A servo system that looks adequate on a catalogue comparison can still fail at the machine. It may trip on acceleration, hunt at standstill, overheat during a production run or stop too slowly when an emergency condition occurs. Knowing how to specify servo drives means defining the actual mechanical duty first, then selecting the motor, drive, feedback and supporting hardware as one coordinated motion system.
For OEMs, integrators and maintenance teams, this approach avoids a common and costly error: selecting a drive from its continuous power rating alone. Servo axes are governed by peak torque, reflected inertia, speed, motion profile, regeneration and control requirements. The application determines the drive, not the other way around.
Start With the Machine Load and Motion Profile
The first task is to describe what the axis must move. This includes the load mass or inertia, transmission arrangement, required stroke or rotation, maximum speed, acceleration and deceleration times, cycle rate, dwell periods and positioning accuracy. A conveyor indexing a modest load, for example, may need substantially higher acceleration torque than a continuously running roller application.
For linear axes, calculate the force needed to overcome gravity, friction, process force and acceleration. Convert this through the screw, belt, rack or gearbox to establish motor torque and speed. For rotary systems, calculate the total load inertia at the motor shaft, including the workpiece, coupling, gearbox and any rotating tooling.
The drive must provide sufficient continuous torque for the thermal duty, as well as enough peak torque for the highest acceleration and process demand. Peak torque is usually available only for a defined period. An axis that reaches its peak rating on every short cycle may look acceptable on paper but can overload the motor or drive over time.
Inertia matching also deserves attention. A high reflected load inertia relative to motor inertia can reduce tuning stability, limit acceleration and increase mechanical resonance. Modern servo drives can manage wider inertia ratios than older systems, but this should not be treated as a substitute for sound mechanical design. Gear reduction can improve torque capability and reflected inertia, although it may introduce backlash, compliance and reduced positioning performance.
How to Specify Servo Drives by Torque, Speed and Duty
Once the motion profile is established, plot torque against time across a complete operating cycle. Include acceleration, constant-speed movement, deceleration, dwell and any process loads such as cutting, winding or pressing. This produces the information needed to check both peak and RMS torque.
The motor and drive combination should satisfy three separate conditions. It must deliver the required maximum speed at the available supply voltage, provide peak torque through the highest-demand portions of the cycle, and remain within continuous thermal limits based on RMS torque. These checks are related but not interchangeable.
A drive’s current capability is particularly relevant. Servo torque is largely proportional to current, so the drive must supply the motor’s required continuous and peak current without exceeding its own overload curve. Check the permitted peak duration and recovery period rather than assuming a nominal overload percentage applies indefinitely.
Do not overlook speed-torque behaviour at the intended operating voltage. Back EMF reduces the available current as motor speed rises. A motor that provides suitable torque at low speed may not meet the required torque at its top speed, especially where rapid acceleration continues into that range.
Check the Regenerative Energy
Decelerating a load returns kinetic energy to the servo drive DC bus. For light, slow axes this energy may be absorbed internally. High-inertia indexing tables, vertical axes, centrifuges and frequent stop-start machines can produce far more regenerative energy than the drive can manage.
Calculate regeneration per deceleration event and across the operating cycle. The result will determine whether an external braking resistor, regenerative unit or shared DC bus arrangement is required. Resistor selection is not simply a wattage exercise: confirm resistance limits, pulse energy, average thermal dissipation, enclosure heat and the required braking performance.
For vertical axes, assess what happens on loss of power or a drive fault. A mechanical holding brake may be required to prevent a suspended load from dropping. The brake is generally a holding device, not a service brake for repeated stopping, so the control sequence and mechanical design must reflect that distinction.
Select Feedback and Control for the Required Accuracy
Feedback choice affects positioning accuracy, repeatability, speed regulation and fault detection. Most servo motors use an integrated encoder, but the application may call for absolute rather than incremental feedback. Absolute encoders retain position information after power loss and can remove the need for a homing cycle, which is valuable where reference movement is impractical or unsafe.
Encoder resolution is only one part of achieved accuracy. Mechanical backlash, belt stretch, screw pitch error, gearbox compliance, bearing condition and thermal expansion can all dominate final position error. For precision applications, consider whether direct load feedback is needed rather than relying only on motor feedback.
The command interface must also suit the control architecture. Pulse train control may be sufficient for a straightforward positioning axis. EtherCAT or another industrial Ethernet motion network may be preferable where coordinated axes, electronic camming, synchronisation or detailed diagnostics are required. Confirm controller compatibility, update rates, network topology and the number of axes before finalising the drive family.
Account for Safety, Supply and Installation Conditions
Servo drives are safety-related equipment even when the machine risk assessment does not require advanced motion safety. At minimum, determine whether Safe Torque Off is required. STO removes torque-producing power from the motor without necessarily removing mains power from the drive, allowing a controlled safety architecture and faster restart where appropriate.
More demanding applications may require functions such as safe stop, safe limited speed or safe direction. These functions must be selected from the machine safety design, including the required performance level or SIL target, not added as an afterthought because a drive happens to offer them.
Supply conditions also affect reliability. Confirm mains voltage, frequency, earthing arrangement, available fault current, upstream protection and whether the application is exposed to poor power quality. Long motor cables can create voltage stress and electromagnetic interference. Output reactors, filters, shielded cable and correct earthing may be needed, particularly on larger motors or installations with sensitive instrumentation nearby.
Installation environment matters just as much. Consider enclosure IP rating, ambient temperature, altitude, contamination, vibration and ventilation. Drives mounted tightly in a control panel need clearances and heat calculations based on actual losses. Derating may be required above the manufacturer’s specified ambient temperature or at elevation.
Specify the Complete Servo System, Not Just the Drive
A usable specification should give the project team enough detail to select, build and commission the axis without assumptions. Along with the chosen drive and motor series, record the supply arrangement, load data, transmission ratio, speed and torque profile, feedback type, brake requirement, control network, safety functions, cable lengths, braking method and environmental conditions.
Where replacement is the objective, do not rely solely on the rating plate of the existing drive. Older equipment may have been oversized, incorrectly configured or operating outside its intended duty. Review the actual machine cycle, fault history and mechanical condition before treating the installed model as the benchmark.
Commissioning should be allowed for at specification stage. Auto-tuning can provide a sound starting point, but high-performance machinery often requires manual adjustment of gains, filters and feed-forward settings. Excessive tuning gains can hide mechanical issues temporarily while increasing wear, noise and instability. The best outcome is a mechanically sound axis with tuning that meets cycle-time and accuracy requirements while retaining reasonable stability margin.
For critical machinery, provide the commissioning team with a documented acceptance test. This may cover repeatability, settling time, following error, peak current, stopping distance, temperature and fault response. It gives maintenance teams a useful baseline for later troubleshooting.
A well-specified servo axis protects more than motion performance. It protects production uptime, operator safety and the ability to support the machine years after commissioning. When the duty is complex or the consequences of failure are high, Tech Source can assist with application-led servo drive specification that accounts for the complete machine and site conditions.