Glossary

The voltage the motor is designed to run at. Terminal voltage for DC motors, line voltage for AC motors; stepper datasheets often give the phase voltage (= phase current × phase resistance).

Sizing tip: The actual drive voltage of a stepper is usually far higher than its phase voltage; the driver limits current by chopping. Higher voltage gives better high-speed torque.

Mechanical output power available continuously at rated voltage and speed, P = T · ω.

The torque the motor can deliver continuously at rated conditions, limited by temperature rise. Handy conversion: T (N·m) = 9550 × P (kW) / n (rpm).

Speed at rated torque.

Input current at rated conditions — RMS line current for AC motors, current per phase for steppers.

Output torque of a DC motor at zero speed, Tₛ ≈ Kt × U / R. Stall current is very high, so it can only be sustained briefly.

The maximum torque available for short periods. Typically about 3× rated for servo motors; for induction motors it is the maximum (breakdown) torque on the characteristic curve.

Sizing tip: When sizing, acceleration torque must stay below peak torque, and the RMS torque must stay below rated torque.

The maximum external torque a stepper can withstand at standstill with rated current, without slipping. The most common stepper specification.

Speed with no load; for DC motors n₀ ≈ U / Ke.

Speed of the rotating stator field of an AC motor, nₛ = 120f / p (f = frequency, p = number of poles). An induction motor runs slightly below it; the difference ratio is the slip s.

The speed on a stepper's torque–speed curve above which torque drops noticeably, because winding current can no longer reach its set value within each step.

Sizing tip: Values on this site are rough estimates from n ≈ U / (2·L·I), for comparison only.

Angle moved per input pulse. 2-phase hybrid steppers are commonly 1.8° (200 steps/rev) or 0.9° (400 steps/rev).

Torque per ampere, T = Kt × I. In SI units it equals the back-EMF constant Ke (V·s/rad).

Voltage induced per 1000 rpm. Ke (V/krpm) = Kt (N·m/A) × 104.72. The hobby-motor KV value (rpm/V) = 1000 / Ke.

DC resistance of the winding — usually line-to-line for BLDC/servo, per phase for steppers. It determines copper loss I²R and stall current.

Limits how fast current builds: higher inductance means torque drops earlier at high speed.

Electrical τₑ = L / R (current response); mechanical τₘ = R·J / Kt² (time to reach 63% of no-load speed from standstill, unloaded).

Number of rotor magnetic poles (pole pairs × 2). Sets the ratio of electrical to mechanical frequency, and the synchronous speed of induction motors.

Ratio of real to apparent power. AC motor input current I = P / (√3 · U · η · cos φ).

Ratio of mechanical output to electrical input. DC-motor efficiency varies with load; peak efficiency is usually around 10–20% of stall torque.

Motor efficiency classes defined by IEC 60034-30-1: IE1 standard, IE2 high, IE3 premium, IE4 super premium. China's equivalent is GB 18613.

Resistance of the rotor to speed changes. 1 kg·m² = 10⁴ kg·cm² = 10⁷ g·cm².

Sizing tip: For servo systems keep load / motor inertia ≤ 10 (≤ 5 for high-response applications).

Maximum permissible winding insulation temperature: class B 130 °C, F 155 °C, H 180 °C.

Enclosure protection per IEC 60529. First digit: solids (0–6); second digit: water (0–8). IP65 = dust-tight and protected against water jets.

The mounting-dimension standard. Steppers commonly use NEMA (NEMA 17 = 42 mm square flange); induction motors use IEC frame numbers, i.e. shaft-centre height in mm.