Difference from three-phase motors
The use of asynchronous electric motors in their pure form with standard connection is only possible in three-phase networks with a voltage of 380 volts, which are usually used in industry, production halls and other premises with powerful equipment and high power consumption. In the design of such machines, the supplying phases create magnetic fields on each winding with an offset in time and location (120˚ in relation to each other), resulting in the resulting magnetic field. Its rotation drives the rotor.
However, there is often a need to connect the induction motor in a single-phase household network with a voltage of 220 volts (for example, in washing machines). If to connect the induction motor will be used not three-phase network, and household single-phase (that is to power through a single winding), it will not work. The reason for this is the alternating sinusoidal current flowing through the circuit. It creates a pulsating field on the winding, which in no way can rotate and therefore move the rotor. In order to turn on a single-phase induction motor is necessary:
- add another winding to the stator by placing it at a 90˚ angle from the one to which the phase is connected.
- For phase shifting, include a phase-shifting element in the additional winding circuit, which is most often a capacitor.
Rarely, a bifilar coil is used for phase shifting. To do this, a few turns of the starter winding are wound in reverse. This is only one of the variants of bifilar, which have a slightly different scope of application, so to study their principle of operation, you should refer to a separate article.
After connecting two windings such a motor is structurally a two-phase motor, but it is usually called single-phase because only one winding acts as a working one.
Wiring diagram for a 220 V collector motor
Wiring diagram for a single-phase induction motor (star connection)
How to choose a capacitor for a three-phase motor?
A non-polar capacitor with a voltage of at least 400 V is used. Either a modern capacitor specially designed for it (Fig. 3), or a Soviet one like MBGCH, MBGO, etc.п. (Fig.4).
So, to calculate the starting and running capacitors for induction motor, enter the data into the form below. You will find these data on the nameplate of the motor, if the data are unknown, you can use the average data in the form by default to calculate the capacitor, but the motor power must be specified.
Our power supply networks are designed to be three-phase. Because the generators that work in power plants have three-phase windings and produce three sinusoidal voltages, shifted in phase with respect to each other by 120 °.
But most of the time we only use one phase. we run one phase wire out of three and connect everything to it. Only in our technology we often see electric motors, and they are by nature three-phase. Well, what is the difference between the phase and the phase? Only by a shift in time. Such a shift can be achieved very simply by including reactive elements in the power supply circuit: capacitances or inductors.
But the winding on the stator is itself an inductance. So all we have to do is add a capacitor, a capacitor, to the motor outside and connect the windings so that one of them in the other one shifts the phase to one side and the capacitor in the third one does the same, only to the other side. And you get the same three phases, only “taken out” from one phase of the supply wires.
The latter circumstance means that we load a three-phase motor with only one of the phases of the incoming supply. Of course it makes an imbalance in power consumption. So it is still better when a three-phase motor is powered by a three-phase voltage, and building a circuit to power it from a single incoming phase is only good if the power of the motor is not particularly great.
But in this case everything should be in moderation, if the capacitance of the starting capacitors is too high nothing bad will happen but the efficiency of starting the motor will be worse. So you should not spend too much money to buy a capacitor with too high a capacity.
Simplifying the formula, we can say that to operate a three-phase motor in a single-phase network, the capacitor capacity for each 0.1 kW of its power should be about 7 μF. So, for a 1.1 kW motor, the capacitor capacity should be 77 μF.
Check during installation
After a suitable starting capacitor has been selected, it must be checked. To do this, proceed as follows:
- First it is necessary to disconnect the power supply from the electric motor.
- The capacitor must be de-energized, because there may still be residual charge in the capacitor. To do this you have to short-circuit its windings.
- Now you need to remove one of the terminals and connect the device to measure the capacitance.
- Connect the probes to the capacitor pins. After that the meter will show the exact value of the capacitance.
When using a multimeter you must first set the main switch to the capacitance measurement mode.
A simplified version can be used for calculations. It is known that the inrush current can exceed the nominal current by a factor of 3 to 8. So you can simply use 2 to 3 times the capacitance of the working capacitor. If the capacity for starting is not enough, just take a more suitable capacitor.
The main purpose of the starting capacitor is obtaining a magnetic field, necessary for to increase the starting torque of the electric motor, as well as for connection to the windings of asynchronous motors supplied from single-phase networks with 50-60 Hz frequency and to convert three-phase motors to single-phase operation.
The capacitor is called a starting capacitor because it is used to equalize torque when starting a motor. At the moment the electric motor starts, the starting current increases sharply and the torque increases at the same time with a lag. It is at this point the motor is under the greatest load, and if you do not use a starting capacitor, the increasing electrical energy will destroy the motor winding.
