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PWM, current and DC motor

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Mike.B

New Member
Hello everyone !

I would like to control the DC motor speed by a proportional command.
To do that, I would like to use a PWM available on a PIC.
My block diagram will look like something as following : http://hpics.li/e766676

My question is : What is the link between the PWM and the current ?
I am a totally beginner in this field. So let's me explain if I am right.
By changing duty cycle, I will change the tension and to some extent the current that the motor needs to work.
So, how can I know which duty cycle, which PWM value matchs with the current to the motor ?

Thank you for your help.
 

crutschow

Well-Known Member
Most Helpful Member
The PWM duty-cycle determines the average voltage the motor sees (e.g. a 50% duty-cycle ideally generates a voltage 50% of the supply voltage), it does not directly control current. The motor current is determined by this voltage and the motor load.
 
If you want to determine the exact relation between PWM and motor current you'll need to find/measure the parameters describing the motor physics. However, dynamical systems are by nature dynamical, so the current is not described simply by a single value. Do you need the current value for any particular purposes?

What is the exact purpose of the motor?
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Motor voltage is essentially proportional to speed when UNLOADED. When loaded, it more like V-Va-I*R. Successful speed controls have been made this way. Va=V(armature).

Torque is basically proportional to current.

PWM allows the motor to have nearly the torque at it's supply voltage rating. (The voltage of the PWM). PWM changes the average voltage, hence for the most part, speed.

The average current is essentially proportional to the load or torque as before. Torque has units of (distance-feet) e.g. ft-lbs.
 

Mike.B

New Member
Hello all !

Thanks for your replies and sorry for taking so long (I had to fix something).
But now, here I am and I will monitor this topic every day :)

I will try to reply your questions as best as I can.

The PWM duty-cycle determines the average voltage the motor sees (e.g. a 50% duty-cycle ideally generates a voltage 50% of the supply voltage), it does not directly control current. The motor current is determined by this voltage and the motor load.
Alright, I get it. It is more understable now how it works ;)

If you want to determine the exact relation between PWM and motor current you'll need to find/measure the parameters describing the motor physics.
What kind of parameters describing the motor physics will I need ?
I can get the datasheet and I can measure some parameters (I have the material)

However, dynamical systems are by nature dynamical, so the current is not described simply by a single value. Do you need the current value for any particular purposes?
What is the exact purpose of the motor?
It will be easier if we can describe current by a single value.
What do you think ? Can we imagine there is an formula that links PWM value, current motor and motor load ?

The motor allows to make turn a wheel thanks to a chain.
Let's me present all the parts :
- Battery provides current to the VFD
- Variable frequency drive that drives and provides current to the motor
- Motor
- Wheel

Currently, I have a mathematical formula. Thanks to it, I can determinate the velocity wheel according to the current provided by the battery (the input is the current and the output is the wheel velocity).

Let's me precise more my problem.
I would like to set a proportionnal command (K*errors)
My setpoint value is a velocity and I can measure the wheel velocity.

To reach this setpoint value in terms of velocity, I command the PWM value.
So for a particular PWM value, I can get the wheel velocity.

I would like to compare my mathematical formula to the reality. But the input of my formula is not PWM value but battery current. That's why I am looking for the link between PWM value and current.
Notice : From battery current, I can find motor current (because I know VDF efficiency)

When loaded, it more like V-Va-I*R. Successful speed controls have been made this way. Va=V(armature).
Where is this formula from ? What is 'R' here ?

The average current is essentially proportional to the load or torque as before.
I agree with that. I have read about it.

I hope my explanations help well to understand my problem. Feel free to ask more if it is necessary.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
It's more like V=Va-I*R

Sorry about the = turned - sign.

Assume for the moment it's a DC motor and no PWM.

The armature would have a resistance R which is measurable.
The current is measurable.
V or the voltage across the motor is measurable.

Va is sort of imagined, but that is the voltage that has to be controlled. It's real too, but you can't measure it.

When you use a motor as a tachometer, current drops to zero and thus speed is proportional to the voltage measured across the motor (no load),

If you really want to control speed, then that's what you should measure. Proportional control should basically work, If you were a low speed motor running under different terrains, then PID control might be needed. Lookup PID control.
 

ClydeCrashKop

Well-Known Member
Most Helpful Member
I always assume there are variables that I would overlook, like inertia and friction. I say build it with overkill and adjust the PWM for the desired physical output.
 

Mike.B

New Member
If you really want to control speed, then that's what you should measure

Thanks for the explanation about the formula.
What should I measure exactly ?

I say build it with overkill and adjust the PWM for the desired phys
ical output.
Yes, I have already ajusted PWM to get the desired physical output.

Have you an idea how I can link PWM value and current ?
Should I prepare some tests ? For example, I will measure current for each PWM value ?
Is there a more efficiency solution ?

Thanks for your help once again
 
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