What should you check when buying a stepper motor and driver?

Choosing a stepper motor and driver involves more than matching numbers on two product labels. Assess the load, speed, acceleration, mechanical transmission and control signals together. A suitable combination must sustain the required motion under the actual load, rather than simply turn on a test bench.

Understand both the motor’s operating principle and how the driver works. The motor produces movement, the driver regulates winding currents, and the controller issues motion commands. A driver with STEP/DIR inputs does not necessarily generate its own motion program.

Define the application first

Before ordering, specify the moving mass, travel distance, operating speed, acceleration time and daily duty cycle. A horizontal slide, vertical lift, rotary table and material feeder impose different loads. Typical applications provide context, but the axis requirements determine the motor choice.

Screw lead, pulley diameter and reduction ratio change the motor speed and torque required. With gears and transmissions, include efficiency, backlash and reflected inertia. A larger motor is not automatically a better choice: rotor inertia, driver-current requirements, heat and cost may also increase.

Distinguish holding torque from running torque

Holding torque is measured with the motor stationary and energised at the specified current. It does not show that the motor can produce the same torque at high speed. Check the torque–speed curve and establish which driver, supply voltage, current setting and winding connection were used to produce it.

Include friction and acceleration torque alongside the load torque, allowing a suitable operating margin for load variation. Speed and position control require appropriate acceleration and deceleration ramps. A motor can run successfully without a load yet miss steps on the actual machine.

Motor construction, phase count and wiring

Stepper motor types include permanent-magnet, variable-reluctance and hybrid constructions. Bipolar and unipolar describe winding arrangements and how current is applied. Listing these as equivalent construction categories is misleading. Step angle is another, separate specification.

Common two-phase motors may have 4, 6 or 8 leads. Four-lead motors are usually wired for bipolar operation; six-lead motors have centre taps, while eight-lead motors expose separate winding sections. Use the manufacturer’s wiring diagram to determine supported series, parallel or half-coil connections. Changing the connection can change current requirements and inductance. Wire colours are not universal across manufacturers. A motor with a different phase count cannot be paired with a two-phase driver merely because its current rating appears suitable.

Phase current and the driver’s Peak/RMS ratings

Check the motor’s rated current per phase for the selected winding connection. The driver’s adjustable output-current range must suit that motor. Its headline maximum current is insufficient: establish whether the setting table specifies peak current or RMS current.

The same number does not represent the same setting under different current definitions. Peak and RMS values differ for sinusoidal current; follow the manufacturer’s method when translating a motor rating into a driver setting. Excessive current increases thermal-management demands, while insufficient current can reduce available torque. Assess standstill-current reduction against the holding force the axis needs.

Supply voltage and power-supply compatibility

The motor’s listed winding voltage and the driver’s supply-input voltage are different specifications. Suitable current-regulated drivers can use a higher DC supply while limiting winding current. Select the supply voltage using the driver’s permitted range, motor characteristics and torque requirements at the intended speed.

Increasing voltage does not automatically increase motor speed; the motion command determines speed. A higher supply can help current build at higher speeds in a suitable system. Do not size the power supply by blindly adding motor phase currents: follow the driver manufacturer’s sizing method and consider axes operating simultaneously. Check the manual’s provisions for supply-voltage rise during deceleration or when the load drives the motor. Treat the maximum voltage rating as a limit, not a target operating value.

Microstepping, resolution and controller pulse capacity

Microstepping can smooth motion and reduce vibration. However, more microsteps do not guarantee a proportional increase in positioning accuracy. Assess resolution, actual accuracy and repeatability separately; backlash, friction and deflection under load affect the result.

Example: A 1.8° motor has 200 full steps per revolution. With the external command setting at 1/16 microstepping, it needs 3,200 pulses per revolution. At 600 rpm, the required frequency is 3,200 × 600 / 60 = 32,000 Hz, or 32 kHz. A 1/32 setting requires 64 kHz at the same speed. This assumes no electronic gearing or other input scaling.

If the PLC’s selected high-speed pulse channel supports only 20 kHz, the theoretical command limit at 1/16 is 375 rpm. Whether the loaded motor can achieve that speed is a separate check. If the driver provides internal microstep interpolation, distinguish its internal subdivision from the number of external command pulses.

Control inputs and feedback

Check the voltage levels, pulse width, direction-signal timing and wiring for STEP/DIR, enable and other inputs. A 5 V input is not directly compatible with a 24 V PLC output on every driver. Assess NPN/PNP wiring logic, common connections and differential-input options against the manual’s diagram. A relay output is not a substitute for a high-speed pulse output.

An open-loop system does not independently verify that the intended position was reached. For closed-loop operation, the motor’s encoder must match the driver’s feedback interface; an arbitrary encoder-equipped motor will not suit every closed-loop driver. Applications requiring direct load-position measurement may need a linear encoder or another separate measuring device. Compare stepper and servo systems using speed, load variation and fault-monitoring requirements.

Mechanical dimensions, environment and protection

Frame designations such as NEMA 17 or NEMA 23 do not, by themselves, specify torque or electrical compatibility. Check the mounting drawing, hole spacing, shaft diameter and length, coupling fit and permitted shaft loads. For vertical axes, holding torque does not replace a suitable safety brake; load behaviour following power loss needs separate design.

Driver operating temperature, enclosure ventilation, dust and moisture matter. Check for overcurrent, overtemperature and overvoltage protection, and establish how the alarm output connects to the control system. Define separate roles for limit switches, homing sensors and the stopping system. An enable input is not a safety function unless the manufacturer has validated it for that purpose.

Checks for new and used equipment

The model number, identification label, wiring diagram and operating manual should be clearly identifiable. On a used motor, inspect the shaft, bearings, leads and connectors; on a driver, inspect terminals, fans and enclosure damage. Maintenance history and available test information are useful. “Working” does not establish adequate torque or full compatibility with your application.

When comparing motor and driver prices, include the power supply, coupling, cables, connectors, cooling and any required controller. Read the sales terms for warranty, returns, product condition and technical-support scope; do not assume the sale includes application design or commissioning. Complete the assessment with an understanding of common faults and the system’s advantages and limitations.