Help! Confused! Controllers and Displays

FatBob

Pedelecer
Apr 15, 2020
138
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Greater Manchester
Sorry, big fat simpleton here. Is there any difference between a "controller" and a "display", as it seems these terms are used interchangeably from what I can tell? E.g. I thought the Bafang C961 was 'just' a display, but elsewhere I've read people referring to it as a controller or "controller display".
 

vfr400

Esteemed Pedelecer
Jun 12, 2011
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Basildon
The display is the control device that you use as an interface to communicate with the controller. the controller is the thing that regulates the motor's speed and power.. In a Bafang BBS01/2, the controller is attached to the motor and the control panel (display) is attached to the handlebars.

Both devices have sophisticated microprocessors in them, so they can do complicated things, and they work independently. They communicate with each other. The sensors (brakes, throttle, hall sensors, speed sensor and PAS are connected to the controller, so it makes some decisions for itself and can work on it's own. It also tells the control panel how it's doing regarding speed, and power and some other things. The control panel converts that data into the display so that you can understand what's happening.

Some manufacturers refer to the control panel as the human interface device (HID).
 
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FatBob

Pedelecer
Apr 15, 2020
138
36
Greater Manchester
If a controller draws at peak 15 A at 36 V (sine wave, unknown frequency & phase difference) can you very naively just multiply these to give an peak instantaneous power rating of 540 W?

When you see power ratings for controllers quoted, what exactly are they measuring?

Apologies for my poor understanding of electrical engineering!
 

sjpt

Esteemed Pedelecer
Jun 8, 2018
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Winchester
If a controller draws at peak 15 A at 36 V (sine wave, unknown frequency & phase difference) can you very naively just multiply these to give an peak instantaneous power rating of 540 W?

When you see power ratings for controllers quoted, what exactly are they measuring?

Apologies for my poor understanding of electrical engineering!
Pretty much. Except of course there will be losses all the way through the system.
To try to understand more play with https://ebikes.ca/tools/simulator.html
Even if it doesn't have you motor it will give a good idea of general behaviour.
 
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Nealh

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Aug 7, 2014
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West Sx RH
Efficacy of motors are about 80% peak so your 540w is less 20% at least at the wheel.
 
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vfr400

Esteemed Pedelecer
Jun 12, 2011
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Basildon
If a controller draws at peak 15 A at 36 V (sine wave, unknown frequency & phase difference) can you very naively just multiply these to give an peak instantaneous power rating of 540 W?

When you see power ratings for controllers quoted, what exactly are they measuring?

Apologies for my poor understanding of electrical engineering!
The 15 amps is the current from the battery, not the curent that goes to the motor. You are therefore taking 15 x 36 = 540w from the battery. That energy/power is then converted into motive power by the motor minus a small switching loss in the controller, which is converted to heat. The motor can convert about 70% when running at normal speed, and drops to 50% for a slow uphil climb and as low as 30% for a very slow climb. The balance of the power is converted into heat and a bit as noise.

The controller reads the current coming from the battery by measuring the voltage drop through a shunt. It regulates the power to the motor according to your settings, but when it reaches 15A, it doesn't allow any power to the motor that would exceed that. The label normally says max current 15A, which is its limit. they sometimes say rated current 7A. The rated current is always half of the max current or the max current is always double the rated current. the rated current is what it's guaranteed to run at continuously without overheating. If you run continuouisly at 15A, the controller will soon overheat. In practice, because of the way motors work, it's not easy to run at max current, except if you have a motor that's too fast for it's power, a crank-drive pedalled with a very low cadence or when you're going too slow up a long hill.
 
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