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For my solar powered bicycle, the Maxun One, I designed a very powerful and lightweight mid-drive motor. I also wanted to investigate whether I could use a Bosch Active Line 3 motor. The Bosch motor weight is 2975g, this is relatively lightweight for a mid-drive motor, but it is still too heavy for my solar bike.

So, I wanted to investigate if the Bosch motor can be reduced in weight, so I disassembled the motor. This is also a great opportunity to see how it works and which electronic parts are used.

Bosch active line 3 disassembly
Bosch active line 3 disassembly

Bosch active line 3 typenumber
Bosch active line 3 typenumber

Bosch Active Line 3 facts

  • On the market since end 2017
  • Max. 250W / 40Nm
  • Max. cadence 100rpm
  • Weight 2975g without plastic covers, metal holder and bolts
  • Weight 3345g with plastic covers, metal holder and bolts

What is striking after opening the motor is that there are only a few parts inside, that is a great achievement of the Bosch engineers:

Bosch Active Line 3 mid-drive motor dismantled
Bosch Active Line 3 mid-drive motor dismantled

Bosch Active Line 3 mid-drive motor inside
Bosch Active Line 3 mid-drive motor inside

Bosch Active Line 3 mid-drive motor taken apart
Bosch Active Line 3 mid-drive motor taken apart

Comparison with Bosch Performance Line CX

Here we can see that the Bosch Performance Line CX is much more complex than the Active line 3, it has two gears, one for the motor and one for the bottom bracket:

Bosch Performance Line CX inside view
Bosch Performance Line CX inside view

Bosch Active Line 3 freewheel

The motor has just one freewheel, used for pedaling without turning the motor. But the motor can’t drive the rear wheel without the pedals turning, this requires an extra freewheel.

Bosch active line 3 freewheel
Bosch active line 3 freewheel
Freewheel
Freewheel


Magnetostrictive torque sensor of the Bosch Active Line

Bosch active line 3 torque sensor
Bosch active line 3 torque sensor

Torque sensor plain bearing
Torque sensor plain bearing
Torque sensor plain bearing
Torque sensor plain bearing

The torque is measured on an extra hollow shaft, which will twist as a result of the torque. This torque is measured magnetically by coils using the inverse magnetostrictive effect.
See more about torque sensors HERE.

Bosch Active Line 3 helical gear

The motor must have a speed above 1000 rpm, otherwise the motor losses will be too high, see the article motor tuning. So, we always need a gear. The previous Bosch mid drive motors have four gears wheels, but the Bosch Active Line 3 has just two gear wheels. That limits the transmission ratio, and so the motor efficiency, unless we take a motor gear wheel with very few teeth. To avoid gear undercutting, the minimum number of teeth of a gear is commonly 12, but that is too much in our situation. The reason that Bosch uses a helical gear is not just noise reduction, more important is that helical gears allows a low number of teeth without undercutting.

The motor gear has just 5 teeth, which is very rare.

Bosch active line 3 motor gear wheel
Bosch active line 3 motor gear wheel

The large gear is made of nylon:

  • Teeth number 71
  • Diameter 115mm
  • Width 18.5mm
  • Helix angle 15º
  • Module m = 112mm / 71 = 1.6

Note that the gear wheel is not standard: https://nl.wikipedia.org/wiki/Modulus_(tandwielen).
The gear ratio is 71 / 5 = 14.2. At a cadence of 100rpm, the motor speed is 1420 rpm which is quite low.

Lubrication

It is remarkable that the gears are nearby the PCB but the electronics remain free of grease. The grease is apparently not being thrown around. This requires a special grease with a certain thickness. Used is “Renolit WTF 2B Bosch Drive Unit Fett” but I don’t know where you can buy it. The small motor gear is fitted with a plastic cap, which ensures that the grease stays on the gear wheels. The nylon plate between the large gear and the cover has the same purpose.

Bosch Active Line 3 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 = 0.109Ω

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, see here:

Bosch active line 3 motor measurement
Bosch active line 3 motor measurement

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 = 38.7V
  • I= 1.00A
  • rpm = 200

Calculation of k:

  • k = U / ω = U * 60 / ( 2 * Π * rpm)
  • k = 1.85

Friction torque Tf

  • Tf = I0 * k
  • Tf = 1.85

Motor construction

  • Stator 18 pole
  • Rotor 14 pole
  • Rotor diameter 46mm
  • Rotor length 46mm
  • Stator outer diameter 84mm
  • Gear ratio 14.2

Bosch motor graphs

Bosch Active Line motor graph 36V
Bosch Active Line motor graph at 36V
Bosch Active Line power motor graph 36V
Bosch Active Line power motor graph at 36V

We see that the speed at 36V is very high: 185 rpm, nobody cycles with a cadence of 185 rpm. This means that the motor runs internally at a much lower voltage! The maximum cadence according to Bosch is 100 rpm, that corresponds to a motor voltage of 19V. A new motor graph is therefore calculated at 19V instead of 36V:

Bosch Active Line motor graph at 19V
Bosch Active Line motor graph at 19V

Bosch Active Line power motor graph at 19V
Bosch Active Line power motor graph at 19V

Normally, the duty cycle of a PWM motor control is 0% for 0V and 100% for 36V. However, at the Bosch Active Line 3, the maximum duty cycle is 19V/36V = 53%. We can also say that the internal motor is a 19V motor.
If anyone has any comments about then let me know.

Just few people cycle with a cadence of 100 rpm on an electric bicycle, therefore a graph is also made at 16V, which corresponds to a cadence of 80 rpm:

Bosch Active Line motor graph at 16V
Bosch Active Line motor graph at 16V
Bosch Active Line power motor curve at 16V
Bosch Active Line power motor curve at 16V

It is not clear why Bosch uses a 19V instead of 36V motor. This reduces the efficiency of the motor controller because the maximum current is greater and therefore the losses in the Mosfets as well.

Permanently sticky glue

Note that there is some permanently sticky glue on the rotor. I suppose, this attracts dirt particles to prevent them from entering the motor.

Permanent sticky glue Bosch Active Line 3 motor
Permanent sticky glue Bosch Active Line 3 motor

Thermal grease

There are four locations on the PCB where thermal grease is applied for cooling electronic parts to the aluminum housing. The used thermal grease is a dry blue paste, I do not know what it is and where you can buy it. Because it is dry, I suppose that you have to apply some extra paste after repair.

Bosch active line 3 PCB thermal grease
Bosch active line 3 PCB thermal grease

Forums

https://endless-sphere.com/forums/viewtopic.php?f=30&t=96269

Part 2

Bosch Active Line 3 printed circuit board dismantled

Bosch active line 3 PCB explained

Do you have any comments about the website or problems with shopping? Please let me know.
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