Speed controller - a device that changes the engine rotational speed. Most often refers to electrical devices.
Why do I need to adjust the speed
Aside from the obvious reasons, when the hardware device requires several speeds, referred to a number of other reasons. For example, at the start of induction motors consume a large current is not able to develop full torque. Consequently, the elevator lifts start to rotate slowly. Meanwhile, the asynchronous motors three phase squirrel cage are now considered the most common type of equipment. In most of the following control methods are applied:
- Changing the slip due to the introduction of dimmers. Normally included in the circuit through phase rotor current collector in the form of rings. The initial conditions are highly dependent on the load on the shaft.
- Changing the frequency of the rotating field is now used more often, but it remains a rarity. Frequency converters change smoothly or steps. Sometimes switching is performed to change the number of winding poles. In the latter case, the rate is increased steps. Applied values: 750, 1500, 3000, 500, 600 and 1000 rpm, respectively, and the number of poles - 2, 4, 6, etc.
- Changing the voltage amplitude. It applies to all types of electric motors. A collector is often made for the change in transmit power by the PWM modulation (in washing machines, for example, changing the angle of the cutoff voltage).
If we take the specific examples, switching coils is considered a simple means, rather than the construction of the frequency converter. For this reason, the method is used more often. Operating modes arbitrarily divided into four groups:
- Engine at any speed gives the same power. Mode characteristic of the majority of machine tools for metal working, wood.
- The power increases in proportion to the speed. Often found in air conditioners.
- Shaft power ahead growth rate: centrifugal pumps and fans.
- high speed power decreases.
In practice, the first two groups are more common: P = const, M = const. Application multirate engines improves usually process:
- Woodworking industry, where the idling speed is very different from working.
- Smooth start-lifts require speed adjustment. Gradually, the platform is gaining momentum.
- Control shaft effort to help protect the equipment. Any food grinder fits the definition. In a food processor is present, in addition to manual and automatic speed control. Thyristor scheme focused on sparking rate and flexibly adjusts modes.
- Installation, the speed of which is influenced by time of day. For example, air conditioning systems.
- Equipment for oil wells, where a single motor is used for a variety of tasks. Here refers steel rolling industry.
- Engines combined with speed regulators.
An additional advantage of the controller of the asynchronous engine speed becomes possible to greatly reduce the rush current. Sometimes capable to 10 times the nominal, causing failure protective automation. Advantageously, in this regard, see Changing the number of poles that is used in marine engines. The machines are often used mechanical speed regulators, similar to say about cars (gearboxes).
In an electric hand tool change advantageously smooth. the speed controller is designed as a potentiometer, which changes power settings transistor or thyristor key by varying the supply voltage. Reverse is made collector engines commutating windings for the electric tool is rarely true, not including screwdrivers. Food processors demonstrate generally range speeds and reverse. Sometimes it is easy one way, sometimes the other, it all depends on a compromise between price and simplicity of the product. In the industry, in addition to these, other factors appear.
Mechanical characteristics of the engine
The motor torque is considered to be the most important characteristic. Typically, the value decreases with increasing speed, if the consumed power of the former. Depending on the formula of torque of the power is not easy to find in the textbooks and the Internet today, it is shown on the screenshot. Each engine marked nominal parameters and limits. This also applies to the torque.
Formula according to torque
The rotational speed depends on said parameter, in addition, they are inversely proportional. Because the driver before entering the rise reduces transmission. For asynchronous electric motor connection is shown in the graph. The shape of the curves shows that the rotational speed of the motor is always greater than the generation. And rightly so, between the rotating field and the rotor will always be a difference. Consequently, for generating voltage 50 Hz frequency axis must do more revolutions (3000 ppm).
Graph shaft speed
From the mechanical characteristics of the springs, which during braking there is a great negative torque, equivalent returns back to the power network. There is known to many reactive component in one of the manifestations. Operating portion becomes ab, where the engine can develop the maximum moment. The mechanical characteristics of the engine used for calculating the circuits of the speed controller. For example, motors with squirrel-cage rotor, loaded on a rheostat, this line 2-4. All speed regulators are measured at the mechanical characteristics.
