PRODUCTS DRIVES & MOTORS


DESIGNING AND SELECTING ROLLER TABLE MOTORS FOR USE IN THE HARSHEST APPLICATIONS


To cope with the extreme heat and dirt found in steel mills, roller tables require special motors with a high dynamic constant as well as a robust overall design, explains Ananda Majumdar of Regal. So how should you go about designing and selecting roller table motors for these environments? Such motors are used throughout the rolling mill on general


conveying duties, but on the reversing mill and associated approach tables the operating duties are extreme with constant stop/starts and instant reversal of directions. In operation the most important issue is the rate of acceleration and deceleration of the stock. This determines the rating of the motors, which is usually


expressed in terms of starting torque. The starting torque requirement, however, depends on the type of stock being rolled. Slab stock is rolled one piece at a time and the starting torque relates to its weight. If the motors are overpowered, the rollers could skid over the surface of the slab, causing imperfections to the surface. However, rod and billet stock is often rolled several pieces at a time, so there is potential for the


pieces to become jammed against one another or against the mill sides. In these cases the motors have to have sufficient torque to reverse and drive through to clear jams. But, this ‘extra’ torque means skidding may occur, causing damage to the stock’s surface. Theoretical maximum and minimum torques for each application are relatively easy to calculate,


based on: roller diameter and length; roller spacing; size, type and weight of stock; acceleration time; and maximum velocity. The skidding torque can also be calculated using the same techniques, and lies between maximum and minimum. For slab rolling applications, motors whose torque characteristics lie between minimum and skidding


torques should be selected. For billets and rod stock, motors that lie between skidding and maximum torque are ideal. Motors that exceed the maximum torque will be overly prone to skidding in use and will also consume excess power, compared to optimally sized motors. Jams and blockages often occur in roller mills, and these can only be cleared by driving and reversal of the


rollers. As it is likely that one or more of the rollers will be in a stall condition for an extended time, they have to be able to withstand the electrical stress this creates. For straightforward applications, international standards generally recommend a capacity to withstand stall conditions for up to three minutes; but for more demanding situations, this can rise to 15 minutes or more. Another key characteristic of roller table motors is the ability to dissipate heat. This relates to the


load, acceleration and frequency of starts, plus a factor to account for the thermal consequences of stalls. Within the industry, this is referred to as the ‘B value’ and is always quoted on technical specifications because of its fundamental importance. So, when designing or specifying a roller table motor, essential criteria include:


• Speed/torque characteristics to match the specific application • Lowest possible rotor inertia • Very high mechanical rigidity to withstand the constant reversals • Robust mechanical design to withstand high loads, shock loads, rapid accelerations and reversals • Large bearings and high-temperature lubrication • Robust electrical design to withstand prolonged stall conditions • Insulation to Class F or H to cope with the high temperatures within the motor • Recognition of the high ambient temperature and suitable derating of motor performance • Duty cycle is likely to be 24x7 over many years, with minimal maintenance The motors will also need to be totally enclosed for protection of operators; and consideration must be


given to cooling. There is also increasing emphasis on energy efficiency as table roller motors will consume a considerable amount of power. The basis of this is often the use of a cast aluminium rotor and careful design of other elements of the electrical engineering. While AC motors are preferred for roller table applications, the use of inverter drives is also well-proven in the field.


Regal/Rotor UK T: 01933 230900 www.rotor.co.uk Enter 207 According to the company,


PowerXL drives are an easy-to-install, user-friendly and energy-efficient solution that can be used in applications such as pumps, fans, conveyor belts, cranes, compressors or lifts. These can be surface mounted directly next to the motor or at the machine. Compliance with the necessary EMC requirements is also easier thanks to the resulting shorter motor cable lengths. The range currently consists of two


models: the DC1 basic model for ratings from 0.37 to 11kW; and the DA1 for ratings from 0.75 to 250kW for more demanding solutions. They are specifically designed to


COMPACT HIGH PERFORMANCE MOTORS


Despite measuring just 22mm in diameter and weighing only 63g, Dunkermotoren’s new BGA 22 3-phase brushless DC motor is said to deliver exceptional power to size and weight ratio. This, the company claims,


produces up to 50% more power than comparably sized brushless DC motors and will be


appreciated in low noise applications, especially when used in conjunction with a gearbox. It supports low nominal speed (around 3,000 rpm) and is suited to medical, pharmaceutical or aviation motion control applications. Examples include powering small pumps in micro-fluidic


handling systems, or wherever optimum flexibility or up-scaling is required. Additional benefits include


long life, high power density and efficiency, high dynamic acceleration and low noise levels.


Dunkermotoren Linear Systems www.dunkermotoren.de Enter 208


14 APRIL 2014 | DESIGN SOLUTIONS


make the benefits of modern VSDs – including energy savings – readily and easily accessible. To achieve this, features include support for Eaton’s SmartWire-DT connection and communication technology, an advanced auto-tune system, and energy optimiser functionality that, with light loads, automatically reduces the voltage applied to the motor and therefore its energy consumption. With SmartWire-DT technology,


prefabricated plug-in cables are used to link the drives and other devices within the panel, reducing wiring time, saving space and almost completely eliminating the risk of wiring errors. The drives also offer connectivity to a


wide range of networks and fieldbus systems and are compatible with IE2, IE3 and IE4 standards.


Eaton Enter 209 www.eaton.uk.com/electrical / DESIGNSOLUTIONS VSD RANGE IS


NOW IP66 RATED Eaton’s robust PowerXL Variable Speed Drives (VSDs), rated up to 7.5kW, are now available with IP66 protection, meaning they can now be installed outside the control cabinet. This simplifies the retrofitting of drives to older installations where there is often insufficient space.


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