1. Definition of Hollow-Cup Motor
A Hollow-Cup Motor is a type of micro DC motor with a rotor structured in a hollow cup shape. Its rotor is hollow inside, and the coil is wound in the air gap between the permanent magnet and the housing, forming a magnetic circuit that is coreless and slotless.
By eliminating the eddy current loss of traditional iron cores, the rotor inertia is significantly reduced, enabling the motor to achieve operational characteristics of high speed, low noise, and high efficiency.
Hollow-cup motors are widely used in scenarios requiring high responsiveness and lightweight design, such as robotics, UAVs (Unmanned Aerial Vehicles), and precision instruments.
2. Working Principle - Lorentz Force and Electronic Commutation
2.1 Basic Principle
When the stator coil is energized, a rotating magnetic field is generated inside the stator.
The permanent magnet provides a constant magnetic field, which interacts with the stator magnetic field to generate Lorentz force, driving the rotor to rotate.
The electronic commutator (or brushes) switches the current direction in real-time according to the rotor position to maintain continuous torque output.
The core working principle of the hollow-cup motor is still the interaction between magnetic field and current. However, due to the hollow-cup rotor, the magnetic circuit is more compact with lower loss.
2.2 Brushed vs. Brushless Commutation
| Commutation Type | Structural Features | Key Advantages | Application Scenarios |
|---|---|---|---|
| Brushed | Carbon brushes + commutator | Low cost, simple drive circuit | Low power, one-time experiments |
| Brushless | Hall sensor or back-EMF commutation | Long lifespan, no sparking, lower noise | High reliability scenarios (aerospace, medical) |
3. Structural Analysis - Function of Each Component
The following schematic diagrams show the structure of a typical brushed hollow-cup motor (left) and brushless hollow-cup motor (right). The key components are numbered in the diagrams for correspondence in the following description.
The components marked in the diagrams, including ① Magnet (Permanent Magnet), ② Hollow-Cup Coil, ③ Commutator, ④ Brush Holder, ⑤ Housing, and ⑥ Ball Bearing, all exist in actual products.
3.1 Function of Key Components
| Number | Component | Function |
|---|---|---|
| 1 | Permanent Magnet | Provides a constant magnetic field, determining the maximum torque and power density of the motor |
| 2 | Hollow-Cup Coil | Wound on the wall of the hollow cup, generates alternating magnetic field and interacts with the permanent magnet |
| 3 | Commutator / Hall Sensor | Switches current direction in real-time to ensure continuous rotation of the rotor |
| 4 | Carbon Brushes (Brushed Type) | Conducts current to the commutator (only for brushed motors) |
| 5 | Housing & Flange | Mechanical support, magnetic circuit closure, and heat dissipation |
| 6 | Ball Bearing | Reduces friction, improves rotation speed and service life |
| 7 | Sealing Ring | Dust and moisture prevention, improves reliability |
4. Key Technical Advantages
| Advantage | Description | Typical Value |
|---|---|---|
| High Power Density | Output power can reach 2-3 times that of motors with the same volume | Above 30 W/cc |
| Low Torque Ripple | Mechanical time constant < 28 ms, some models < 10 ms | - |
| Low Noise & Vibration | Slotless structure and low rotor inertia result in low noise | Noise < 30 dB |
| High Efficiency | Efficiency ranges from 70% to 90%, some products > 90% | 70% - 90% (> 90% for some products) |
| Fast Response | Acceleration time < 5 ms, enabling high-acceleration motion | - |
| Long Lifespan (Brushless) | Operation time up to 20,000 hours or more; brushed type approx. 2,000 hours | Brushless: > 20,000 h; Brushed: ~ 2,000 h |
5. Design and Selection Key Points
Power/Speed Requirement: Select appropriate rated power and speed range based on load torque and maximum speed.
Drive Method: Brushed type is suitable for low-cost and low-power applications; brushless type is suitable for scenarios requiring high reliability and long lifespan.
Size Limitation: The diameter of hollow-cup motors is usually 10 mm - 40 mm; select according to assembly space.
Heat Dissipation Solution: For high-power applications, consider housing heat sinks or forced air cooling.
Control Precision: If closed-loop position control is required, it is recommended to select models with Hall sensors or optical encoders.
Comprehensive evaluation should be conducted based on three key indicators: power density, response speed, and lifespan during selection.
6. Typical Application Scenarios
6.1 Robotics and Flexible Grippers
Humanoid Robot Dexterous Hands: The high torque density and fast response of hollow-cup motors enable the fingers to achieve micron-level position control and high-speed grasping.
Flexible Joints: Embedding hollow-cup motors in robot joints enables backlash-free and low-inertia motion control.
