Servo Drive Selection and Sizing: Power, Regeneration and Cable

Key Takeaway

Servo drive selection follows four numbers: continuous and peak current, supply voltage, and regeneration. Size for continuous current first, verify peak current for acceleration, and match the regeneration resistor to the deceleration energy.

Figure 1: Servo drive sizing – work backwards from the load

1. Load: Force from the Process
2. Motor: Torque and Speed
3. Amplifier: Current and Regeneration
4. Cable: Voltage Drop and Shielding
5. Selection Summary
Related Reading
Sources

1. Load: Force from the Process

What the load needs

The drive is sized for the load, not the motor. The motor is sized for the load, and the drive is sized for the motor. Work backwards from the process.

Process force

Determine the force the process requires at the point of work: a press force, a web tension, a cut force, a part acceleration force. This force is transmitted through the drive train (screw, belt, rack) to the motor shaft as torque.

torque_at_shaft = force × radius_or_lead

Friction and gravity

Add friction force (sliding, rolling, seals) and gravity force (for vertical axes, the full weight of the moving mass). These are always present, even during constant-speed motion.

Duty cycle and RMS

The drive must handle the RMS current over the duty cycle, not just the peak. A move profile with 20 percent acceleration, 60 percent cruise, and 20 percent deceleration has a different RMS current than one with continuous acceleration.

2. Motor: Torque and Speed

Rated and peak torque

The motor must produce rated torque continuously and peak torque for acceleration and deceleration. Peak torque is typically 2 to 3 times rated torque and is available for a limited time (usually a few seconds).

Speed needed at the shaft

The motor speed is determined by the required load speed and the drive train ratio:

motor_rpm = load_rpm × gear_ratio

For a belt-driven axis with a 20 mm pulley moving at 1 m/s:

pulley_rpm = 1 m/s / (π × 0.020 m) × 60 = 955 rpm

Through a 3:1 gearbox, motor rpm = 955 × 3 = 2865 rpm.

Motor inertia

The motor rotor inertia must be small enough compared to the reflected load inertia for stable tuning. The inertia ratio (load referred to motor / motor rotor) should be below 10:1 for general axes and below 5:1 for high-acceleration axes.

3. Amplifier: Current and Regeneration

Continuous and peak current

The amplifier (drive) must supply continuous current for the RMS load and peak current for acceleration and deceleration. Peak current is typically 2 to 3 times continuous current and is available for a limited time.

motor_torque = torque_constant × current

For a motor with torque constant 0.8 N·m/A needing 2 N·m peak torque:

peak_current = 2 N·m / 0.8 N·m/A = 2.5 A

Regeneration duty

During deceleration, the motor acts as a generator and pumps energy back into the drive DC bus. If the bus voltage rises too high, the drive faults on overvoltage. A regeneration resistor dissipates this energy as heat.

Regeneration power depends on the deceleration energy:

regen_power = 0.5 × J_total × ω² / decel_time

For a 0.01 kg m² total inertia decelerating from 3000 rpm (314 rad/s) in 0.2 s:

regen_power = 0.5 × 0.01 × 314² / 0.2 = 2467 W

This is peak regen power; the average over the duty cycle is much lower.

Single or three phase

Single-phase drives are common up to about 1-2 kW. Three-phase drives are used for higher power. Match the drive input to the available supply.

Brake resistor sizing

The brake resistor must dissipate the peak regen power without overheating. Resistor wattage rating should exceed the average regen power over the duty cycle, and the resistance value must match the drive specification (too low draws excessive current, too high cannot dissipate enough power).

4. Cable: Voltage Drop and Shielding

Voltage drop and length

Motor power cable voltage drop reduces the torque available at the motor. Cable voltage drop:

V_drop = 2 × I × L × resistance_per_meter

Keep voltage drop under 3 percent of the motor voltage. For long cable runs, use larger gauge cable or higher voltage.

Shielded motor power cable

Motor power cables carry fast-switching PWM current that radiates noise. Use shielded motor cable with the shield bonded to the drive and motor at both ends. Do not run motor cable next to signal or encoder cable.

Encoder feedback cable

Encoder cable carries low-level signals and must be shielded twisted pair. Run encoder cable separate from motor power cable. If they must cross, cross at 90 degrees.

Separation from VFD output

Keep servo motor cable separate from VFD (variable frequency drive) output cable. VFDs generate high-frequency noise that can couple into servo motor cable and cause encoder errors or current loop instability.

5. Selection Summary

Work the decision in this order:

Load.Determine process force, friction, gravity, and duty cycle.
Motor.Select motor torque (rated and peak) and speed for the load.
Amplifier.Size continuous and peak current for the motor; size regeneration resistor for deceleration energy.
Cable.Size cable for voltage drop; use shielded motor cable; separate from VFD output.

The servo drive is the bridge between the control signal and the motor. Size it for the actual duty cycle, not the nameplate, and it will run cool and reliable.

Sources

Key Takeaways

Size for continuous current first, verify peak current for acceleration and deceleration.
Regeneration resistor must dissipate deceleration energy; size for average power over the duty cycle, not peak.
Motor cable voltage drop under 3 percent; use shielded motor cable; separate from VFD output.
Continuous current rating, not peak, determines thermal loading of the drive.
Match drive input voltage and phase to available supply; single-phase up to ~1-2 kW, three-phase above.