When you turn to the work of any device, machine or device for some time in these processes occur, which are called non-stationary or launchers.The most famous examples of life - to pull away, let's say, a freight truck, train, quite clearly shows that the initial power boost is usually required more than the effort in the future.
The same phenomena occur in electrical devices: lamps, motors, solenoids, etc.Launch processes in these devices depend on the status of work items: filament lamp, the state of magnetization of the core of an electromagnet coil, the degree of ionization electrode gap in discharge lamps, etc.For example, consider a filament lamp.It is well known that in the cold state, it has a much lower resistance than at its
heated to 1000 deg.in operation.Try to calculate the resistance of the filament
for 100-watt light bulbs - about 490 ohms, and ohmmeter measured in idle state, this value is less than 50 ohms.And now the most interesting - count inrush current, and you'll understand why when the bulbs burn.
It turns out that when the current reaches 4-5 A, which is the power consumption of more than 1 kW.So why is the 100-watt light bulbs do not burn, "without exception"?Yes, because, being heated, the filament light bulb has
growing resistance that in the steady state becomes permanent, great initial value and limits the operating current at about 0.5 A.
motors are widely applied in the art, so the knowledge of the characteristics of theirstarting characteristics is of great importance for the proper operation eletroprivodov.Slip and torque on the shaft - the main affecting the starting current settings.First binds the rotational speed of the electromagnetic field with the rotation speed of the rotor and decreases the speed of one set to the minimum value, and the second determines the mechanical load on the shaft, the maximum at the beginning of starting and rated after complete dispersal.The asynchronous motor is started, equivalent to a short-circuited transformer secondary winding.Because of its small
resistance motor starting current abruptly reaches a tenfold excess of its nominal value.
supply current to the coils leads to an increase in the saturation magnetic field of the rotor core, the emergence of the emfself-inductance, resulting in increased
inductive circuit resistance.The rotor starts to rotate, and the slip ratio decreases, i.e.motor accelerates.At the same time with the growth of the inrush current resistance is reduced to a steady-state value.
problems, causes an increased inrush currents arise
due to overheating motors, electrical network overload at the moment
start, occurrence of shock loads in the connected mechanical mechanism, such as gears.There are two classes of devices to address these issues in modern technology - soft starters and frequency converters.
Their choice - an engineering problem with the analysis of many
operational characteristics.The load in real conditions of use of electric motors is divided into two groups: Pump and Fan and general.Soft starters are mainly used for loads of fan groups.Such regulators limit the inrush current to a level not higher than 2 nominal values, instead of 5-10 times the ordinary start-up, by changing the voltage of the windings.
most widely used in the industry received AC motors.However, their simple design and low cost has a downside - the heavy start conditions, which are facilitated by means of frequency converters.A particularly valuable feature of frequency converters
support the inrush current of the induction motor
for a long time - a minute or more.The best examples of modern converters are intelligent devices that perform not just managing the start, but the start-up and optimization for any given performance criteria: the magnitude and consistency of the starting current, slip, shaft torque, an optimal power factor, etc.
