Insufficient Current to Motors

[rahulpwns]

hey guys,
I am trying to use the motor from an rc car for my robot, but i don't know how much current is required for it. also, i do not know how to test the current with a multimeter (it would be great if you could explain how to do that). my arduino motor shield supplies a max of 1.2 amperes to a single motor. will my robot move with 1.2 amperes of current? i know you cannot provide excessive current to the motor, but is it ok to provide an amount that is less than what is required?
i am a beginner at hardware (i generally only do programming)

[newInRobotics]

Hi rahulpwns,

i do not know how to test the current with a multimeter (it would be great if you could explain how to do that).

Well, You set multimeter to A setting and connect probes so that multimeter is in series with motor. When motor is on and spinning freely, You measure No Load current, when motor is on and You stall it (stop it by Your fingers so it does not spin), You measure Stall current.

You can always use Google to get some more details 

my arduino motor shield supplies a max of 1.2 amperes to a single motor. will my robot move with 1.2 amperes of current? i know you cannot provide excessive current to the motor, but is it ok to provide an amount that is less than what is required?

If motor requires more current than Arduino shield can provide (and there is no overcurrent protection), Arduino shield will be destroyed. There is no damage to motor if driver is capable to provide more current than necessary as long as voltage is not higher than recommended by motor manufacturer. You can limit/regulate motor current and that would not affect motor in any negative way appart from loss of RPM and torque.

[rahulpwns]

Well, You set multimeter to A setting and connect probes so that multimeter is in series with motor. When motor is on and spinning freely, You measure No Load current, when motor is on and You stall it (stop it by Your fingers so it does not spin), You measure Stall current.

thanks! but, what is no load current compared to stall current? how do i use these measurements to determine how much current my arduino motor shield needs to have?

You can limit/regulate motor current and that would not affect motor in any negative way appart from loss of RPM and torque.

how do i limit/regulate motor current? what component does this?

[newInRobotics]

thanks! but, what is no load current compared to stall current?

• No Load current is exactly what it says it is - current drawn by a motor when there is no load on it; plain motor, without anything on it, spinning freely.
• Stall current is, as well, exactly what it says it is - current drawn by a motor when it is loaded so much that it can spin no more.

how do i use these measurements to determine how much current my arduino motor shield needs to have?

If You need motor to change directions quickly, it will draw almost double the Stall current, so have Your Arduino shield capable of double the Stall current of Your motor (if motor draws 10A when stalled, have Your shield capable of 20A).

how do i limit/regulate motor current? what component does this?

• Current is limited by a resistor in series with a motor.
• Current is regulated by IC such as LM317 or circuit such as BJT Current Mirror
• More advanced current limiting/regulation can be achieved by microcontroller driven PWM with current sense feedback. It's like Buck Converter circuit plus current sensing and adjustment feature.
• If current regulation is required for motor control, You can use something likeL298N H-bridge with microcontroller driven PWM and current feedback. You can always make H-bridge Yourself from BJTs or MOSFETs and use it instead of L298N.

[rahulpwns]

Current is limited by a resistor in series with a motor.

http://www.sparkfun.com/tutorial/Ardumoto%20shield/reshoot06.jpg
in this picture where would i place the resistor (the motor is being powered by the blue terminals)?

Current is regulated by IC such as LM317 or circuit such as BJT Current Mirror[/li][/list]
More advanced current limiting/regulation can be achieved by microcontroller driven PWM with current sense feedback.

The motor shield already has PWM with it's h-bridge, so i think i don't have to worry about that. I was planning to control the motor through pwm. would there be any problem with this, since the motor already draws more current than the board supplies.

[newInRobotics]

would there be any problem with this, since the motor already draws more current than the board supplies.

As mentioned earlier, motor driver board has to be rated for double the motor stall current, otherwise You play with fire as it might burn, unless it has overcurrent protection.

http://www.sparkfun.com/tutorial/Ardumoto%20shield/reshoot06.jpg
in this picture where would i place the resistor (the motor is being powered by the blue terminals)?

You put resistor between one of motor terminals and a blue terminal. With resistor in series, Your motor will loose some of RPM and torque.

Ardumoto board uses L298 H-bridge IC, hence You can either drive 2 motors with stall current of around 1.5A or 1 motor with stall current of around 3A.

Current sensing technique (which is more efficient than use of resistor as motor does not loose torque) can only be used when driving single motor as all grounds are merged to one on Ardumoto shield.

[rahulpwns]

i measured the currents for the motors:

Acceleration/Deceleration Motor
No Load- 185mA
Stall- 1.85A

Left/Right Motor
No Load- ~300mA
Stall- ~600mA (since this motor turns the car both ways, i just measured the no load current for each direction then added them.)