The starting capacitor allows reactive energy to escape from the motor winding and accumulate in this capacitance until the motor reaches operating frequency and power.
Starting capacitors are used in compressors, pumps, washing machines, refrigerators, starters, air conditioners, split systems and other equipment where reactive currents compensation is necessary.
What is the difference between a starting capacitor and a working capacitor
Start capacitors and run capacitors are used for starting and running asynchronous motors in single-phase AC circuits.
Starting capacitor is designed for short-term work. at the moment of motor starting. After the engine has reached operating frequency and power, the starting capacitor is turned off and the motor runs due to phase shift in the working windings. Therefore, run time of a start capacitor should be very short, about 3 seconds, as long run time of a start capacitor, can lead to its additional overheating and the electric motor as a whole that is fraught with failure of circuit elements.
This is necessary for those motors whose circuitry provides for this starting mode. For other motors, only when there is a load on the shaft that prevents the rotor from rotating freely.
The run capacitor is designed for a large number of running hours and is connected to the circuit at all times, acting as a phase-shift circuit for the motor windings. Because the capacitor and the motor winding create an oscillating circuit, an overvoltage is generated on the capacitor at the moment of the phase change from one cycle phase to the next which is greater than the supply voltage. This must be taken into consideration when selecting the duty capacitor.
|Operating capacitor||Starting capacitor|
|Use||Operating winding circuit in asynchronous motors||In the starting circuit|
|Functions performed||Creation of a rotating electromagnetic field for electric motor operation||Phase displacement between the start winding and the run winding, motor start under load|
|Connection||In series with the auxiliary winding of the electric motor||Parallel to the running capacitor|
|Operating time||Continuously||At startup until the motor speed reaches the desired mode|
|Capacity||Approximately 6 to 7 uF for each 100W of motor power||For each 100W of electric motor power about 12-18 uF is required|
|Capacitor type||CBB60, CBB61, CBB65, CD60, MBGO, MBGH, MBGV and similar with voltage 1.15 times the supply voltage||CBB60, CBB61, CBB65, CD60, MBGO, MBGH, MBGV and the like with voltages 2 to 3 times the supply voltage|
Star and delta connection of a three-phase electric motor to a single-phase network
The basic diagrams for connecting a three-phase motor to a single-phase system are “star” и “delta“.
To connect the starting capacitor to the asynchronous motor, a button is used, which switches the starting capacitor for the time required for the motor to reach the necessary power and speed.
Operating capacitor permanently connected to the motor circuit and does not need to be disconnected.
Types of capacitors, comparison of capacitor series, what are the
The most common series of starting capacitors: CBB60, CBB61, CBB65, CD60, MBGO, MBHG, MBGV.
Differentiated by series data dielectric type (polypropylene, metal or paper), form и case material (rectangular or cylindrical body, metal or plastic), to the nominal range of containers и voltages.
How to Test Motor Start and Motor Run AC Capacitor of ac fan and compressor
|Type||Characteristic||Case||Capacitance, μF||Operating voltage, V||Deviationcapacity||Tangent angle of loss, max||ResistanceInsulation between pins, Mohm·μF|
|CBB60||metallopropylene sealed||cylindrical plastic||1. 150 μF||450, 630 В||±5%||0,002||3000|
|CBB61||metal-propylene sealed||rectangular plastic||1. 50 μF||450, 630 В||±5%||0,002||3000|
|CBB65||Metal/Propylene hermetically sealed||cylindrical metal||4. 150 μF||450, 630 В||±5%||0,002||3000|
|CD60||electrolytic sealed||cylindrical metallic||50. 1500 μF||220. 450 В||±5%±10%±20%||0,15||3000|
|MBGO||metallic hermetically sealed single layer||rectangular metal||0,25. 30 μF||160. 630 В||±10%±20%||0,025||240;60|
|MBGP(KMBG)||paper-metal sealed single-layer||rectangular metal||0,1. 30 uF||160. 1500 В||±10%±20%||0,025||240;60|
|MBGT||Same, heat-resistant||rectangular metal||0,1. 20 uF||160. 1000 В||±10%±20%||0,025||240;60|
|MBGCH||Same, for higher frequencies||rectangular metallic||0,25. 10 μF||250. 1000 В||±10%±20%||0,025||240;60|
|MBGV||As above, high capacitance||rectangular metal||60. 200 μF||500, 1000 В||±5%±10%||0,025||240;60|
In general, paper-metal capacitors have only one advantage. they Better tolerance of short-term current overloads. But it is 100% certain that polypropylene capacitors also polypropylene capacitors also work reliably and are becoming more and more popular by the day. This technology allows charging in a smaller volume and at a much lower cost. In this regard, polypropylene starting capacitors are increasingly used in equipment as an alternative to metal capacitors, because of their decent quality, better performance and lower price.
