1 Introduction
With the application of new power electronic devices and high-performance microprocessors, as well as the development of motor control technology and fieldbus technology, frequency converters are evolving towards modularization, digitalization, intelligence, and networking. Siemens' 6SE70/6SE71 series vector (VC) frequency converters and ABB's ACS800/ACC800 series direct torque (DTC) frequency converters both possess modularization, digitalization, intelligence, and networking characteristics, representing the pinnacle of high-performance engineering frequency converters. Addressing the characteristics of potential energy loads, both the 6SE70/6SE71 and ACS800/ACC800 series frequency converters are designed with mechanical braking control, commonly known as brake control. The main function of this control is to lock the motor and driven equipment at zero speed when the drive unit stops or is not powered, ensuring the safety of the potential energy load, drive unit, and equipment.
2. Purpose and Application of Mechanical Braking Control Function in Frequency Converter
2.1 Purpose of Mechanical Braking Control Function
For potential energy loads, due to gravity, the load cannot stop in mid-air without a dedicated braking device. Therefore, a mechanical brake must be installed on the motor shaft, commonly an electromagnet brake or a hydraulic electromagnetic brake. Most brakes are normally closed, meaning that when the coil is de-energized, the brake holds the shaft by spring force; when the coil is energized, it releases. When the load begins to lift or stop, the actions of the brake and the motor must be closely coordinated. Because the brake's action from tightening to loosening, and vice versa, takes time (approximately 0.6 seconds, varying depending on the motor's capacity), while the generation or disappearance of the motor's torque is immediately apparent upon energization or de-energization, problems easily arise in the coordination of their actions. If the motor is energized but the brake has not yet released, it will cause severe motor overload; conversely, if the motor is de-energized but the brake has not yet tightened, the load will inevitably slide down, resulting in a slippage.
2.2 Application of Mechanical Braking Control in General-Purpose Frequency Converters
2.2.1 General-purpose frequency converter mechanical braking control
Since the initial application of frequency converters in potential energy load fields, their mechanical braking control function has become a key focus in design and maintenance. Due to the inherent limitations of general-purpose frequency converters, the available resources are not abundant. In practical applications, the following control methods are generally used:
(1) The mechanical braking control signal is formed by the definable switch output signal and fault output signal of the frequency converter;
(2) The operating signal and fault output of the frequency converter are used to form the mechanical braking control signal;
(3) The inverter’s operating signal, switch output signal, fault output signal, and current analog signal are input to the PLC. The PLC’s programming control function is used to design the mechanical braking control conditions, and the PLC output signal controls the mechanical brake.
Of the three methods mentioned above, the first two are generally used in simple systems with low control requirements, while the third is used in slightly more complex systems with higher control precision requirements, and generally requires a PLC system. In the mid-1990s, during the frequency conversion upgrade of the oxygen lance control system at Jinan Iron and Steel Group's No. 1 steelmaking plant, the designers fully utilized the signals available from the frequency converter to control the mechanical braking, a typical example of a PLC system participating in mechanical braking. The control concept was as follows: the PLC system collected the frequency converter's operating signals, definable switch output signals, fault output signals, and analog current signals. The definable switch output signals were defined as outputs greater than 3Hz. Using the analog current signal and the PLC's comparison function, a current threshold value for switching the mechanical braking was set. The PLC system then designed the conditions for switching the mechanical braking based on these conditions, controlling the mechanical brake coil through the PLC's output. The electrical schematic diagram of the PLC system participating in the mechanical braking of a general-purpose frequency converter is shown in Figure 1.
2.2.2 Mechanical Braking Control Process of General-Purpose Frequency Converter
The following example, using the configurable switching and current signals of a general-purpose frequency converter as the main signals for mechanical braking control, illustrates the specific working process of mechanical braking control during start-up and shutdown:
(1) Mechanical braking control process during load start-up
A "lifting start frequency" fsd is set. When the inverter's operating frequency rises to fsd, the lifting will pause. To ensure that the inverter can control the lifting of the load and prevent slippage after the brake electromagnet is released, the inverter will start detecting the current and set the current detection time tsc at the same time the operating frequency reaches fsd; a "release command" is issued. When the inverter confirms that there is a sufficiently large output current, it will issue a "release command" to energize the mechanical brake controller; a holding time trd is set for frd, and the length of trd should be slightly longer than the time required for the mechanical brake controller to fully release from energization; the inverter will then increase its operating frequency to the required frequency.
