How to Select a Servo Drive: 8 Engineering Checks

1. Define the motion profile

Start with the axis motion, not a motor power rating. Record travel, target speed, acceleration and deceleration times, dwell time and cycles per hour. On a vertical axis, gravity must also be treated as a continuous load torque.

This profile lets you check peak torque at the worst acceleration point and continuous thermal loading through RMS torque over the complete cycle.

2. Verify the torque-speed envelope

Calculate load, friction and acceleration torque for the same time segments. Peak torque must remain inside the short-duration limit of the drive-motor pair, while RMS torque must remain inside its continuous operating curve. Matching only the catalogue kW rating does not replace these checks.

• Use the worst-case supply voltage and mains tolerance.
• Check maximum speed on the loaded torque-speed curve.
• Calculate regenerative deceleration energy separately.

3. Keep an approved motor-drive pairing

The drive current must cover the motor continuous and peak requirements, and the feedback type and resolution must match an interface supported by the drive. Two motors with the same power can have different current, inertia and encoder requirements, so do not infer compatibility from power alone.

4. Match the supply and cabinet conditions

Single- or three-phase supply, nominal voltage, short-circuit capacity, earthing arrangement and cabinet temperature are part of the selection. The drive rating must remain sufficient at the real cabinet temperature and mounting clearance, not only on a room-temperature bench.

5. List feedback and I/O requirements

State the need for incremental or absolute feedback, brake output, limit and home sensors, probe input and an external encoder at the start of the project. If absolute position is required, also assess homing behaviour after power loss and battery maintenance.

6. Select communication for the machine architecture

EtherCAT is a strong candidate for coordinated multi-axis motion and tight synchronisation, CANopen for more modest cycle and data requirements, and Modbus RTU for simple command and monitoring. Before choosing a protocol name, verify controller master support, cycle time, axis count and commissioning tools.

7. Do not leave safety and regeneration until later

The machine risk assessment determines whether STO or more advanced safety functions are required. On vertical and high-inertia axes, a mechanical holding brake, dynamic braking and a regenerative resistor perform different jobs and must not be assumed to replace one another.

8. Validate the selection with the real cycle

The final choice must be checked against the manufacturer pairing table, torque-speed curve, derating conditions, braking calculation and cable-length limits. During commissioning, compare measured torque, speed and DC-bus data with the calculation; if they differ, validate the mechanical model before simply increasing drive size.