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 pushes a button and the motor accelerates quickly according to 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 at which it can work.
On a two-speed motor, there are two speeds available for the crane to operate in – usually in a 3:1 share range. As a result, if the crane is equipped with a two-speed motor, it is capable of running 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 reduces the acceleration at start-up and reduces the torque exerted on the overhead crane drive system during acceleration. However, this method of manipulation generates a lot of heat and is not recommended for use in high-use/high-load cycle cranes as it can quickly overheat the motor.
Wound rotor motor control and DC control
Years ago, process cranes and other high-performance crane systems used an effective but appropriately cluttered motor control system called wound rotor control. This contains 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 acceleration of the motor by changing the resistance on the motor rotor.
These systems work brilliantly, but troubleshooting problems can be difficult due to all the cabling and clutter 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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