Timber Lake Playhouse (TLP) is a 371-seat theatre that sits on a 10-acre campus, which includes housing for its staff, plus the associated shops and administration facilities needed to put on multiple performances every summer. 

From an engineering perspective, what makes TLP especially interesting is its semi-thrust stage—extending into the audience—featuring a 36-foot diameter section of the floor that rotates under electromechanical control. This moving floor, known simply as a “revolve” in theatre terms, allows scenery to be set up backstage, then rotated into place for quick scene changes. While the basic idea of a revolving stage seems fundamental, there has been significant progress in power transmission technology since TLP’s founding over 60 years ago. 

Changing Technologies in 2017

Originally, the revolve system was powered by an electro-hydraulic drive, consisting of a 5hp electric motor as the input power source, driving a hydraulic pump. In turn, the hydraulic fluid was used to operate a hydraulic motor which turned the stage through a large gearbox. This worked for decades, but eventually became unreliable and difficult to maintain. 

To improve this situation, I volunteered to upgrade the system to full electrical control. Having spent a summer working at the Playhouse, and now being employed as a controls engineer for ShowFab, which specializes in set building and entertainment fabrication, I possessed the right combination of experience and expertise needed to complete the upgrade.

After removing the original hydraulic drive, an electrical-motor-driven gearbox was installed. Pulleys and sheaves, interconnected in part with a continuous loop of aircraft cable and tensioned by weights to establish the necessary friction, transferred the rotational motion to the stage. This robust mechanical setup quietly handles the system’s massive torque and moderate speed requirements without taking away from the experience of the play itself.

To control the 240V three-phase electrical motor in both directions and at varying speeds, I incorporated an AutomationDirect GS2 AC variable frequency drive (VFD). Because the site happened to have electrical power available in a somewhat uncommon 240VAC open delta 3-phase format, the VFD was readily configured to operate the motor, with a small user-friendly control panel, to spin the massive stage turntable in both a clockwise and counterclockwise direction without issue.

Motor Control Upgrades for 2025

This GS2 drive setup worked great for nearly two decades, but eventually the VFD began experiencing some intermittent faults, which were easily cleared but created a nuisance for users. In 2025, the team realized it was time for a refit. Again, I was able to volunteer my time. We took this opportunity to provide a few safety and performance upgrades, as well.

For the upgrade, I selected the latest version of the AutomationDirect VFD family, the GS23, which includes the same functionality as the older unit but also offers many enhanced features. Of special importance for this project, the newer VFD could also accept a single-phase input to produce a three-phase output needed for velocity control, forward/reverse operation, and compatibility with the existing motor. The form factor of the new drive closely matched that of the older unit, allowing a new enclosure subpanel to be fabricated in advance to support a quick change-out.

While heavy industry commonly uses three-phase power to enable superior load carrying capacity for a given electrical load, the rotating stage was the last remaining piece of equipment at TLP that required three-phase power at the time of installation. With the new drive in operation, the three-phase requirement could be dropped, simplifying the facility’s electrical distribution system.

Enhanced Operator Interface and Safety

In addition to the VFD panel, the system includes an operator console with a direction switch, run/stop buttons, a rotation speed potentiometer, indicator lights, and a fault reset button, all sources from AutomationDirect.

Within the VFD panel, the latest upgrade now includes an AutomationDirect Reer MOSAIC safety controller to monitor emergency stop (e-stop) buttons located at both the operator console and the stage manager’s position. If either e-stop button is pressed, the safety controller activates the VFD’s industry-standard safe torque off (STO) function, which causes the drive motor to coast to a stop. 

Another unique feature of the design regards how the cooling fan for the VFD enclosure is controlled. The goal was to turn the fan on whenever the VFD is started, and then to continue to run the fan for a period of time after the VFD is stopped to provide the desired cooling. This could be achieved with a hardwired relay, or with a small programmable logic controller (PLC), or by using the PLC function built into the G23 VFD. However, in this case it was determined that a practical way to achieve this control would be to use a safety controller auxiliary output. Designers appreciate it when they have multiple options for creating a solution.

With these new improvements, the Timber Lake Playhouse is set up to reliably entertain audiences for decades to come, rotating the stage smoothly in either direction for scene transitions. What is going on behind—or more literally, below—the scenes may be interesting to stagehands and engineers, but when everything runs smoothly, the audience can get lost in the story itself.

For more information: 

AutomationDirect 

GS23

Safety Controls

ShowFab 

Timber Lake Playhouse 

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