Selection of a working capacitor for a three-phase electric motor

Selection of a working capacitor for a three-phase electric motor

To answer the question of how to choose a capacitor for asynchronous motors and how capacitors differ from each other, we will assemble a stand from a conventional three-phase motor with a power of 250 W. As a load we use a standard generator from a VAZ car.
Selection of a working capacitor for a three-phase electric motor

Let's connect three different capacitors through the machines. Turning on/off the machines will make it possible to check the capabilities of the capacitors.
Selection of a working capacitor for a three-phase electric motor

Selecting a capacitor


For the experiment, we will select three capacitors with capacities of 10, 20 and 50 microfarads. Our task is to try to start the electric motor from each capacitor in turn.

10 µF capacitor


When connected to a 220 V network and the first capacitor with a capacity of 10 microfarads is turned on, the electric motor turns on only after a push by hand. There is no automatic start.
Selection of a working capacitor for a three-phase electric motor

Conclusion: for a 250 W electric motor, a capacitor capacity of 10 microfarads is not enough.

20 µF capacitor


When you try to start an electric motor from a capacitor with a capacity of 20 microfarads, the motor starts up automatically.
Selection of a working capacitor for a three-phase electric motor

Conclusion: with a capacitor capacity of 20 microfarads, the electric motor started without problems.
Selection of a working capacitor for a three-phase electric motor

50 µF capacitor


Selection of a working capacitor for a three-phase electric motor

When continuing the experiment with a capacitor with a capacity of 50 microfarads, the electric motor starts automatically, but operates with a high noise level and simply shakes.
Conclusion: The capacitance of the last tested capacitor is large for the installed electric motor.
When selecting a capacitor for a low-power three-phase electric motor, give preference to a device with a rated capacitance (as in our experiment) corresponding to the motor power. A small capacitor does not start the electric motor; a capacitor that is too large causes the motor to heat up and make a lot of noise during operation. A capacitor with a capacity of 20 microfarads proved to be optimal in the experiment; it immediately started the engine and did not cause it to overheat.

Conclusion


To start a three-phase electric motor in a 220 V network, the working capacitor is selected based on the engine power. With an increase in power for every 100 W, the capacity should increase by 7-10 microfarads. For example, for a 0.5 kW motor, you can select a capacitor with a capacity in the range of 35-50 μF.
You also need to take into account such a parameter as the rated voltage of the device (that is, the voltage that the capacitor can withstand). It is recommended to use capacitors with parameters that are 100% higher than the actual voltage applied to the device. For this example it is 450 V.

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Comments (9)
  1. Guest Andrey
    #1 Guest Andrey Guests July 31, 2018 08:44
    1
    Can you attach a wiring diagram?
  2. Nikolai Nikolaevich
    #2 Nikolai Nikolaevich Guests 31 July 2018 18:30
    4
    Not professional! With the correct selection of a capacitor for a three-phase asynchronous electric motor, it is necessary to achieve equal voltages and currents in the phases under load. An approximate selection is calculated as follows: C working is equal to 4800 multiplied by the phase current (from the electric motor plate) and divided by 220. C starting is equal to C working multiplied by 2 --- 2.5
  3. Guest Anatoly
    #3 Guest Anatoly Guests 31 July 2018 20:28
    4
    Take 8 microfarads for every 100 watts and you won’t go wrong. This has been known for a long time. But if the start-up is difficult, then the Mkf is increased by 30-40%, and after start-up it is turned off.
  4. Guest Victor
    #4 Guest Victor Guests 31 July 2018 21:08
    4
    You need to make a calculation that depends on the connection of the windings (star or delta), power, efficiency, ideally calculate and install two capacitors - starting and working. And your research, at best, is only suitable for a low-power, low-speed sharpening machine.
  5. Guest Alexander
    #5 Guest Alexander Guests 31 July 2018 21:35
    3
    Old electricians always selected 100 watts - 7 microfarads
  6. Guest Evgeniy
    #6 Guest Evgeniy Guests 31 July 2018 23:10
    2
    90 µF per 1 kW of power for start-up, then turn off 2/3 for operation.... Thus, a 30 µF per 1 kW capacity condenser remains in operation. The connection is better in a "triangle" - higher efficiency... The entire three-phase system has been working for 40 years from 02 , up to 5 kW according to this calculation of the condensers. “High-speed” engines of 3000 t/rpm are better to start with inductive “shift”, and leave capacitive for work. Active phase shift does not provide high efficiency and increases energy consumption. Something like this...
  7. Guest Mikhail
    #7 Guest Mikhail Guests August 1, 2018 09:56
    3
    Why bother with such nonsense when there are formulas for calculating the starting and operating capacitor? If, of course, the engine dates back to the Battle of the Ice, and all the nameplates have been torn off from it, then yes.
  8. Manuel
    #8 Manuel Guests 9 August 2018 19:08
    1
    I used nichrome resistor instead of Conder. Electrolytic capacitors can be used by connecting them in series. Minus to minus, and positive conclusions as final. But the total capacity will be half as much. For example, 2 el. 100 mkf conductors The total capacitance for a series connection will be 50 mkf.
  9. Sergey Natalevich
    #9 Sergey Natalevich Guests 16 March 2019 19:05
    1
    I propose 2 schemes for connecting a 3-phase electric motor to a single-phase network: