Yamaha PW-SE

Details: Last Updated: 25 March 2020

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Yamaha PW-SE facts

Max. 250W
Max 70Nm
Max. cadence 110rpm
Weight 3500g

Yamaha PW-SE tear down

There are several circuit board versions of the Yamaha PW SE

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.

Tuning the Yamaha PW-SE motor for more power

The Yamaha PW-SE motor has room for thicker copper wire. Rewinding the motor with thicker copper wire gives a higher motor efficiency and therefore a higher permissible power.

Broken nylon helix gearwheel repair

Broken nylon helix gear Yamaha ebike PW motor

If you have a broken gear in your Yamaha e-bike motor, let me know. I am currently testing new nylon gears for the Yamaha PW-X2.

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.

Replacing the thermal gap filler pads

I have looked if there are thermal pads which do not always have to be replaced after repairing the motor. The thermal pad must be elastic and has to rebound to the original shape after the pressure is removed. The Bergquist Gap Pad TGP 1000VOUS meets these requirements. The viscoelastic nature of the material also gives excellent low-stress vibration dampening and shock absorbing characteristics. The required thickness is 1mm for both the Bosch and Yamaha mid-drive motors. The thermal conductivity is 1 W/mK with 1mm.

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.

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

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.