What Type of Control Systems Did Variable Frequency Drives Replace

Prior to the introduction of variable frequency drives, overhead crane operators controlled their cranes using contactor-based controls. For lower duty cycle cranes, the systems were equipped with either a single-speed or 2-speed motor. For higher duty cycle cranes, they used what was called a wound rotor motor.

Single-Speed or 2-Speed Motors

On a single speed motor, the operator pushes a button and the motor accelerates as fast as it can, depending on the load. As soon as the button is released, the brakes set and the crane stops. So, if your crane is equipped with a motor that operates at 50 fpm (feet per minute), then that’s the only speed that it can work at.

On a 2-speed motor, there are two speeds available for the crane to operate at—typically within a 3:1 ratio. So, if the crane is equipped with a 2-speed motor, it can operate at 40 fpm or 120 fpm, but there are no other speed options in between.

These lower duty cycle applications can be equipped with “Soft Start” technology, also known as a Reduced Voltage Starter. This technology reduces voltage to the motor which results in a reduction in acceleration at startup and reduces the amount of torque being applied to the overhead crane’s drivetrain during acceleration. However, this control method generates a lot of heat and is not recommended for high-use/high duty cycle cranes, because it can quickly overheat their motors.

Wound Rotor Motor Controls and DC Controls

Years ago, process cranes and other high-performance crane systems were using an effective but fairly complicated motor control system called wound rotor controls. This consisted of windings on the rotor of the motor, which are then brought out to slip rings, then brought out to brushes, and then to resistor banks. Using this system, you could control the acceleration of the motor by changing the resistance on the rotor of the motor.

These systems worked well, but were a headache to troubleshoot problems due to all of the wiring and complexity built into the system. Other high duty cycle cranes used high performance speed, or DC controls, to provide the operator with more motor controls for the crane. These are still popular in mills today, but they’re also a complicated design and inefficient use of the controls.

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