Servo Motors Selection Guide
A servo motor is a motor whose position, speed and torque are precisely controlled through encoder feedback and a servo drive. It is selected by torque, speed and drive compatibility.
What is a servo motor? A servo motor is a motor whose position, speed and torque are precisely controlled by working together with a feedback element on it (an encoder) and a servo drive. The drive brings the motor to the commanded position and speed based on the information it receives from the encoder. It is used in CNC machines, robots, packaging machines and positioning axes; it is not connected directly to the mains and requires a compatible servo drive.
This guide explains what to look at when choosing products in the Servo Motors category.
What to look at when choosing a servo motor
- Torque and speed: Rated torque is the torque the motor can deliver continuously, while peak torque is the torque it can deliver for a short time during acceleration. Power, torque and speed are related; at the same power, a lower-speed motor delivers higher torque. For example, the 1 kW Delta motors in the category are listed with a rated torque of 3.18 Nm at 3000 rpm and 4.77 Nm at 2000 rpm. Compare the continuous torque the load requires over its speed profile with the rated torque, and the torque it requires during acceleration with the peak torque.
- Inertia matching: As the load inertia reflected to the motor shaft grows relative to the rotor inertia, tuning the axis becomes harder. For this reason manufacturers offer low-, medium- and high-inertia motor series; do the sizing and the permitted ratio according to the manufacturer data.
- Flange and shaft: The flange size and shaft diameter must match the gearbox or coupling. The category has flange sizes such as 40, 60, 80, 100, 130 and 180 mm; for Delta, GMT and Isıso products, the flange and shaft diameter are written in the product name or data.
- Brake: On vertical axes, a model with a brake is chosen to prevent the load from slipping when the motor is deenergized; the conditions of use of the brake are defined in the manufacturer data.
- Encoder: Product names include resolutions of 17, 20, 22 and 23 bits and incremental and absolute types. Choose according to whether the axis can take a reference at each power-up and which encoder type the drive supports.
- Supply and drive: The "compatible system supply" in the data of GMT products (single-phase 220 V AC or three-phase 380 V AC) is the mains supply of the drive the motor works with. Match the motor with a servo drive from the same series; to buy the motor and drive as a single item, see the Servo Kits category.
- Protection rating: IP54, IP65 and IP67 values appear in the product data. The first digit, 5, indicates a dust-protected housing and 6 a dust-tight one; the second digit indicates protection against splashing water for 4, water jets for 5 and immersion in water for 7. Check in the manufacturer data whether the value covers the shaft exit.
How to read the codes in the product name
- Delta ECMA and ECM-E3: Of the four digits that follow the series letters and the encoder digit, the first two indicate the flange and the last two the power. A motor with the code ECMA-C20604 has a 60 mm flange and 400 W, and a motor with the code ECMA-E21315 has a 130 mm flange and 1.5 kW. In the codes in this category, an R after the digits is a motor without a brake and an S is a motor with a brake. The speed and encoder information is written out in the product name.
- GMT: In the code S2M060F0400ADB, 060 indicates the flange size and 0400 the power (W); in the letters after the power, B is the model with a brake and X the model without one (ADB and ADX, FDB and FDX, BH and XH).
- Schneider Electric BCH16 and BCH18: The product name contains the power, the housing code (such as H60-14 or M80) and the speed or encoder information (23bit ABS). For products with BSH and BMI codes and some BMH products, the name gives only the series; for power and torque, refer to the manufacturer's data sheet.
Difference between a servo motor and a stepper motor
A stepper motor is driven without feedback in most applications; if it loses steps under load, the controller does not notice and a position error occurs. In a servo system, the drive continuously compares the actual position it reads from the encoder with the command and corrects the deviation. If simpler positioning is enough, see the Stepper Motor Systems category; for pump, fan and conveyor drives where only speed adjustment is needed, see the variable frequency drives used with induction motors.
Brands in this category
- Schneider Electric: servo motors with BCH16, BCH18, BSH, BMH and BMI codes; also power, brake and encoder connector kits for BCH16 motors.
