电动卷扬机的控制——机械专业毕业设计论文外文翻译（中英文翻译、外文文献翻译）

电动卷扬机的控制——机械专业毕业设计论文外文翻译（中英文翻译、外文文献翻译） 电动卷扬机的控制——机械专业毕业设计论文外文翻 译(中英文翻译、外文文献翻译) 英文原文Electrical Winch Controls by Tom Young The form of motor control we all know best is the simple manual station with up and down pushbuttons. While these stations may still be the perfect choice for certain applications，a dizzying array of more sophisticated controls is also available. This article addresses the basic electrical requirements of the motors and user interface issues you will need to address before specifying，building or buying winch controls. To begin with，the manual control stations should be of the hold-to-run type，so that if you take your finger off of the button the winch stops. Additionally，every control station needs an emergency stop (E-stop) that kills all power to the winch，not just the control circuit. Think about it―if the winch isn’t stopping when it should，you really need a failsafe way to kill the line power. It’s also a great idea to have a key operated switch on control stations，especially where access to the stations is not controlled. Safe operation by authorized personnel must be considered when designing even the simplest manual controls. Controlling Fixed Speed Motors The actual controlling device for a fixed speed winch is a three phase reversing starter. The motor is reversed by simply switching the phase sequence from ABC to CBA. This is accomplished by two three-pole contactors，interlocked，so they can’t both be closed at the same time. The NEC requires both overload and short circuit protection. To protect the motor from overheating due to mechanical overloads a thermal overload relay is built into the starter. This has bi-metallic strips that match the heating pattern of the motor and trips contacts when they overheat. Alternatively，a thermistor can be mounted in the motor winding to monitor the motor temperature. Short circuit protection is generally provided by fuses rated for use with motors. A separate line contactor should be provided ahead of the reversing contactor for redundancy. This contactor is controlled by the safety circuits: E-stop and overtravel limits. This brings us to limit switches. When you get to the normal end of travel limit the winch stops and you can only move it in the opposite direction (away from the limit). There also needs to be an overtravel limit in case，due to an electrical or mechanical problem，the winch runs past the normal limit. If you hit an overtravel limit the line contactor opens so there is no way to drive off of the limits. If this occurs，a competent technician needs to fix the problem that resulted in hitting the overtravel limit. Then，you can override the overtravels using the spring return toggle switch inside the starter―as opposed to using jumpers or hand shooting the contactors. Variable Speed Requirements Of course，the simple fixed speed starter gets replaced with a variable speed drive. Here’s where things start to get interesting! At the very least you need to add a speed pot to the control station. A joystick is a better operator interface，as it gives you a more intuitive control of the moving piece. Unfortunately，you can’t just order any old variable speed drive from your local supplier and expect it to raise and lower equipment safely and reliably over kids on stage. Most variable speed drives won’t，as they aren’t designed for lifting. The drive needs to be set up so that torque is developed at the motor before the brake is released，and (when stopping) the brake is set before torque is taken away. For many years DC motors and drives provided a popular solution as they allowed for good torque at all speeds. The large DC motors required for most winches are expensive，costing many times what a comparable AC motor costs. However，the early AC drives were not very useful，as they had a very limited speed range and produced low torque at low speeds. More recently，as the AC drives improved，the low cost and plentiful availability of AC motors resulted in a transition to AC drives. There are two families of variable speed AC drives. Variable frequency inverters are well known and readily available. These drives convert AC to DC，then convert it back to AC with a different frequency. If the drive produces 30 Hz， a normal 60 Hz motor will run at half speed. In theory this is great， but in reality there are a couple of problems. First，a typical 60 Hz motor gets confused at a line frequency below 2 or 3 Hz，and starts to cog (jerk and sputter)，or just stops. This limits you to a speed range of as low as 20:1―hardly suitable for subtle effects on stage! Second，many lower cost inverters are also incapable of providing full torque at low speeds. Employing such drives can result in jerky moves，or a complete failure to lift the piece―exactly what you don’t want to see when you are trying to start smoothly lifting a scenic element. Some of the newer inverters are closed loop (obtain feedback from the motor to provide more accurate speed control) and will work quite well. The other family of AC drives is flux vector drives. These units require an encoder mounted on the motor shaft allowing the driv