There is a plate on the electric motors, which indicates the main technical characteristics of the unit: power, speed, etc. However, manufacturers do not talk about such a parameter as inrush current. This is an important characteristic that has a significant impact on the operation of the power unit. A good electrician should be able to measure this and know what to do with the values.
Content
- Definition of the concept
- Calculation features
- Practical use
Definition of the concept
The starting current of the engine is the electric current consumed by the power unit at the moment of start. Its indicator is several times higher than the value of the rated current and when choosing equipment it is extremely important to take this parameter into account. Here it is appropriate to compare with a car, during acceleration of which much more fuel is spent in comparison with driving at a constant speed. This phenomenon is typical for various electrical equipment:
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Submersible pumps are distinguished by the most difficult start, and their starting electric current can exceed the rated one by 9 times. - Refrigerators - at start-up, the current exceeds the nominal 3.33 times.
- Microwave ovens - the indicator of the starting electric current is 2 times higher than the nominal value.
This is due to the fact that at the moment the electric motor is turned on, a strong magnetic field is created in its winding, which is necessary to spin the rotor. That is why the starting electric current indicator significantly exceeds the nominal value. Its value is influenced by various factors:
- The presence of a load on the shaft of the power unit.
- Rotational speed.
- Connection diagram, etc.
Calculation features
Determination of the value of the starting current of the electric motor is carried out in two stages. First you need to calculate the nominal electric current, the following formula is used for this:
Then you can proceed to determining the starting current indicator, using the formula:
Knowing this value, you can easily select circuit breakers, thereby ensuring reliable protection of the closing line. The electric motor passport indicates the value of the current at the rated load on the shaft of the power unit. For example, if the motor has an inscription 13.8 / 8 A, then when it is connected to a 220 V network and rated load, the current will be 13.8 A. When connected to a 380 V network, the current will be 8 A.
If you know the rated power of the power unit, you can easily find out its rated current. To do this, you have to use the formula:
Sometimes the power factor of a motor may not be known. In such a situation, it is worth using a simple ratio - 2 A / 1 kW.
For example, if the rated power of the motor is 15 kW, then it will consume about 30 A. The error in this calculation is minimal.
Practical use
Power drives will only be operated correctly if the starting characteristics were taken into account in their selection.
High starting current is a serious hazard to electrical equipment. If you do not take measures to limit it, serious problems are possible.
The starting current can damage not only the motor itself, but also other electrical equipment installed with it on the same line. To solve the problem the following methods can be used:
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Start the power unit at idle - the load is applied only after the motor has switched to operating mode. - When connecting, use a delta-star circuit.
- Use autotransformer start - voltage is supplied to the motor through an autotransformer, which allows achieving a smooth increase in current strength.
- Use starting resistors.
- The use of frequency controllers and thyristor soft start devices.
With the help of soft starters based on thyristors, it is possible to reduce the starting current by half. Moreover, they can work with both asynchronous and synchronous electric motors. In the case of three-phase asynchronous motors, frequency converters are widely used. They allow you to change the frequency of the electric current, providing not only a smooth start of the motor, but also the frequency of rotation of its rotor. These are efficient devices, but at a high cost. It should be remembered that frequency converters create noise in the network, which can be eliminated by a line filter.
You can also use the starting circuit of the power unit with switching the windings from star to delta.
Time relays are often used to solve this problem. However, remember that this method is not suitable for all electric motors.
For example, this method does not apply when connecting asynchronous electric motors designed for a voltage of 220-380 V.
Now on the market there are more modern devices - soft starters. They are based on microprocessors and are highly efficient. The only drawback of these devices can be considered only the high cost.
