TECHNICAL SUPPORT
Published 2026-04-09
When aservomotor orservomechanism receives less current than its operating requirement, it cannot deliver the intended torque, speed, or positional accuracy. Current insufficiency is not a single failure but a symptom that appears under several distinct electrical, mechanical, or configuration-related conditions. This article describes the most frequent real-world scenarios whereservocurrent becomes insufficient, explains why each scenario leads to undercurrent, and provides actionable steps to diagnose and resolve the issue. All cases are based on common field observations, without referencing any specific brand or manufacturer.
Scenario:A servo system is rated for 10A continuous current, but the power supply unit (PSU) or battery pack can only deliver 6A continuously. When the servo attempts a moderate acceleration or holds a static load, the voltage drops, current limits are hit, and the servo behaves erratically.
Why it happens:The power source’s maximum output current is lower than the servo’s peak or even continuous demand. The source enters current limiting or voltage sag, effectively starving the servo.
Common evidence:
Servo moves slowly under load, stalls easily, or triggers low-voltage alarms.
Power supply feels excessively hot or cycles on/off.
Battery voltage drops sharply (e.g., from 12V to 9V) as soon as the servo moves.
Action:Calculate the servo’s peak and continuous current from its datasheet. Add 30–50% margin. Select a power source rated for at least that continuous current. For battery systems, ensure the battery’s C‑rate and capacity support the required current without sagging below the servo’s minimum operating voltage.
Scenario:A servo is placed 5 meters away from its power supply. The installer uses AWG 26 wires (very thin). Under normal operation, the servo draws 4A, but the cable resistance causes a 1.5V drop. At peak demand of 8A, the voltage at the servo terminals falls below its undervoltage threshold, causing reset or erratic motion.
Why it happens:Cable resistance (R = ρ·L/A) creates a voltage drop proportional to current. The servo receives lower voltage, and because power = voltage × current, to maintain required torque, the servo tries to draw even more current — but the cable limits it, resulting in insufficient effective current at the motor.
Common evidence:
The servo works fine when very close to the supply, but fails when moved farther.
Wires feel warm or hot to the touch.
Voltage measured at the servo’s connector is significantly lower than at the supply terminals.
Action:Measure voltage at the servo under peak load. If drop exceeds 5% of nominal voltage, upgrade to thicker wire (lower AWG number). Keep cable runs as short as practical. For long distances, use a local capacitor bank or move the power supply closer to the servo.
Scenario:After months of vibration, a screw terminal on the power distribution board becomes slightly loose. Contact resistance increases from 0.01Ω to 0.5Ω. At 5A, that loose connection drops 2.5V and dissipates 12.5W of heat. The servo experiences intermittent current starvation, especially during peak demands.
Why it happens:High-resistance connections limit current flow and produce heat. The servo’s drive sees fluctuating supply voltage and cannot deliver the commanded current. In many cases, the drive’s current regulation loop saturates trying to compensate, but the physical current remains insufficient.
Common evidence:
Intermittent servo twitching, unexpected stops, or “stall” faults without mechanical binding.
Discolored, melted, or charred connectors/terminals.
The problem temporarily improves if you wiggle the wires or reseat connectors.
Action: Inspect all power connections from the supply to the servo drive and from the drive to the servo motor. Torque screw terminals to manufacturer specification. Clean any corrosion with contact cleaner. Apply dielectric grease for vibration-prone environments. Re-crimp or replace damaged connectors.
Scenario: A switching power supply is rated for 15A, but its transient response is poor. When the servo demands a sudden 12A step (e.g., quick reversal), the supply’s output dips to 8V for 50ms before recovering. The servo’s drive detects undervoltage and shuts down or loses position.
Why it happens: Servos draw highly dynamic currents. A supply with slow control loop or insufficient output capacitance cannot maintain voltage during fast load steps. The instantaneous current delivered is limited by the supply’s ability to sustain voltage, leading to functional insufficiency even though average current rating seems adequate.
Common evidence:
The servo fails only during rapid acceleration/deceleration or direction changes.
With slow, smooth moves, the system works perfectly.
An oscilloscope shows voltage drops of >20% during load steps.
Action: Use a power supply designed for servo or motor drive applications — these specify transient response and have larger output capacitance. Add a low‑ESR electrolytic capacitor bank (e.g., 2000–5000µF per 10A) close to the servo drive to provide local energy during transients.
Scenario: A servo drive has configurable peak and continuous current limits. The installer inadvertently sets the continuous limit to 3A even though the motor is rated for 8A. Under moderate load, the drive artificially restricts current to 3A, causing the motor to lose torque and stall.
