How to Select a DC Servo Motor: Voltage, Torque, Speed and Encoder

Selecting a DC servo motor requires more than matching voltage or rated power. The motor must provide the required speed and torque throughout the operating cycle while matching the available power supply, feedback system, mechanical interface and installation space.

This guide provides a practical selection sequence for DC servo motors used in AGV, AMR, mobile robot and automated drive systems.

1. Start With the Available DC Supply Voltage

Confirm the available DC bus or battery voltage before comparing motor power and torque. A motor designed for a different nominal voltage may require a different drive configuration and may not deliver the expected speed or torque from the available supply.

For battery-powered mobile robots, the nominal system voltage should be checked together with the actual operating voltage range and the requirements of the motor drive electronics.

2. Determine the Required Motor Speed

Motor speed should be derived from the required mechanism or wheel speed and the transmission ratio.

For a geared drive:

n_motor = n_output × i

where i is the gearbox reduction ratio.

A motor should not be selected only because its rated speed is high enough. Rated operating speed, maximum permitted speed and the gearbox input-speed limit should all be checked together.

3. Calculate the Required Continuous Torque

The continuous torque requirement should come from the real mechanical load. For a wheel-driven system, required wheel torque can be estimated from:

T = F × r

where F is tangential force and r is effective wheel radius.

If a gearbox is used, the motor-side torque requirement depends on reduction ratio and drivetrain efficiency.

T_motor ≈ T_output / (i × η)

This provides a useful preliminary value, but acceleration, grade, rolling resistance and application-specific loads must also be considered.

4. Check Power, Torque and Speed Together

Motor power, torque and rotational speed are related by:

P = T × ω

with:

ω = 2πn / 60

This relationship is useful for checking whether catalogue values are internally consistent and whether a proposed motor operating point is realistic.

5. Rated Torque and Peak Torque Serve Different Purposes

Rated torque is the primary value for normal continuous operation. Peak torque is intended for short-duration conditions such as acceleration, deceleration or temporary load disturbances.

A motor with sufficient peak torque can still be undersized if its continuous operating point exceeds the rated torque. The permitted duration and repetition frequency of peak operation should therefore be confirmed for the actual duty cycle.

6. Select the Feedback System

Servo systems use position or speed feedback to control motor motion. Encoder type and resolution should be evaluated according to the required speed regulation, positioning accuracy and control architecture.

Higher encoder resolution alone does not guarantee higher machine accuracy. Gearbox backlash, coupling compliance, structural deflection and controller tuning also contribute to the final system performance.

7. Check the Mechanical Interface

Confirm the motor flange, pilot diameter, shaft diameter, shaft length, mounting-hole pattern and coupling or gearbox interface before final selection.

Installation space should also include cable routing, connector access and the space required to assemble or service the motor.

8. Consider Duty Cycle and Thermal Loading

AGV and AMR drives often operate with repeated starts, stops, acceleration, braking and direction changes. These conditions are different from continuous steady-speed operation.

Review the continuous operating torque, acceleration demand, cycle frequency, operating time and expected peak events. Thermal loading can become the limiting factor even when the motor has sufficient short-term peak torque.

9. Worked Example: 48V, 400W Servo Motor

Consider the JY Robot 48V DC servo motor platform with the following rated data:

At 3000 r/min:

ω = 2π × 3000 / 60 ≈ 314.16 rad/s

The torque corresponding to 400 W at this speed is:

T = 400 / 314.16 ≈ 1.27 N·m

This agrees with the stated rated torque and provides a useful consistency check between rated power, speed and torque.

The motor also has a stated peak torque of 2.54 N·m. This peak value should not be treated as a continuous operating torque. The actual duration and frequency of peak loading must be considered when validating the duty cycle.

10. Check Size and Installation Envelope

The reference motor uses an approximately 62 × 62 mm mounting flange, with a body length of approximately 90 mm and a total weight of approximately 1.2 kg.

These dimensions can help during preliminary packaging studies, but the detailed drawing should be checked before designing mounting plates, couplings or gearbox interfaces.

11. Information Needed Before Final Motor Selection

• Available DC supply voltage
• Required motor or output speed
• Required continuous torque
• Required acceleration or peak torque
• Gearbox ratio, if used
• Duty cycle and start/stop frequency
• Encoder and feedback requirements
• Motor flange and shaft dimensions
• Available installation space
• Operating environment

48V DC Servo Motor Reference

For the motor platform used in the worked example, see the 48V DC servo motor for AGV/AMR.

If the motor is used with a reduction gearbox, the motor operating point and gearbox ratio should be checked together. See the planetary gearbox selection guide for ratio, torque and backlash considerations.

Final motor selection should be based on the complete load profile, mechanical transmission and operating cycle rather than a single rated value.