哈哈刚刚学会使用伺服电机原理和应用-全面实用的工控技术信息-PLC技术网-可编程控制器技术门户

哈哈刚刚学会使用伺服电机原理和应用

日期：2008-4-10 18:10:01 来源：本站整理
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伺服电机原理和应用

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Servomotors are available as AC or DC motors. Early servomotors were generally DC motors because the only type of control for large currents was through SCRs for many years. As transistors became capable of controlling larger currents and switching the large currents at higher frequencies, the AC servomotor became used more often. Early servomotors were specifically designed for servo amplifiers. Today a class of motors is designed for applica-tions that may use a servo amplifier or a variable-frequency controller, which means that a motor may be used in a servo system in one application, and used in a variable-frequency drive in another application. Some companies also call any closed-loop system that does not use a stepper motor a servo system, so it is possible for a simple AC induction motor that is connected to a velocity controller to be called a servomotor.

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Some changes that must be made to any motor that is designed as a servomotor in-cludes the ability to operate at a range of speeds without overheating, the ability to operate at zero speed and retain sufficient torque to hold a load in position, and the ability to operate at very low speeds for long periods of time without overheating. Older-type motors have cooling fans that are connected directly to the motor shaft. When the motor runs at slow speed, the fan does not move enough air to cool the motor. Newer motors have a separate fan mounted so it will provide optimum cooling air. This fan is powered by a con-stant voltage source so that it will turn at maximum RPM at all times regardless of the speed of the servomotor. One of the most usable types of motors in servo systems is the permanent magnet (PM) type motor. The voltage for the field winding of the permanent magnet type motor can be AC voltage or DC voltage. The permanent magnet-type motor is similar to other PM type motors presented previously. Figure 11-83 shows a cutaway picture of a PM motor and Fig. 11-84 shows a cutaway diagram of a PM motor. From the picture and diagram you can see the housing, rotor and stator all look very similar to the previous type PM motors. The major difference with this type of motor is that it may have gear reduction to be able to move larger loads quickly from a stand still position. This type of PM motor also has an encoder or resolver built into the motor housing. This ensures that the device will accurately indicate the position or velocity of the motor shaft.

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FIGURE 11-83 Typical PM servomotors.

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FIGURE 11-84 Cutaway picture of a permanent magnet servomotor.

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11.11.5.1 Brushless Servomotors The brushless servomotor is designed to operate without brushes. This means that the commutation that the brushes provided must now be provided electronically. Electronic commutation is provided by switching transistors on and off at appropriate times. Figure 11-85 shows three examples of the voltage and current waveforms that are sent to the brushless servomotor. Figure 11-86 shows an example of the three windings of the brushless servomotor. The main point about the brushless servomo-tor is that it can be powered by either ac voltage or dc voltage.

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