Today, I'd like to discuss issues related to inverter-driven lifting loads. Lifting loads are actually a common term for potential energy constant torque loads. You can easily imagine their practical applications, such as: crane main and auxiliary hooks, winches, elevators, escalators, stacker crane lifting mechanisms, mine cages, loading trolleys, bucket elevators, tower cranes, port loading and unloading bridges, gantry cranes, and material handling equipment in vehicle factories, etc.
We previously introduced the classification of mechanical loads, including potential energy constant torque loads. Here, we'll review the characteristics of this type of load:
1. Constant load torque.
2. The load torque direction is always downward.
3. The characteristic curves are located in the first and fourth quadrants.
4. When lowering heavy objects, there is energy feedback.
For this type of load characteristic, regardless of the industry or operating conditions, the two most critical issues for the electrical drive system consisting of a frequency converter and a motor in hoisting equipment are:
(1) Handling of potential energy
(2) Brake control
I. Treatment of Potential Energy
1. The energy conversion process during the lowering of the heavy object
a. When a heavy object is lowered, its gravitational potential energy is converted into kinetic energy.
b. The heavy object pulls the motor in reverse through mechanical mechanisms such as wire ropes and speed reducers (the motor rotor speed exceeds the speed output by the frequency converter), so that the motor is in a power generation state, and the kinetic energy of the heavy object is converted into electrical energy.
c. Electrical energy flows to the DC circuit through the diodes in the inverter bridge of the frequency converter.
d. Due to the limited capacitance of the DC link, electrical energy cannot be absorbed indefinitely.
2. Handling of DC power:
a. If the frequency converter is equipped with a braking unit and a braking resistor, the braking unit can be turned on by the CU unit to connect the braking resistor, so that electrical energy is converted into heat energy generated by the resistor.
b. If the inverter's rectifier bridge has an energy feedback function, the rectifier unit can be controlled by the CU unit to feed energy back to the grid.
c. Timely processing of motor feedback energy to ensure that the frequency converter does not experience overvoltage faults.
d. Whether the electrical energy in the DC link is consumed by heat or fed back to the grid for reuse requires comprehensive consideration of the equipment's operating conditions and the investment budget for the frequency converter.
II. Brake Control:
For motor brake control, Siemens frequency converters offer brake control methods, such as sequential control, controlled via BICO interconnect parameters. When the motor control method is vector control, the brake opening and closing condition is the set output torque threshold value; if the motor control method is V/F mode, the brake opening and closing condition is set as a percentage of the speed.
For Siemens' S120, G130, and G150 series frequency converters, an extended brake function is also provided. The extended brake control function is more powerful and can add some more complex state parameters to control the brake.
Of course, the motor brake can also be controlled by an external PLC.
III. Precautions for Commissioning Inverters with Lifting Loads
1. Debugging of the brake logic and the timing of the opening and closing delays (P1216, P1217).
2. In a system where multiple frequency converters control multiple motors, the braking actions must be consistent.
3. The brake coil must not be connected to the inverter output; it must be correctly connected to the corresponding power supply.
4. The parameters of the braking unit and braking resistor are set correctly.
5. The braking resistor will generate heat during operation, so it is necessary to ensure that there is sufficient heat dissipation space around the braking resistor.
6. Optimize the velocity loop and consider acceleration pre-control.
7. Various limiting protections, such as torque, current, and power.
8. For passenger elevators, comfort needs to be considered. This can be achieved by adding a curved curve and installing a weighing sensor to adjust the speed loop PI parameter.
9. This type of load usually requires starting under load, which requires the frequency converter to output large torque at low frequency, and the low frequency compensation voltage needs to be adjusted.
10. In typical lifting applications, limit and over-limit protection, system overspeed protection, and wire rope over-winding protection are installed on the track or wire rope drum to prevent the heavy object from hitting the top or falling to the bottom.
11. For vertical passenger elevators, multiple mechanical and electrical protections should be installed, such as mechanical speed limiters, overload protection, and shaft safety clamps.
IV. Common Problems and Solutions for Boosting Loads
1. Rollout: It is necessary to check the brake delay time, the torque for opening and closing the brake, or the logic of the external control of the brake, and whether there is a problem with the mechanical actuator of the brake.
2. Overcurrent: The brake opens too late, and the motor is in a stalled state; the brake opens too early when the load is lifted for the second time from a suspended state, and the load begins to fall. The output torque of the motor is insufficient to overcome the downward torque of the load. The brake engages too early during the motor deceleration process; the lifting weight exceeds the rated load; the speed loop parameters are unreasonable.
3. Overvoltage: The braking resistor may not have sufficient power or the parameters may not be set correctly.
4. The braking resistor is overheating: Check the resistor power selection to see if it meets the peak and average power curves of the load feedback power and whether it meets the repetition cycle of the resistor's own load.
Please refer to the following figure for the load curves of the braking module and braking resistor:
