Modern electric drive systems based on Variable Frequency Drives (VFDs) provide high efficiency and flexibility in controlling electric motors. However, when working with inertial loads, a specific problem arises: during braking, the motor switches to generator mode and returns energy to the network. This excess energy can lead to emergency failures and even damage to equipment. An effective and reliable solution to this problem is the use of a braking resistor.
What is a braking resistor and why is it needed?
A braking resistor is, in essence, a powerful heating element designed to dissipate excess electrical energy generated by the motor during braking, in the form of heat. When the rotational speed of the motor rotor exceeds the rotational speed of the stator's magnetic field, which happens during forced deceleration due to load inertia, the motor begins to operate as a generator. It converts the kinetic energy of mechanical motion into electrical energy.
This energy is returned to the frequency converter, specifically to its DC link. As a result, the voltage across the DC link capacitors begins to rise rapidly. If not controlled, this will lead to the activation of overvoltage protection (usually an "OV" - Overvoltage error) and an emergency stop of the drive. The braking resistor serves precisely to "dump" this excess energy, ensuring controlled and safe braking.
Principle of Dynamic Braking System Operation
The dynamic braking system, whose key element is the resistor, operates according to a simple but effective algorithm:
Braking Start: The control system commands a reduction in motor speed. Due to load inertia (e.g., a heavy rotating drum or a descending elevator cabin), the motor's speed decreases slower than the magnetic field's speed in the converter.
Generator Mode: The motor enters generator mode, and recovered energy begins to flow into the VFD's DC link.
Voltage Rise: The voltage across the capacitors in the DC link (DC bus) begins to rise above the nominal level.
Braking Module Activation: A comparator built into the frequency converter monitors the voltage level. When the voltage reaches a certain threshold (e.g., 700-750 V for a 380 V network), the control system opens a power transistor (IGBT), known as the braking switch or braking module.
Energy Dissipation: The braking switch connects the resistor in parallel with the DC link capacitors. Current begins to flow through the resistor, which instantly converts this electrical energy into heat and dissipates it into the environment.
Voltage Stabilization: As a result of "dumping" the energy, the voltage in the DC link drops to a safe level. When the voltage falls below a certain threshold, the braking switch closes, disconnecting the resistor. This process repeats cyclically throughout the braking time, maintaining the voltage within permissible limits.
Thanks to this process, the frequency converter does not shut down due to an error and provides smooth and intensive braking of the drive according to the set pace.
When is it necessary to use a braking resistor?
The necessity of installing a braking resistor depends on the load characteristics and the requirements for braking dynamics. Let's consider typical cases where its use is mandatory.
| Equipment Type | Nature of Work and Justification |
| Hoisting and conveying mechanisms (cranes, elevators, winches) | When lowering a load, the motor operates in regenerative mode, and gravity spins it up. Without a resistor, lowering will be uncontrolled, and the VFD will constantly trip due to overvoltage. |
| Conveyors (especially inclined or with heavy loads) | Intense braking is required for quick and precise stopping of a long or loaded conveyor. Also, when an inclined conveyor operates "downhill", a constant braking torque arises. |
| Centrifuges, separators, mixers | These mechanisms have very high inertia. For quick completion of the work cycle or emergency stop, a huge amount of kinetic energy needs to be dissipated. |
| Winding and unwinding machines | Precise control of braking torque is required to maintain constant material tension and quickly stop rolls. |
| Machine tools (spindles, feed mechanisms) | For quick tool changes or precise positioning, rapid stopping of the spindle, which has significant inertia at high speeds, is necessary. |
| Engine test benches | When simulating various operating modes, including intensive braking, the resistor allows dissipating the energy produced by the tested engine. |
How to choose the right braking resistor: key parameters
Proper selection of a braking resistor is critically important for the reliable operation of the entire system. An incorrectly chosen resistor can be inefficient or, worse, lead to the failure of the converter's braking module. Selection is based on two main parameters: resistance (Ohm) and power (Watt).
1. Resistance (R, Ohm)
The resistance value determines the current that will flow through the resistor during braking. This value is strictly regulated by the frequency converter manufacturer. The technical documentation for the VFD always specifies the minimum permissible resistance (R_min).
If the resistance is too low (R < R_min): Too much current will flow through the resistor, exceeding the maximum current of the braking switch (IGBT transistor). This will inevitably lead to overheating and burnout of the switch. The cost of VFD repair in such a case can be very high.
If the resistance is too high: The braking current will be insufficient to effectively dissipate energy. The voltage in the DC link will not decrease fast enough, and the converter may still trip due to an overvoltage error. Braking will be sluggish and will not meet the specified parameters.
Selection Rule: The resistance of the selected resistor (R) must be slightly greater than or equal to the minimum permissible value specified in the VFD's manual: R ≥ R_min.
2. Power (P, Watt)
The resistor's power indicates how much thermal energy it can dissipate over a long period without overheating above the permissible temperature. This parameter depends on the intensity and duration of braking cycles.
Peak power: The maximum power the resistor can dissipate for a short period (several seconds).
Average power: The power that the resistor can dissipate during continuous operation or frequent braking cycles. This parameter is usually specified as the nominal power of the resistor.
The selection of power depends on the duty cycle (DC, %), which shows the ratio of braking time to the total operating cycle time. For an approximate calculation, you can use a formula that takes into account the peak braking power (P_peak) and the duty cycle (DC):
P_avg = P_peak * (DC / 100)
In practice, if precise calculations are difficult, empirical rules can be followed:
- Light and infrequent braking (DC < 10%): The resistor's power can be 5-10% of the motor's power.
- Medium intensity and frequency braking (DC ≈ 10-30%): The resistor's power is chosen in the range of 15-50% of the motor's power.
- Heavy and prolonged braking (load lowering, DC > 40%): The resistor's power can be equal to or even exceed the motor's power (50-150%).
It is always better to choose a resistor with a power reserve, as this will increase the system's reliability and prevent its premature failure due to overheating.
Connection and Installation
The braking resistor is connected to special terminals on the frequency converter. They are usually marked as PB and P+ or BR and +. It is important to strictly follow the connection diagram provided in your VFD's operating manual.
Most modern VFDs of low and medium power (up to 15-30 kW) have a built-in braking module. For more powerful models, an external braking module may be required, which is purchased separately.
During installation, it should be remembered that the resistor heats up significantly during operation (temperature can reach 200-400 °C). Therefore, it must be installed in a well-ventilated area, at a safe distance from flammable materials and other electronic components, ensuring free air convection.
Conclusion
A braking resistor is an integral element of a modern electric drive operating with inertial loads. It ensures safe and controlled braking, protects the frequency converter from emergency shutdowns due to overvoltage, and extends the service life of the equipment. Correct calculation of the resistor's resistance and power based on load characteristics and technological process requirements is the key to stable and efficient operation of the entire drive system.
