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How A Wright Crane Works
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This is just a short article about how Acco-Wright cranes work. I'm going to explain using  a crane with two hoists, two trolleys, and a bridge, which runs off of a 480 volt primary voltage, drawing 30 amps of power. To some this article may be confusing, and to others that just like to hear about the way things work may be more intrigued. Let's see if you can follow me.

The first thing that the power will run through is a 30 amp Main disconnect panel (Seeing as the crane draws 30 amps) Since 480vac is 3 phase, there will be three fuses inside of the panel, one for each leg of 240. The fuses are designed so that if there is a fault on the circuit, such as when a leg (one of the three phases) makes contact with a ground, a link inside of it will melt and seperate causing the circuit to become open, or dead. The reason for this is because when a leg goes to ground or shorts out to another leg, it draws more amps (Current) and when the amps go up, so does the heat.

After the power leaves the disconnect, it makes it's way to the Bus Rails. The crane is equipped with 3 - 4 collector shoes (depending on whether or not it pulls a ground from the bus rails) that travel along the rails. The shoes are insulated from the frame of the crane to prevent shortage to ground and electrocution of personnel. Wires run from the shoes down into and control panel, into an additional disconnect on the crane itself. We'll call the side that comes out of the disconnect, the "Load Wires" and the wires that go into the disconnect, the "Line Wires" Most on-board disconnects open and close a set of contacts with a turning motion.

As the wires leave the load side of the disconnect, two wires will enter the primary side of the transformer and all three will go to the "main contactor". On most cranes, The contactors, or starters, have 110-120vac coils on them thus the need for a "Step down transformer". I will explain coils a little later on. The step down transformer takes in the 480vac and turns it into 110vac. This particular control transformer controls the starting and stopping circuit. Coming off of the transformer, there are two wires. One is called X1 and it is known as your "Hot Wire" and the other is X2 which is your "Neutral wire". This is called your secondary power or control power. The neutral wire goes directly to the main contactor coil, and the Hot wire goes through an "Emergency Stop" or "E-Stop" switch and heads back to power the hot side of the Main transformer coil. When the E-stop is pulled out the main contactor closes, and when the E-Stop is pushed in the main contactor will open.

In a contactor, you normally have 3 sets of contacts, again for each of the three legs of 3 phase 480vac. When control power is applied to the contactor coil the coil turns into a live magnet and draws in a spring loaded metal plunger which causes the contacts to close and the power to be transferred from one side of the contactor to the other. Just imagine if you had 3 men standing in front of you, with their arms wide open, and you gave them a signal and they clamped their hands together, you would be working in the same way a contactor does. you're the magnet, and they're the contacts.

When the main contactor is closed, forming a closed circuit, the primary voltage gets transferred through and sent to a set of three fuses for hoist one, then jumped one by one to another set of three fuses for hoist two, and another set for bridge. after leaving the individual crane function primary voltage fuses, it leaves the control panel and travels down wire ribbon known as "festoon" to power each hoist.

Once the power gets to each hoist it then goes through another control transformer and to the line side of the trolly and hoist contactors. Here's where it get's a little tricky. The hoise and trolly are on forwarding / reversing contactors, meaning it's a set of two contactors, six contacts, and only one can pull in at a time. The line side power is jumped across from one contactor to the other, but reversing in order. this is called reverse phasing. reverse phasing is when you have three legs of 480, lets call one L1. the next L2, and the third L3. when applied to a motor in that order the motor will run forward, but when you switch L1 and L3, it will run in reverse. so basically in one directional contactor the lineside will have L1 - L2 - L3, and the other contactor will have on the lineside L3 - L2 - L1 causing it to run in reverse. The same applies to the trolly circuit.

As mentioned earlier, there is also another control transformer on each hoist. This transformer provides the control power for the hoist and trolly contactor coils.  From the secondary side of this transformer, X2 will go to both of the trolly contactor coils, and through the hoist limit switch before it heads towards the neutral side of the hoist contactor coil. X1 will follow the festoon back to a pendant station and power the lineside of the hoist and trolly buttons, and 4 more wires from the loadside (one for forward hoist, forward trolly, reverse hoist, and reverse trolly) will head back down the festoon and to the hot side of the four contactors. (2 for hoist, 2 for trolly)

Now when someone asks you, you know a little about the electrical layout on cranes. :)

 


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