The energy stored in the motor and the inertia load must be dissipated. If the frequency converter ramps down quickly while still providing power to the motor, the motor may act as a generator, feeding energy back into the inverter. For more details, refer to the parameter list. Some features may not be available on all drive types—such as the MM410, 420, and 430 models, which do not include a built-in brake unit.
It is important to note that using a variable speed drive as an emergency stop is strictly prohibited (EN602049.2.5.4).
1. OFF1: This is the standard and default parking method. When a stop command is issued, the inverter reduces the output frequency to zero at the ramp rate set by P1121. However, if the motor and load have high inertia and system losses are low, the inertial energy may return to the inverter, causing the DC voltage to rise. In such cases, the inverter automatically adjusts the ramp time via P1240-3 to limit the voltage increase. In extreme situations, an overvoltage trip (F0002) will occur to protect the system. To maintain control, consider increasing the ramp rate (P1121) or exploring alternative braking options.
2. OFF2: With OFF2, the inverter immediately stops its output, allowing the motor and load to coast to a stop. This method is ideal when using an external mechanical brake to prevent the drive from blocking during the ramp-down phase. OFF2 is typically controlled by a digital input with a reverse sensor, meaning it’s active low and fail-safe. If you need to disconnect the motor (e.g., for safety), use OFF2 before opening the contactor to avoid alarms or faults.
3. OFF3: OFF3 offers a faster shutdown than OFF1 on older drives. On the MM4 series, only one ramp-down time is available, set by P1135. OFF3 is also usually active low.
4. DC Brake: Applying DC current to the motor generates a braking torque. If the motor is stopped, this torque can hold the motor in place, potentially replacing a mechanical brake. The DC braking function is configured via parameters P1230-4, with various timing and frequency options available. For detailed guidance, consult the parameter list and FAQ7734180. However, frequent or prolonged use of DC braking can cause the motor to overheat due to the feedback of DC current. Additionally, DC braking does not control the motor speed, so stopping time depends on factors like load, loss, and inertia. Calculating the exact braking torque is also challenging.
5. Composite Brake (P1236): Similar to OFF1, the composite brake adds a DC component when there's excessive energy returning to the inverter. During the ramp-down phase, the inverter applies both a falling frequency and a DC current, resulting in a combined braking effect. This helps dissipate energy more effectively. However, under sensorless vector control (P1300=20-23), the composite brake is not functional. The figure below illustrates how the composite brake combines OFF1 and DC braking techniques.
6. Kinetic (or Resistance) Braking: When OFF1 is used and excess energy is returned to the inverter, a braking resistor controlled by a brake transistor or IGBT can dissipate the energy. On MM440 units with frame size F, the brake unit is built-in, and an external resistor can be connected to manage the DC bus voltage. Choosing the correct resistor is critical to protect the brake transistor; refer to FAQ 7800906 for guidelines. For example, each resistor has a minimum resistance value to prevent damage. When the braking function is enabled via P1237, the brake cycle can be limited to protect the resistor. Standard resistors supplied with MM4 units have a 5% braking cycle, and this setting must be used accordingly. For applications requiring higher braking cycles, consult FAQ 7800906 for proper resistor selection. Some resistors also feature thermal switches for added protection, which can trigger alarms or trips.
Kinetic Brake Cycle Calculation: A 5% braking cycle means the resistor can handle 12 seconds of full power followed by 228 seconds of cooling. If braking duration is shorter than 12 seconds or power is less than 100%, multiple braking events can occur within 240 seconds. The inverter calculates the i²t of the resistor. For higher braking cycles (e.g., P1237 = 2), the allowed energy increases proportionally. For example, a 5-second braking event at 50% power per minute would require a larger resistor than the theoretical recommendation. It's advisable to install a larger resistor and adjust the braking cycle accordingly.
Example: A 7.5kW inverter that brakes 5 times in 60 seconds, with 2 seconds of braking at 50% power each time, equates to 10 seconds of total braking in 240 seconds. At half power, this represents 8% of the cycle. Using a 750W resistor and setting P1237 = 2 (10%) would help manage the braking process. For more examples, see FAQ7800906.
Kinetic Brake Alarm and Overload: Once the inverter determines that the resistor has absorbed the maximum allowable energy for the braking cycle, it limits the braking cycle to the setting in P1237. For instance, after 12 seconds of 100% braking with P1237 = 1 (5%), the power to the resistor is reduced to 5%. If the load starts at 50%, this happens after 24 seconds. An alarm (A0535) indicates when the braking cycle reaches 95% in 10 seconds (or 42% in 20 seconds). Under continuous high-load conditions, a low braking cycle setting may lead to alarms or damage. In such cases, ensure the resistor is correctly sized or use a safe brake via the alarm signal. Alternatively, a voltage threshold measurement (set above normal operation but below trip level) can activate a relay (P2172, P731 = 53.7/8).
Kinetic braking is highly effective for high-inertia loads, though the braking power is limited to 100% of the inverter's capacity, which includes short-term overload capability.
7. Mechanical Brake Control: The frequency converter includes a feature that simplifies the control of an external mechanical brake. Parameters P1215–7 allow the built-in relay to control the external brake, enabling the motor to engage and release the brake. The mechanical brake operates in conjunction with OFF1, offering a smooth and reliable stopping mechanism.
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