The mechanical braking control process for load start-up is shown in Figure 2.
(2) Mechanical braking control process when the load stops
Set a "stop start frequency" fbs
When the inverter's operating frequency drops to fbs (e.g., 3Hz), the inverter will output a "frequency arrival signal" and issue a power-off command to the mechanical brake controller.
Set the duration of an FBS to tbb
The length of the TBB should be slightly greater than the time required for the mechanical brake controller to fully engage from the start of release.
The inverter reduces the operating frequency to 0.
The mechanical braking control process when the load stops is shown in Figure 3.
2.3 Application of Mechanical Braking Control in Engineering-Type Frequency Converters
2.3.1 Methods of Mechanical Braking for Engineering-Type Frequency Converters
With the development of high-performance microprocessor technology and the application of DSP technology in frequency converters, frequency converters not only possess superior control performance but also powerful digital programming processing capabilities. Siemens 6SE70/6SE71 frequency converters and ABB ACS800/ACC800 frequency converters both incorporate mechanical braking control functions internally. The mechanical braking control logic is integrated into the frequency converter application, allowing users to implement complex mechanical braking control functions through simple definitions and hard-wired connections. The mechanical braking control concept of engineering-type frequency converters is as follows: the frequency converter uses various signals it collects, such as current, torque, speed, and fault signals, to internally calculate and generate a signal to switch the mechanical braking controller. This signal directly controls the mechanical braking controller, while simultaneously generating control signals (such as internal enable and inverter enable) to control the frequency converter's operation in coordination with the mechanical brake's action. This function can also utilize the mechanical brake controller's detection elements to check if the mechanical brake is functioning correctly. Figure 4 shows an application principle diagram of mechanical braking control in an engineering-type frequency converter.
2.3.2 Control process of mechanical braking in engineering-type frequency converters
The mechanical braking control function of an engineering-type frequency converter is used as an example to describe the operation process of the mechanical brake when the load starts and stops. The mechanical braking control process of the engineering-type frequency converter is shown in Figure 5.
(1) Mechanical braking control process during load start-up
Set a "starting torque when the brake is open" ts
When the inverter start command and the external speed set enable signal are available, the inverter starts to work. In order to ensure that the motor has sufficient torque when the mechanical brake is engaged and the load does not slip, a threshold torque value ts for opening the brake is set. The threshold signal can also be a current signal.
Issue "brake release command"
When the frequency converter confirms that there is a sufficiently large output torque (current), and all other logic conditions are met, it will issue a "brake opening command" to energize the mechanical brake controller.
Set a "brake opening time" tod
Once the inverter confirms that the mechanical brake has been opened, after the "brake opening time" tod, it controls the internal given enable through the control function, causing the inverter to start accelerating along the speed curve to the required speed.
The mechanical braking control process for load start-up is shown in Figures 5, 1 to 4.
(2) Mechanical braking control process when the load stops
Set a "brake closing speed" ncs
The inverter sets a "brake closing speed" ncs. To ensure that the motor has sufficient torque when the brake is closed and the load does not slip, a threshold torque value ncs for closing the brake is set. The threshold signal for brake closing is generally a speed signal.
Issue "brake closing command"
When the inverter stops running, after the external enable and external speed signals stop, the speed decreases along the inverter speed curve. When the inverter detects NCS (such as 5% of rated speed), the inverter will output a "brake closed" signal and issue a brake power-off command.
Set a "brake closing time" tcd
Once the inverter confirms that the mechanical brake is closed, after the "brake open/close" time clock (TCD), it immediately stops working by blocking the inverter's internal enable (motor excitation) and the inverter enable through the control function.
The mechanical braking control process when the load stops is shown in Figures 5 to 7.
2.4 Comparison of Mechanical Braking in General-Purpose Frequency Converters and Engineering-Type Frequency Converters
Compared with the mechanical braking control function of general-purpose frequency converters, the mechanical braking control function of engineering-type frequency converters has the following three main advantages:
2.4.1 Control methods and concepts
Due to inherent limitations, general-purpose frequency converters have limited resource availability. The output points provided by these converters are only at the same level of control signals for designing mechanical braking functions. In contrast, engineering-grade frequency converters, with their programmability, can directly or programmatically provide various signals to the mechanical braking function. Engineering-grade frequency converters have dedicated mechanical braking modules that use RS trigger controllers, prioritizing mechanical braking conditions. This is the most reasonable and safest approach for mechanical braking. Therefore, the control methods and concepts of mechanical braking functions in engineering-grade frequency converters are more rational.
