How to Select Servo Systems

Servo systems offer incredible levels of speed, accuracy, and flexibility in automated gear when the right servo is selected for the application. Sadly, selecting the incorrect servo can result in difficulty in tuning, poor accuracy, or underwhelming overall performance. With a lot of various servos to select from, how do you make sure that the proper 1 was selected? Luckily, the procedure of selecting the proper servo motor and drive, recognized as sizing, may be broken down into eight easy actions.


The first and easiest factor to consider is the available power for the equipment. Servos are available in 100 VAC, 200 VAC, and 400 VAC models, and are compatible with single phase or three-phase power.

Motion Profile

For equipment that performs a repetitive operation, plot out the required motor speeds throughout the cycle. Be sure to allow for acceleration and deceleration time; servomotors are not magic, and cannot make step changes in speed. For non-repetitive operations such as milling, calculate the peak speed and acceleration required for the application.


Torque is how much “muscle” it takes to rotate a mechanism, and comes from three different sources: accelerating the mechanism’s inertia, friction, and external forces such as pressing against an object or gravity. This is the most difficult part to calculate accurately, but is also the most forgiving part of the selection process. Calculate the inertia of each component of the system and add the values. The formulas for calculating rotational inertia of various shapes are readily available on the Internet. Multiply the acceleration by the load inertia to calculate the load’s acceleration torque. Calculate friction forces for sliding loads, gravitational forces for vertical loads, and any external forces. Multiply each force by the radius it is acting on (known as the “moment”) to calculate the torque. Calculate the peak torque by adding up all the torque values in the worst-case scenario. This is typically when the fastest acceleration is occurring or when there is the most mass on the machine. Add up the torque values from external forces, gravity and friction to calculate the continuous torque requirement. Ideally, the continuous torque requirement would use a root-mean-squared (RMS) calculation, but this is tedious without the help of a software tool.

Inertia Ratio

Calculating an Inertia Ratio is often overlooked by newcomers to servo sizing, but is arguably the most important factor in determining the performance of a servo system. Inertia Ratio is the ratio of the load’s inertia divided by the motor’s rotor inertia divided by the square of the gear reduction. To use a boxing analogy, if torque is how strong the fighter is, inertia ratio is the weight class. Not only does the servo system need to have enough torque to move the load, it must have the ability to accurately control the load. In an ideal world, the mechanism would be mechanically rigid and torque from the motor would be transferred smoothly and without delay to the load. In the real world, couplings flex, belts and chains stretch, and gears have backlash. These imperfections can be minimized but not completely eliminated. When the motor begins to move, the machine winds up like a spring, and begins to push back on the motor with some tiny delay. This spring effect is magnified with large inertia ratios. Servo systems use the feedback from the built-in encoder and the PID algorithm in the amplifier to accurately position the motor, but this tiny spring delay can cause oscillations and loss of control if the loop gains are too high. Reducing the gain will stop the oscillation, but at the cost of responsiveness. While basic servo drives may require inertia ratios of 3:1 or smaller, high performance servo drives have auto-tuning, vibration suppression, resonance filters, and disturbance compensation functions that allow up to a 30:1 inertia ratio without sacrificing performance. A ratio of 1:1 will give excellent performance, but usually results in an oversized motor. Ratios less than 1:1 waste power with no performance advantage.

Servo Drive and Options

Once the servomotor has been selected, choose a servo drive rated for the correct input voltage and with sufficient output current to drive the servo motor. Servo drives can be controlled via several different interface types. These interfaces include pulse-and-direction digital control, analog control, and other servo networks. A servo network provides high-speed control and feedback, reduced wiring, and superior diagnostics capabilities compared to the other interfaces. Finally, choose any options such as keyed motor shafts, shaft seals, holding brakes for vertical loads, or external braking resistors.

Selecting the best servo system for an application is a skill that improves with practice. When in doubt, it’s a good idea to verify your results with the manufacturer or distributor.

Fasttobuy Delta Servo Motor Drive Kit Recommendation

Delta 400W Servo Motor Drive kit ECMA-C10604RS+ASD-A2-0421-L

ECMA-C10604RS+ASD-A2-0421-LBrand Name:Delta
Model Number:Delta servo motor 400W ECMA-C10604RS+A2 ASD-A2-0421-L
Type:Servo Motor, dc servo motor
Output Power:400W
Protect Feature:Drip-proof
Certification:CCC, CE, ROHS, UL
Ac Voltage:200v220v460v500v600v
Place Of Origin:Taiwan
Efficiency:IE 3
Name:Three-phase Asynchronous Motor
Max Torque:3.8n.m
Motor shaft:pinion shaft
Main specs: Delta servo motor 400W ECMA-C10604RS+A2 ASD-A2-0421-L

Delta 1KW 4.77NM Servo Motor Drive Kit ECMA-E11310RS+ASD-A2-1021-L

ECMA-E11310RS+ASD-A2-1021-LSpeed: 3000/5000r/min
Phase: Three-phase
Place of Origin: Taiwan
Brand Name: Delta
Model Number: Delta servo motor 1KW ECMA-E11310RS+ASD-A2-1021-L
Application: CNC
Type: Servo Motor, dc servo motor
Frequency: 50hz60hz
Output Power: 1KW
Protect Feature: Drip-proof
Certification: CCC, CE, ROHS, UL
AC Voltage: 200v220v460v500v600v
Efficiency: IE 3
Name: Three-phase Asynchronous Motor
Weight: 8kg
max torque: 14.3n.m
Motor shaft: pinion shaft
Main specs: Delta servo motor 1KW ECMA-E11310RS+ASD-A2-1021-L

Delta 200W Servo Motor Drive Kit ECMA-C20602RS+ASD-B2-0221-B

ECMA-C20602RS+ASD-B2-0221-BBrand Name: Delta
Model Number: Delta servo motor with drive 200W ECMA-C20602RS+ASD-B2-0221-B
Type: Servo Motor, dc servo motor
Frequency: 50hz60hz
Output Power: 200W
Protect Feature: Drip-proof
Phase: Three-phase
Certification: CCC, CE, ROHS, UL
Ac Voltage: 200v220v460v500v600v
Place Of Origin: Taiwan
Efficiency: IE 3
Application: CNC
Weight: 8kg
Speed: 3000/5000r/min
Name: Three-phase Asynchronous Motor
Max Torque: 1.9n.m
Motor shaft: pinion shaft
Main specs: Delta servo motor with drive 200W ECMA-C20602RS+ASD-B2-0221-B

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