Braking of an Asynchronous Motor and Its Principles
When braking an asynchronous motor, energy is transferred back to the frequency converter, which operates in generator mode. The frequency converter attempts to return the system to its normal state by reducing the motor's slip by increasing the output frequency.
When the motor operates under a small non-inertial load, braking occurs due to losses within the motor itself, operating at approximately 20% of its rated power. This method is suitable when the kinetic energy is small, and the braking time is not critical.
Emergency Braking
For emergency (rapid) braking, a braking resistor is usually employed, which has the following functions:
- Provides continuous energy consumption for braking coming from the motor.
- Dissipates the braking energy, converting it into thermal energy.
This mode is activated when the rotational speed of the shaft decreases, particularly for inertial loads. Equipment such as ventilation systems, conveyors, and cranes operate similarly.
Generator Mode
If the reduction in the overall rotational speed of the motor occurs significantly slower than the decrease in frequency at the converter, the device switches to generator mode. In this case, the mechanical energy of the motor's rotation is converted into electrical energy. The generated electrical energy is accumulated in special capacitors in the DC circuit, which can lead to a voltage increase. This, in turn, may cause a breakdown or even destruction of the capacitor.
Solution to the Energy Recovery Problem
To address this issue, it is advisable to install a rectifier in the design of the frequency converter. This will enable the implementation of the energy recovery process, in which all energy is transmitted to the electrical grid. However, the cost of such equipment significantly increases.
There are also frequency converters that provide for the use of a single DC bus, allowing energy to be transmitted to other drives. However, achieving normal operation of both drives (one in braking mode, the other in driving mode) can be difficult or even impossible.
Use of Braking Resistors
Therefore, the best solution is to use specialized braking resistors if energy accumulation during braking is planned in the operational process.
The determination of the minimum resistance value of the braking resistor depends on the allowable current value of the braking switch in the frequency converter circuit. The maximum values of resistance and power of the braking resistor directly depend on the maximum amount of energy released during the braking of the drive.