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How to Choose the Right AC Servo Motor for Your CNC Machine
Learn how to select an AC servo motor for CNC machine axes by comparing torque speed inertia feedback drive compatibility and operating conditions.
Case description
How to Choose the Right AC Servo Motor for Your CNC Machine
Choosing an AC servo motor for a CNC machine involves more than selecting a motor by power rating. The motor must accelerate the actual machine load, reach the required feed rate, maintain torque during cutting, and work correctly with the selected drive and CNC controller.
A suitable servo motor for CNC machine axes is selected from the complete motion requirement. That includes the table mass, screw lead, transmission ratio, cutting force, travel distance, acceleration target, duty cycle, and axis orientation. A motor that appears adequate at standstill may still lack the required torque at speed or operate too close to its thermal limit during production.
Begin With the Motion Requirement
The first step is to define what each axis must do during a real machining cycle. A light X axis on a small milling machine does not have the same requirements as a Z axis carrying a heavy spindle head or a large lathe carriage.
Collect these machine details before selecting an AC servo motor:
- Moving mass of the table, saddle, spindle head, or carriage
- Required axis travel and maximum feed rate
- Acceleration and deceleration target
- Ball screw lead or transmission ratio
- Horizontal or vertical axis arrangement
- Expected cutting force and friction
- Number of motion cycles per minute or per shift
- Required positioning and repeatability performance
A short rapid movement with frequent reversals can require substantial acceleration torque. A long feed move may place greater emphasis on continuous torque and motor temperature. Both conditions matter when selecting a servo motor for CNC machine use.
Convert Linear Axis Speed Into Motor Speed
For a ball-screw axis, the screw lead links linear movement to motor rotation. The required motor speed can be estimated from the desired axis feed rate.
For example, consider a milling axis with:
Machine requirement | Example value |
Ball screw lead | 10 mm per revolution |
Required rapid feed | 12,000 mm per minute |
Direct motor connection | 1 to 1 ratio |
At 10 mm per revolution, the motor must rotate at approximately 1,200 rpm to achieve 12,000 mm per minute. If the machine uses a belt reduction or gearbox, the motor speed changes according to the transmission ratio.
This calculation is only the start. The selected AC servo motor must also provide sufficient torque at the required speed. Motor catalogs normally show rated speed, maximum speed, rated torque, peak torque, and torque-speed characteristics. Do not choose a motor from wattage alone.
According to Mitsubishi CNC servo selection guidance, motors with the same power rating can have different rated speeds and torque characteristics. This is why the machine’s actual speed requirement must be checked against the motor data.
Check Continuous Torque and Peak Torque
Servo sizing requires two torque checks. Continuous torque covers the average demand throughout a complete machine cycle. Peak torque covers short periods of acceleration, deceleration, cutting load, or direction reversal.
A motor can use peak torque for a limited period, but it cannot operate at its maximum rating continuously without creating excessive heat. The duty cycle matters.
For a typical CNC axis, torque demand may include:
- Friction from ball screws, bearings, and guideways
- Cutting force transferred through the machine structure
- Acceleration torque for moving mass
- Gravity-related load on vertical axes
- Transmission losses from belts, gearboxes, or couplings
The continuous demand should remain within the motor’s continuous operating capability. Peak demand should remain within the permitted short-time range. Servo sizing references commonly identify inertia, speed, peak torque, and RMS torque as the core checks for a motion application. Heidenhain’s servo selection overview explains the distinction between continuous RMS torque and intermittent peak torque.
An AC servo motor that is too small may overheat, trigger overload alarms, or slow the machine cycle. An oversized motor can add cost, physical weight, and unnecessary drive capacity.
Evaluate Load Inertia and Transmission Design
Inertia is the resistance of a load to changes in speed. It affects how much torque the AC servo motor needs to accelerate and decelerate the machine axis.
The motor rotor has its own inertia, while the table, ball screw, pulley, gearbox, fixture, and workpiece contribute load inertia. The reflected load inertia should be evaluated against the manufacturer’s recommended range for the selected motor.
A high inertia ratio can make the axis more difficult to tune, increase settling time, and reduce motion stability. A suitable transmission ratio can improve the motor’s operating conditions, but it also changes speed, torque, and reflected inertia.
