128 Industrial Robotics Fundamentals Copyright Goodheart-Willcox Co., Inc. on the machine) increases, the motor speed decreases. The opposite is also true. As the mechanical load decreases, the motor speed increases. The amount of cemf increases or decreases in direct relation to the speed of rotation. Since the cemf fl ows against the supply voltage, the actual working voltage of fl the motor increases as the cemf decreases. When the working voltage increases, more current fl ows through the rotor windings. The torque of the motor is directly fl proportional to the amount of current fl owing through the armature. Therefore, fl torque increases as armature current increases and decreases as armature current decreases. The amount of torque also varies with changes in load. As the load on a motor increases, torque increases to handle the greater load. The increase in torque causes the motor to draw more current from the power source. When a motor starts, it draws a very large initial current (compared to the cur- rent it draws at full speed) because of the absence of cemf. To reduce the starting current of a motor, resistors wired in series with the armature circuit are often used. Once the motor reaches full speed, these resistors are bypassed by an automatic or manual switching system. This allows the motor to produce maximum torque. Horsepower is a measure of the amount of work performed over a specifiedfi amount of time. The horsepower rating of a motor represents the power of a motor. It is based on the amount of torque produced at the rated full-load values and is a very common rating for electric motors used for robotic applications. As torque or the work requirement increases for any application, the horsepower of the motor used to drive the machine must also increase. This can be expressed mathematically as: hp = 2πST 33,000 hp = ST 5252 (hp = Horsepower rating, π = Constant, S = Speed of the motor expressed in rpm, T = Torque developed by the motor expressed in ft/lbs.) Motor Operating Characteristics Mechanical Load L d Increases Increases Decreases Decreases Increases Decreases Decreases Increases Increases Decreases Rotor Current Torque CEMF Motor Speed Goodheart-Willcox Publisher Figure 6-8. This illustration depicts the effect of mechanical load on the operating characteristics of a dc motor.
Industrial Robotics Fundamentals: Theory and Applications, 3rd Edition Page 128 (144 of 479)
Powered by Tizra® |
Extracted Text (may have errors)
128 Industrial Robotics Fundamentals Copyright Goodheart-Willcox Co., Inc. on the machine) increases, the motor speed decreases. The opposite is also true. As the mechanical load decreases, the motor speed increases. The amount of cemf increases or decreases in direct relation to the speed of rotation. Since the cemf fl ows against the supply voltage, the actual working voltage of fl the motor increases as the cemf decreases. When the working voltage increases, more current fl ows through the rotor windings. The torque of the motor is directly fl proportional to the amount of current fl owing through the armature. Therefore, fl torque increases as armature current increases and decreases as armature current decreases. The amount of torque also varies with changes in load. As the load on a motor increases, torque increases to handle the greater load. The increase in torque causes the motor to draw more current from the power source. When a motor starts, it draws a very large initial current (compared to the cur- rent it draws at full speed) because of the absence of cemf. To reduce the starting current of a motor, resistors wired in series with the armature circuit are often used. Once the motor reaches full speed, these resistors are bypassed by an automatic or manual switching system. This allows the motor to produce maximum torque. Horsepower is a measure of the amount of work performed over a specifiedfi amount of time. The horsepower rating of a motor represents the power of a motor. It is based on the amount of torque produced at the rated full-load values and is a very common rating for electric motors used for robotic applications. As torque or the work requirement increases for any application, the horsepower of the motor used to drive the machine must also increase. This can be expressed mathematically as: hp = 2πST 33,000 hp = ST 5252 (hp = Horsepower rating, π = Constant, S = Speed of the motor expressed in rpm, T = Torque developed by the motor expressed in ft/lbs.) Motor Operating Characteristics Mechanical Load L d Increases Increases Decreases Decreases Increases Decreases Decreases Increases Increases Decreases Rotor Current Torque CEMF Motor Speed Goodheart-Willcox Publisher Figure 6-8. This illustration depicts the effect of mechanical load on the operating characteristics of a dc motor.