[rahulpwns]

sorry, forgot to say this in post above.

how do i determine the amount of ohms my resistors need to be?
i don't know exactly how to use ohms law to determine the resistance, but i know it involves using the voltage and current to find the resistance.

Thanks you a lot! you have been really helpful to me so far!

[newInRobotics]

Left/Right Motor
No Load- ~300mA
Stall- ~600mA (since this motor turns the car both ways, i just measured the no load current for each direction then added them.)

Is this Left/Right motor is part of rack and pinion steering system? If so, it will be stalled when maximum steer angle is reached. You should measure stall current by actually stalling the motor. Have a look at the Acceleration/Deceleration Motor measurements, stall current is 10 time higher than no load current. Do You think it's sensible to assume that stall current of motor which no load current is 300mA is only twice as high?

how do i determine the amount of ohms my resistors need to be?
i don't know exactly how to use ohms law to determine the resistance, but i know it involves using the voltage and current to find the resistance.

Ohm's law is the one be used.



Well, if You need motor to run at 12V and no more than 1A, You put these values into last equation and get value of resistance. So: R = 12V/1A = 12ohm.

I still suggest using either more powerfull H-bridge, or add 4 extra MOSFETs/Transistors per motor to utilize full power of them and not to be afraid to burn something.

[rahulpwns]

what values would i use in ohm's law: no load or stall?
i will be supplying the motor shield so it should look like this...
R = 9v/1.85A (if i was to use stall current)
...right?
the steering is rack and pinion and i did turn the wheels to the maximum steering angle.
should i physically hold the wheel and resist it from turning to measure stall current?

[newInRobotics]

Good morning 

what values would i use in ohm's law: no load or stall?

Neither. You use value that You want Your motor current draw would be limited to.

i will be supplying the motor shield so it should look like this...
R = 9v/1.85A (if i was to use stall current)
...right?

I'm not sure I get it. If by that You mean that You are going to add resistor between power supply and arduino shield, then NO, You should not do that. You should add resistor between shield and motor (just add resistor to one of motor terminals.

the steering is rack and pinion and i did turn the wheels to the maximum steering angle.
should i physically hold the wheel and resist it from turning to measure stall current?

If rack and pinion stop motor spinning, then yes, You were measuring stall current.

[rahulpwns]

Neither. You use value that You want Your motor current draw would be limited to.

Can you clarify that a little or provide an example?

I'm not sure I get it. If by that You mean that You are going to add resistor between power supply and arduino shield, then NO, You should not do that. You should add resistor between shield and motor (just add resistor to one of motor terminals.

yea i was going to place the resistor between the motor and the motor shield.

If rack and pinion stop motor spinning, then yes, You were measuring stall current.

the motor does not stop spinning with the rack and pinion, but the wheels don't turn any further.

[newInRobotics]

Can you clarify that a little or provide an example?

If You want motor to draw no more than 1A from 6V battery, You have to use Resistor with R = V / I = 6V / 1A = 6ohm

the motor does not stop spinning with the rack and pinion, but the wheels don't turn any further.

How is that possible? If wheel reach their turning limit, hence rack cannot move any further, so pinion cannot spin any further as well stalling the motor. How come that wheels turn no more and motor keeps spinning?

[rahulpwns]

the motor does not stop spinning with the rack and pinion, but the wheels don't turn any further.

How is that possible? If wheel reach their turning limit, hence rack cannot move any further, so pinion cannot spin any further as well stalling the motor. How come that wheels turn no more and motor keeps spinning?

The wheels stop turning, but the motor continues to rotate. not sure exactly how it happens, because the mechanism is covered.

Also, where could i find a 6 ohm resistor? or a pack of resistors from 1 - 20 ohms? i have been looking and i simply cannot find one.

[newInRobotics]

Almost always I get my stuff from eBay 

[bens]

Also, where could i find a 6 ohm resistor? or a pack of resistors from 1 - 20 ohms? i have been looking and i simply cannot find one.

Try digikey.  However, using a resistor to limit motor current is certainly not an optimal approach.  For one, if you expect 1A to be flowing through your 6 Ohm resistor, you will need a resistor that can dissipate 6 Watts (P = I^2*R), which starts getting bulky and expensive.  For reference, the standard through-hole resistors you usually see lying around are rated for 1/10 or 1/4 W.  Another reason this is bad is that the resistor wastes power, and the more load you put on your motor, the weaker your motor is going to be, which is usually not the behavior you want.  The better approach is to limit your motor controller's maximum speed so that your motor voltage is effectively less than 6 V (or to get a more appropriate motor controller for your motor).

- Ben