Transmission choice | Potential benefit | Item to check |
Direct coupling | Simple mechanical structure | Motor must provide required torque directly |
Belt reduction | Can increase available output torque | Check belt stiffness, ratio, and motor speed |
Gear reducer | Can support higher load torque | Check backlash, efficiency, and reflected inertia |
Larger ball screw lead | Can increase linear speed | Requires higher motor torque at the screw |
Mitsubishi’s CNC documentation identifies load inertia ratio, short-time acceleration torque, and continuous effective torque as key elements of servo motor capacity selection. Mitsubishi servo sizing guidance also notes that excessive inertia can make parameter adjustment more difficult.
Match the Motor With the Servo Drive and Controller
An AC servo motor operates as part of a matched system. The CNC controller issues motion commands, the servo drive regulates current and motion, the encoder provides feedback, and the motor moves the machine axis.
Before ordering, confirm:
- Motor voltage and power rating
- Compatible servo drive model
- Encoder type and feedback connection
- Controller command interface
- Available power supply
- Brake requirement for vertical axes
- Motor flange size and shaft dimensions
- Cable length and connector type
- Required fault and enable signals
A motor and drive should not be paired only because their power ratings are similar. The drive must support the motor’s encoder, electrical characteristics, and intended operating mode.
SZGH offers AC servo motor and driver product information across several frame sizes. The final motor-drive combination should be confirmed using the machine’s axis data and the supplier’s matching documentation.
Consider Encoder Resolution and Machine Feedback
The encoder provides motion feedback to the servo system. It helps the drive compare commanded movement with actual motor movement and respond to position or speed error.
Encoder resolution alone does not determine finished-part accuracy. The ball screw, coupling, gearbox, guideway condition, machine rigidity, spindle cutting load, and workholding also affect the result.
When selecting a servo motor for CNC machine use, ask:
- What feedback device is included with the motor?
- Is the feedback compatible with the selected servo drive?
- Does the application need a brake or absolute position function?
- Will the machine use motor feedback only, or additional axis feedback?
- Are feedback cables shielded and routed away from electrical interference?
For example, the SZGH 08100DC 1000W servo motor lists a rated speed of 2,500 rpm, rated torque of 4 Nm, peak torque of 12 Nm, and a 2,500 PPR encoder. These figures should be compared with the required axis speed, load, and drive compatibility before they are applied to any particular machine.
Check Installation and Operating Conditions
The final AC servo motor selection must account for the real machine environment. A correctly sized motor can still encounter problems if it is installed in a hot, contaminated, poorly ventilated, or high-vibration location.
Review these conditions:
- Ambient temperature around the motor and cabinet
- Oil mist, coolant, dust, and metal chip exposure
- Cabinet ventilation and drive cooling
- Motor mounting rigidity
- Coupling alignment
- Cable protection and strain relief
- Vertical-axis holding requirement
- Daily operating hours and production duty cycle
A motor selected near its continuous operating limit may work during a brief test but become too hot during a full production shift. Include the expected duty cycle in the motor selection review.
FAQ
How do I choose the AC servo motor power for a CNC axis?
Start with the required axis speed, moving mass, transmission design, acceleration, cutting load, and duty cycle. Then check continuous torque, peak torque, motor speed, and inertia ratio against the manufacturer’s data.
Is a higher wattage servo motor always better?
No. A larger motor may cost more, require a larger drive, and add unnecessary inertia or size. Select a motor that meets the actual motion requirement with suitable operating margin.
Why does a vertical CNC axis often need a servo motor with a brake?
A vertical axis may move under gravity when power is removed. The brake requirement depends on the machine design, load, transmission, and safety arrangement.
Can I use any servo drive with an AC servo motor?
No. The servo drive must be compatible with the motor voltage, current, encoder type, feedback connection, and operating mode.
What information should I send for servo motor selection?
Provide the axis type, moving mass, travel, speed, acceleration, ball screw lead or gearbox ratio, load direction, cutting force, existing controller model, and available power supply.
Conclusion
The right AC servo motor is selected from the axis motion requirement, not from motor power alone. Check speed, continuous torque, peak torque, inertia, transmission design, drive compatibility, encoder feedback, and operating environment before confirming a model.
For support selecting a servo motor for CNC machine axes, contact the SZGH technical team with your machine drawings, axis data, existing drive details, and production requirements. Review the SZGH AC Servo Motor Driver Catalog for available motor and drive options.
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