When the motor is decelerating, the energy stored in the motor and the load must be dissipated. If the frequency converter ramps down too quickly while still supplying power to the motor, the motor may act as a generator, feeding energy back into the inverter. This can cause the DC bus voltage to rise. To prevent overvoltage faults, the inverter automatically adjusts the ramp time using a voltage controller (P1240-3). However, if the system has high inertia and low losses, this could lead to an overvoltage trip (F0002), which will shut down the drive. In such cases, it’s recommended to extend the ramp time (P1121) or use additional braking methods.
OFF1 is the standard parking method. When a stop command is issued, the inverter reduces the output frequency at the rate set by P1121. If the motor and load have high inertia, the energy from the load may return to the inverter, increasing the DC voltage. The inverter will then adjust the ramp time to manage this. In extreme situations, an overvoltage fault may occur. It's important to ensure that the ramp rate is sufficient to avoid this.
OFF2 is used when you want the motor to coast to a stop without active braking. This is typically controlled via a digital input with a reverse sensor. OFF2 is often used with external mechanical brakes to prevent the drive from being blocked during deceleration. Before disconnecting the motor (e.g., using a contactor), it’s advisable to use OFF2 to avoid unexpected alarms or faults.
OFF3 is a faster stopping method compared to OFF1, especially on older drives. It uses a single ramp-down time setting (P1135). OFF3 is also usually active low, making it suitable for fail-safe applications.
DC braking applies a direct current to the motor, creating a braking torque. This can help hold the motor in place when stopped. The settings for DC braking are found in parameters P1230-4. While DC braking can replace mechanical brakes in some cases, it should not be used for long periods, as it can cause the motor to overheat due to the energy being dissipated in the windings.
Composite braking (P1236) combines OFF1 with DC braking. When the motor frequency is reduced, a DC component is added to the output, allowing for more effective energy dissipation. This helps control the motor speed and reduce the risk of overvoltage. However, composite braking does not work under sensorless vector control (P1300=20-23).
Kinetic or resistance braking is used when excess energy is returned to the inverter. A braking resistor is connected to the DC bus, and the brake transistor or IGBT controls the energy dissipation. On MM440 units with frame size F, the brake unit is built-in, and an external resistor can be used to manage the DC voltage. Choosing the correct resistor is critical to protect the brake transistor. For example, the resistor must meet minimum resistance requirements to avoid damage.
Braking cycles are managed through parameter P1237. A 5% braking cycle means the resistor can handle 12 seconds of full power before needing cooling. For longer or more frequent braking, a larger resistor may be required. The inverter calculates the i²t of the resistor to determine how much energy can be absorbed safely. If the braking cycle exceeds 5%, the resistor must be selected carefully according to guidelines in FAQ 7800906.
For example, a 7.5kW inverter that brakes five times per minute at 50% power for 2 seconds each time would require a 750W resistor with P1237 set to 2 (10%). This ensures the resistor can handle the energy without overheating. Braking resistors may also include thermal switches to trigger alarms or trips if the temperature becomes too high.
If the braking cycle is set too low (e.g., 5%), and the inverter is subjected to continuous high-load braking, it may trigger alarms or even trip, leading to loss of braking capability. In such cases, it’s crucial to install the correct resistor or use an alarm signal to activate a safe mechanical brake. Alternatively, a voltage threshold measurement (P2172, P731 = 53.7/8) can be used to operate a relay before the inverter trips.
Mechanical brake control is also available via parameter P1215-7, which allows the inverter to control an external mechanical brake. This feature simplifies the process of engaging and releasing the brake, especially when used in conjunction with OFF1.
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