Capacitor for electric motor: how to choose and use, calculation of capacity for starting and working, connection and operation

Many owners often find themselves in a situation, when you need to connect such a device in the garage or cottage, as a three-phase asynchronous motor to various equipment, which can be an emery or drilling machine. This raises a problem, since the source is designed for single-phase voltage. What to do here? In fact, this problem is quite easy to solve by connecting the unit according to the schemes, used for capacitor. To implement this idea, requires a working and starting device, often referred to as phase shifters.

Capacity selection

To ensure the proper operation of the motor, you need to calculate certain parameters.

For working capacitor

To find the effective capacity of the device, it is necessary to perform calculations according to the formula:

• I1 - rated stator current, for measurement of which special pincers are used;
• Networks - mains voltage with one phase, (V).

After performing the calculations, the capacity of the working capacitor in the IFF will be obtained.

It may be difficult for someone to calculate this parameter according to the above formula. However, in this case, you can use another scheme to calculate the capacity, where you do not need to perform such complex operations. This method allows you to easily determine the required parameter based only on the power of the induction motor.

That's enough to keep in mind, what 100 The watt of power of the three-phase unit should correspond approximately 7 uF capacitance of the working capacitor.

The calculations need to monitor the flow, which is fed to the phase winding of the stator in the selected mode. It is considered inadmissible, if the current is more important, than the nominal figure.

For starting capacitor

There are situations, when the electric motor has to be included in the conditions of big loading on a shaft. Then one working capacitor will not be enough, so you have to add a starting capacitor to it. The peculiarity of his work is that, that it will work only during the start-up of the device no more 3 seconds, what the SA key is used for. When the rotor reaches the level of rated speed, the appliance switches off.

If due to negligence the owner left the starters on, this will lead to the formation of a significant skew in the currents in the phases. In such situations, there is a high probability of engine overheating. When determining the capacity should be based on that, that the value of this parameter should be in 2,5-3 times the capacity of the working capacitor. Acting in a similar way, you can achieve that, that the starting torque of the engine reaches the nominal value, as a result, there are no complications during its launch.

To create the required capacity, capacitors can be connected in parallel and in series. It should be borne in mind the operation of three-phase units with a capacity of not more than 1 kW is allowed in that case, if they are connected to a single-phase network in the presence of a working device. And here you can do without a starting capacitor.

A type

After calculations it is necessary to define, what type of capacitor can be used for the selected circuit

The best option, when a similar type is used for both capacitors. Usually the operation of a three-phase motor is provided by paper starting capacitors, dressed in an airtight steel case type MPGO, MBGP, KBP or MBGO.

Most of these devices are made in the form of a rectangle. If you look at the case, then there are their characteristics:

• Capacity (uF);
• Operating voltage (V).

Application of electrolytic devices

Using paper starting capacitors, you need to keep in mind the following negative point: they are quite large, while providing a small capacity. For this reason, for the efficient operation of a three-phase low-power motor has to use a large number of capacitors. If desired, paper can be replaced by electrolytic. In this case, they need to be connected in a slightly different way, where additional elements must be present, represented by diodes and resistors.

However, experts do not recommend the use of electrolytic starting capacitors. This is due to their serious shortcomings, which is manifested in the following: if the diode does not cope with its task, the device will start selling AC power, and this already threatens its heating and subsequent explosion.

Another reason is, that today on the market you can find improved with metallized coating polypropylene starting models of alternating current type SVV.

Most often, they are designed to work with voltage 400-450 St.. they should be preferred, considering, that they have repeatedly shown themselves on the good side.

High-voltage

Considering different types of starter rectifiers for three-phase motor, connected to a single-phase network, this parameter should also be taken into account, as operating voltage.