As a starting method is known in the electrical engineering methods that serve the starting current of electric motors to reduce. The individual starting processes are adapted to the respective motor type according to the network conditions, the level of the starting current and the run-up time.
When starting up an electric motor, the inrush current is considerably higher than the rated current . These current peaks put a strain on the power grid , which can lead to disruptive network drops in weak networks. For this reason, only motors with a pick-up current of a maximum of 60 amperes may be started directly in the public network ( DOL = direct online). In the case of single-phase motors, direct starting is permitted for motors with a rated power of up to 1.4 kW. In accordance with the technical connection conditions ( TAB ) of the electricity supplier, special starting methods must be used for motors with a higher starting current in order to limit the starting current. The starting procedures to be used must be agreed with the respective network operator.
The individual procedures
One differentiates between the starting procedures
- Classic tempering process
- Electronic starting procedure
- Starting aids
Classic tempering processes
Direct switching on is one of the classic starting methods, but is only permitted for smaller electric motors. There are two options for reducing the starting current:
- Increase in resistance
- Decrease in tension
To increase the resistance, either active resistances as starting resistors or starting resistors or special starting chokes are connected in series with the stator winding. Another possibility is to divide the stator winding into partial windings. To reduce the voltage, either special starting transformers are used or the stator windings of the motor are interconnected differently.
Electronic starting procedure
There are two different methods of electronic starting procedures:
In soft starters, the starting current is limited by means of phase control . The starting current is regulated by means of voltage and frequency setting by means of starting converters. It is the most demanding device-based starting process.
Under certain start-up conditions z. B. Heavy starting or with certain motors, additional measures are required to start the motor.
Starting aids for heavy starting
If the driven machine has a high moment of inertia, it takes a long time to reach the operating speed. One speaks of heavy starting. In the start-up phase, the motor is at risk from high operating currents. A coupling can be arranged between the motor and the driven machine so that the motor quickly reaches operating speed and the driven machine only gradually takes over this speed. One speaks of starting couplings. Centrifugal clutches , hydrodynamic clutches and induction clutches are suitable as starting clutches.
For very large three-phase motors or for certain motors, small starting motors are used which bring the large motor up to speed in the unloaded state, and the larger motor is only switched on after it has started up. This starting aid can only be used when idling.
Starting three-phase motors
Different starting methods are used for the respective three-phase motors.
Starting of squirrel cage motors
A large number of options are available for starting squirrel cage motors.
- Direct starting
- Star-delta starting
- Partial development start-up
- Starting with a stand starter
- Starting with a starting transformer
- Starting using electronic starting procedures
- Starting with a starting motor
On the public grid, the direct tempering is only for smaller cage motors possible. Motors with current displacement rotors and motors with resistance rotors are an exception . The start-up times for normal start-up are between 0.2 and 5 seconds, and start-up times of up to 30 seconds are possible for heavy start-ups. The direct starting is suitable for heavy starting.
The star-delta starting is the most frequently used starting method. With this method, the starting current is reduced to a third compared to direct starting. The disadvantage is that the starting torque is also reduced to a third. The run-up time is 2 to 15 seconds under normal conditions; With a heavy start, run-up times of up to 60 seconds are possible. The star-delta starting is not suitable for heavy starting due to the reduced torque.
Partial development start-up
For these starting methods, special wound motors with split coils are required. This process is used for large refrigeration system compressors with special motors.
Starting with a stand starter
This starting procedure generally corresponds to the start-up of DC motors. With the stator starter, either starting resistors or starting chokes are switched into the stator circuit. The stator starter is switched to the open star point either before the stator winding or when the motor is star- connected. These stator starters are also called star point starters. After the motor has started up, the stator starter is bypassed. Stator starters with resistors are available as stepped switchable fixed resistance starters or as liquid starters. Due to the voltage loss at the resistors, the starting resistors reduce the stator voltage and thus the inrush current. The disadvantage here is that the torque drops at the same time as the square of the voltage drop.
Instead of the starting resistor , a special choke coil is often connected to the motor line. The circuit variants are the same as for the starter with a starting resistor. Short-term network dips, especially in weak networks, are avoided by using starting chokes. In addition, disturbing network perturbations are reduced. The disadvantage is the power factor shift that results from the additional inductance. In practice, the starting current can hardly be reduced below 50% with this starting method. It is not suitable for heavy starting. The start-up time for normal start-up is between 2 and 20 seconds. However, this method is always advantageous when the switching process must not lead to a temporary current interruption and the associated switching current peaks.
