How does my controller know when to cut the assistance?

[kitchenman]

I was asked this question today and I didn't know the answer... ... I've got a Tongxin hub and a Cytronex set up ... I think the answer is the pedelec sensor but I dont know why! ... If its not the pedelec sensor then it must be something in the hub? ...
Thanks ..

 

[Blew it]

If there is no electronic switching such as that fitted to Tonaro and Bosch, The motor will run out of steam when the back EMF voltage from the motor hall sensors equals the forward voltage of the battery....or something like that.

 

[kitchenman]

I dont think I have any hall sensors!
I dont think I'm going to learn this unless someone draws me some pictures or comes up with an analogy that I can relate to! ...

 

[amigafan2003]

I wont try to explain back EMF (which you get even on sensorless motors - i.e. through the phase wires) but basically as a motor spins, it generates voltage. The faster it spins the more voltage it generates. When this generated voltage equals the input voltage (i.e. the voltage from the battery) then the controller will cease to provide current for the motor to spin.

This is why when you put a higher voltage battery pack on a bike with no other mods you go faster - for example say at 175rpm at the hub equals 15mph then the output voltage from the hub @ 175rpm/15mph will equal 36v. If you're using a 36v battery then assistance will cease at 36v. If you use a 44v battery instead then the hub needs to spin @ 19mph or 207rpm to generate the 44v to equal the batteries voltage so now assistance stops at 19mph.

The only way to increase the speed of your bike is to use a higher voltage battery or use a hub motor that achieves the input voltage at a higher rpm. That's why you'll see motors rated at different rpms even though they have the same voltage - achieved by different "windings". Increasing the amps of the motor will not increase top speed - it will only increase torque i.e. hill climbing ability and acceleration.

Make sense?

 

[kitchenman]

Yes. Thanks for taking the time to explain that for me. So, the speed is governed by my kit. I have a 175 rpm capable hub in a 36 volt system therefore I am restricted to 15 mph. I was wondering why there are THREE wires connecting the controller to the hub. I guess the third wire enables the controller to know the voltage generated by the motor.

 

[NRG]

No, the motors are 3 phase brush less DC motors.

The controller pulses each phase in turn making the motor rotate, the motors are not straight DC brushed motors (some are but they have two wires)



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[kitchenman]

No, the motors are 3 phase brush less DC motors.

The controller pulses each phase in turn making the motor rotate, the motors are not straight DC brushed motors (some are but they have two wires)



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So, how is the voltage being generated by the motor detected by the controller?

 

[NRG]

The controller does not detect the voltage. As the motor spins a voltage is induced in the motor coils. This voltage (back EMF) is in opposition to the applied voltage that is spinning the motor. As the motor spins faster the BEMF increases until it equals that of the applied voltage. The motor at this point ceases to spin any faster.

When I get back to my laptop I'll post a few links that will explain how it works...



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[piotrmacheta]

A good way of looking at this is to consider 2 containers linked together with a hose (bottom to bottom). If you fill one container with water to a set level then the water will flow into the 2nd container up to the level of the water in the 1st. If you maintain the level in the 1st container constant then it's like a battery supplying the water to the 2nd container (the motor). The water level in the 2nd container cannot go higher than the 1st therefore a limit. Voltage can be considered analogous to a height of water whereas current is analogous to the diameter of the pipe (ie how quickly it fills up the 2nd container.

 

[NRG]

There are a number of animations on the web if you google search, here's two:

Brushless DC Motor Animation

and the one of Flecc's site that can be found on numerous sites...

http://users.tinyworld.co.uk/flecc/4-pole-bldc-motor031102.swf

Both show the use of Hall sensors, the motors work the same way even if sensorless except the controller has to detect the BEMF pulse in the third phase (unused at that moment) to understand where the rotor is instead of relying on the feed back from the hall sensors.

 

[amigafan2003]

NRG explained back emf better than I could even think of doing!

My "then the controller will cease to provide current for the motor to spin" statementwas a bit of a band aid for explaining something I couldn't put into words.

 

[kitchenman]

NRG explained back emf better than I could even think of doing!

My "then the controller will cease to provide current for the motor to spin" statementwas a bit of a band aid for explaining something I couldn't put into words.

Thats fine. I wouldn't have got to this level of understanding without your help .. thanks.