# Speed

## Concept

It is represented by the symbol "n"; its international standard unit is rps (revolutions per second) or rpm (revolutions per minute), and it is also expressed as RPM (revolutions per minute, mainly It is adopted by Japan and Europe, and our country adopts international standards). When the unit is r/S, the value is equal to the frequency, that is, n=f=1/T, and T is the period of circular motion. The linear velocity corresponding to a point on the circle is: v=2π*R*n, where R is the radius of rotation corresponding to the point. Common speeds are: rated speed and maximum speed. The international unit of the centrifuge is g, and the formula for rotating speed r/min to g: RCF=1.12*10^(-5)*r*(r/min)^2

### Rated speed

Rated speed refers to the maximum speed under the condition of rated power. Usually when leaving the factory, it is marked on the obvious part of the product as the main parameter of the product.

### Maximum speed

The maximum speed is the maximum value that the speed can reach under certain conditions. For example, the spindle speed of the motor in the hard disk is the maximum speed that the hard disk platters can reach under normal working voltage conditions. The speed of rotation is one of the important parameters indicating the grade of the hard disk. It is one of the key factors that determine the internal transmission rate of the hard disk, and it directly affects the speed of the hard disk to a large extent. The faster the rotation speed of the hard disk, the faster the hard disk can find files, and the transmission speed of the relative hard disk has also been improved.

## Features

The spindle motor of the hard disk drives the platters to rotate at a high speed, generating buoyancy to make the head float above the platters. To bring the sector of the data to be accessed below the head, the faster the speed, the shorter the waiting time. Therefore, the rotational speed determines the speed of the hard disk to a large extent. The rotation speed of ordinary hard disks for home use is generally 5400rpm and 7200rpm. Server users have high requirements for hard disk performance. The SCSI hard disks used in servers basically use 10000rpm, and even 15000rpm, whose performance is much higher than that of household products. A higher speed can shorten the average seek time and actual read and write time of the hard disk, but with the continuous increase of the hard disk speed, it also brings negative effects such as temperature rise, motor spindle wear, and working noise. The speed of notebook hard disks is lower than that of desktop hard disks, which is affected to a certain extent by this factor. The internal space of the notebook is narrow, and the size of the notebook hard disk (2.5 inches) is also designed to be smaller than that of the desktop hard disk (3.5 inches). The increase in temperature caused by the increase in speed puts higher requirements on the heat dissipation performance of the notebook itself; the noise becomes larger. , And must take necessary noise reduction measures, these have put forward more requirements on the notebook hard disk manufacturing technology. At the same time, the increase in speed, while the others remain unchanged, means that the power consumption of the motor will increase, the more electricity is consumed per unit time, and the working time of the battery is shortened, so that the portability of the notebook will be affected. Therefore, notebook hard drives generally use relatively low-speed 5400rpm hard drives.

In mechanical equipment, speed is an important technical parameter. The following table lists the speed ranges of commonly used industrial machinery:

 Project equipment name td> Speed ​​range (r/min) Application 1 Turbo expander 10000 ~ 450000 Air separation, deep refrigeration 2 Dental drill td> 200000 ~ 400000 Medical 3 p> Air motor 0 ~ 150000 Pneumatic tool 4 Turbocharger 0 ~ 100000 Car, ship 5 DC motor 0 ~ 20000 Electric tools, precision machinery< /td> 6 Engine 0 ~ 10000< /p> Vehicles, ships 7 < p>AC motor 1500 ~ 3000 Machinery, power equipment 8 Reducer 0 ~ 500 td> Mechanical equipment

## Development

The rotation speed changes with the increase of the hard disk motor. Fluid dynamic bearing motors have completely replaced the traditional ball bearing motors. Liquid bearing motors are usually used in the precision machinery industry. It uses mucosal fluid oil bearings and replaces balls with oil films. In this way, direct friction of the metal surface can be avoided, and noise and temperature can be reduced to a minimum; at the same time, the oil film can effectively absorb vibration, so that the anti-vibration ability is improved; it can also reduce wear and increase life.