How to choose the capacitor capacity for an electric motor (calculator)
Starting and working capacitors for electric motors based on The required capacity and voltage rating. With the aid of an online calculator you can calculate the capacity of the starting and working capacitor for three-phase motors in connection with the motor windings in the “star” or “delta“and its connection to a single-phase network.
When selecting the capacity of the run capacitor it is recommended that use more than one High capacitor capacity working capacitor, and several The capacitors are connected in parallel and have a lower capacity. The capacitance can be selected by connecting or disconnecting additional capacitors in parallel (the total capacitance in this case is equal to the sum of the capacitances of the capacitors connected).
Rated starting capacitor voltage must be chosen so that in the process of operation the operating voltage does not exceed the capacitor parameters by more than 10%.
Practice shows that about 6-7 μF is required for each 100 W of motor power. With a properly dimensioned capacitor, the capacity of a three-phase motor connected to a single-phase network should not drop by more than 30%.
The voltage of the operating capacitor for connection to the asynchronous motor must be chosen with a factor of 1.15, t.е. For 220V the working voltage of the capacitor must be 2201,15=250V.
To connect the starting capacitor to an induction motor, a factor of 2 to 3 is taken in the voltage calculations. For a 220V circuit, the starting capacitor voltage should be 400-500V. This will provide the necessary voltage reserve during operation.
Recommendations for connection
Before connecting the capacitors make sure that there is no of the accumulated charge. Since the capacitor stores the accumulated charge for a long time, after each shutdown it is necessary to conduct its discharge. Some capacitors are designed with a built-in discharge resistor. The resistance of the discharge resistor must be chosen so as to After 50 seconds completely discharge any residual voltage from the capacitor.
To prevent accidental contact with live parts under voltage, they should be insulated with a cover or fence. Capacitor case must be securely fastened. during operation under the influence of vibrations and shocks capacitors can be displaced and get into the working devices.
220V is a life-threatening voltage. In order to comply with the rules of safe operation of electrical installations of consumers, preserving the life and health of persons operating devices, the application of switching circuits should be carried out by a specialist.
What type to use
The requirements for capacitors for starting electric motors are simple:
- The value of the capacitance is sufficient to start the motor;
- rated voltage must be selected by 10-15% higher than the connected voltage;
- the bipolar circuit must be operated with the type of current applied.
There are small nuances for electric machines, which differ in the principle of operation.
To work with a three-phase electric motor
In this case the part performs a phase shift in the winding of an induction machine, and its capacitance must be high. Creation of starting torque and further operation under load require a more accurate selection of this element characteristic.
Connection to a single-phase motor
Starting capacitors are used here for connection of an auxiliary winding. It is designed to start the motor and can be switched on permanently, through a bipolar, or for a short time without it.
Part selection features
The selected starting capacitors correspond to the applied voltage. Their value of capacitance must not allow the motor to overheat during operation and must not allow it to start easily when switched on. There are no special difficulties with the selection of elements.
To ensure the reliable operation of the electric motor, starting capacitors are used.
The greatest load on the electric motor is exerted at the moment of its start. It is in this situation that the starting capacitor begins to work. Also note that in many situations starting is under load. In this case, the load on the windings and other components is very great. What is the design that reduces the load?
All capacitors, including starting capacitors, have the following features:
This design is a combination of 2 conductors separated by a dielectric. The use of modern materials significantly increases the capacitance index and reduces its overall dimensions and increases its reliability. Many have impressive performance figures and are no more than 50 millimeters in size.
Design and purpose of capacitors
This element of the circuit consists of two plates (cladding). The layers are arranged with respect to one another so that a gap is left between them. When a capacitor is placed in an electric current loop, charges are built up on the electrodes. Because of the physical gap between the plates, the device has a small conductivity.
Warning! This gap may be air or filled with dielectric. Paper, electrolyte, oxide films are used as a dielectric.
The main feature of such a bipolar capacitor is its ability to accumulate the energy of an electric field and instantly give it back to the load (charge and discharge).
The first prototype for a container was the Leiden jar, created in 1745 in Leiden by the German von Kleist. The can was lined with copper foil inside and outside. That is how the idea of making the windings came about.
Two-pole symbol on the diagrams and drawings. two vertically arranged lines (like the facings) with a gap between them.
How to replace the starting capacitor
Starting and working capacitors are used for start and operation of electric motors operating in 220 V single-phase circuit.
This is why they are also called phase shifters.
Installed between the supply line and the motor starter winding.
Capacitor symbol on drawings
Graphic designation on the circuit is shown in the figure, the letter C and the serial number in the circuit.