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Published 2026-04-04
The output torque of aservomotor is directly proportional to its operating current. When mechanical load increases, the motor draws more current to generate the necessary torque. This relationship is fundamental toservooperation: higher torque demands always result in higher current consumption. Understanding this allows you to correctly size power supplies, prevent overheating, and ensure reliable performance in your projects.
Inside every servo, a DC motor drives a gear train. The torque produced by the motor is given by the equation:Torque = Torque Constant (Kt) × Armature Current. This linear relationship means that for a given motor, doubling the torque requires approximately double the current. However, the overall servo system—including control electronics, friction losses, and back EMF—introduces minor deviations, but the core principle remains:current rises as torque rises.
1. No-load condition– The servo shaft rotates freely without resistance. Current is minimal (typically 100–300 mA for standard hobby servos at 5V) because only friction and inertia need to be overcome.
2. Light to moderate load– As load increases, current rises gradually. For example, a common micro servo (roughly 9g size) drawing 200 mA at no load may draw 400–600 mA when holding a moderate weight (e.g., 0.5 kg·cm torque).
3. Stall condition– When the output shaft is prevented from moving, the motor tries with maximum force. Current spikes to its highest value, known as stall current. At stall, the servo produces its maximum rated torque. For a typical standard servo rated at 3–5 kg·cm at 5V, stall current can reach 1.2–2.0 A. For larger servos (15–25 kg·cm),stall current often exceeds 3–5 A.
Consider a standard-sized servo (not a specific brand) used in a robot arm joint. At rest with no load, it draws 150 mA at 5V. When the arm lifts a 200g weight at a 10 cm distance (torque = 0.2 kg × 10 cm = 2 kg·cm), the current increases to 800 mA. If the arm is blocked mid-motion, causing a stall, current jumps to 1.8 A while torque reaches the servo’s rated stall torque of 2.5 kg·cm. This pattern is consistent across all servo types: micro, standard, and high-torque models. The exact numbers vary, but the proportional relationship remains.
While the motor’s internal torque–current relationship is nearly linear, the servo’s output torque at the horn depends on:
Gear reduction ratio– Higher reduction multiplies output torque but increases reflected current demand proportionally.
Voltage– Higher supply voltage increases no-load speed and stall torque, but also raises stall current (Ohm’s law: I = V/R, motor resistance R constant). For example, a servo rated at 3 kg·cm stall torque at 4.8V may produce 4 kg·cm at 6V, but stall current will increase by roughly 25%.
Temperature– Hot windings increase resistance, slightly reducing current for the same torque, but this also reduces efficiency and risks thermal damage.
Control signal (PWM)– The servo’s internal controller attempts to hold position; under load, it drives the motor harder, increasing current.
Based on the direct torque–current relationship, follow these actions to avoid failures:
1. Always measure or look up stall current– Do not rely solely on rated torque. Multiply stall current by the number of servos active simultaneously to size your power supply. For a typical 5 kg·cm servo, expect 1.5–2.0 A stall current. For a 15 kg·cm servo, expect 3.5–5.0 A.
2. Add a safety margin to power supplies– Use a supply rated for at least 150% of the calculated peak total current. For example, two servos with 2 A stall current each (4 A total) need a 6 A supply. Insufficient current causes voltage drops, servo jitter, or resets.
3. Prevent prolonged stalling– A stalled servo draws maximum current continuously, overheating the motor and damaging gears. Implement mechanical stops or current monitoring in your control code. If a servo draws high current for more than 2–3 seconds without motion, cut power or reverse direction.
4. Use separate power for servos and logic– Servo current spikes cause voltage dips that can reset microcontrollers. Always power servos from a dedicated battery or regulator, and keep the control signal lines with a common ground but isolated power.
5. Estimate current from torque when specifications are missing– If a servo only lists stall torque (e.g., 4 kg·cm at 5V), you can approximate stall current by comparing to similar known servos. For a standard-sized servo at 5V, a reasonable rule of thumb isstall current (A) ≈ 0.4 × stall torque (kg·cm). For 4 kg·cm, that gives 1.6 A. Verify with actual measurement.
“Higher voltage reduces current for the same torque”– False. For the same mechanical output torque, higher voltage actually reduces current because the motor draws less current to produce the same torque (since torque = Kt × I, Kt is fixed). However, stall current increases with voltage because the motor can spin faster and produce higher torque before stalling. The relationship is nuanced: at a given torque below stall, higher voltage reduces current; at stall, higher voltage increases current.
“Current is constant during holding”– False. Holding torque requires continuous current. Under static load, a servo draws steady current equal to the torque demand divided by Kt. If the load is high, holding current is high. Do not assume holding current is low.
Core repeated finding: Servo output torque and operating current are directly linked—more torque requires more current. The relationship is approximately linear from no-load to stall, though gear efficiency and voltage cause minor variations. Always use stall current ratings for power supply design, avoid prolonged stalling, and measure actual currents for critical applications.
Immediate actions to take:
Check your servo’s datasheet for both “stall torque” and “stall current” (or measure stall current with a multimeter).
Ensure your power supply can deliver at least 1.5× the sum of stall currents of all servos.
Implement software or hardware current limiting to prevent overheating.
Never power servos directly from a microcontroller’s 5V pin.
By respecting the torque–current relationship, you will achieve reliable servo operation, longer component life, and successful project outcomes.
Update Time：2026-04-04
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