Abstract: This paper introduces the characteristics, shortcomings and composition of inverter energy-saving braking, and focuses on the optimization selection method of inverter energy-saving braking unit and braking resistor.
1. Inverter energy consumption braking
The method of regenerative braking involves installing a braking unit assembly on the DC side of the frequency converter, which dissipates regenerative electrical energy in a braking resistor to achieve braking, as shown in Figure 1. This is the most direct and simple method for handling regenerative energy. It dissipates regenerative energy through a dedicated regenerative braking circuit in a resistor, converting it into heat energy. This resistor is called resistive braking.
Figure 1. Connection method of energy-saving braking, braking unit, and braking resistor.
Regenerative braking is characterized by its simple circuitry and low cost. However, its drawback lies in the fact that during braking, as the motor speed decreases, the kinetic energy of the drive system also decreases. Consequently, the motor's regenerative capacity and braking torque also diminish. Therefore, in drive systems with high inertia, it often fails to stop at low speeds, resulting in a "creeping" phenomenon that affects stopping time or stopping accuracy. Thus, regenerative braking is only suitable for stopping general loads. Regenerative braking consists of two parts: a braking unit and a braking resistor.
(1) Braking unit
The function of the braking unit is to activate the energy-dissipating circuit when the DC circuit voltage Ud exceeds a specified limit, allowing the DC circuit to release energy as heat after passing through the braking resistor. Braking units can be divided into two types: built-in and external. Built-in units are suitable for low-power general-purpose frequency converters, while external units are suitable for high-power frequency converters or operating conditions with special braking requirements. In principle, there is no difference between the two; both braking units act as a "switch" to activate the braking resistor, and they include a power transistor, a voltage sampling and comparison circuit, and a drive circuit.
(2) Braking resistor
Braking resistors are carriers used to dissipate the regenerative energy of electric motors as heat. They include two important parameters: resistance value and power capacity. In engineering, corrugated resistors and aluminum alloy resistors are commonly used. Corrugated resistors feature vertical corrugations on the surface, which facilitates heat dissipation and reduces parasitic inductance. They also use a highly flame-retardant inorganic coating to effectively protect the resistance wire from aging and extend its service life. Aluminum alloy resistors have superior weather resistance and vibration resistance compared to traditional ceramic frame resistors. They are widely used in demanding and harsh environments, are easy to install securely, easy to attach heat sinks, and have an aesthetically pleasing appearance.
The regenerative braking process is as follows: When the motor decelerates or reverses under external force (including being driven), the motor operates in generator mode, feeding energy back to the DC circuit and increasing the bus voltage; the braking unit samples the bus voltage, and when the DC voltage reaches the set conduction value of the braking unit, the power switch of the braking unit turns on, and current flows through the braking resistor; the braking resistor converts electrical energy into heat energy, the motor speed decreases, and the DC bus voltage also decreases; when the bus voltage drops to the set cutoff value of the braking unit, the switching power transistor of the braking unit turns off, and no current flows through the braking resistor.
The wiring distance between the braking unit and the frequency converter, and between the braking unit and the braking resistor, should be as short as possible (wire length less than 2m), and the conductor cross-section should meet the discharge current requirements of the braking resistor. During operation, the braking resistor will generate significant heat; therefore, it should have good heat dissipation. Heat-resistant wires should be used for connecting the braking resistor, and the wires should not touch the braking resistor. The braking resistor should be securely fixed using insulating baffles, and the installation location should ensure good heat dissipation. When installed inside a cabinet, the braking resistor should be mounted on top of the frequency converter cabinet.
2. Selection of Braking Unit
The braking torque of a variable frequency speed control system can be calculated using the following formula:
Where: MZ is the braking torque; GD is the moment of inertia of the motor; GDˊ is the moment of inertia of the motor load referred to the motor side; VQ is the speed before braking; VH is the speed after braking; MFZ is the load resistance torque; and Tj is the deceleration time.
Under normal circumstances, when braking an electric motor, there is a certain amount of internal loss, which is about 18% to 22% of the rated torque. Therefore, if the required braking torque is less than 18% to 22% of the rated torque of the motor, there is no need to connect a braking device.
When selecting a braking unit, the maximum operating current of the braking unit is the sole criterion for selection, and the calculation formula is as follows:
Where: IPM braking current instantaneous value; UD braking unit DC bus voltage.
3. Optimized selection of braking resistor
The resistance value of the braking resistor in a variable frequency speed control system can be calculated using the following formula:
Where: RZ is the braking resistance value; UZ is the braking unit operating voltage value; and Me is the rated torque of the motor.
During the operation of the braking unit, the rise and fall of the DC bus voltage depends on the constant RC, where R is the resistance of the braking resistor and C is the capacitance of the internal capacitor of the frequency converter.
If the braking resistor value is too large, braking will be slow; if it is too small, the braking switching element will be easily damaged. Generally, when the load inertia is not too large, it is assumed that a maximum of 70% of the energy is consumed by the braking resistor during motor braking, and 30% of the energy is consumed by various losses in the motor itself and the load. In this case, the braking resistor is:
In the formula: P is the motor power (kW); UC is the voltage on the bus during braking (V); R is the braking resistor (Ω).
When the three-phase voltage is 380V, UC≈700V; when the single-phase voltage is 220V, UC≈390V; and the resistance value of the braking resistor when the three-phase voltage is 380V is:
The resistance value of the braking resistor in single-phase 220V is:
The power dissipation of the braking resistor in low-frequency braking is generally 1/4 to 1/5 of the motor power. For frequent braking, the power dissipation needs to be increased. Some small-capacity frequency converters have internal braking resistors, but during high-frequency or heavy-load braking, the heat dissipation of the internal braking resistor is insufficient, making it prone to damage. In such cases, a high-power external braking resistor should be used. All braking resistors should be low-inductance resistors; the connecting wires should be short and use twisted-pair or parallel wires. Low-inductance measures are taken to prevent and reduce inductive energy from being applied to the braking switch transistor, causing damage. If the circuit has high inductance and low resistance, it will damage the braking switch transistor.
The braking resistance is closely related to the flywheel torque of the electric motor, and the flywheel torque of the electric motor changes during operation. Therefore, it is difficult to accurately calculate the braking resistance. Usually, an approximate value is obtained by using an empirical formula.
RZ>=(2×UD)/Ie
Where: Ie is the rated current of the frequency converter; UD is the DC bus voltage of the frequency converter.
Since the braking resistor operates on a short-time duty cycle, its nominal power rating in a variable frequency speed control system can generally be calculated using the following formula, based on the resistor's characteristics and technical specifications:
PB = K × Pav × η%
In the formula: PB is the nominal power of the braking resistor; K is the derating factor of the braking resistor; Pav is the average power consumed during braking; η is the braking utilization rate.
To reduce the variety of braking resistor values, inverter manufacturers often provide braking resistors with the same resistance value for several different motor capacities. Therefore, the braking torque obtained during braking varies considerably. For example, Emerson's TD3000 series inverters provide 3kW, 20Ω braking resistors for motors with capacities of 22kW, 30kW, and 37kW. When the braking unit is activated at a DC voltage of 700V, the braking current is:
IB=700/20=35A
The power of the braking resistor is:
PB0 = (700)²/20 = 24.5kW
4. Conclusion
Braking units and braking resistors used in variable frequency speed control systems are essential for the safe and reliable operation of systems with regenerative energy and requiring accurate stopping. Therefore, when selecting a variable frequency speed control system, the braking unit and braking resistor should be optimized. This can not only reduce the chance of failure in the variable frequency speed control system, but also enable the designed variable frequency speed control system to have high dynamic performance indicators.
