1、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 art
2、icle addresses the basic electrical requirements of the motors and user interface issues you will need to address before spcifying， building or buying winch controls. 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 winc
3、h 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 isnt stopping when it should， you really need a failsafe way to kill the line power. Its also a great idea to have a key operated
4、 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 re
5、versing 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 cant both be closed at the same time. The NEC requires both overload and short circuit protection. To protect the motor from overheat
6、ing 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 ci
7、rcuit 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 t
8、o 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 con
9、tactor 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 ha
10、nd shooting the contactors. Variable Speed Requirements Of course， the simple fixed speed starter gets replaced with a variable speed drive. Heres 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， a
11、s it gives you a more intuitive control of the moving piece. Unfortunately， you cant 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 wont， as they arent designed for lifting
12、. 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 r
13、equired 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 availabil
14、ity 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 6
15、0 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 su
16、itable 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 dont want to see when you are trying to start smoothly lifting a
17、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 drive to precisely
18、 monitor the rotation of the armature. A processor determines the exact vector of magnetic flux (thus flux vector drive) required to rotate the armature the next few degrees at a given speed. These drives allow an infinite speed range， as you can actually produce full torque at zero speed. The preci
19、se speed and position control offered by these drives make them a favorite in high performance applications. PLC-based controls provide system status as well as control options. This screen give the operator full access to Carnegie Halls nine stage floor lifts. PLC Based Systems A PLC is a programma
20、ble logic controller. First developed to replace the relay based industrial control systems of the 50s and 60s， these controls are at home in rugged， industrial environments. These are modular systems with a great variety of I/O modules allowing semi-custom hardware configurations to be assembled ea
21、sily at a reasonable price. These include position control modules， counters， A/D and D/A converters and all sorts of solid state or hard contact closure outputs. The great variety of I/O components and the modular nature of the PLC make this an effective way to build custom and semi-custom control
22、systems. The greatest drawback to PLC systems is the lack of really great displays to tell you what they are doing or to help you program them. Monochrome and medium resolution color displays are the norm， as the primary use for these components in on a factory floor. One of the first major PLC systems used in a large entertainment venue is the complex lift and wagon system at the original MGM Grand (now Ballys) in Las