2.4.2 Control performance and accuracy
General-purpose frequency converters use PLC control for their mechanical braking function, allowing it to consider more conditions. However, the PLC collects data from the frequency converter via communication and then performs mechanical braking logic control. The system takes at least one cycle to execute the PLC program and calculate the result, which is then used to control the mechanical braking controller via the output module. Even at 200ms, this is unacceptable for high-precision loads (such as elevator systems). For engineering-grade frequency converters, the computation time for mechanical braking logic control is merely a hardware calculation time. Engineering-grade frequency converters allow for maximum control performance and precision in their mechanical braking function.
2.4.3 Safety Performance
The mechanical brake control of general-purpose frequency converters only collects signals from the frequency converter without providing any information or control to it. The mechanical brake control function of engineering-type frequency converters provides two control signals: the internal setpoint enable of the frequency converter and the inverter enable. These two signals control the start of the internal setpoint of the frequency converter and the stop of the inverter. When the mechanical brake is opened and closed, the frequency converter cooperates with the operation of the mechanical brake to protect the safety of the frequency converter and the load. The mechanical brake function of engineering-type frequency converters gives the frequency converter higher safety performance.
3. Features and mechanical braking function of Siemens 6SE70 frequency converter
3.1 Features of Siemens 6SE70 Frequency Inverter
The Siemens 6SE70 fully digital vector control frequency converter belongs to the Simovert MasterDriv series and is an outstanding high-performance frequency converter product from Siemens. Its fully digital design and powerful software control, monitoring, recording, and protection functions ensure the stability and reliability of its operation. Its selectable control functions include voltage-frequency control, vector control, and scalar control, suitable for various load characteristics. Flexible parameter settings allow for easy modification of parameters and control modes. Compatible communication functions enable communication with other control systems. Through parameter selection, the frequency converter can be controlled locally or remotely via a fieldbus communication network.
3.2 Mechanical braking function of Siemens 6SE70 frequency converter
The Siemens 6SE70 series frequency converter has a very powerful mechanical braking function, which fully demonstrates the programmable advantages of engineering-grade frequency converters. The mechanical braking function can be controlled in three ways: control without external mechanical brake, control with external mechanical brake but without mechanical brake detection, and control with external mechanical brake and mechanical brake detection. Moreover, the mechanical braking function can be used to control the frequency converter's setpoint enable and inverter enable to quickly stop the frequency converter. Its control principle block diagram is shown in Figure 6.
3.3 Implementation of Mechanical Braking Function in Siemens 6SE70 Frequency Inverter
3.3.1 Principle of Mechanical Braking Control Function
Siemens mechanical brake control utilizes the RS flip-flop principle. The "S" operation (opening the mechanical brake) is an "AND" operation, requiring all conditions to be met before the brake can be opened. The "R" operation (closing the mechanical brake) is an "OR" operation, requiring only one condition to be met before the brake can be closed. Furthermore, the RS flip-flop has reset priority, which in practical applications means brake closure priority. This control method is safer for mechanical brake control. Two internal switching signals are obtained through the RS flip-flop, which can be connected to the inverter's switching output points to control the mechanical brake controller. Simultaneously, the mechanical brake control function generates internal inverter enable and inverter enable control signals, allowing selection of the control signals to turn the inverter and inverter on and off as needed.
3.3.2 Components of Mechanical Braking Control
Figure 6 shows the block diagram of the mechanical braking function of a Siemens frequency converter, which can be divided into seven parts: mechanical braking on/off conditions, mechanical braking off/on control signals, control setpoint integral enable signal, inverter enable signal, alarm signal for mechanical braking on/off detection function, and alarm signal for mechanical braking off detection function. The main parts are described below:
(1) Conditions for activating mechanical braking
The logic for activating the mechanical brake is "AND," meaning all conditions must be met before the mechanical brake can be activated. This function has four conditions: motor excitation is a fixed condition, while the other three can be set. Parameter p608 can set two switching control signals; generally, one inverter operating signal (b104) is sufficient. The most important setting in this function is the mechanical brake activation threshold control. Parameter p610 defines the selected reference quantity, often a current signal (kk242). p611 is a percentage of the reference signal's rated value, defined according to actual needs, typically around 20%. The principle of the mechanical brake activation conditions is shown in part 1 of Figure 6.
