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... as your partner not just your supplier. |
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Wound Rotor Motors The wound rotor motor, an induction motor like the well-known squirrel cage, has the additional advantage of variable speeds and torques. This unique feature is due to its rotor construction, allowing varying resistance from almost short-circuit condition (its internal resistance only) to an open-circuit condition (infinite external resistance). Motor Controllers With a variety of control systems available, the motor characteristics can be varied to give high or low torque with relatively low current over a wide speed range. The simplest controller is the drum type, providing five or more resistors in the rotor circuit through the slip rings. A much more sophisticated type is the AC Thyrister static control for bridge and trolley application, which incorporates stepless control of torque from zero to maximum. Other types are reactor-resistor, full magnetic control and resistors, and liquid rheostats. The latter is used on much larger motors than MAC ratings and gives stepless control. Applications and economics determine selection of the type used. Brakes are employed with wound rotor motors to stop the motor in case of power failure and to assist in slowing or holding the load. In the event of power failure, load stall, or other reason for stopping, the controller is designed so that all the secondary resistance must be in the circuit before restarting to prevent damaging the slip rings with high current. General Squirrel cage induction motors are basically constant speed machines and are most frequently used as such. Since the amount of rotor resistance determines the speed-torque shape, it would necessitate an almost unlimited variety of rotor punching, end rings and conductor materials to duplicate the multitude of speed-torque curves possible with the wound rotor. This would be economically unsound and physically impractical. Another important consideration is that the current through the rotor resistance produces heat, which must be dissipated, or the motor would overheat, damaging the rotor and/or stator. The squirrel cage motor can be designed with several types of speed-torque characteristics as exemplified by NEMA Designs B, C and D (high slip). Also the stator winding, single or two individual, can be reconnected to give 2, 3, or 4 speeds. These speeds, however, are in discrete steps of two to one for each winding, thus limiting the possible speed range. In spite of its limitations, this simple and rugged motor services the majority of jobs in industry. As stated above, the rotor resistance represents loss and heat in the motor under running conditions. The wound rotor has a decided advantage over the squirrel cage in this area, since the internal losses remain the same at a fixed torque regardless of the speed change produced by the secondary control resistors. The additional loss due to speed reduction is outside the motor in the secondary resistors, where it can be dissipated with relative ease. Consider the following: for a motor running at half speed (50 percent slip) with the required secondary resistance, only approximately 4 percent (full load slip with collector rings short-circuited) of this loss is in the motor. The other 46 percent are in the external resistors and do not contribute to the motor heating. However, the lower speed decreases the motor cooling or ventilating capacity and must be considered in applying and selecting drip-proof motors. The wound rotor motors can be grouped into three types or categories, even though the basic design distinction is minor. NEMA has established breakdown torques for specific horsepowers, as well as the secondary volts and amperes. The following types are supplied: General Purpose Motors are designed for constant or adjustable speed continuous duty with the proper control. These motors are used on fans, blowers, pumps and compressors requiring variable torque and wide range of speed. Crane and Hoist Motors are intermittent or short-time rated adjustable speed machines designed for severe service; i.e., frequent starting and stopping, reversing and plugging, high starting torque and low starting current. Typical applications are conveyors, hoists, lift bridges, transfer tables, cranes, etc. The breakdown torque is slightly higher for crane and hoist motors than for general-purpose type. Even though these motors are suitable for constant torque and variable speed applications, it may be necessary to force ventilate some motors on very severe duty cycle applications. The force-vent system can consist of an auxiliary unit with a separate motor blower mounted on the wound rotor motor or piped air supplied by the customer’s system. Special Purpose Motors
are other than the
above types, requiring specific motor features, such as physical ruggedness
and/or electrical characteristics. One example is our AC Mill Motors which are
designed per AISE standards
for rugged applications. Another is our drip-proof motor with an enclosed
collector to protect it from detrimental
atmosphere. |
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