Direct start
It involves connecting the stator windings to the electrical network without “intermediaries”. Suitable for motors with squirrel-cage rotor. These are low-power motors in which, when the stator windings are connected directly to the electrical network, the resulting starting currents do not cause overheating, which can damage the equipment.
In asynchronous motors, the ratio of winding inductance to resistance (L/R) is small. And the smaller the power of the device, the smaller it is. Therefore, during startup, the resulting free current quickly decays and can be neglected. Only the current strength that is established as a result of the transient process will be taken into account.
figure (a) is a diagram of a magnetic starter, indicated by the letter K. Technically, this is an electromagnetic switch, often used when starting squirrel-cage electric motors. It is necessary for automatic acceleration according to a natural mechanical characteristic (denoted by M ) from the beginning of the launch (point P ) until the moment when M becomes equal to the moment of resistance ( Ms ).
Picture (b) shows a graph of the starting current versus the initial torque. Based on it, the acceleration acceleration is equal to the difference between the abscissas of the graphs M and M(c) . In this case, if Mstart is less than Ms , then the electric motor will not be able to accelerate. Mstart value for acceleration for a motor with a squirrel-cage rotor, use the formula (slip coefficient s is equal to unity):
The ratio of Mstart to the nominal ( Mnom ) is a value defined as a multiple of the initial torque. Denoted kpm . The coefficient for engines with a squirrel-cage rotor is in the range from 1 to 1.8 and is established by GOST.
Example. If kpm = 1.4, and Mnom = 5000 N*m, then direct start should begin with MP = 7000 N*m.
Attention! The standards established by GOST must not be exceeded. This leads to an increase in active resistance on the rotating element of the motor.
Direct engine starting has the following advantages:
- Cheapness;
- Simplicity;
- Minimal heating of the windings during startup.
Disadvantages of the method:
- The value of Mstart is up to 300% of Mnom ;
- The starting current is up to 800% of the rated current (see graphs below).
Even with the listed disadvantages, direct starting remains the most preferable for asynchronous electric motors with a squirrel-cage rotor, because provides high energy performance.
Frequency regulation
Frequency regulation refers to the use of a frequency-controlled drive. This device regulates the rotor speed of the electric motor. The design of the frequency converter includes an inverter and a rectifier. The advantages of starting an engine through frequency regulation include a large selection of values for adjusting the number of revolutions, an increase in the service life of the motor, maximum starting torque and saving electrical energy compared to other methods of starting the motor.
Frequency regulation also has disadvantages. This is the relatively high price of converters for powerful motors, as well as the high level of interference that is observed in the vicinity of these devices.
Undervoltage starting
Suitable for starting high-power electric motors, but also optimal for medium-power analogues if the voltage in the operating network does not allow accelerating the motor using direct starting.
There are three ways to reduce voltage:
- Switching the stator windings from triangle (normal circuit) to star (starting circuit). Starting starts from star, and when the rated frequency is reached, it switches to delta. In this case, the voltage supplying the phases of the stator windings drops by 1.73 times. This allows phase currents to decrease by the same amount, and linear currents are reduced by three times.
- Starting with additional resistance leading to a drop in voltage on the stator winding (Figure a). At the time of start-up, reactors or resistors are included in the electrical circuit (reactive and active resistance, respectively).
- Start with connection via a step-down transformer with several automatically switched stages (Figure b).
The main advantage is the ability to accelerate the engine at almost the same voltage that is necessary for normal operation. The only disadvantages include a drop in MP and Mmax (maximum moment). These values are directly proportional to the voltage: the lower the Volt, the smaller the torque. Therefore, the motor will not start with a load.
Connecting the rotor to the rheostat during switching on
The method is suitable for switching on motors with a wound rotor. If the rotor circuit includes a rheostat, then the active resistance increases. In this case, point K in figure a below moves closer to O and is designated K` . This does not lead to a decrease in Mmax , but it does provide an increase in Mstart . At the same time, the critical slip increases, and the dependence of the moment on s shifts to the zone of large slips. The number of revolutions shifts to the zone of lower rotational frequencies (Figures b and c).
Typically, the rheostat used to start the motor has from 3 to 6 stages (see figure a below). The starting resistance gradually decreases, which ensures a large Mstart . Initially, the motor is driven according to the fourth characteristic, illustrated in Figure b . It matches the resistance of the starting rheostat and provides maximum starting power.
Torque ( Mvr ) decreases with increasing speed. At a certain minimum value, it is necessary to turn off part of the rheostat so that Mvr increases again to the maximum (see the third characteristic). But the revolutions are increasing, so Mvr decreases again. Then another part of the rheostat is turned off, and work on the second characteristic begins. When the rheostat of a wound-rotor motor is turned off completely, the starting process is completed. The motor continues to operate according to characteristic 1.