Relevant cases can be seen in the brushless hollow-cup motor structure diagram used in Beihang University's BH-985 dexterous hand.
6.2 UAVs and Model Aircraft
High-Speed Rotor Drive: The lightweight feature of hollow-cup motors significantly reduces the inertia of the aircraft, improving climb rate and maneuverability.
Attitude Control: Using hollow-cup motors in the attitude loop of quadrotors enables millisecond-level response and improves wind resistance.
6.3 Medical and Precision Instruments
Miniature Pumps and Syringes: The low noise and high-precision positioning of hollow-cup motors are suitable for infusion pumps and blood circulation pumps.
Optical Focusing Mechanisms: In microscopes and optical scanners, hollow-cup motors can achieve sub-micron-level focusing.
In 2022, the artificial blood pump developed by the University of Shanghai for Science and Technology adopted a dedicated hollow-cup motor, achieving high torque and low power consumption blood drive.
6.4 Aerospace
Attitude control gyros and miniature thrusters: The high power density and low electromagnetic interference (brushless type) of hollow-cup motors meet the harsh aerospace environment requirements.
7. Market Status and Development Trends
7.1 Market Scale
In 2024, the domestic hollow-cup motor market scale exceeded 10 billion RMB, with a compound annual growth rate of approximately 15%. The main driving forces come from three key segments: robotics, UAVs, and medical care.
7.2 Future Technical Trends
| Trend | Key Technology | Expected Impact |
|---|---|---|
| High Power Density | Permanent magnet materials with higher magnetic flux density (NdFeB) | Single-unit power exceeds 100 W |
| Intelligent Drive | Integrated MCU + sensor closed-loop control | Realizes adaptive speed regulation and fault diagnosis |
| Modular Integration | Hollow-cup motor + planetary reduction gearbox | Directly outputs low-speed and high-torque, reducing system complexity |
| Lightweight Materials | Aluminum alloy, carbon fiber housing | Further improves power-to-weight ratio |
| Environmental-Friendly Manufacturing | Lead-free soldering, low-energy consumption processes | Complies with green manufacturing policies and reduces costs |
Industry reports indicate that brushless hollow-cup motors will account for more than 60% of the market share in high-reliability scenarios (aerospace, medical care).
8. In-Depth Case Analysis
8.1 Beihang University BH-985 Dexterous Hand
Structure: Adopts brushless hollow-cup motor + Hall sensor, achieving 0.1° position resolution.
Performance: Maximum torque per finger is 0.35 Nm, response time < 4 ms.
Advantages: The low inertia of the hollow cup ensures stability during high-speed grasping, with noise below 30 dB.
The structure diagram is from the project demonstration page of Beihang University.
8.2 Artificial Blood Pump (University of Shanghai for Science and Technology)
Requirement: Continuous, stable, and low-shear stress blood delivery.
Scheme: Customized brushed hollow-cup motor combined with planetary reduction gear, achieving stable speed of 150 rpm under 0.5 W output power.
Result: Blood damage rate < 0.5%, operation time exceeds 10,000 hours.
Detailed structure and experimental data can be found in the paper "Design of Hollow-Cup Motor for Artificial Blood Pump".
9. Practical Selection: From Requirement to Product
The following is a selection process to help engineers quickly locate the appropriate hollow-cup motor:
Clarify Requirements
Load torque (Nm)
Maximum speed (rpm)
Operating voltage (V)
Whether closed-loop control is required
Screen Suppliers (Major domestic manufacturers: Zhengyuan Motor, Leadshine Technology, Yunhongda, etc.)
Check power density and efficiency curves in product manuals.
Evaluate Drive Scheme
Brushed type → Simple PWM drive
Brushless type → Requires FOC (Field-Oriented Control) or BLDC driver chip (e.g., TI DRV8305)
Thermal Design
Calculate power loss and determine whether heat sinks or air cooling are needed.
Prototype Verification
Test torque, response time, and noise level through test benches.
Bulk Procurement
Negotiate MOQ (Minimum Order Quantity), delivery time, and quality assurance with suppliers, and sign a technical support agreement.
This process combines industry best practices, which can significantly reduce R&D risks and improve product reliability.
10. Conclusion
With its unique advantages of high power density, low inertia, and fast response, hollow-cup motors have become core drive components in high-end applications such as robotics, UAVs, and medical instruments.
With the continuous advancement of permanent magnet materials, intelligent drive chips, and lightweight manufacturing, the performance boundaries of hollow-cup motors are constantly being broken. In the future, they will play a key role in broader scenarios such as aerospace and intelligent manufacturing.