The Problem of Starting Currents in Electric Motors
When starting any electric motor, there is a sharp increase in the current in the rotor and stator windings, which in most cases significantly exceeds the nominal value. This can lead to overheating of the windings and contacts, which in turn causes an increase in thermal and electromagnetic losses. As a result, during startup, the efficiency of the motor significantly decreases. If the motor is frequently turned on and off as part of a unit, the energy consumption increases substantially.
Additionally, the heating of the electrical circuit elements leads to faster wear of the equipment. This causes the operational characteristics of the motor to deviate from the calculated values, shortens the lifespan of individual drive elements, and increases the number of malfunctions and failures. All these factors lead to additional maintenance and repair costs, as well as unforeseen expenses due to downtime associated with the need for preventive and repair work.
Ways to Reduce Starting Currents
In order to reduce the starting currents in modern electric drives, one of the following solutions is used:
- soft starters (SS);
- frequency converters (FC).
A soft starter regulates the supply voltage to the electric motor, limiting the torque on the drive shaft and the starting currents in the windings. The values of torque and current are roughly proportional to the square of the applied voltage. Thanks to this, for systems with low mechanical loads (for example, pumps), it is possible to reduce the starting currents to 1.5-2 nominal values. For powerful drives and systems with gear reducers, the starting currents can reach 3-4 times the nominal values.
The frequency converter in the initial startup period operates similarly to the soft starter. At low shaft rotation frequency, the starting current is proportional to the voltage supplied from the converter to the motor input. However, as acceleration occurs, the amplitude-frequency characteristic of the FC-motor system differs significantly from other starting methods. The form of this characteristic can change depending on the tuning of the motor and the converter. Some frequency converters support vector control methods, allowing the starting currents in the windings to not even exceed the nominal values – the motor smoothly accelerates until it reaches the normal operating mode.
Advantages of Frequency Converters
The use of frequency converters allows to significantly limit starting currents, eliminating most of the associated problems. Additionally, the frequency method of regulating the operating modes of electric drives has many additional advantages:
- the possibility of significantly reducing energy consumption;
- the presence of numerous protections against emergency situations;
- extensive opportunities for automation of the drive operation depending on parameter values;
- the ability for convenient remote monitoring and maintenance.
It is not surprising that the application of frequency converters in systems based on synchronous and asynchronous motors is increasing. They are used by both professionals in large enterprises and enthusiasts in private households. The reduction in the cost of converters and their expanded functional capabilities due to the latest technologies only strengthen this trend, making frequency converters a common device, similar to a car or an ATM.