Servomotor Efficiency by John Mazurkiewicz Baldor Electric
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Servomotor Efficiency By John Mazurkiewicz Baldor Electric In industry we are finding that products must deliver greater performance and reliability while using less electricity. With the cost of electricity steadily increasing, energy efficiency is becoming increasingly relevant. This is also true in motion/position control applications, where servomotors are widely used. You may not be aware that a properly designed servomotor is already highly efficient. It’s how you apply and use the servomotor, which determines whether it is used in an efficient or inefficient manner. Properly designed servomotors Let’s look at what makes a servomotor. First of all, when comparing a servomotor to other motors, which provide the same torque, notice that a servomotor is smaller in diameter. For example, as shown in the photo a 1 horsepower AC induction motor has a 5.6 – 7.6” diameter, an SCR rated motor has a 4.7” OD, a brush–type servomotor is 4” OD, and brushless servomotors is 3.5” sq. The servomotor has intentionally been designed to have a smaller diameter. This is to reduce the rotor inertia, to provide the fast response, which is demanded in motion control applications. Secondly, the servomotor has a feedback device. This added distinguishing feature allows for closed loop control and improved accuracy. Third, in the manufacturing of the servomotor – extra care is taken, and more wire is wound onto the laminations. The space between the teeth of the lamination is stuffed with wire – the slot fill ranges 75 – 80% (most motors are less than 65%). This becomes tough to accomplish in production, however it provides extra torque, and greater efficiency in the servomotor. Servomotor efficiency curve Today, you will find manufacturers advertising efficient servomotors. If you have a motor, say it is advertised to have a “high and improved efficiency of 70%” – it was not designed as a servomotor. Servomotors are already designed to be over 85% efficient. It’s how the servo is applied and operated, which uses it in an efficient or inefficient manner. So let’s look at how your application measures up. A servomotors’ performance curve provides information about speed, torque, voltage, and thermal conditions. Figure 1 shows a typical servomotor speed–torque curve and thermal line. The speed–torque curve informs us about voltage and speed. For a given load torque and voltage on the motor, it is possible to determine the operating speed. To learn how to read and interpret these curves, refer to the appendix “How to Read Motor Speed Torque Curves”. The thermal line shows the area at which the motor is running at its maximum design condition of 155 °C. In other words, if the motor is operated continuously at any point to the left of the thermal line, the motor will operate cooler than 155 °C; and if operated continuously on the right of the line, the motor will overheat. The efficiency curve provides the servomotors’ input power vs. delivered power over the operating range. Efficiency is easiest and most accurately obtained from testing, thus it is typically not provided. From the efficiency curve of Figure 1, this servomotor attains over 88% efficiency at 38% of torque (continuous stall), and increases to attain a high of 88.8%. Efficiency remains high, and is 87.6% at 90% of continuous stall torque. As you can see, the servomotor is designed to be efficient over a wide operating range. Figure 1 – Servo performance curve Figure 2 – Pushing the limit RPMx1000 Efficiency % RPMx1000 Efficiency % 5 300VDC 5 Thermal Line Thermal Line 90 Efficiency 90 Curve 4 80 4 80 Efficiency Curve Speed-Torque 70 Speed-Torque 70 3 3 Continuous Intermittent Operation Operation 2 2 1 1 38% 90% 2xContinuous 100 200 300 400 500 600 100 200 300 400 500 600 Torque (oz-in) Torque (oz-in) In motion control applications, the servomotor should be operated within this continuous range to obtain high efficiencies. High torque applications A servomotor in an application is typically being utilized for its high peak torque capability, thus providing rapid acceleration. To get these high torques, the motor is required to deliver between 2 – 3, or more, times its continuous torque. And when the motor is operated at these higher torques, the efficiency is somewhat less. As can be seen from the extended efficiency curve of Figure 2, the point at which the motor is delivering 2 times the continuous torque, efficiency is about 76%. Although slightly lower, the good news is that the servomotor is operated in this area intermittently. And for the average of the applications duty cycle, the motor is being operated in the higher efficiency range. Applying other voltages Servo motors are designed to have a range of voltages applied. This is how speed is varied in the application. Change the voltage, and you vary the speed. This motor is designed to operate with a bus voltage of 300 VDC, and provide a continuous stall torque of 275 oz–in. Modern digital servo controls PWM (pulse width modulate) the voltage, thus varying motor speed. A PWM signal which has a 66% duty cycle would apply 2/3 voltage to the motor, thus reaching 2/3 speed. And when the voltage is changed to 2/3 x 300 = 200 VDC, the efficiency curves changes. Figure 3 shows the re–plot of the speed–torque and efficiency curves. This shows that efficiency reaches 83% (at 38% torque) and decreases slightly to 81% when it crosses the thermal line. The reason efficiency has declined, is attributed to the decline in the applied voltage (from 300 VDC to 200 VDC). The result is a decline of input power, therefore decline in efficiency (recall that efficiency is input power vs. delivered power). However as you can see, the overall efficiency of the servo motor still is quite high. Figure 3 – Voltage and frequency relationship RPMx1000 Efficiency % 5 Thermal Line 90 4 80 200VDC Efficiency Curve 70 at 200VDC 3 Speed-Torque 2 1 38% 90% 100 200 300 400 500 600 Torque (oz-in) Efficiency guidelines Use the following as guidelines to obtain maximum efficiency from your servomotor: Servomotors have a very flat efficiency curve over the design range. However operating at 38% of torque, results in a very cool operating motor, but if this really your operating point, you should investigate downsizing to a smaller, less expensive motor. When operating at more than 90 – 95% of continuous torque, use caution to not exceed the thermal line, otherwise you will overheat the motor. Connect and make use of the motors’ internal thermal switch – this will protect the motor. Keep in mind that when the motor is supplying high acceleration torques of 2 – 3 times continuous, is that these demands are intermittent. And for the average over the duty cycle, the servomotor is being used in the higher efficient range. The servomotor is a product operating at design efficiencies higher than the standard motor. Appendix How to read motor speed–torque curves In constant speed applications, motors are defined in terms of horsepower (which is torque at base speed). However servomotors normally operate over a wider speed range, and the curves show continuous torque (defined as torque which will not overheat the motor), and peak torque (defined as intermittent, acceleration torque). It is also necessary to know the current and voltage required for the motor to operate. The curves have a scale that shows current required for any torque, and voltage required for any speed. As an example, an application requires a continuous torque of 30 lb–in at a speed of 3600 RPM. The peak torque required for acceleration is 80 lb–in. The curve in Figure 4 show that this motor will work for this application. The bus voltage required is 300 VDC. The continuous and peak currents required is 7 and 18 amps. From this information, a control is selected that can deliver these currents with a 230 VAC input (300 VDC bus). Note that “Rated” conditions refer to measurement points, and are selected as an easy and convenient “reference” or “measurement ” point for the manufacturer. Manufacturers select a “rated voltage”, operate with a “rated torque”, to verify that “rated speed” is reached. Note that any voltage may be applied to a servomotor. However the design limits must be observed. And those are: 1) maximum speed (RPM) limit and 2) demagnetization (max torque/current) limit. Figure 4 – Servomotor application example Motor Peak 35 160 18 Current capability 140 16 30 300VDC 14 120 25 12 100 20 10 Peak Current 80 (example) 160VDC 8 (Nm) Torque 15 60 Torque (lb-in) Torque 6 10 Current (A) 40 4 Continuous Current Continuous Torque (example) at Speed (example) 5 20 2 0 0 0 1 234 567 Speed (RPM x 1000) Max Motor Speed.