- Delta Electronics: ECMA motors listed together with the ASDA-A2 series, and the ECM-E3 motors of the ASDA-E3 series.
- GMT: the S0M series with a 17-bit encoder and the S2M series with a 23-bit absolute encoder; models with and without a brake.
Isıso is listed with servo motors coded ASMS, ASMH and ASMG.
Order notes
- Products are sold per piece.
- Most Schneider Electric servo motors and the Isıso models are listed as made-to-order items; for these products, the delivery time is confirmed before the order.
- Ready-made motor and encoder cables are in the Servo Motor Cables category; confirm before ordering whether the cables and connectors come with the motor.
- You can reach the product directly by typing the manufacturer code (such as ECMA-C20604RS or S2M060F0400ADB) into the search.
Servo Motor and Drive Selection Comparison Table
| Servo Capacity Class | Power and Rated Torque Range | Inertia and Brake Option | Typical Industrial Application | Category Link |
|---|---|---|---|---|
| Low-Power / High-Speed Servo Sets | 100 W – 400 W · 0.32 – 1.27 Nm | Low inertia · 3000 rpm · With or without 24V DC holding brake | Labeling, pick-and-place, compact Cartesian and packaging axes | Servo Motors |
| Medium-Power Industrial Servo Motors | 750 W – 1.5 kW · 2.39 – 7.16 Nm | Low / Medium inertia · 2000–3000 rpm · Braked for vertical axes | CNC feed axes, flying shears, filling machines and vertical Z-axes | Servo Motors |
| High-Torque Heavy-Duty Servo Motors | 2.0 kW – 7.5+ kW · 9.55 – 47+ Nm | Medium / High inertia · 1500–2000 rpm · Absolute/incremental encoder | Press feeders, heavy indexing conveyors, bending and textile machinery | Motor and Drive Systems |
Frequently asked questions
How is the torque of a servo motor calculated from power?
The relationship between power, torque and speed is P = M × 2π × n / 60; P is in watts, M in Nm and n in rpm. To find the torque, you divide the power by 2π × n / 60: a 400 W, 3000 rpm motor gives a rated torque of about 1.27 Nm, and a 750 W, 3000 rpm motor about 2.39 Nm. This is the continuous torque; compare the peak torque needed during acceleration separately from the manufacturer data.
When is a servo motor with a brake needed?
A model with a brake is chosen on axes where the load can move under its own weight while the motor is deenergized, for example vertical axes and Z axes with a lead screw drive. The brake on the motor is a holding brake that engages when its power is cut and keeps the stationary shaft in place. Slowing the axis down is done by the drive. For the brake supply and its control with the drive, follow the connection conditions in the manufacturer data.
What is the difference between a 17-bit and a 23-bit encoder?
The bit value indicates how many positions the encoder divides one shaft revolution into; each additional bit doubles this number. Therefore a 23-bit encoder resolves one revolution 64 times more finely than a 17-bit encoder. High resolution gives smoother rotation at low speeds; the positioning accuracy of the axis is also determined by the mechanical transmission. The motor's encoder must be of a type the drive supports; check the compatible encoder information in the drive's product data.
Is homing needed with an absolute-encoder servo motor?
An absolute encoder reports the shaft position the moment power is applied, while an axis with an incremental encoder is sent to a reference point at each power-up. The GMT S2M series in the category has a 23-bit absolute encoder; the manufacturer states that in this series the position information is retained even when the system is shut down and no referencing is needed. There are also encoders that use a battery to store the number of turns while deenergized; for other series, confirm from the manufacturer data how the position is stored.
Can a servo motor be used on a single-phase 220 V network?
A servo motor is connected not to the mains but to the servo drive; the mains voltage is applied to the drive's input. In the data of the GMT products in this category, the "compatible system supply" field shows whether the drive the motor is paired with is supplied with single-phase 220 V AC or three-phase 380 V AC. Small-power models are mostly in single-phase systems and large-power models in three-phase systems; rely on the value on the product page.