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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

Yamaha PW SE tear down
Yamaha PW SE tear down

Yamaha PW SE tear down
Yamaha PW SE tear down

Yamaha PW SE tear down
Yamaha PW SE tear down

Yamaha PW SE tear down
Yamaha PW SE tear down

Yamaha PW SE PCB
Yamaha PW SE PCB

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.

Prevent condensation in an ebike motor with a pressure balance element
Prevent condensation in an ebike motor with a pressure balance element

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.

Yamaha PW SE motor graph measurements
Yamaha PW SE motor graph measurements

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.

Yamaha PW SE motor graph versus Bosch Active Line 3
Yamaha PW SE motor graph versus Bosch Active Line 3

The Yamaha motor has 4 gear wheels. Including an assumed loss of 5% per gear (10% total), we get the following graph:

Yamaha PW SE motor performance efficiency graph with gear loss
Yamaha PW SE motor performance efficiency graph with gear loss

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

Bosch Active Line motor performance efficiency graph 24V with gear loss
Bosch Active Line motor performance efficiency graph 24V with gear loss

Connectors

This connector is the JWPF type from JST:

JST JWPF 2mm pitch wire to wire waterproof connectors
JST JWPF 2mm pitch wire to wire waterproof connectors


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

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