Launching using this method is characterized by a change in Mvr from the maximum to the minimum value. The resistance in this case decreases stepwise along a broken curved line (highlighted in bold on the graph). The rheostat parts are turned off automatically or manually.
The advantage of starting an electric motor with a wound rotor using a rheostat is the ability to turn it on at Mstart close to Mmax . Inrush currents are minimal. The change in current strength is illustrated in Figure c.
There are plenty of shortcomings. The first is the difficulty of inclusion. Secondly, this is the need to use not at all cheap motors with a wound rotor. The nature of the operation is worse than that of analogues with a squirrel-cage rotor with the same power value - this is the third disadvantage. This explains why electric motors with a wound rotor are used mainly in case of difficulties with starting other motors.
Star-triangle
The star-delta circuit involves a two-stage safe connection of the electric motor:
- First, the motor is started within the “Star” circuit, which involves the use of low starting currents. For some time the engine is powered according to this circuit and smoothly picks up speed.
- After reaching a certain number of revolutions per minute, the motor switches to the “Triangle” circuit, which requires high starting currents for operation. Here the engine reaches its design power.
To implement this starting circuit, you will need a three-pole switch, three contactors, a thermal relay and a time relay. The benefits of this type of start are similar to those of the soft start described above.
Starting a single-phase motor
To turn on an asynchronous motor powered from a single-phase network, auxiliary winding is used. It should lie perpendicular to the working stator winding. But to create a rotating magnetic field, one more condition must be met. This is the phase shift of the current flowing through the auxiliary winding relative to the current arising in the working winding.
To ensure a phase shift at the time of connection to a single-phase network, a special element is included in the electrical circuit of the auxiliary winding. This could be a resistor, capacitor or inductor. But only the first two are common elements.
After accelerating the motor to a frequency equal to the established frequency, the additional winding is turned off. This can be done manually or automatically. At the beginning, the motor operates according to a two-phase characteristic, and after setting the frequency, according to a single-phase characteristic.
Applying resistance at start
The method is applicable for asynchronous motors connected to a single-phase network and having a primary additional winding with a squirrel-cage rotor. This is the name given to a split-phase motor, the electrical circuit of which has a high active resistance.
To start a motor powered from a single-phase network, a starting resistor is needed, connected in series with the additional winding. Then the phase shift is 30 degrees. This is enough for overclocking. Below is a diagram according to which an ohmic phase shift is achieved.
Instead of a resistor, you can use an additional winding of high resistance but low inductance. In this case, the winding has few turns, which are made from wire of a smaller cross-section, in contrast to what is used for working winding.
In Russia, motors come off the assembly line connected to a single-phase network, equipped with a phase shift resistor. Their power varies in the range of 18-600 W. The motors are designed for networks with a voltage of 127, 220 or 380 Volts and alternating current with a frequency of 50 Hz.
Launching an EPT with sequential excitation
A special feature of using such a circuit for starting DC motors is the serial connection of a variable resistance and an excitation coil to the motor.
In this case, as in the previous one, a current of the same rating will flow through the circuit of both coils. This method is characterized by good starting characteristics, but on the condition that the electric motor shaft will be under load at this moment. And one more feature of an EPT with sequential excitation: the shaft rotation speed during startup will be adjusted depending on the load. This scheme is ideal for electric transport - trams, trolleybuses, with some modifications - on electric trains.
Schematic diagram of starting an EPT with sequential excitation:
Using a capacitor
The method differs from the previous one in that a split-phase motor, when connected to a single-phase line, has a high resistance only at the time of start-up.
To ensure the highest value of Mstart, a circular and rotating magnetic field is required. To do this, the currents in the working and additional windings are shifted by 90 degrees. Only a capacitor can provide such a bias. Its use helps to achieve good starting performance of an asynchronous motor powered from a single-phase electrical network.
The choice of starting method for an asynchronous electric motor depends on which network it is connected to: single-phase or three-phase. The power of the motor and its design also influence.
More on the topic: - Connection diagrams for asynchronous and synchronous single-phase motors - Connection diagrams for an electric motor via capacitors - Reversible diagram for connecting an electric motor - Do-it-yourself soft start of an electric motor - What is the difference between asynchronous and synchronous motors - Reversible connection of a single-phase asynchronous motor with your own hands - How to check an electric motor - Electric motor repair
Other launch methods
It is also worth listing other quite popular schemes:
- regulation of a reversible motor with two starters (or one reversible) and three buttons;
- for a reversible motor using 2 starters and 3 keys, of which 2 have paired contacts.
All control options are, one way or another, similar to each other, but despite this, each of them is unique in its own way. It is worth choosing the exact option, focusing on the performance of the motor that will be driven.