## Measurement

Electronic rotational speed measuring instrument, composed of rotational speed sensor and meter (display). Most of the output pulse signals can be matched with voltage and current input type pointer meters and digital meters through frequency-current conversion, or directly sent to PLC; the methods of frequency-current conversion include resistance-volume method, charge pump method and application-specific integrated circuit method. The first two methods are also used in magnetoelectric tachometers. Most ASICs are a combination of resistance-volume method and charge pump method. When there are certain requirements on display accuracy, reliability, cost and flexibility of use, the pulse frequency calculation type tachometer can be directly used. Frequency calculation methods include timing counting method (frequency measurement method), fixed number timing method (week measuring method) and synchronous counting timing method.

The timing counting method (frequency measurement method) has an error of ±1 in measurement, and the error is relatively large at low speed; the fixed timing method (week measurement method) also has an error of ±1 time unit, at high speed , The error is also very large. Synchronous counting and timing method combines the advantages of the above two methods, and achieves high accuracy in the entire measurement range, and more than five ten thousandths of measuring speed meters basically use this method. In order to ensure the reliable performance of the generator, the output electromotive force of the motor tachometer generator has the characteristics of high slope, linear characteristics, small no-signal area or low residual voltage, small output voltage asymmetry during forward and reverse rotation, and low temperature sensitivity. . In addition, the DC motor tachogenerator requires a small AC component of the output voltage at a certain speed, and the radio interference is small; the AC motor tachogenerator requires a small output voltage phase change within the range of the operating speed.

Motor tachometer generators are widely used in various speed or position control systems. In the automatic control system, it is used as a component for detecting the speed of the motor, to adjust the motor speed or to improve the stability and accuracy of the system through feedback; in the solving device, it can be used as a derivative, integral component, or as an acceleration or delay signal. To measure the speed of various moving machinery in swing or rotation and linear motion. Motor tachometer generators are divided into two types: DC and AC. There are two types of DC motor tachometer generators: permanent magnet type and electromagnetic type. Its structure is similar to that of a DC generator. The permanent magnet type uses high-performance permanent magnet excitation, which is less affected by temperature changes, small output changes, high slopes, and small linear errors. This kind of motor developed rapidly in the 1980s due to the emergence of new permanent magnet materials. The electromagnetic type adopts the separate excitation type, which is not only complicated but also because the excitation is affected by factors such as power supply and environment, the output voltage changes greatly, so it is not used much. DC motor tachogenerators made of permanent magnet materials are also divided into limited-angle motor tachogenerators and linear motor tachogenerators. They are used to measure the speed of rotation or linear motion respectively, and their performance requirements are similar to those of DC motor tachogenerators, but the structure is somewhat different. There are three types of AC motor tachogenerators: coreless rotor asynchronous motor tachogenerator, cage rotor asynchronous motor tachogenerator and synchronous motor tachogenerator. ①Tachogenerator of hollow-cup rotor asynchronous motor: mainly composed of inner stator, outer stator and cup-shaped rotor rotating in the air gap between them. The field winding and the output winding are embedded in the stator, and the electrical angle is 90° apart from each other in space. The cup rotor is made of non-magnetic material. When the rotor is not rotating, the magnetic field generated by the cup-shaped rotor current after excitation is perpendicular to the axis of the output winding, and the output winding does not induce electromotive force; when the rotor is rotating, the magnetic field generated by the cup-shaped rotor coincides with the axis of the output winding and is in the output winding. The magnitude of the induced electromotive force is proportional to the speed of the cup-shaped rotor, and the frequency is the same as the frequency of the excitation voltage and has nothing to do with the speed. The output voltage phase is also reversed when reversed. The cup-shaped rotor is the key to transmitting the signal, and its quality has a great effect on the performance. Because its technical performance is superior to other types of AC motor tachogenerators, the structure is not very complicated, and at the same time it has low noise, no interference and small size. It is the most widely used AC motor tachogenerator. ②Cage rotor asynchronous motor tachometer generator: similar to AC servo motor, because of the poor linearity of the output, it is only used for occasions with low requirements. ③Synchronous motor tachometer generator: an alternator with permanent magnets as the rotor. Because the output voltage and frequency change with the speed at the same time, and the rotation direction can not be judged, it is inconvenient to use. It is rarely used in the automatic control system. It is mainly used for the direct measurement of the speed.