine's number of

revolutions per minute.

The revolution counter can return data in the form of revolutions per minute, often with a needle

sliding along a graduated scale. The scale may have shaded zones indicating safe operating

speeds. High speeds may endanger the equipment and the scale could include green, yellow, and

red zones so an operator knows how well the system is functioning. If the revolution counter starts to

enter the red zone, the operator can make adjustments to slow down the revolutions per minute to

reduce the risks of damage or injury.

It can also operate with a simple rolling ticker, clicking over a new number with each revolution. The

counter can display digitally or mechanically. This displays the total number of revolutions, but does

not provide a time frame. In the case of a device like an odometerin a car, a revolution counter

tracking rotational speed moves a counter reflecting distance traveled. The revolution counter will

turn more slowly at lower speeds, and will speed up at higher ones, tracking overall distance.

The sensitivity of a revolution counter can vary. Some are capable of very detailed measurements,

including fractions of a revolution, while others are simpler. Generally, the more complex, the more

expensive. More complex devices are also more fragile. Their sensitivity may be ruined if they are

handled incorrectly or not maintained properly. Housing the device in a protective casing can help

keep it reliable and functional. Periodic calibration may be important to make sure the

measurements stay accurate.

Scientific suppliers, as well as companies selling machine parts, usually sell revolution counters. It ispossible to special order certain products for particular applications, or to buy generic models

designed for a wide range of uses. Most devices come with a guarantee, and should not fail within a

set time period or number of revolutions. If a device breaks or fails to register data properly, the

company will send a replacement at no charge.

A tachometer is an instrument designed to measure the rotation speed of an object, such as a gauge

in an automobile that measures the revolutions per minute (RPMs) of the engine's crankshaft. The

word is derived from the Greek words tachos, meaning "speed," and metron, meaning "to measure."

This device traditionally is laid out with a dial, a needle that indicates the current reading and

markings that indicate safe and dangerous levels. Digital tachometers have become more common,

however, and they give numerical readings instead of using dials and needles.

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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 knownfactor 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),

andpermanent 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.

The basic construction of all tachometer generator types closely follows that of conventional electric

motors with arotorrevolving 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 secondaryvoltage. 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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