(2) Conditions for closing mechanical brakes
The condition for closing the mechanical brake (r) is "OR"; the mechanical brake will close if any one of the conditions for closing the mechanical brake is met. There are a total of 6 conditions in this function, with power outage being a fixed condition and the other 5 conditions being configurable. Parameter p609 can set four switching control signals; generally, the inverter's non-running signal (b105) is sufficient. The most important setting in this function is the mechanical brake closure threshold control setting. Parameter p615 defines the selected reference quantity, commonly using the speed signal (kk148 or kk91). p616 is the percentage of the reference signal's rated value, defined according to actual needs, generally around 10%. Note parameter p617; using this parameter can change the mechanical brake closure signal from a falling edge signal to a wide pulse signal, usually defined as 0.5s. The mechanical brake closure threshold signal is also valid only when one of the stop signal, fault-free signal, or parameter p614 is constant 1. The principle of the mechanical brake closure condition is shown in part 2 of Figure 6.
(3) Switch mechanical brake control signal
After processing by the RS flip-flop, two mechanical brake control signals are obtained: an on mechanical brake control signal (b0275) and a off mechanical brake control signal (b0276). The principle of switching the mechanical brake control signals is shown in part 3 of Figure 6.
(4) Control the given integral enable signal
Siemens frequency converters can select three mechanical braking function control modes via parameter p605. When no mechanical braking is selected (p605=0), the given integral enable signal (b0277) is constant 1. When mechanical braking without detection information is selected (p605=1), when the mechanical braking signal is opened, the internal given enable is activated after the time set by parameter p606. When stopped, when the mechanical braking signal is closed, the internal given enable is locked after the time set by parameter p606. When mechanical braking with detection information is selected (p605=2), when the mechanical braking signal is opened, after the time set by parameter p606 and a brake opening signal is detected, the internal given enable is activated. When the mechanical braking signal is closed, after the time set by parameter p606 and a mechanical brake return information is detected, the internal given enable is locked. The control word p564 for this function must be defined as b0277. The principle of the mechanical braking control given integral enable is shown in part 4 of Figure 6.
(5) Control inverter enable signal
The inverter enable signal mainly comes from the mechanical brake threshold control signal in the mechanical brake closure condition. After system startup, the inverter enable signal (b0278) is 1. When the inverter is selected as no mechanical brake (p605=0), the inverter enable signal is immediately blocked when the mechanical brake closure threshold control signal condition is met. When the mechanical brake without detection information mode is selected (p605=1), the internal inverter enable signal is blocked after a delay following the time set by parameter p607 when the mechanical brake closure condition is met. When the mechanical brake with detection information mode is selected (p605=2), the internal inverter enable signal is immediately blocked after the mechanical brake return signal is detected following the time set by parameter p607 when the mechanical brake closure threshold control signal condition is met. The control word p561 for this function needs to be defined as b0278. The principle of mechanical brake control for inverter enable is shown in part 5 of Figure 6.
3.3.3 Output of mechanical braking control signal
Siemens frequency converter mechanical brake control signals can be output in three ways:
(1) Use the x101's digital output point (+24V);
(2) Expand the switch signal output (+24V) of the eb2 board;
(3) Output is achieved using terminals 4 and 5 (a pair of passive 220V terminals) of the x9 terminal. In practical applications, the control voltage of the mechanical brake controller is generally 220V. If the first two outputs are used, the control signal still needs to be converted by an intermediate relay. Adding an intermediate link increases the control time and the point of failure.
Of the three output methods mentioned above, the best approach is to directly use the control signal x9(4,5) to participate in the mechanical brake controller.
4. Application of the mechanical braking function of Siemens 6SE70 frequency converter in the auxiliary gun of Jinan Iron and Steel Group [4]
4.1 Jinan Iron and Steel Group's Auxiliary Gun Lifting Frequency Conversion Control System
The Jinan Iron and Steel Group's No. 3 steelmaking project is a key project for the group's expansion and strengthening. Its converter control system utilizes Danieli-Connors' sub-lance technology. The sub-lance electrical drive control system includes the sub-lance lifting drive system, the sub-lance rotation drive system, and the APC system (automatic probe loading and unloading device).
4.1.1 Secondary gun lifting drive equipment
The auxiliary lance hoisting device is located on the winch platform and includes a 55kWAC motor, DC brake, gearbox, winch drum, speed generator for speed control, and two pulse generators for height measurement (the second pulse generator is installed for safety reasons). If an unacceptable error occurs between the pulse generators, the lance can only be lowered and raised at low speed. In case of an emergency, the auxiliary lance can be lifted from the converter via the emergency DC battery; no other actions are permitted. The process requirements for auxiliary lance hoisting control are shown in Table 1.
