Prior to the introduction of frequency converters, overhead crane operators operated the cranes using contactor-based controls. For low-cycle cranes, the system is equipped with a single or two-speed motor. For cranes with higher load cycles, they use so-called wound rotor motors.
Single-speed or two-speed motors
On a single-speed motor, the operator presses a button and the motor is able to speed up as quickly as possible depending on the load. As soon as the button is released, the brake is aborted and the crane is aborted. So, if your crane is equipped with a motor that runs at 50 feet per minute (feet per minute), this is the only speed it can operate.
On a two-speed motor, there are two speeds available for the crane to operate in - usually in a 3:1 ratio range. So, if the crane is equipped with a two-speed motor, it can run at 40 fpm or 120 fpm, but there are no other speed options in between.
These low duty cycle applications can be equipped with a "soft start" skill, also known as a buck starter. This skill reduces the voltage of the motor, which then reduces the acceleration at start-up and reduces the torque exerted on the overhead crane drive system during the acceleration process. However, this method of handling generates a lot of heat and is not recommended for high-use/high-load cycle cranes, as it can overheat the motor quickly.
Wound rotor motor control and DC control
Years ago, process cranes and other high-performance crane systems used a useful but rather messy system of motor control called wound rotor control. This includes the windings on the rotor of the motor, which is then taken to the slip ring, then led out to the brushes, and then to the resistor bank. With this system, you can control the speed of the motor by changing the resistance on the motor's rotor.
These systems worked well, but troubleshooting problems was difficult due to the wiring and clutter of everything built into the system. Other high-load cycle cranes use high-performance speed or DC control to provide the operator with more crane motor control. These are still popular in today's factories, but they are also an inefficient use of messy planning and manipulation.
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