# DC controller questions



## brian_ (Feb 7, 2017)

GE11 said:


> BAttery current is different form Motor amps. But how much difference in these two current values can there be?


Hugely different.

The battery voltage is essentially constant, but the voltage to the motor goes from nearly zero (at zero motor speed) to the full battery voltage (at whatever speed where the available voltage becomes the limiting factor). Only a fraction of the battery voltage is needed to push that current through the motor against the winding resistance (requiring very little voltage); most of the voltage to the motor is used to push against the back-EMF due to motor rotation.

The power into the controller (from the battery) must be not much more that the power out of the controller (to the motor) - assuming that the controller isn't terribly inefficient - so if you multiply current and voltage between the battery and controller, it can't be much more than what you get by multiplying current and voltage controller and motor. 

So, consider a motor at low speed and with the accelerator fully applied so the controller is delivering current at the set limit. If the motor voltage at this low speed is only half of battery voltage (for instance), motor current must be almost twice the battery current.

This might be useful: 4QD - Battery current and motor current



GE11 said:


> I’m trying to understand how you can build more current with the free wheeling diode and motor inductance.


This previous forum discussion might be useful, but gets into AC power and so becomes less directly applicable: Controller Current vs Battery Current


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## Russco (Dec 23, 2008)

GE11 said:


> I have an open revolt controller. Has anyone tried to change and see if the battery limit amps work??
> How does the controller measure and sense the battery current if it does not have a Hall effect sensor on the battery post??
> 
> Also,
> ...



A controller feeding battery volts to motor volts acts like a transformer.

Power In = Power Out

Power = Volts x Amps

At full speed and 100% Duty Cycle, if the motor current = 200 amps, the battery current = 200 amps.

At half speed and 50% duty cycle, if motor current = 200 amps, the battery current = 100 amps.

At 10% speed and 10% duty cycle, if motor current = 200 amps, the battery current is 20 amps.

Just like a transformer.


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## GE11 (Oct 24, 2011)

Ok,

So if I have my current limit set to a full 500Amps and I open the throttle to 5%. Should the motor see the full 500 amps until the armature speeds up to the “5% voltage” that corosponds to the throttle being opened? Would this define the controller as voltage control with 500A current limit?


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## brian_ (Feb 7, 2017)

GE11 said:


> So if I have my current limit set to a full 500Amps and I open the throttle to 5%. Should the motor see the full 500 amps until the armature speeds up to the “5% voltage” that corosponds to the throttle being opened? Would this define the controller as voltage control with 500A current limit?


Yes, something like that would be voltage control, but why would you be using voltage control? Normal would be torque or current control, so I assume that in this example you would get 5% of 500 amps (25 amps) to the motor, regardless of what voltage is required to drive the motor at 25 amps.


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## Russco (Dec 23, 2008)

GE11 said:


> Ok,
> 
> So if I have my current limit set to a full 500Amps and I open the throttle to 5%. Should the motor see the full 500 amps until the armature speeds up to the “5% voltage” that corosponds to the throttle being opened? Would this define the controller as voltage control with 500A current limit?


Sort of, in theory. In practice, not really, with such a low armature voltage. The motor will need 5 volts just to magnetize the field and armature achieving almost no speed, like 20 RPM. At .02 ohms for the field and armature resistance, ohms laws verifies 5 volts is useless.

See the Curtis Manual, 1231, at:

https://cdn.curtisinstruments.com/products/manuals/1209B_manual_en.pdf

Section 6, near the end.

Also more information in a Google Search: Jones Chopper.


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## brian_ (Feb 7, 2017)

Okay, I had a look at the linked Curtis manual. Section 6 is the glossary - I don't know what section of that would address the question.

The interesting thing in the manual is a chart which suggests that 'throttle' position determines PWM duty cycle directly (rather than controlling measured current). That's seems simplistic to me, and is sort of close to voltage control. The resulting current will change as the motor speed (and so back-EMF) changes, and the motor's response to 'throttle' position will be excessively sensitive at low speeds. Hmm... I think I see where some people get the idea that EVs have huge acceleration - with a controller like this (which doesn't control current except to limit it) and a brushed DC motor, I would expect they are getting nearly full torque from small pedal movements, but only at low speed.


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## Russco (Dec 23, 2008)

brian_ said:


> Okay, I had a look at the linked Curtis manual. Section 6 is the glossary - I don't know what section of that would address the question.
> 
> The interesting thing in the manual is a chart which suggests that 'throttle' position determines PWM duty cycle directly (rather than controlling measured current). That's seems simplistic to me, and is sort of close to voltage control. The resulting current will change as the motor speed (and so back-EMF) changes, and the motor's response to 'throttle' position will be excessively sensitive at low speeds. Hmm... I think I see where some people get the idea that EVs have huge acceleration - with a controller like this (which doesn't control current except to limit it) and a brushed DC motor, I would expect they are getting nearly full torque from small pedal movements, but only at low speed.


The Curtis 1221 and 1231 were designed in the 1980's, which, I believe, use voltage control. Newer controller designs use torque or current control, which gives excellent smooth starting from a stop.

The last couple of pages in the Curtis Manual describe their controller operation.


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## GE11 (Oct 24, 2011)

So what I want from my controller is purely voltage controller. Kinda like a dc to dc buck converter. Is there a way to set the Paul and Sabrina that way?? If it is, how does the controller sense the voltage?? Looking at the latest schematic does not show a sense point where it is senseing the Motor voltage...

Thanks for the great responses!


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## brian_ (Feb 7, 2017)

Russco said:


> The Curtis 1221 and 1231 were designed in the 1980's, which, I believe, use voltage control. Newer controller designs use torque or current control, which gives excellent smooth starting from a stop.


From the current lineup of Curtis series motor controllers, it appears that they haven't designed a new traction motor controller for a very long time - only pump motor controllers. The manual for more recent models (1204M/05M/09M/21M) maps 'throttle' position to "throttle command", rather than directly to PWM duty cycle, which is encouraging. Unfortunately, it says even less about how the controller actually works than the earlier manual, and doesn't explain what "throttle command" actually means. Disturbingly, and typical of Curtis documentation, it persistently refers to "speed", even though the controller has no speed measurement method; it refers to "current" only in the context of high-current and low-current wiring connections, hazards, and current limits, never as a controlled variable.



Russco said:


> The last couple of pages in the Curtis Manual describe their controller operation.


I get it now - you're referring to the appendices, which are after Section 6, not part of it.



GE11 said:


> So what I want from my controller is purely voltage controller. Kinda like a dc to dc buck converter. Is there a way to set the Paul and Sabrina that way?? If it is, how does the controller sense the voltage?? Looking at the latest schematic does not show a sense point where it is senseing the Motor voltage...


Why? Are you not using the motor and controller to drive a car?

If you're looking for some sort of cruise control (constant speed setting), voltage control would not be the way to do it.


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