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More Speed, Less Weight

by John Mazurkiewicz
Product/ Marketing Manager
Baldor Electric Co.

Servos improve machine performance. With the advantage of a wider performance range-that is, higher speed capability and faster acceleration capability-servos will enhance a machine's performance by empowering it to produce more parts. Servos have speed range capability beyond the typical 1,800 revolutions per minute and even the 3,600 rpm based speed of other technologies. Servos offer speeds up to 7,000 rpm, 12,000 rpm and even higher. And higher speeds mean moving faster.

Where traditional inverter and vector technology provide peak torques of 150 to 200 percent, servos can attain 200 percent, 300 percent, and even up to 400 percent. This allows very high acceleration torques, which in turn move the load and get it into position very fast. Faster acceleration means faster positioning-and this relates to a machine's throughput. It can increase the number of parts produced, and increasing production helps reduce part costs.

Think of completing a task or making a product. In the time a vector motor can make one part, a standard brushless servo motor can make four, and a low inertia brushless servo motor can make 20.

Save weight and power

For a given amount of output torque, a servo motor is smaller than the alternatives. For example, a typical 1 hp (0.74 kilowatts) ac induction motor is 7 to 8 inches (177 - 203 millimeters) in diameter, an SCR motor is 4.5 inches (114 mm) in diameter, dc servos are 4 inches (102 mm) in diameter, and ac brushless servos are 3.5 inches (90 mm) square. This makes it easy to fit servo motors into tight confining locations.

Smaller size also leads to less weight. Figure 3 compares weights of similar horsepower (kW) and servo motors, and reveals that servo motors are lighter than induction motors-often 40 to 50 percent lighter and sometimes up to 70 percent lighter.

This becomes very important in applications that move a load that also includes the servo motor. As an example, if your arm was a robot arm, there would be one motor in your wrist, a second in your elbow, and a third in your shoulder. When your shoulder moves, it must carry two motors (elbow and wrist motors). Imagine that the wrist motor weighs 40 pounds (18 kilograms), and the elbow motor weighs 60 pounds (27 kg). The shoulder motor must then be much larger in order to handle and move the other motors as well as the load in your hand. It could possibly weigh upward of 100 pounds (45 kg).

Now let's see what would occur if lighter-weight servo motors were used. The servo motors would still provide the same amount of torque (or horsepower, or kilowatts). If you can use a smaller motor in the wrist-for example, a 10-pound motor (4.5 kg)-then the elbow may be downsized to a smaller motor, possibly 15 pounds (6.8 kg); now the shoulder motor may be only 25 pounds (11 kg). The smaller the servo motor, the less power required to move it. If the motor can be downsized, you save on weight, and you save overall power.

Reduce part costs and deliver efficiency

The smaller servo motor also has the benefit of lower rotor inertia. Because of high-flux magnets, the torque delivered is equivalent to that of larger motors. Lower rotor inertia means faster acceleration and faster positioning, thus improving a machine's throughput. By improving the speed of the machine, it will increase the number of parts produced, and increasing production rates helps to reduce part costs.

Servo motors are designed to be highly efficient. For example, in the production of a servo motor the amount of wire inserted between the slots of the laminations, termed "slot fill," is 75 to 80 percent. By comparison, a typical ac induction motor slot fill is less than 65 percent.

As shown in figure 4, the servo motor efficiency curve is quite flat in the area between 40 percent and 90 percent of the motor's continuous stall torque. And the efficiency is above 85 percent throughout this range. Unlike hydraulic and pneumatic systems that use energy continuously (to keep the pump going to maintain pressure), the servo motor uses energy only when energized, i.e., commanded to make a move. For example, injection molding machines have replaced hydraulics with servos to cut down on overall energy used, to improve the equipment's overall efficiency. The servo motor design inherently provides a high power conversion factor, i.e., efficiency, thus making the best use of power and energy.

Best long-term productivity investment

Closed loop servo operation provides controllability that improves accuracy and product reliability. Controllability and accuracy means better part quality. And by improving the quality of the part being produced, this leads to lowering of rejects. This saves money, improving the bottom line.

Servos make parts faster, thus improving productivity, improving the machines' reliability and piece part quality. Servos represent the best long-term productivity investment-a winning combination.

Figure 2: Servo motors get a load into position very fast, thus maximizing machine operation.

Figure 3: Servo motors are lighter in weight.

Figure 4: Servo motors provide high efficiency over a wide operating range to make the best use of power/energy.

For more information, visit www.baldor.com.

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