4.1.2 Secondary gun hoisting frequency conversion control system
The auxiliary gun hoisting electrical control system consists of two sets of frequency converters: a normal mode frequency converter and an emergency mode frequency converter. The normal mode primarily handles the manual, semi-automatic, and computer-controlled operation of the auxiliary gun; the emergency mode handles operation in emergency situations. The auxiliary gun hoisting drive motor is 55kW. For normal mode, a Siemens 75kW frequency converter (6SE7031-2EF60) is selected, along with a 50kW braking unit (6SE7028-0EA87-2DA0). The feedback device is a pulse encoder (1024p/r). Because the distance to the equipment is nearly 100m, a digital tachometer DT1 (6SE7090-0xx84-3DB0) is used to amplify and repair the signal. The emergency mode uses a Siemens 75kW frequency converter (6SE7031-2EF60) without a braking unit or feedback device. Both the normal and emergency mode frequency converters are equipped with input and output reactors to eliminate harmonics and suppress voltage spikes.
4.2 Application of Mechanical Braking Control Function in Sub-gun Frequency Converter System
In the secondary gun lifting control, the normal mode uses an external mechanical brake with mechanical brake signal detection, while the emergency mode uses an external mechanical brake without mechanical brake detection. Moreover, the emergency mode does not have a braking unit. In this mode, the existing mechanical brake signal is used to control the inverter's enable signal and the inverter signal enable signal to control the inverter.
4.2.1 Normal Mode
In normal mode, the conditions for activating the mechanical brake are that the inverter's operating signal and the inverter current equal to 20% of the motor's rated current. The mechanical brake is only activated when both conditions are met to ensure sufficient torque in the motor and prevent load slippage. The conditions for deactivating the mechanical brake are that the inverter stops and the motor speed equals 5% of the rated speed. Using a speed detection signal to deactivate the mechanical brake offers better control safety than using a detected current signal because, for a dual-loop control system, the speed loop is stable, and the speed signal changes steadily during system shutdown.
In normal mode, the inverter's mechanical brake control uses a mechanical brake detection function. A limit switch (x101, 6 digital inputs) is installed on the external mechanical brake to detect whether the mechanical brake is in place. When the mechanical brake is engaged, if the mechanical brake engagement condition is met but the mechanical brake detection signal does not arrive within 1.0s, it is considered a fault state, the inverter is locked (inverter signal enable), and alarm a036 is triggered. When the mechanical brake is disengaged, if the mechanical brake disengagement condition is met but the mechanical brake signal remains unchanged within 1.6s, it is considered a fault state, the inverter is locked (given enable), and alarm a037 is triggered.
In the normal mode of the secondary gun, the mechanical brake control signal is directly used to control the mechanical brake contactor via the x9 (4,5) output.
4.2.2 Emergency Mode
The mechanical braking control of the inverter in emergency mode is basically the same as that in normal mode. Since no braking unit is used in emergency control, the speed threshold for closing the mechanical brake is very low to avoid large feedback voltage impacting the inverter. Emergency mode uses a control method without a mechanical brake detection signal. This method applies the principle that if the mechanical brake signal of the control switch does not activate within a defined time, the inverter's setpoint enable and the inverter signal enable are blocked. In the secondary emergency mode, the mechanical brake control signal directly participates in the mechanical brake contactor control using the x9 (4,5) output.
4.3 Application of secondary gun mechanical braking control parameters
The mechanical braking control parameters of the frequency converter in normal mode and emergency mode are selected with mechanical braking detection signal mode and without mechanical braking detection mode, respectively. The mechanical braking control parameters of the frequency converter are shown in Table 2.
5. Conclusion
The mechanical braking control function of the engineering-type frequency converter can effectively meet the requirement that the action of the brake and the motor must be closely coordinated when the potential energy load starts to rise or stop. The realization of this function ensures the safety of the potential energy load, the transmission unit and equipment. However, it should be noted that in various safety regulations and related standards (IEC 61800-2), the frequency converter is defined as a complete drive unit and a basic drive unit, and is not listed as a safety device. Therefore, safety cannot rely entirely on the performance of the brake control function of the frequency converter, but should be strictly implemented according to special safety regulations [3].
