Using a single-phase network to power a three-phase motor. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Using a single-phase network to power a three-phase motor. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electric motors

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There are many ways to start 3-phase motors from a single-phase network. I will tell you about the methods that I have repeatedly used in practice.

The most "old-fashioned" method is to start a 3-phase motor using a cord wound around the motor shaft. But this method is applicable only to start the engine without a load, which is connected after the engine "spins up". At the same time, only 35 ... 40% of the power is removed from the motor shaft, which the motor could give when connected to a 3-phase network. Another of the "old-fashioned" methods is the use of two engines with similar power. One of them (motor-generator) is used for phase shift (Fig. 1).

It is launched with a rope and when it "unwinds" with the SA2 switch, the working engine is turned on. At the same time, the working engine develops almost full power (about 90%). So you can turn on several engines, each previous one provides a phase shift for the next one.

The most common phase shift method is with a capacitor. But there is a feature here: you need a starting and running capacitors.

The starting capacitor has a larger capacitance than the working one. Of course, you can do without a starting capacitor, but in this case, the engine must be started without load. You can connect the load when the engine picks up speed.

If the engine is started under load, then the starting capacitor Sp is required. It is connected (Fig. 2, 3) in parallel with the working capacitor with the SB1 start button, which is released as soon as the engine picks up speed. Then only the working capacitor Cp remains in operation. But even with this method of turning on a three-phase motor, it is impossible to obtain the full nameplate power on the shaft (about 60% of the power develops).

Starting and running capacitors must be non-polar, preferably paper, with an operating voltage of at least 500 V. I know from my practice that capacitors with a lower operating voltage often fail (swell and even explode), especially "old men". Paper capacitors such as MBGO, KBGI, etc. it is better to take square ones (of which you will have to collect a battery of capacitors), it is more convenient to mount them in square boxes.

In the literature there are recommendations on the use of electrolytic (polar) capacitors connected in opposite directions. But this cannot be done! Such a pair of capacitors quickly heats up and explodes. In extreme cases, they can be used as starting ones (short-term operation), their operating voltage must be at least 500 V.

In parallel with the starting capacitor, it is necessary to include a 1 W resistor with a resistance of 100 ... 510 kOhm. This resistor is used to remove the residual charge from the starting capacitor.

The capacitor method has an advantage - the possibility of reversing the engine. This is done using the SA1 switch (Fig. 2, 3). First, the engine is stopped, and then SA1 is switched and plugged back into the network.

The table shows data on working and starting capacitors for motors of different power when the windings are connected in a "star" and "triangle":

If you have an engine of greater power, then Cp can be calculated using the formulas for the "star" and "triangle" respectively:

Ср=2800I/U (uF),

Ср=4800I/U (uF),

where the current I is calculated from the given power P (W), the efficiency η and the power factor cosϕ according to the formula:

I=P/1,73Uηcosϕ.

This data is in the passport for the motor or on the nameplate attached to the motor housing. The capacity of the starting capacitor should be 2,5-3 times greater than that of the working one.

If the engine has stopped during an overload, then you can start it again by pressing the start button SB1. If there is no resistor on the starting capacitor to remove the residual charge, then this charge can be removed as follows: after stopping the engine and disconnecting it from the mains, you must press the SB1 button, and the capacitor Sp will be discharged through the motor winding.

Author: O.G. Rashitov

See other articles Section Electric motors.

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Comments on the article:

Anatoly
Good afternoon, to help you expand the capacity of the capacitors of the starting capacitor for a three-phase motor with a pressure of 5.5 kW.

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