Understanding Magic Pie Throttle

[Albike]

I am new to electric bikes, and recently I bought a used one. So far I know from (gasoline powered) motorcycles that the throttle controls the energy supply to the engine. But it seems this is different on an e-bike. Can it be that the throttle setting tells the motor what speed I want, and then the Magic in the controller allocates the according Amperes?

Sorry if this question sounds stupid.

[Bikemad]

Hi andto the forum.

The throttle signal output voltage basically controls the speed of the motor (rpm).
The motor starts to turn with a throttle signal voltage of ~1.25V and full throttle is reached at ~3.25V as shown below:



If you apply full throttle at low rpm, the motor will draw high current, which will automatically reduce as the motor rpm increases until the rpm matches the expected rpm for the throttle output voltage, or maximum achievable road speed for the power being produced is reached.

The maximum achievable speed at full throttle is dependant upon many factors including the following:

• battery voltage
• available current
• the total combined weight (bike, motor, battery, luggage and rider etc.)
• grade/incline
• road surface
• tyre type (and pressure)
• windspeed (headwind, sidewind or tailwind)
• aerodynamic resistance (drag coefficient)
• frictional losses (rolling friction coefficient)

I would say that an ebike's throttle works pretty much like a gasoline powered bike's throttle, the more throttle you apply, the greater the power output from the motor, causing quicker acceleration and a faster top speed. 

Alan
 
EDIT:
Revised throttle graph added

[Albike]

Thanks for the explanation.

The reason for my question is that the previous owner of my bike put an adjustable resistor after the throttle and calibrated it so that the bike does not reach more than 25 km/h (maximum in my country so that it is not considered an unlicensed moped). He did that because he had no USB cable to set the maximum RPM in the controller. So effectively I cannot apply full throttle now, because the resistor limits the throttle voltage, and thus my bike cannot go faster than 25.

So far so good. What still puzzles me: Would the bike reach its 25 km/h faster if I could apply full throttle, or does it come out to the same with the current setup?

[Bikemad]

If your maximum throttle is restricted, then your acceleration will also be reduced by a similar amount.

Setting the maximum speed limit by adjusting the maximum motor rpm would make a lot more sense, as this would allow full throttle acceleration right up to the set rpm limit.

Alternatively, if the resistor has been added in line with the signal wire, as per my simple throttle restrictor circuit:


It should be possible to install a switch directly across the resistor to bypass the restriction and allow full throttle signal to the controller for off road use. 

Alan
 

[Albike]

Thanks for the good explanation.

The bypass switch looks tempting, but since I use my bike mainly in the city, I rather opt for the legal path. I live in Austria, and when e-bikes were introduced police had no clue, you basically could do what you wanted. But after some nasty accidents happened with senior citizens riding tuned bikes, police got their acts together. Now Vienna has bicycle police people in special uniforms with short pants and sleeves riding bio-bikes, and these guys know their job. They can tell whether a cyclist is pedaling with or without motor assist (or not pedaling at all), and if that is beyond 25 km/h, they know that the bike is tuned. And if they know there is a switch to be found they will find it. Btw., the bike unit is very popular among police people, and there is a waiting list for getting in.

So I need a USB cable to limit the rpm. My bike has this connector. What is the cable I need called, and where can I order it (preferably within the EU)?

[Bikemad]

After looking at your connector I am puzzled as to which controller you have, as that connector is different to all of mine.

It appears to be a 6 pin connector, but the nearest I have to that is a five pin connector on one of my early MPIII controllers:



The early MPIII controllers required a USB cable that connected to the motor harness via the same 8 pin Connector that the control harness plugs into:



Later MPIII controllers required a USB cable with a ten pin connector to plug into the motor harness:



The MP4 and MP5 controllers have a separate five pin connector that is used for the USB programming lead, Smart LCD Display and Bluetooth dongle:



Please Note: Bluetooth dongle will only work with MP5, SP5 and MP Edge controllers.

You will need to identify which model of controller you have before you can determine which USB programming cable you need to purchase.

If your motor runs very quietly under load, it will have a vector controller (MP4 or MP5) but if there is noticeably loud ticking noise under load, it is more likely to be an MPIII (or earlier).

The original Magic Pie and MPII controllers used a non symmetrical pin pattern on the 8 pin connector on the motor harness, and these would require a connector on the USB programming lead that looks like this:



Once you have identified your controller, you may be able to purchase the correct USB cable from a local GM dealer.
If you can't get one locally, the Goldenmotor.ca website shows the USB programming cables for MPIII (8 and 10 pin versions) and the MP4/5.

If your motor is an MPII or earlier, you may have a problem finding a dealer who still has stock of the original non symmetrical 8 pin version of the programming cable. 

Alan
 

[Albike]

Actually mine is five pin. What appears to be a sixth hole in the center is a shallow dimple in the plastic. So I have exactly what is shown in your first picture, the early MPIII.

Did I understand correctly that the 5 pin connector is not where I plug in the USB, but instead I have to look for a different seperable connection which then will have 8 or 10 pins? What is the 5 pin connector for?

[Albike]

Oh well, just forget it. My bike does not have the 8 or 10 pin connector anymore, neither the 4-in-1 harness. The builder obviously had his own ideas:


So no real way to connect USB anymore 

Anyway, the resistor made the bike road legal, and I will not bother anymore.

[Bikemad]

Actually mine is five pin. What appears to be a sixth hole in the center is a shallow dimple in the plastic. So I have exactly what is shown in your first picture, the early MPIII.

Did I understand correctly that the 5 pin connector is not where I plug in the USB, but instead I have to look for a different seperable connection which then will have 8 or 10 pins? What is the 5 pin connector for?

The 5 pin connector is "For Meter Display":



I only have this 5 pin connector on one of my MPIII controllers, the other does not have this additional lead and connector.

I have yet to see a GM meter display that has this style of connector. The connector on the BAC-601 Smart LCD Display is of different gender to this and would therefore be unable to connect to it. Perhaps GM were originally intending to produce a display unit specifically for use with the MPIII, but upgraded the Magic Pie to vector controllers before the meter went into production. 

From the picture in your previous post, it looks like the Magic Pie was fitted as an upgrade to an existing ebike, and the 8 or 10 pin connector that would have been used for the programming cable was obviously removed during the upgrade/installation.

The housing in the frame where all of those wires now terminate would have originally contained a small external controller.

Alan