A change in speed control strategy ramps up production for steel company
TSA Manufacturing in Omaha, Neb., provides steel products and services to the construction, precast concrete, and structural steel markets. One machine—a thread roller that makes long steel threaded rods up to 1 in. diameter used in bridge construction—required extensive maintenance and replacement of worn-out parts, and was out of service for repairs too often.
The thread roller had variable sheave pulley speed controls, which were the source of numerous problems due to a preponderance of moving parts. The controls also required constant operator interaction to change speeds and make other adjustments.
The belt used in the pulley has a “V” cross section that matches the angled face of the pulley flanges. As the flanges open or close, the belt moves up or down to increase or decrease machine speed. Operators control speed with manual switches that open or close the flanges. This type of speed control is fairly standard on many older machines.
Unfortunately, the belts and pulleys were frequently wearing out because they were constantly being adjusted, and they required maintenance several times a year, depending on how many hours TSA ran the machine to fill orders for threaded rods.
The TSA team decided to upgrade the speed controls by using a variable speed drive (VSD), but they weren’t sure how to size the new motor and drive. They also wanted to make sure the upgrade didn’t introduce undue complexity to machine operation.
Sizing for speed
The existing motor operated at a fixed speed of 1760 rpm. Technical specialist Mike Daniel of Industrial Motion Technologies had TSA run the machine through several operations while he monitored speed and power requirements with a tachometer and multimeter.
His calculations indicated that removing the variable sheave pulley and replacing it with a VFD would require the new motor to operate at 2400-plus rpm to meet operating requirements. The power and speed requirements dictated replacing the existing motor with a slightly larger motor than the existing 25 hp motor—specifically a new 30 hp motor and matching heavy-duty rated Yaskawa F7 VFD.
The new motor selected was a 1760 rpm Marathon motor rated to run at 1.5 times speed, providing a maximum speed of more than 2600 rpm. After monitoring power, amperage, and motor temperature, TSA increased the maximum frequency to 90Hz without overloading or overheating the VFD or the motor.
Lakeland Engineering in Omaha revised the automation system after the variable sheave pulley was replaced with the new motor and VFD. The operators were all familiar with the existing machine automation system, and if the automation system was drastically modified, the operators would require retraining before production could resume. With some wiring modifications, programming, and setup, Lakeland was able to make the automation system conversion appear seamless to the operators while keeping downtime to a minimum.
Lakeland used the existing pushbuttons, pilot lights, and joystick speed control, wired those components into the VFD inputs, and programmed the system so the VFD speed control would operate in a similar manner as the variable sheave pulley system as far as the operators were concerned.
While the original goal was merely to duplicate the existing speed controls and reduce maintenance problems, the production rate doubled and maintenance issues were eliminated. Maintenance manager Marty Wilmer says the time to thread a rod was reduced to 13 seconds from the previous 30 seconds, an improvement of over 50%. TSA used to run the machine for five to eight days to fill orders, but since the conversion was made, it now needs to run the machine for only two to three days to fill the same number of orders.
Once the new motor and VFD were installed and operating, TSA converted to high-speed cutting dies and was able to far exceed the operating parameters of the original system. Downtime and maintenance issues, which had been a large expense, have been eliminated since the conversion.