Continuously variable transmission now firmly established in the automotive industry. Their feature is considered to be a smooth adjustment of speed to the needs of the driver. V-belt transmission is performed through a system of cones. Their number and parameters of successful operation of the device depends.
In industry, there are variable speed electrical, mechanical or hydraulic. About the manual control is no question because of the large capacity. For example, horsepower does not even reach a kilowatt. Pontiac 700 horses not cool when compared to hydroelectric turbine, developing Megawatts. Of course, the power plant does not contain a variator shaft rotation speed controller. It shows an example for the creation of presentation of the facilities that appear in the production.
Stepless speed regulators - satellite indispensable tools for precision machining and quality parts. It was not until the advent of electronic controllers. Most industrial CVTs operate with a multiplication factor to 5, inclusive. Spread spectrum complicates the design velocity jump, which greatly reduces the benefits of the technology. Reduced switching reliability. do the following in practice:
- Starts commutated motor with two poles at a speed ratio of 1 to 4.
- The moment is transferred to the executive part of the machine through a CVT with an identical range.
- The result is an installation where all the values of overlap. Like used in the automotive variator.
Acceptable described in the kinematic chain to add other reduction gears, belt or wedge-shaped, producing a further increase in the range.
induction motors, limited slip clutches ratio of 1 to 4 and allow to change speed smoothly. Efficiency of such a device for sliding underestimated value (in the literature denoted S). By sliding is meant the difference between the rates of rotation of the rotor and the drive equipment. If the difference in the 80% (maximum) efficiency will be only 20%. power loss occurs entirely on the clutch drum.
The method finds limited applications. It is used in a combined embodiment: the combination of the multi-motor with clutch relatively narrow range. As a result, the efficiency increases significantly. To better illustrate this, consider an example:
- When using the sliding sleeve with adjusting the range 1 to 5 the efficiency is 20%.
- Engine power 5 kW four of them will be scattered by the friction force in the clutch. Belt slip around the drum, reducing the transmission rate.
This is the simplest way to change the speed of the motor, but expensive to assemble. It is often used in outdoor fans like the most invisible. As a result of a couple of buttons at the front allow to perform the necessary manipulations. Usually this type of motors show two, three or four speeds. As a rule, it is a model with short-circuited rotor.
Less technical solutions described in that lost power at low speeds, using only part of the winding. In practice, the engine is hard to find the number of windings of more than two. Analysis shows that in this case is half consumed more copper and steel - by 30-40%. Simultaneously, an effect of reducing the efficiency of the power factor due to gravity (reactance increases). This is seen by the example of asynchronous motors 220 with the start winding, invariably switched off when the rotor attains momentum. This is to reduce the proportion of reactive power.
Two separate windings and provide benefits, otherwise they are not used. The circuit switching is obtained easier, and the technical characteristics of the motor as close as possible to specify. In engines with a ratio of speeds 1 to 2 necessarily applicable only winding. The two coils are found in trohskorostnyh models. Particularly complicated three-phase motors, where the same number of revolutions occurs denominations different number of pins, depending on the circuit type (triangle, star, etc.).
It is in the process of assembling pay attention when designing. For example, for four speeds with a minimum number of terminals required to take the ratio as 1 to 2: 500, 750, 1000, 1500 or 500, 1000, 1500, 3000. Otherwise, the number of contacts in the relays is greatly increased. As a result, most of the multi-speed motors shows two windings.
Often changing shaft speed resistance are used in the circuit of three-phase stator. On start-up time is recorded extremely high current, to repay the rheostat is operated at full. As the acceleration difference between the fields is reduced, the resistance becomes smaller. This increases the flux linkage, shaft comes gradually to the nominal mode. Ultimately resistor in parallel across the closed switch on and off. Initial data for the calculation of the values will be:
- Engine's type. Read labeling.
- operation. For example, long. The data is taken from the passport.
- Rated power is usually listed on the label.
- The rated voltage is specified separately for the stars and triangle, for example, 220 and 380 V.
- Rated current. Usually indicated line (between the phases).
- Shaft speed.
- Sometimes indicated: connection diagram of windings of the rotor, the nominal EMF on its open-loop terminals at nominal voltage, nominal rotor current (operating).