Joe Howard
Technical Education Committee Member
Good Fruit Electric

What is a LeTourneau? It is a brand of heavy equipment named after the inventor R.G. LeTourneau. Many consider R.G. LeTourneau to be one of the most prolific inventors of earthmoving and materials-handling equipment in history. For those involved in the paper industry, you may already be familiar with LeTourneau's contributions, such as the JC40 cranes or the log stackers commonly found in wood yards. Notably, the log stackers (Figure 1) were among the first electric drive vehicles, featuring a diesel-electric generator and AC/DC generator system. The wheels were powered by DC motors connected to a planetary gearbox, while lift, tilt and steering were operated by unique AC motors integrated into a gearbox with variations in size depending on the model.

This article will delve into the AC motors responsible for the tilt and lift functions of the log stackers. These motors possess distinct characteristics, including a high-resistance rotor. The rotor's construction is noteworthy, featuring one end ring made of copper and the other of steel (Figure 2). The rotor bars are equally unique with copper bars attached to stainless steel paddles welded to the steel end ring. The connection on the copper end ring is conventionally brazed, while the steel end ring increases rotor resistance creating what electromechanical professionals refer to as a "Beyond Design D" motor.

But why does this design work and why was it a suitable choice? Initially developed in the 1930s and 1940s, LeTourneau's log stackers required a motor with high slip that would be gentle on the equipment. Given the technological limitations of the time, such as the absence of soft starts and variable frequency drives (VFDs), the high-resistance, high-slip rotor offered a unique solution.

Admittedly, one downside was the increased temperature on the rotor, though this was mitigated by the short operational periods of the motor. Nonetheless, there are tales of observers witnessing the glowing red hue of these motors against the night sky in the woodyard.

If you encounter one of these motors, you should utilize original equipment manufacturer (OEM) bars and blades. The OEM bars are connected to the stainless steel paddles utilizing a special welding process. The motors operate at very high temperatures, and it is recommended that insulation material at or above 220°C (428°F) be utilized in the rewind process.

Categories: AC motors

Tags: Rotors AC Rotor design

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