More Info
Additional reference information for homebuilt BLDC motors including termination types, winding combinations, and practical tips.
Delta vs Wye (Star) Termination
Theoretically, the ratio between Delta and Wye is 1.732 (√3). However, in practice this number is closer to 1.8.
A motor with a given number of turns, terminated Wye, yields ~1.8 times more Kt (torque per amp) than a similarly wound motor terminated Delta, while the Kv (RPM/volt) is ~1.8 times lower.
- Delta Kv is 1.8× higher than Wye
- Delta Kt is 1.8× lower than Wye
- With Wye termination, 1.8× fewer turns are needed to achieve the same Kv as Delta
Key Relationships
Important: If a motor is rewound to have the same Kv but with the other type of termination, the RPM and torque will be the same as before the rewind.
Delta Termination
Higher RPM, Less Torque
Each phase end connects to another phase start:
A-end → B-start, B-end → C-start, C-end → A-start
Wye (Star) Termination
Lower RPM, More Torque
All phase ends connect to a common neutral point.
ESC connects to the three phase starts.
Winding Diagram Table
This table shows common slot/pole combinations and their winding quality. Colors indicate suitability for use as a motor.
| Slots | 4P | 6P | 8P | 10P | 12P | 14P | 16P | 18P | 20P | 22P |
|---|---|---|---|---|---|---|---|---|---|---|
| 6 | AaBbCc | - | AaBbCc | AaBbCc | - | AaBbCc | AaBbCc | - | AaBbCc | AaBbCc |
| 9 | - | AbCaBc | AbCaBc | AbCaBc | ABCABCABC | AbCaBc | AbCaBc | - | AbCaBc | AbCaBc |
| 12 | AaAaBbBbCcCc | - | AaAaBbBbCcCc | AaBbCcAaBbCc | - | AaBbCcAaBbCc | AaAaBbBbCcCc | - | AaBbCcAaBbCc | AaBbCcAaBbCc |
| 15 | - | - | - | AbCaBc... | - | AbCaBc... | AbCaBc... | - | AbCaBc... | AbCaBc... |
| 18 | - | - | - | - | - | - | AaAaBbBbCcCc... | - | AaBbCcAaBbCc... | - |
| 21 | - | - | - | - | - | AbCaBc... | - | - | AbCaBc... | AbCaBc... |
| 24 | - | - | - | - | - | - | - | - | AaBbCcAaBbCc... | - |
| 27 | - | - | - | - | - | - | - | - | - | - |
| 36 | - | - | - | - | - | - | - | - | - | - |
Legend
- A, B, C - Clockwise (CW) winding for each phase
- a, b, c - Counter-clockwise (CCW) winding for each phase
- Blue - Good combination
- White - Works
- Red - Works, but not very well
- Black - Does not work
Practical Tips
Winding Direction
Example: AaBbCc (6 teeth)
- Phase A: Wind tooth 1 CW, continue to tooth 2 CCW
- Phase B: Wind tooth 3 CW, continue to tooth 4 CCW
- Phase C: Wind tooth 5 CW, continue to tooth 6 CCW
The alternating direction ensures proper magnetic field generation for smooth rotation.
Measuring Rm (4-Wire Method)
Equipment needed:
- Ammeter (capable of 5A+)
- Voltmeter (digital preferred)
- 10-20 Ω (10W) resistor
- 12V battery (e.g., 3S-4S LiPo)
Connect battery through resistor and ammeter to motor leads. Measure voltage drop directly across motor leads. Rm = V / I
CD-ROM Motor Winding
Typical 9N12P CD-ROM motors use the simplest winding pattern:
ABCABCABC - All teeth wound the same direction
Usually terminated Wye (Star). This is an excellent motor type for beginners to practice rewinding.
ESC RPM Limits
Electronic Speed Controllers (ESCs) have maximum RPM limits based on their timing update rate.
- Standard ESCs: ~50,000-80,000 RPM
- High-speed ESCs: 100,000+ RPM
Check manufacturer specs before using high-Kv motors with high cell counts.
Useful Formulas
| Formula | Description | Units |
|---|---|---|
| Kv = RPM / V | Velocity constant (no-load) | RPM/V |
| Kt = 60 / (2π × Kv) | Torque constant (SI conversion) | N·m/A |
| Torque = Kt × (I - Io) | Output torque | N·m |
| P_copper = I² × Rm | Copper (resistive) losses | W |
| P_iron = V × Io | Iron (magnetic) losses | W |
| Km = Kt / √Rm | Motor constant (quality metric) | N·m/√W |
| Rm_T = Rm_20 × (1 + 0.00393 × ΔT) | Resistance at temperature T | Ω |