Why it happens: The drive’s software or DIP‑switch settings impose a lower current ceiling than the motor and power supply can provide. This is a logical, not electrical, insufficiency — the hardware is capable, but the configuration starves the servo.
Common evidence:
The servo never draws more than the configured limit (measured with a clamp meter).
No voltage drop or heating of cables/power supply.
The problem disappears when the current limit is raised to the motor’s rated value.
Action: Verify all current‑related parameters: peak current limit, continuous current limit, torque limit, and any “power reduction” or “economy” modes. Set them according to the motor and load requirements. Ensure the drive’s firmware is not in a derated test mode.
Scenario: A servo motor has been overheated multiple times. One of the three phase windings develops a partial short (e.g., inter‑turn short). The drive attempts to push current, but the shorted winding draws excessive current locally without producing proportional torque. The drive’s overcurrent protection triggers or the effective torque‑producing current becomes insufficient.
Why it happens: Damaged windings change the motor’s electrical characteristics. Some current is wasted as heat rather than torque. The drive may limit total current to protect itself, leaving insufficient healthy current for the load.
Common evidence:
The motor runs hot even at no load.
Current on each phase is unbalanced (e.g., 2A, 2A, 5A) when measured with a clamp meter.
The servo makes unusual buzzing or growling noises.
Resistance between phases differs by more than 10%.
Action: Disconnect the motor and measure phase‑to‑phase resistance with a milliohmmeter. All three readings should match within 10%. Check insulation resistance to ground (should be >10MΩ). If winding damage is confirmed, replace the motor.
Scenario: A servo rated for 2Nm continuous torque is coupled to a mechanism that requires 3Nm to move at the required speed. The drive tries to supply the necessary current (say 10A) but the power supply is only 8A capable. The servo stalls, and the drive reports current insufficiency or overload.
Why it happens: The load demand exceeds the system’s combined capability (motor + drive + supply). The current required for the commanded torque is physically higher than what the power source or drive can deliver. This is a sizing, not a fault, issue.
Common evidence:
The servo can move the load very slowly but fails at required speed/acceleration.
The power supply voltage dips severely under load.
The drive hits its software current limit before the load moves.
Action: Re‑evaluate the load torque‑speed curve. Measure actual current during operation. If current exceeds the motor’s continuous rating for more than a few seconds, either reduce the load (friction, inertia, duty cycle) or upgrade to a larger servo system (motor + drive + supply).
Scenario: A single 30A power supply runs three servos on a shared DC bus. When two servos accelerate simultaneously,the instantaneous current demand exceeds the supply’s peak capability. Without a local capacitor bank, the bus voltage collapses, and all three servos experience current insufficiency.
Why it happens: Servo drives reflect pulsed current draw. A power supply alone cannot smooth these pulses; it relies on bulk capacitance. Without adequate capacitance, the supply’s current limitation is constantly triggered, resulting in insufficient delivered current during peaks.
Common evidence:
Multiple servos work fine one at a time but fail when moving together.
Voltage measured on the DC bus shows spikes and deep troughs.
Adding a large capacitor bank (e.g., 10,000µF) solves the problem.
Action: Install a low‑inductance capacitor bank (electrolytic + film capacitors) as close as possible to the servo drives. Typical rule: 1000–2000µF per 10A of peak current. For high‑performance systems, use a DC link module designed for multi‑axis applications.
Servo current insufficiency is rarely caused by the servo motor alone. In over 80% of field cases, the root cause is one of the following: an undersized power supply, long/thin cables, loose connections, poor transient response, misconfigured current limits, or missing decoupling capacitance. Only a small fraction stems from actual motor winding damage.
To resolve servo current insufficiency:
1. Measure – Use a clamp meter (DC, with peak‑hold or oscilloscope function) to record actual current at the servo under the failing condition. Compare with the motor’s datasheet rating.
2. Check voltages – Measure voltage at the servo terminals during peak load. A drop >5% points to cables or connections.
3. Inspect – Visually and thermally check all power connectors, terminals, and cables. Tighten or replace as needed.
4. Verify configuration – Review all current limits in the servo drive. Ensure they match the motor and load.
5. Add capacitance – If voltage sags but cables are adequate, install a capacitor bank near the drive.
6. Test power supply – If all else fails, replace the power supply with one rated for at least 150% of the servo’s peak current, with fast transient response.
Following this systematic approach will eliminate current insufficiency in the vast majority of servo applications, ensuring reliable torque, speed, and positional accuracy.
Update Time：2026-04-09
Contact Kpower's product specialist to recommend suitable motor or gearbox for your product.
