The benefits of the Yamaha PW-SE mid-drive motor
For my solar bike, I want to use the best mid-drive motor. My choice is the Yamaha PW-X motor for the following reasons:
- Zero cadence support
- Relatively low weight of 3.1 kg
- Any battery can be used on the Yamaha PW-X (with a simple hack)
- The efficiency of the Yamaha PW-X motor is the highest of all mid-drive motors 1*)
1*) This is due to the large gear ratio. The internal motor therefore has a high speed. Read more in this article.
Instead of the Yamaha PW-X motor, I had a Yamaha PW-SE motor available. Therefore, to calculate the efficiency, I have done measurements on the Yamaha PW-SE motor.
Yamaha PW-SE facts
- Max. 250W
- Max 70Nm
- Max. cadence 110rpm
- Weight 3500g
Yamaha PW-SE tear down
Zero cadence support with two freewheels
Most mid-drive motors (like the Bosch) have just one freewheel, this is used for pedaling without turning the motor. With just a single freewheel, the motor can never drive the rear wheel without the pedals turning, this requires an extra freewheel.
The Yamaha has 2 freewheels, so we can do both:
- Pedaling without turning the motor
- The motor can drive the rear wheel without the pedals turning
A disadvantage of the Yamaha is that gear wheel 3 and 4 will rotate during pedaling without motor assistance, which will give some loss.
Yamaha PW-SE gearwheels
| Gear 1 | Gear 2 | Gear 3 | Gear 4 | |||||||
| Material | steel | Material | nylon | Material | steel | Material | nylon | |||
| Teeth number | 8 | Teeth number | 56 | Teeth number | 16 | Teeth number | 101 | |||
| Diameter [mm] | 10,8 | Diameter [mm] | 61 | Diameter [mm] | 19 | Diameter [mm] | 105 | |||
| Width [mm] | 22 | Width [mm] | 22 | Width [mm] | 12 | Width [mm] |
12 |
The gear ratio is 101/16 * 56/8 = 44.2
The motor speed of the Yamaha PW-SE at a cadence of 100 is 4420 rpm.
Pressure-balance-element
An adhesive pressure balance element is provided to prevent condensation in the motor housing. It looks like it is a GORE® Adhesive Protective Vent.
Liquid gasket
Attention: the Yamaha uses liquid gasket. Once the motor has been opened, this must be renewed.
Yamaha PW-SE motor curve
Just 3 motor parameters are required to create the motor curve: R, k and Tf. These will be measured and calulated here.
Motor resistance R
This is the resistance between two motor terminals:
R = 50.8mΩ
Motor constant k
The motor constant k is measured with an external motorcontroller KU63 at a voltage of 36V, which is directly connected to the motor. Here, the motor constant k relates to the motor-gearbox combination. The voltage, no load current and speed at the output axis are measured, fortuitous it are round numbers:
- U = 39.2V
- Iο= 0.90A
- rpm = 131
Calculation of k:
- k = U / ω = U * 60 / ( 2 * Π * rpm)
- k = 2,857
Friction torque Tf
- Tf = Iο * k
- Tf = 2,572
Yamaha PW-SE motor Yamaha PW SE motor performance efficiency graph with gear loss
We can see that the speed of the output axis at 36V is 120 rpm.
You can download the Excel sheet that is used at GitHub here.
The Yamaha motor has 4 gear wheels. Including an assumed loss of 5% per gear (10% total), we get the following graph:
Efficiency mid drive motor Yamaha PW-SE vs Bosch Active Line 3
We can see below that the Yamaha PW-SE has a better efficiency than the Bosch Activ line 3, this is due to the higher internal motor speed. But the Yamaha has more gear wheels, which gives higher mechanical losses. To get the same cadence as the Yamaha (120rpm), the Bosch graph is measured at 24V, see the article here.
Connectors
This connector is the JWPF type from JST:
Forums
https://www.pedelecforum.de/forum/index.php?threads/yamaha-pw-se-mid-drive-motor-tear-down.63476/
https://endless-sphere.com/forums/viewtopic.php?f=30&t=100440
