Managing Overcharging Issues in the Wabco 111A Charging System

[MikePhua]

Managing Overcharging Issues in the Wabco 111A Charging System
The Wabco 111A and Its Electrical Legacy
The Wabco 111A motor scraper was developed during the mid-20th century by the Westinghouse Air Brake Company, a pioneer in earthmoving equipment. Known for its electric steering system and robust mechanical drivetrain, the 111A was widely used in large-scale earthworks and infrastructure projects. Its electrical system, however, was unconventional by today’s standards, relying on a transformer-rectifier setup rather than a modern alternator.
The original charging system used a flux bridge transformer paired with a selenium rectifier to convert AC to DC and maintain battery charge. This system was designed for 24V operation, typically using four 6V batteries in series. Over time, many machines were retrofitted with air-cooled diode rectifiers and two 12V batteries, introducing new challenges in voltage regulation and current control.
Symptoms of Overcharging and Battery Damage
Operators have reported that the transformer outputs over 30 amps to the batteries at high idle with no load. This excessive current leads to battery overheating, acid boil-off, and eventual shorting. Even after removing all shims from the flux bridge—a method traditionally used to reduce output voltage—the current remains too high.
This suggests that the transformer is no longer regulating properly, or that the replacement rectifier is allowing too much current to pass. In some cases, the diode rectifier may be leaking AC ripple into the DC circuit, further stressing the batteries.
Terminology Clarification

• Flux Bridge: A magnetic core assembly in the transformer that controls output voltage by adjusting the air gap with shims.
  
• Selenium Rectifier: An early type of rectifier using selenium plates to convert AC to DC; now largely obsolete.
  
• Air-Cooled Diode Rectifier: A modern solid-state replacement for selenium rectifiers, using silicon diodes and heat sinks.
  
• AC Ripple: Alternating current components that remain in a DC circuit due to incomplete rectification, harmful to batteries.
  

Root Causes and Technical Analysis
Several factors contribute to the overcharging issue:

• Rectifier Mismatch: Modern diode rectifiers may not match the impedance characteristics of the original transformer, leading to uncontrolled current flow.
  
• Flux Bridge Saturation: If the magnetic core is saturated or improperly shimmed, voltage regulation becomes ineffective.
  
• Absence of Voltage Regulation: Unlike alternators, the original system lacks a feedback loop to adjust output based on battery state.
  
• Battery Configuration Change: Switching from four 6V to two 12V batteries alters the load characteristics and may increase charging current.
  

In one documented case, two Wabco 111As were retrofitted with air-cooled rectifiers and continued to charge at 30V despite shim adjustments. While the machines operated without major issues, the batteries required frequent maintenance due to fluid loss and sulfation.
Recommended Solutions and Modernization Options
To address the problem:

• Test Each Diode: Disconnect and test individual diodes for leakage or reverse current using a multimeter.
  
• Install a Voltage Regulator: Add a solid-state regulator between the rectifier and battery to limit voltage to 27.5–28V.
  
• Switch to Alternator: Retrofit a 24V alternator with built-in regulation. Positive ground units are available for compatibility.
  
• Use Deep-Cycle Batteries: These tolerate higher charging currents and reduce boil-off risk.
  
• Monitor Battery Temperature: Install thermal sensors to detect overheating and trigger alarms or shutdowns.
  

A Story from the Field
In Texas, an excavation contractor operating a Wabco 111A noticed his batteries boiling dry every few weeks. After replacing the selenium rectifier with a diode unit, the problem persisted. Eventually, he installed a 24V alternator from a military surplus truck, bypassing the transformer entirely. The result was a stable 28V output, longer battery life, and reduced maintenance. The transformation cost under $500 and eliminated years of electrical headaches.
Conclusion
The Wabco 111A’s original charging system, while innovative for its time, struggles to adapt to modern components without careful tuning. Overcharging caused by transformer and rectifier mismatches can be mitigated through diode testing, voltage regulation, or full alternator conversion. With thoughtful upgrades, these classic machines can continue operating reliably in today’s demanding environments.