Servo motor selection is not done by reading a power value from a catalog. Selection is a calculation chain that starts from the mechanical properties of the load and continues with the motion profile. When any link in this chain is skipped, the result is either an oversized and expensive system or an axis that struggles in the field, overheats and cannot hold its position.
1. Define the mechanical structure
First, how the load is coupled to the motor must be clarified. Common mechanisms:
- Ball screw: Linear motion, high accuracy.
- Belt-pulley: Long distances, lower rigidity.
- Rack-and-pinion: Long-stroke linear axes.
- Gearbox + rotary table: High torque, low speed.
- Direct drive: No transmission element, no backlash.
Each mechanism has its own transmission ratio, efficiency and backlash behavior. These are inputs to the calculation.
3. Derive the motion profile
A position-time or speed-time graph is drawn. A typical trapezoidal profile has three parts: acceleration, constant speed, deceleration. A dwell time is added to these, and one cycle is completed.
Two values that come out of this profile are decisive:
- Maximum speed (n_max): The motor speed in the constant-speed section. Calculated with the gearbox
- Acceleration (α): As the acceleration time shortens, the required torque increases.
ratio and the mechanism transmission.
Pushing the cycle time directly increases the motor size. In practice, questioning the cycle time that is actually required often makes it possible to work with a motor one size smaller.
4. Torque calculation
The required torque is the sum of three components:
Acceleration torque: T_a = (J_total) × α, the product of total inertia and angular acceleration. It is usually the largest component.
Friction torque: Losses from guides, bearings, belts and the gearbox.
Gravity / process torque: The weight of the load on vertical axes, and the process force in operations such as cutting or pressing.
These three components are summed at the moment of acceleration, and the peak torque is found. The peak torque must remain below the motor’s instantaneous maximum torque curve.
However, what is truly decisive is the RMS (effective) torque. The root-mean-square of the torques in the different sections throughout the cycle is taken and compared with the motor’s continuous torque rating. The motor must be able to sustain the RMS torque continuously; otherwise it is thermally overstressed.
5. Verification on the speed-torque curve
The calculated operating points (speed, torque) are plotted on the motor’s speed-torque curve. There are two regions: the continuous operating region and the short-term (intermittent) operating region. The constant-speed section must stay in the continuous region, and the acceleration peak point in the intermittent region.
A safety margin is added to the calculation. Friction increasing over time, changes in lubrication, mechanical aging and a drop in supply voltage consume this margin.
6. Drive and supply
The drive is selected to meet the motor’s continuous and peak current. The supply voltage determines the maximum speed the motor can reach: as the voltage drops, the usable torque at high speed decreases. Because the motor acts like a generator during deceleration and returns energy to the DC bus, a braking resistor must be calculated in applications with high inertia and frequent stops.
7. Encoder, brake and mechanical details
Encoder type: An incremental encoder requires homing at each power-up. An absolute encoder retains the position even when de-energized; multiturn types also store the number of turns. If the homing time affects the cycle time, an absolute encoder is preferred.
Brake: On vertical axes and in places where the load must not drop when power is cut, a holding brake is essential. A servo brake is for holding, not for dynamic stopping.
Protection class and shaft structure: In washdown environments the IP class, in belt drives the radial load capacity, and on precision axes the choice of a keyed/keyless shaft affect the selection.
Common mistakes
- Looking only at peak torque and not calculating RMS torque
- Using raw load inertia instead of reflected inertia
- Not accounting for gearbox efficiency and backlash
- Ignoring the need for a braking resistor
- Overlooking the effect of cable length on the encoder signal and voltage drop
overlooking
Related product families: Servo Motor, Servo Drive, Servo Set, Encoder, Servo Cable, Braking Resistor
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