In AC motor drive systems utilizing variable frequency drives (VFDs), some VFDs are externally connected to braking resistors, while others may seem not to be connected. So, the question arises: what is the purpose of a braking resistor, and why connect one? Today, we'll delve into the topic of braking resistors in VFDs.
To understand how braking resistors function, we need to start with the working principle of electric motors.
As we know, the speed formula for three-phase asynchronous motors is: n=60f/p(1-s). Here, "n" represents the motor speed, "f" denotes the frequency of the AC power source, "p" signifies the number of motor poles, and "s" stands for slip (the ratio of the difference between the synchronous magnetic field speed n0 and the motor speed n to the synchronous magnetic field speed n0). For a finished motor, the number of poles "p" remains constant. Therefore, we can only change the speed of the asynchronous motor by altering the frequency of the AC power source, which is the basis for speed control in AC variable frequency drives.
Three-phase asynchronous motors operate in two modes—motor operation and generator operation.
- Motor Operation:
When the three-phase asynchronous motor is energized, it generates a rotating magnetic field (synchronized with the magnetic field of the three-phase AC power, hence termed synchronous magnetic field). This rotating magnetic field cuts through the rotor windings, inducing an electromotive force (EMF) in the rotor windings, thereby producing current in the rotor windings. The rotor windings generating current experience electromagnetic force in the rotating magnetic field, causing electromagnetic torque, and thus the three-phase asynchronous motor starts rotating. In this scenario, the speed of the asynchronous motor "n" is always less than the speed of the rotating magnetic field "n0", indicating that the motor is driven by the magnetic field and is in the motor operation state. In the motor operation state, the direction of electromagnetic torque aligns with the direction of motor operation.
- Generator Operation:
In the motor operation state, if external force increases the speed of the motor "n" further, at a certain point when "n" exceeds the speed of the rotating magnetic field "n0", the direction of the EMF induced in the rotor windings will reverse, causing the rotor current to flow in the opposite direction. As a result, electromagnetic torque opposite to the direction of motor operation is generated. In this situation, electromagnetic torque acts as braking torque, and the asynchronous motor enters the generator operation state. Typical scenarios of generator operation include rapid descent of heavy loads in cranes, rapid descent of electric vehicles on slopes, and rapid stopping.
In AC variable frequency speed control, the VFD changes the motor speed by altering the frequency of the output power supply (while concurrently adjusting voltage). Increasing the frequency accelerates the motor, while decreasing the frequency decelerates the motor and stops it.
In a drive system with significant load inertia, the VFD decelerates the motor and stops it by reducing the frequency. At the moment of frequency reduction, the speed of the synchronous magnetic field also decreases. However, due to mechanical inertia, the rotor speed does not instantaneously decrease. At this point, the motor speed "n" exceeds the speed of the synchronous rotating magnetic field, and the motor enters the generator operation state. Electromagnetic torque acts as braking torque, causing the motor speed to decrease, and the regenerated energy returns to the DC bus after rectification. In VFDs without regenerative energy feedback units, regenerated energy cannot be fed back into the grid but can only be absorbed by the VFD's own capacitors. This can cause a rapid increase in capacitor voltage, known as "pumping voltage". Pumping voltage leads to a sharp increase in DC voltage, which may cause component breakdown and damage to the VFD.
To address the issue of pumping voltage, braking resistors are used in VFDs without regenerative energy feedback units to dissipate regenerated energy. When the voltage of the DC bus rises to a threshold, the braking unit opens the circuit of the braking resistor, and the regenerated energy is dissipated in the form of heat in the braking resistor. Braking resistors come in different power ratings and should be selected according to the actual situation. Many VFDs have a small built-in braking resistor, but if there is significant regenerated energy, an external braking resistor needs to be connected.
The following image depicts the appearance of a braking resistor for SEW variable frequency drives:
Another method to mitigate pumping voltage is by integrating regenerative energy feedback units into the variable frequency drive (VFD). In VFDs equipped with regenerative energy feedback units, regenerated energy is fed back into the grid, effectively achieving energy savings. However, VFDs with integrated regenerative energy feedback units are usually more expensive, and due to cost considerations, they are less commonly used in equipment.
By now, you should have a clear understanding of why variable frequency drives need to be connected to braking resistors. Even those VFDs where braking resistors are not externally visible typically have a small braking resistor integrated internally.