Choose wisely
A variety of factors can complicate the task of picking the right servomotor.
Though there is little standardization in the servomotor manufacturing industry, motor manufacturers work hard to overcome this limitation by making their product offerings flexible. Regardless of your application complexity, it is always good to talk to a factory rep to confirm what will work best for your design.
Derating motor torqueMotor torque ratings change when motors operate under ambient temperature conditions different than those listed in the specifications. The approximate change is calculated from the equation: 
For example, consider a servomotor rated at 24.8 Nm when operated at 40°C ambient temperature. The rated maximum temperature rise of the motor is 100°C. Ambient temperature in the area of operation was specified to be 65°C. First, calculate the maximum temperature of the motor by adding the rated motor temperature rise to its ambient specification: 100 + 40 = 140°C maximum motor temperature. Subtract the specified ambient temperature from the maximum motor temperature to determine the actual temperature rise permitted for the motor in this application: 140 - 65 = 75°C actual temperature rise. Plug the numbers into the equation to calculate the new torque rating of the motor: 
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POSITION FEEDBACK DEVICES | ||||
| Parameter | Resolver | Incremental encoder | Sine encoder | Smart Feedback Device (SFD) |
| Mechanical shock and vibration resistance | Excellent | Fair to good | Excellent | Excellent |
| Temperature rating | Typically same as motor windings: up to 170° | 80 to 115°C | Typically same as motor windings: up to 170 °C | –20 to 115°C |
| Resolution | Dependent on resolver to digital (R/D) converter, typically 12 to 16 bits (4,096 – 65,536 counts for single speed type) | 500 to 20,000 lines (2,000 – 80,000 counts) or more per revolution (4,000 lines is typical minimum for velocity control) | 500k to 2 million counts depending on amount of interpolation of the fundamental number of lines of resolution | > 16 million based on 224 built-in interpolation of integral single speed resolver |
| Accuracy | 10 to 20 arc-min | 3 to 5 arc-min; can be much less Stegmann Endat encoders are 45 arc-sec | 20 to 60 arc-sec | 9 to 16 arc-min |
| Speed | 12,000 rpm or more | Typically 7,000 rpm maximum | 12,000 rpm or more however can be limited by an input frequency limitation of the amplifier | 12,000 rpm or more |
| Output | Analog requiring R/D converter or interpolation in software | Digital output | Analog — requires converter or interpolation in software | RS-485 |
| Dynamic response | Good, signal conversion results in some phase delay | Good to excellent, related to resolution of device | Excellent due to high resolution | Excellent – update every 51.2 sec |
| Commutation method | Direct based on absolute feedback of resolver (motor poles must be evenly divisible by resolver poles) | Requires additional Hall-effect devices or commutation tracks to initialize motion until absolute position can be determined | Direct based on absolute feedback nature of sine encoder | Direct based on the absolute feedback nature of the device |
| Distance from controller | Up to 75 m (typical) | Up to 30 m (typical) | Up to 40 m (typical) | Up to 75 m or more (typical) |
| Cable conductors | Three pair | Seven pair | Five pair | Two pair – two for power, two for RS-485 |
| Cost | Low | Low to moderate | Moderate to high | Low to moderate |
This table provides a brief operating summary of the four most 1common feedback devices used with servomotors. | ||||
MAKE CONTACT Danaher Motion, (866) 993-2624, danahermotion.com
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© 2008 Penton Media Inc.
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