Eddy-current starters - Automatic and reliable starting devices for electrical motor
Until recently, most popular starters in Poland were these of resistance or liquid type. In practice, they cause many maintenance problems, because of long time of starting, high failure frequency, need of frequent maintenance and also environmental threat. Eddy-current starters, offered by Partner Serwis, thanks to their modern solutions, eliminate above-mentioned inconveniences.
Wide offer includes production of starters to inductive squirrel-cage and slip-ring motors and also to synchronous motors with asynchronous start-up. Large, initial values of starting currents cause creation of large dynamic forces, dangerous because of their threat of mechanical elements’ damage inside the motor (especially windings).
Apart from dynamic influence, large amount of heat, which is created in the motor, cause insulation and electric connections’ damages, especially in places, where there is hard to carry the heat, like rotor. Apart from starting current, percussive character of breakaway torque can cause bearing, clutch damages and even damage of the whole propelled machine. To these kinds of dangers, one must be prepared especially at large and medium capacity motors, and also in case of frequently repeated start-ups.
Large values of starting currents cause voltage drop in supply network, particularly adverse because of industry being highly saturated with elements of precise automation and computer science. The main task of an eddy-current startter is to limit the starting current, turn down the initial breakaway torque in squirrel-cage and synchronous, with asynchronous start-up, motors, or increasing the breakaway torque in case of slip-ring motors.
The principle of operation of an eddy-current starter is based on using the phenomenon of creation of rotary currents in solid ferromagnetic materials, influenced by time-changing magnetic flux. The flowing starting current will create variable magnetic field of current’s frequency, in the core of a starter. Part of energy, taken from the power source, during the start-up will be accumulated in the eddy-current starter, in the form of heat energy and as a magnetic field’s energy. An energy receiver, which a starter is, can be described by the parameters of symbolic resistance Rr2 and reactance Xr2 of the core (secondary side) and resistance Rr1 and reactance Xr1 fo the winding (primary side). Expressions setting electric parameters of the core of the starter are complex, and they are determined basing on the electromagnetic field analysis for nonlinear ferromagnetic environment, created by the steel elements of columns’ cores (solid ferromagnetic material).
The analysis is made basing on the Maxwell’s equations for linear environment with Nejman’s alterations, including variablity of magnetic permeability inside the core’s material. Depending on the motor’s type, the starter is connected to synchronous squirrel-cage motor stator’s network, or into the network of slip-ring motor’s rotor. Connected in series with the motor, it easens the effects of the start-up by lowering the supply voltage.
Declining starting current in angular velocity function, and also increase of the motor’s capacity value, as the slip of the motor decreases, at constant starter’s power factor, causes gentle increase of the voltage on the motor’s clamps. This phenomenon has a practical meaning, because it influences the motor torque’s increase in last stage of the start-up, and the lowering adverse current surges and brekaway torque in the moment of starter’s shorting.
It is especially useful in case of using eddy-current statory starter to asynchronous start-up of a synchronous motor, where voltage increase in the last phase of a strat-up eases the motor’s synchronization. The constructor featured variability of motor’s power factor at the designing level, which allows to propose optimal start-up systems, considering as well expenses as technical factors.
In order to limit the starting current and increase the breakaway torque to the value, determined by the type of a start-up, start-ups of slip-ring motors are performed using the eddy-current starters, implemented into rotator network. After plugging the motor into the network, in solid ferromagnetic elements of the starter’s core, the losses of active and reactive power occur, caused by the rotational currents, which are effects of passage of motor’s starting currents in starter’s winding.
Because of the fact, that losses of active and reactive power in the core, simultaneously the substitute pramaeters of resistance and reactance of the starter, are dependent on frequency and value of the starting current (magnetic permeability of core’s elements), that is why they change automatically during the start-up. It results in obtaining stepless characteristic’s course M=f(s) and I2=f(s).
Considering the torque, eddy-current starter provides constant or monotonely changeable value of power transmission system’s acceleration. Character of starter’s resistance and reactance changeability, thus shaping the characteristics of breakaway torque and starting current of the motor with, as well, statory starter, as the rotary starter, are carried out by the use of eddy-current coils’ taps change.
Besides, this change guarantees accomplishment of the series of type of every starter, for motors of different capacities and electric parameters.
The main part of the construction is a an eddy-current segment of the starter. It consists of three winded collumns, connected by shackles (upper and lower) on the pattern of a transformer (picture 3).The core of each collumn made of solid ferromagnetic components. Depending on the kind of start-up of the machine and a drive, starters are constructed as one- or two-section, with an additional resistance section or without such a section.
For the operation of the starter as a whole, dedicated control system is responsible. This system, among others, controls the contactors which switch the specific sections of a segment during the start-up of the motor.
- Automation of a motor’s start-up process,
- Fluent and stepless start-up,
- Automatic adjust of a course of a start-up to current machine’s load,
- Adjusting the starter to series of types for few motors of different capacities and parameter’s,
- Elimination of additional cooling element, for example, oil or water,
- Simple construction, good mobile reliability,
- Long-standing use without a need of maintenance,
- Competitive price considering other start-up systems.
The knowledge about power transmission systems, essential at designing the starters allows to propose optimal start-up systems, as well considering technical as the price factors. Equipping the starter with electrical apparatus is performed for individual order of the employer, depending on the place and conditions of work. Offered control systems allow controlling by master systems.
Eddy-currnet starters can be used in motors without capacity limit in power transmission systems of light, medium and heavy start-up, providing simultaneously few start-up procedures of a motor, one after another. Because of its construction, starters can be used for many years without maintenance. Massive steel construction of the starters’ core and reinforced heat insulation allow to use them in hardest conditions without failure occurence.
Above-mentioned features and good points of eddy-current starters caused the popularity of implemention it in drives of:
- Band conveyors,
- Ball mills,
- Jaw crushers,
- Cogging mills,
- Pumps,
- Ventilators
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- Presses,
- Steel mill shears,
- Centrifuges
- Winches,
- And others.
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Starters for motors of 400 – 2000 kW capacity have certificate, issued by the State Mining Authority to use in underground mining plants.
GE-115/05
GE-25/07
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