Introduction
We often hear from engineers and product designers facing a common issue: multiple brushed DC gear motors controlled by a single switch fail to stay synchronized. One motor runs faster, another slower, and over time the gap widens. This phenomenon is known as motor desynchronization.
This article will help you understand why it happens, which applications truly require synchronized motors, and how to select the right solution.
1. Why Do DC Gear Motors Have Speed Variation?
Even identical models of brushed DC gear motors usually have ±10%–±15% speed variance. The main reasons include:
- Manufacturing tolerances – Differences in windings, magnets, or gear machining.
- Gearbox friction variations – Assembly precision and lubrication differences.
- Brush and commutation differences – Contact pressure and resistance vary.
Even with careful quality control, it is impossible to make two motors exactly the same.
2. Why Do Multiple Motors Fall Out of Sync?
Even if the motors themselves are well-matched, they can still fall out of sync in real-world applications. Common causes include:
- Load differences – Each side of the mechanism may experience different friction or weight.
- Power supply differences – Variations in wiring length or thickness can lead to unequal voltage.
- Thermal effects and aging – Motor characteristics change over time as they heat up.
- Open-loop control – Without feedback, the controller cannot adjust to actual operating conditions.
A single switch without feedback cannot guarantee motor synchronization.
3. Open-Loop vs. Closed-Loop Control
- Open-loop control: Sends commands without checking the outcome. For example, applying the same PWM signal to multiple motors. Advantages: simple and low-cost. Disadvantages: errors accumulate over time.
- Closed-loop control: Uses sensors (like encoders) to measure speed or position in real-time, compares it to the target, and adjusts outputs accordingly. Can compensate for load variations and maintain synchronization. Advantages: precise and robust. Disadvantages: higher cost and complexity.
Analogy:
- Open-loop = “Give the command and hope for the best.”
- Closed-loop = “Monitor and correct continuously.”
4. Can Motors Stay Synchronized Throughout the Entire Process?
- Limit switches: Ensure start and end positions are aligned, but motors can drift during the procedure.
- Individual closed-loop control: Each motor is more stable, but phase differences may still accumulate.
- Closed-loop with electronic synchronization (master-slave control, virtual master axis, electronic gearing, or cross-coupling):
✔️ Can maintain synchronization from start to finish, which is standard in industrial multi-axis systems.
5. Applications That Require or Don’t Require Synchronization
No synchronization needed:
- Fans or blowers
- Independent pumps
- Toy cars (differential absorbs speed differences)
- Segmented conveyor lines
Synchronization required:
- Dual-screw lift platforms
- Two-sided automatic doors
- Stage lift platforms
- Printing, packaging, or textile machines (rollers must align)
- CNC machines, 3D printers, robotic joints
Rule of thumb: If motors drive the same mechanism or must maintain relative alignment → synchronization is required.
6. Which Motor Types Are Suitable for Synchronization?
| Motor Type | Synchronization Feasibility | Requirements | Precision | Typical Applications |
|---|---|---|---|---|
| Brushed DC Motor | ✔ Possible | Encoder + closed-loop | ±1% speed / ±0.5° position | Automatic doors, conveyors |
| Brushless DC Motor | ✔ Strong | FOC + sensor | ±0.1%–0.5% | Robotics, industrial automation |
| Stepper Motor | ✔ Strong | Pulse sync or closed-loop | ±1 step | 3D printers, basic CNC |
| Servo Motor | ✔ Excellent | Built-in closed-loop | ±0.01° | CNC, robotics, printing |
| Induction Motor | △ Limited | Vector control + encoder | Near servo-level | Large conveyors, hoists |
7. How Closed-Loop Control Works
Closed-loop systems are usually layered:
- Current/torque loop (inner) – controls torque, responds fastest
- Speed loop (middle) – adjusts torque based on speed error
- Position loop (outer) – generates speed commands based on position error
Key elements:
- Sensors: Optical/absolute Encoders for high precision, Hall sensors for basic feedback
- Algorithms: PID with feedforward, filtering, anti-windup
- Communication: Real-time protocols like EtherCAT or CANopen ensure multi-motor synchronization with minimal latency
8. Practical Engineering Advice
- General precision needs: Brushed DC + encoder + MCU closed-loop, or BLDC + simple FOC
- High precision needs: Servo motors with bus synchronization
- Low budget: Open-loop with limit switches—ensures start/end alignment only
9. Conclusion
- Manufacturing tolerances make motor uniformity impossible.
- Open-loop control only guarantees synchronization at start/end points, not during motion.
- Closed-loop control with feedback significantly improves consistency.
- Full-process multi-motor synchronization requires closed-loop control + electronic coordination.
- Application requirements dictate whether synchronization is necessary, and motor type determines achievable precision and cost.
At HOTEC MOTOR, we provide customized DC gear motor solutions tailored to suit your need. Contact us to find the right motor for your project.