Starting with a starting transformer
With this starting process, a starting transformer is connected in front of the stator winding , with which the motor voltage is reduced. Autotransformers with a limited duty cycle are used as the starting transformer. After the motor has started up, the transformer is switched off. With this method, the starting torque of the motor is significantly greater than when connecting with a stator starter. So that the motor can start up safely, the voltage is reduced so that the motor delivers a starting torque that is 10 to 15% above the torque requirement of the machine to be driven. The run-up time for normal start-up is between 2 and 20 seconds; for heavy-duty start-up, the run-up time can be 60 seconds. Since the starting method only puts a low load on the network, it is particularly suitable for high-voltage motors or for motors with high outputs in so-called soft voltage supply networks. Heavy start-up is only possible to a limited extent due to the long start-up times. The high investment costs for the transformer are also disadvantageous.
Starting using electronic starting procedures
Both soft starters and starting converters are suitable for starting three-phase squirrel cage motors. The run-up time under normal conditions is between 0.5 and 10 seconds, with a heavy start it increases to up to 60 seconds. Soft starters are only suitable for heavy starting to a limited extent. With a starting converter, the starting torque of the motor can be adapted to the load requirements within wide limits.
Starting with a starting motor
In the case of squirrel cage motors with high output, starter motors are used for starting. To do this, the large squirrel cage motor is run up to the synchronous speed of the large motor with a smaller DC motor or a slip ring motor. Then the large squirrel cage motor is connected to the mains, the starter motor is decoupled and switched off. The started large motor now falls back into asynchronism. A high switching current is avoided by this starting method. The additional expenditure on switching devices is disadvantageous.
Starting of slip ring motors
In slip-ring motors annealing is mainly by means of rotor starter. The switching of the individual resistance levels usually takes place automatically. Hand-operated starter switches are used for smaller engines. However, switching to the next starter stage too early often results in increased motor current consumption. Since the starting resistors are not designed for continuous operation, they can heat up to an unacceptably high level. This starting process is only used for large slip ring motors. Smaller slip ring motors are started by means of a starting converter. Larger slip-ring motors are still started with the classic starting method due to network perturbations. Slip ring motors are very well suited for heavy starting.
Starting of three-phase synchronous motors
Three starting methods are suitable for three-phase synchronous motors:
- Starting with a starting motor
- Starting with a cage
- Starting with a starting converter
When the drives are idling, the synchronous motor can be started up with a smaller three-phase asynchronous motor as a starting motor. The synchronous motor is then synchronized with the mains. The starting motor is then uncoupled and switched off. Heavy starting is not possible with this method. Some synchronous motors have an additional cage installed. Starting with this starter cage is comparable to starting with a starter motor. The cage winding of the launch cage also acts as a damper winding during operation . A heavy start is also not possible with this. If three-phase synchronous motors have to be started under load, this is possible using a starting converter. The three-phase synchronous motor is started with increasing frequency .
Starting of AC motors
Large AC motors are started either with starting resistors or soft starters.
Starting of DC motors
Direct switching on is only possible with smaller, permanent magnet excited DC motors . With larger machines, direct switching on would not only place a heavy load on the mains, but the high inrush current can damage the motor. Therefore, it is necessary to lower the voltage. This is done on the network with constant direct voltage , for example in direct current railways, by means of stepped switchable starting resistors that are connected directly in front of the armature winding. The starting resistor is dimensioned so that the starting current is limited to the nominal motor current. Although the armature current is reduced by the armature circuit series resistor, the starting torque of the motor is also reduced at the same time. The disadvantage here is the power loss that arises in the resistors. In the case of very large motors, the starting resistance would be enormous, which is why a variable DC voltage is generated in these machines using a Leonard converter . The disadvantage here is the low efficiency of the converter and the high acquisition and maintenance costs. Nevertheless, Leonard converters, e.g. B. on construction cranes also for speed control, still used today. In modern DC drives, the starting current is limited by means of a converter supply. A variable direct voltage is generated with the help of a thyristor control. Since the conversion into direct voltage takes place directly from the three-phase network, this method is also called direct supply.
Other starting procedures
In addition to the starting methods that are required to limit the starting current, there are also methods that serve other purposes. The KUSA circuit enables three-phase squirrel cage motors to start smoothly and without jerks. In order to generate a higher starting torque in three-phase squirrel cage motors that are operated with the Steinmetz circuit, a starting capacitor is connected in parallel to the operating capacitor during the start-up phase .
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