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Ace Industries, Inc. Cranes and Hoists
Ace tips on adjustable (variable) frequency drives for overhead hoists and cranes

AC type adjustable frequency drives (AFD) also called inverters, variable frequency drives, etc., are recommended as standard epuipment on all Ace bridge motions unless conditions dictate contra. These drives are the most significant advancement in bridge cranes over the last several decades. Their rapid development grew out of the oil embargo days. Simply stated, they control the electrical input to AC motors causing them to run at various adjustable speeds using less energy. They are useful to save energy costs in all types of industrial applications but have found another use in industries such as hoists and cranes by providing the highly desirable characteristics of smooth starting and stopping. The “ramped” acceleration and deceleration available with an AFD minimizes load swing and wear on brakes and other components. Positioning loads is simplified with Ace standard infinitely variable speed on the bridge motion. This essentially means you can precisely regulate the speed anywhere between barely moving and full speed without any of the “bumping” of the pushbutton used when an AFD is not installed. Every time you “bump” or quickly depress and release the push button a surge of current up to 6 times the normal current is applied to the motor. The extra 500% does no useful work and is transferred into heat energy which will damage the motor to some degree depending on the severity of the abusive use.

These AFD are not standard on Ace hoist and trolley motions although they are sometimes desirable. Cost is still a consideration and the slower speeds normal for these motions as compared to the bridge, often influence a customer to opt for the standard single or two speed instead of an AFD. The day is approaching quickly when AFD will be a cost effective alternative to 2 speed. We always recommend analyzing the AFD benefit for any motion before ruling it out for your application. We feel that AF devices pay for themselves many times over in almost all applications in a very short time due to improved worker efficiency, and reduced wear and tear on equipment.

Two-step infinitely variable control is normally used on a bridge or trolley (traverse) motion. With this type of operation, a two-speed pushbutton insert is normally used. When the operator depresses the two-speed button to the first step, the crane quickly accelerates to the programmed slow speed and holds there. Once the button is depressed to the second step, the motor begins to accelerate toward full speed over a programmed ramp up time. When the operator observes that the speed is proper for his need, he returns the button to the first step and the crane motion holds the speed it had attained immediately before being returned to the first step. To speed up again, he depresses the button to the second step again, and ramp up continues until full speed is attained. If the button is released, the motor begins the programmed ramp down to stop. If the button is depressed to the first step during this time, the motor will continue operation at the speed it had ramped down to at the moment the button was depressed to the first step. When 2 step infinitely variable is used on a hoist motion, the sequence is the same except when the button is released, the brake is immediately applied instead of the ramp down to a stop. For practical purposes with a hoist motion, the difference this makes is that it is not possible to slow the load without stopping first.

Three-step infinitely variable control is normally used on hoist motions, and is described in the following sequence. With this type of operation a three-speed insert is normally used. The operator presses in the button to the first step and the motor rapidly accelerates to slow speed and holds. To accelerate, the operator presses the button further past the second to the third step, and acceleration begins. When the desired speed is attained, the operator drops back to the second step where the attained speed is held. To slow down, the operator can drop back to the first step and ramp down begins to slow speed. To hold, he depresses to second step, and so on. Whenever the button is released, the brake is immediately applied and the hoist stops. First step is slow or decelerate, second step is hold, third is accelerate to high speed, and button release immediately applies the brake and stops the hoist with no ramp down time. A two-speed insert can be used for hoist motions, but for safety reasons, when the button is released, the brake is activated and the hoist stops immediately, rather than ramp down.

There are two often overlooked cautions to observe when applying AFD. First, the ambient temperature should not be over 114 degrees fahrenheit, and second, the electrical power needs to be clean, or free from spikes and surges. We stock a complete line of drives and accessories and are experts in their application. We will help you make any necessary decision in this regard. These devices are surprisingly affordable and easy to retrofit. There are no contact points to wear and when applied correctly will last indefinitely. If you pay for the replacement of two magnetic contractors; you could probably have paid for an AFD.

Ace Industries is a Performance Plus Center for Electromotive Systems, Inc. - the recognized leader in AF drives for hoists and cranes. Electromotive and Ace Industries both maintain 24 hour 7 day hotlines for drive problems. Recently Crane Mart, a division of Columbus McKinnon, has introduced a new drive for their cranes and hoists called Space Vector. We stock a full line of this drive and maintain a hotline for it also. In addition to these inverters, others by Power Electronics, Square D, Mitsubishi, and many other manufacturers can be serviced and installed by our experts.

CAUTION: These tips are provided as a starting point in the selection process and are not universally applicable. Please consult our experts for the solution to your specific application problem.