Soft start-stop system calms cable-drive spasms

June 1, 2000
Flux vector adjustable-speed drive eliminates jarring starts and stops of a high-inertia cable system, boosts productivity, lowers power requirements, and reduces component wear and tear

The abrupt accelerations and decelerations of a cable-driven hopper system produced a chronic maintenance headache for Brooks Construction Co., Fort Wayne, Indiana. The company, a producer of hot-mix paving products, uses a cable to pull a 6-ton hopper loaded with paving materials up a 45-deg incline to a position over one of four heated storage bins which, in turn, dispenses the materials to trucks for transport to the job sites, Figure 1. Each bin holds a different type of paving material.

The drive system

Prior to the installation of a flux vector drive, the hopper was driven by a motorbrake system, Figure 2. The 694-ft long cable is pulled by a winch driven by a 60 hp, 1,750 rpm, ac squirrel cage induction motor. The winch is driven through a parallel- shaft speed reducer with a fail-safe type friction brake on the input shaft.

After loading the hopper with material from a batch mixer, the motor would be started and the brake released jerking the hopper into motion up the incline. Hopper position over a predetermined bin (depending on type of paving material contained) was controlled by an Allen-Bradley programmable logic controller (PLC). A signal from a resolver on the winch system indicates to the PLC the hopper position as it approaches the bin designated for unloading. As the hopper approaches (at 330 fpm) the proper unloading position, the brake would slam on and fingers, pushed by the hopper, would open both a door in the bottom of the hopper and one at the top of the bin.

After unloading, the brake was released and gravity pulled the hopper down the incline. Near the bottom of the incline, the brake once again slammed on stopping the hopper under the batch plant.

The problems

Such spasmodic starts and stops under heavy load led to many problems. “Obviously, brake wear was extremely high requiring expensive rebuilding several times a year,” observes Larry Rice, Brooks’ electrical maintenance supervisor. “Also, cable stretch due to the high acceleration and deceleration forces required cable replacement about once a year and periodic resolver adjustment. Frequent starts of the motor at locked rotor current created an overheating problem, so we’d get about three years out a motor. And, to top it off, contact wear in the part-winding motor starter required frequent contactor maintenance.”

At first, Mr. Rice looked at a dc regenerative drive and motor to replace the acmotor, mechanical-brake arrangement. The motor could provide braking and control the hopper acceleration and deceleration. Such systems are frequently used for controlled stopping of high-inertia loads.

“But Jerry Clary, application specialist for Complete Drives Inc., a Fort Wayne distributor, pointed out that we would need spare dc controls and a spare dc motor as backup to prevent loss of production in the event of a breakdown,” points out Mr. Rice. “We already had a spare ac motor for our existing system.”

The solution

The solution, proposed by Mr. Clary and adopted three years ago by Brooks Construction, was to use a Thor Technology Series 7000, 60-hp, 460-V, 3-phase flux vector drive with two Thor B100 25-hp dynamic braking modules to control the existing 60-hp ac motor, Figure 3. The existing controls were left intact and used as a backup should a breakdown occur.

A local motor shop removed the motor fan assembly and installed an ac powered blower on the fan end for cooling. Also, a Drive Control Systems 60-pulse per revolution magnet wheel was mounted on the original fan shaft for feedback to the drive.

Mr. Rice re-programmed the PLC so the hopper is pulled up the incline with a motor speed of 2,000 rpm. When it reaches a certain position, the drive slows to a creep speed and then stops when the hopper is positioned directly over the appropriate bin designated by the operator. When commanded to return down the incline, the B100 braking modules limits the motor speed during descent to 2,000 rpm until the hopper nears the stopping position where the drive is again commanded to run at creep speed then stop when the hopper is in position under the batch plant.

There are only two conditions when the fail-safe brake is engaged. First, in the event of a power failure, the brake is automatically engaged. The second braking instance involves the drive’s ability to provide full output torque from the ac motor at zero speed. Therefore, when the drive is stopped, it holds the hopper in position. After a programmed interval, the drive turns off the power to the fail-safe brake causing it to engage and hold the load. Then, the drive shuts off power to the motor to prevent excessive heating.

When the drive receives a command, it energizes in the “Run at Zero Speed Mode,” develops holding torque, applies power to the brake to release it, and then runs in the chosen direction.

The benefits In addition

to many component-maintenance benefits and power savings, the system also runs faster. Maximum speed of the old system was 1,750 rpm; the flux vector drive system’s maximum speed is 2,000 rpm. Now, after delivering a load of material to the bins, the hopper quickly returns to the batch plant for reloading. This eliminates the need to hold a freshly-made batch of material while waiting for the hopper to return.

Brake. Since the brake is only used when the motor is stopped, it no longer is subjected to the strain of frequent jerky stops and starts. “Before we installed the new equipment, our brake linings needed replacing about three times a season at a cost of from $1,500 to $3,000 per set,” says Mr. Rice. “In over three years’ of operations with the new system, we have not had to replace brake linings.”

Cable. Also, the soft start-stop feature of the new drive system has eliminated the cable-stretching problem. “Previously, we had to replace the cable once a year on average. Now, after three years, it still looks good and we are saving several thousands of dollars in replacement cable costs, plus the downtime and labor involved,” comments Mr. Rice.

Motor. Because the motor no longer starts at high inrush currents, it runs cooler, even on hot, summer days. The motor has not been rewound since installing the new drive system.

Gearbox. The abrupt starts and stops of the original system increased backlash in the gearbox. Estimated cost to rebuild the gearbox was $30,000. Since the flux vector drive system was installed, apparent gearbox wear has not progressed, and no rebuilding is anticipated.

Energy. The new drive system has also dramatically reduced electrical power costs. “We’re based on demand so whatever our highest demand is, that is how we’ve averaged for the month,” explains Mr. Rice. “We drove that demand down and are saving several thousands of dollars in power charges per year.”

The drop in power usage is due to the smoother operation of the new system. As Mr. Clary points out, “Before, energy was being eaten up in braking and in starting which produced a large inrush current. Now, the hopper starts and stops gradually and we no longer generate that huge power requirement.”

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