I’m Giving It All She’s Got

Hi, I’m John Lange. For 27 years, I was an electrician for the North Carolina Department of Transportation. I was responsible for maintaining three of the state’s most critical drawbridges. Now I am contracting electrical work for private and non-governmental drawbridges…at least I was when I wrote this. Who knows what future-me is up to now?

I recently had to explain to a bridge operator how a variable resistance, wound rotor motor drive system actually works. He was bringing his swing bridge in on a windy day when, during the final half of the closing movement, the wind began to slow the span down. In response, the operator moved the drum controller to higher and higher speed notches, but instead of speeding up the bridge slowed even further.

Worried that he might not be able to fully close the bridge, he called for help. I advised him to drop back to the second speed notch. And what happened then? The span slowly but steadily completed its journey and seated properly.

These wound rotor motor systems have been in use for more than a century and are still found in bascule and swing bridges all over the country. While they’re not ideal for applications that require precise or constant speed control, they are well-suited for tasks like opening and closing drawbridges. Situations where variable torque and gradual movement are more important than precision speed.

So how does a wound rotor motor work?

It starts with the stator (the stationary part of the motor) containing windings that are energized with three-phase power to create a rotating magnetic field. This field then induces current into a second set of windings in the rotor (the part of the motor that spins). The rotor reacts by trying to follow the rotating magnetic field, causing it to turn in the same direction.

What makes the wound rotor motor unique is that external resistors can be inserted into the rotor circuit. This is done through slip rings and brushes, which connect the rotor to a set of electrical switches. These switches open or close based on the position of a lever in what’s called a drum controller.

When resistance is added to the rotor circuit, motor speed is reduced, but torque increases. This is especially helpful when starting a heavy load like a bridge span. At this point, the motor is working the hardest but the added resistance limits the inrush of current, reducing strain on wiring and allowing the use of smaller motors and cables.

As resistance is removed from the rotor circuit, the motor speed increases and torque decreases. Once the bridge is already in motion and we no longer need as much torque, we can remove resistance to pick up speed without drawing more current.

At the highest speed notch, with all resistance removed, the motor runs at full speed but provides the least amount of torque. So, when you need to slow the span down, you reintroduce resistance in steps, which allows for smoother control of the bridge’s movement.

That windy day, the operator had the right intention but misunderstood how the system works. By increasing the speed notch he was actually removing resistance and decreasing torque, exactly the opposite of what he needed. He was thinking of the controller as a gas pedal: the more you give it, the harder the motor works. But a better analogy is a gear shifter on a ten-speed bike. When you hit a hill (or a gust of wind), you need to downshift to a lower gear, which gives you more torque to keep moving. He was shifting up, making the hill harder to climb.

It’s worth noting that not all systems are identical. Some drum controllers begin with a “D” or drift notch, which releases the brake without energizing the motor. Others start directly at “1,” which begins motor movement. In some designs, torque is highest at notch 2 or 3, depending on how much resistance is inserted. Too much resistance can sometimes reduce torque as well. But in general, lower speed notches provide higher torque, making them the better choice when you’re fighting wind, current, or other obstacles.

Understanding how wound rotor motor systems respond to resistance can make all the difference in successfully operating a bridge, especially in challenging conditions. These century-old systems might seem simple, but they reflect a smart, mechanical approach to variable torque control that still gets the job done today. For bridge operators, knowing when to “downshift” can mean the difference between a stalled span and a smooth, confident close.