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The Case of the Culpable Contactor
Myron J. Boyajian, President, Engineering Consultants -- Design News, April 7, 2008
The U.S. Navy used electric-powered forklifts in the ammo holds on its aircraft carriers. These stand-up rider, end-control, straddle-type forklifts were propelled by two linked, steerable motor/wheel units mounted to the underside of the truck frame by heavy-duty bearings.
The Scene of the Crime
Back in the day, Navy personnel reported some of the forklifts that were manufactured by my employer stopped so rapidly drivers were slammed against the driver's compartment or almost thrown out of the compartment.
The Investigation
Unable to fix the problem, battle-readiness was compromised and the pressure was on to find a fix. A forklift exhibiting rough plugging was shipped to our factory for analysis. While moving from the receiving dock to our lab, the truck did, indeed, slam our technician against the compartment wall as he stopped the vehicle. As a staff engineer, a liaison between engineering, sales and production, I was asked to help resolve this mystery.
I lead off with questions regarding brake modulation, pad material and other causes of rough braking, but quickly learned the problem wasn't severe braking per se, but was actually violent plugging. Plugging is reversing motor voltage to power it to a stop and rather than braking, is commonly used for rapid slowing and reversal in short, choppy work cycles. Driver fatigue is reduced by eliminating leg and foot movement to apply brakes. Except this truck plugged like no other! I drove this machine in reverse at a modest speed and pushed the combined speed/direction control handle forward. The truck plugged to a stop in the blink of an eye and unceremoniously dumped me on the shop floor. I did a quick-step to recover, but slipped and fell, getting a chipped elbow in the process. Recovering from pain and embarrassment, I proceeded with troubleshooting in more measured steps.
The Smoking Gun
With raised drive wheels, I noted the operation of the contactors that controlled drive motor motion. Soon, a pattern emerged and the cause of the violent plugging became evident. Because two drive motors were used, our designers used a dual-drive contactor system with separate contactors for each motor. In sharp turns, power to the inside motor would be cut to prevent wheel spin. Plugging has special requirements. When plugging direct current motors that are connected to a common electrical supply and are mechanically linked (via the drive wheels and floor), each motor will try to act as a generator with the other as a load. This regenerative effect can result in instant and violent reversal and possible mechanical damage. A plugging contactor electrically ties the two motor armatures together during plugging so the speed control can limit plugging current in both motors. As the motors stop or begin to rotate normally in the selected direction, the plug contactor drops out to restore independent control of each motor.
During my tests, the motors correctly stopped and started as the wheels were moved through their steering arc and I saw the plugging contactor pick up and drop out during plugging. Motor control should have been normal. With a bright light and close examination, I found a heavy deposit of corrosion on the plugging contactor tips, possibly due to salt-air contamination, that prevented an electrical tie between the armature circuits during plugging. I cleaned the contactor tips and plugging returned to normal.
As I pondered this situation and its fix, two questions came up — why have a plugging contactor and more to the point, why have a dual motor control at all? Separate motor controls were the norm for dual-drive motor/wheel units on the same rigid axle. Cutting power or even reversing the inside motor results in a sharp or even zero radius turn. Here, a plugging contactor is necessary. However, the Navy truck's drive wheels were steerable, not fixed, so even with Ackerman steering geometry the motor speeds were nearly always identical, obviating the need for a dual motor control and plugging contactor. I never got an answer as to whether our designers failed to analyze the situation before picking a control system or blindly followed the control supplier's recommendation. I recommended replacing the plugging contactor with a solid bus bar to prevent violent plugging. The Navy contract ended, so no trucks were converted. However, the Navy was alerted as to this fix.
| Author Information |
| Myron J. Boyajian, P.E., (mboyajian@sbcglobal.net) is president of Engineering Consultants, a consulting service for forensic and design activities. Cases presented here are from his actual files. |
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