Fig. 1 - VARIABLE AND HIGH OVERLOAD TORQUES ARE REQUIRED FOR MANY
APPLICATIONS SUCH AS THIS WOODYARD CRANE
Wound rotor (or slip ring) motors have been the staple of high overload torque applications since the 1930s. This technology, based on inserting external resistances into the induction motor rotor circuit for induction motor torque control, has been successful for many years. Prior to the advent of variable frequency drives (VFDs), there was no other way for an induction motor to develop variable and high overload torques (200-250%) as required for many diverse industrial applications such as woodyard crane hoists, gantry or trolley drives, chippers, beaters or kiln drives.
While this technology is still present in many large motor applications, typically in the cement industry, there are still quite a number of vintage wound rotor applications in the 50 to 500 hp ranges that can be good candidates for retrofit to a VFD.
Barriers to retrofitting wound rotor motor applications are typically lack of knowledge of the motor’s unique characteristics and lack of available documentation of the motor’s speed torque curves. In addition, many end users who have applications with these motors also have limited engineering staff and are apprehensive to adapt to new technologies available to them. Thus, the goal of this article is to demonstrate that outdated technology can and has been replaced with more robust, smoother operating and lower cost of operation technology. In addition, this conversion can be performed with limited information, as is the usual case.
The wound rotor motor is an induction machine where the rotor bars are terminated in shaft-mounted slip rings to which external impedances can be con-
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