An asynchronous three-phase motor can be connected without much damage to a conventional single-phase electrical network through capacitors. With their help, the launch and achievement of the required modes of operation with such a power system is ensured. Distinguish between working and starting capacitors.
Content
- Differences between them
- Connection methods
- Working conditions
Differences between them
They consist in their purpose, capacity, method of connection, as well as in working conditions. The first difference is that the worker (first) the capacitor serves to shift the phases. As a result, a rotating magnetic field appears between the windings, which is necessary to drive a motor without mechanical load. Such an electric motor is installed, for example, in a grinding machine.
Starting (second) provides an increase in the starting torque of the motorunder mechanical stress, so that it more easily reaches the desired mode. The resources of one worker may not be enough, because of which the rotor of the engine simply will not start rotating. The use is justified in conjunction with machine tools, lifting mechanisms, pumps and similar heavy devices. It can also be used with a more powerful three-phase motor if there is not enough worker to start it reliably.
The capacitance of both capacitors will also differ. It is directly proportional to the power of the electric motor and inversely to the mains voltage. Depending on the connection diagram of the windings, a correction factor is introduced. The capacity of the launcher can be twice that of the worker.
Connection methods
The first capacitor, in the most common case, is connected to the rupture of one of the induction motor windings, which is also often called "auxiliary". The other is connected directly to the electrical network, and the third remains unused. The type of this scheme is called "star". There is also a delta connection. It differs in both the connection method and the complexity.
The second capacitive element, in contrast to the working one, is connected in parallel with the last one through a button or a centrifugal switch. In the first case, control is carried out by a person, and in the second, by the drive itself. Both of these switches briefly close this circuit at the time the electric motor is started, and after it enters the operating mode, they open it.
Working conditions
They are different for each of the capacitors. Since the first of them is permanently connected to the motor winding, this circuit forms an elementary oscillatory circuit. Because of this, at certain moments, a voltage is generated at its terminals that exceeds the input by two and a half to three times. This circumstance should be taken into account when selecting, it is necessary to focus on parts designed for 500-600 volts.
Starting capacitors for electric motors - 220 V work in other, less harsh conditions, in contrast to the working ones. The voltage applied to this capacitive element exceeds the main voltage by about 1.15 times. It joins the circuits from time to time, which also has a positive effect on its working conditions, and significantly extends its service life.
The most commonly used domestic paper or oil-filled capacitors brands MBGO or MBGCH. Their advantage is their resistance to high AC voltages. But there is also a disadvantage - the large size. Oxide capacitors can be used as an alternative. They are not connected directly, but through diodes, according to certain schemes.
Conventional electrolytic capacitors used in various devices, and designed for considerable operating voltages, are suitable for asynchronous motors only as starting motors. This is due to the fact that a large reactive power passes through them due to the low resistance of the windings. Connecting capacitive elements with violations or deviations from the circuit will lead to damage or boiling of the electrolyte, which can harm the motor and personnel.
So, you can deduce from this a few tips, how to distinguish a starting capacitor from a working one:
- The first of these plays a supporting role. It is connected in parallel with the worker during the start of the motor - for a few seconds to facilitate the start.
- The second of them is permanently connected, providing the necessary phase shift, as a result of which a three-phase motor can operate from a single-phase network.
If you mix up the capacitors, then there will be serious problems. The capacity of the worker should also not be too large, otherwise the motor will heat up, and the increase in power and torque from this will increase slightly.
