I am trying to power 2 identical brushed DC motors (they are dynamite 15T motors for reference) with minimal loading (connected to plastic propellers) using a 5000mAh 8.4V NiMH battery. I have connected my arduino uno to an L293 motor shield h bridge, however the motors are not turning.
When the battery is connected directly to the motor leads, the propellers turn quite rapidly, which indicates to me that the motors and battery are each individually working correctly. When connected to the motor shield however, the motors do not recieve enough power to even move, and appear to be stuck in deadband even when maximum signal amplitude is requested.
The battery leads are connected to the external power bus on the motor shield, and the jumper plug has been removed to ensure that the power is being drawn from the NiMH battery and not the 9V battery pack powering the arduino through the barrel connector.
In order to verify that the code was working properly I measured the outputs of the motor shield using a multimeter to ensure that a voltage was being supplied to the motors as specified in the code. The AFMotors library was imported and used being that the shield is a V1. A 64KHz pwm signal was supplied to the motors at varying signal amplitudes in order to verify the code. There was a measurable voltage as well as an audible hum of the motors that matched the timing in the code, indicating that the code is correct. It was verified that the voltage supply of the batttery was around 9.6V, however even when sending an analog signal of 255 only a maximum of 0.5V was delivered to either motor.
I am wondering why only 0.5V is being supplied when it should be closer to 8V? I have looked at all of the available documentation and I cannot seem to determine how much current the motors are drawing, so that may be a problem, and if that is the case, then what is my best approach to be able to control these motors using this battery with an Arduino?
If you mean 15 turn 540-size car/buggy motors then on 8.4V there's a fair chance they are drawing 20A or more depending on the size of the "plastic propellers". I've had even standard 22T motors drawing that on "plastic propellers". They will definitely be trying to draw a lot more than the roughly 1A maximum for a L293 because their no-load current is over that.
Try a different motor shield with a more practical current limit. I'd suggest at least 10A.
Thank you, that is what I figured. I apologize for the unspecificity of the propeller description, it is 12" long and approximately 1" in width at the widest section near the center. They are not particularly heavy due to being made of what appears to be ABS plastic. Do you know of any good shields or other motor drivers that you would reccomend? I am considering using one of these two:
If the motors will be turning in only one direction, there is no need for a motor shield or driver. A single, logic level MOSFET capable of handling 100 A peak (or more) should do for one motor.
Look for MOSFETs with the lowest possible Rds(on) at a gate voltage of 3-4V. It should be in the range of a few milliOhms.
You still haven't confirmed exactly what motor you have.
But if it is a 15 turn 540 that prop is way too big. At most it will run about a 6 or 7 inch prop at probably over 30A. It's a very high Kv motor intended for use in cars with lots of gearing, certainly not for driving big props.
BTW the load imposed by a propeller has basically nothing to do with the weight of the prop, it's all about the amount of air that it moves and that's related to diameter (length) and pitch (roughly angle of blades).
I do not necessarily need bi-directional control of the motors, so a MOSFET may be a better option. Would this be a suitable choice for one:
If not, would you be able to provide a link to a better one?
As far as the motor type, it is indeed a 540 type 15 turn DC motor (part number DYN1172). The propeller turned at a sufficient speed when the motor was powered with the current battery, so when you say that the propeller is too large, what does that mean? Will the current prop simply draw more electrical current than a 6" or 7" one resulting in less run-time, or could it result in a more severe failure of the motor?
Bracksonjennecke:
As far as the motor type, it is indeed a 540 type 15 turn DC motor (part number DYN1172). The propeller turned at a sufficient speed when the motor was powered with the current battery, so when you say that the propeller is too large, what does that mean? Will the current prop simply draw more electrical current than a 6" or 7" one resulting in less run-time, or could it result in a more severe failure of the motor?
Assuming the motor is not geared a 12" prop on that motor is likely to be trying to draw well over 30A with estimated losses in the motor of around 200W. It will run for a short time before either the windings or the brushes burn out. The only thing that might save it is if the battery voltage drops so much that it reduces the losses but then that won't do the battery much good.
No, this is an RC motor, just get the right brushed-motor ESC for the motor. The current requirements
of RC motors are vast compared to any normal motor, the simplest and cheapest way to drive them is
with the standard RC components for the job. And the relevant LiPo battery for power.
Stall current for most RC motors is in the 100's of amps range, working currents anything from 5 to 50A
is commonplace.
The 15T in the name means 15 turns on the armature, ie the motor electrically is a short thick piece of copper
wire with a very small inductance - its not going to work with Arduino PWM rates due to the low inductance,
and requires switching devices capable of perhaps 100A pulses.
Your typical RC ESC uses many very low on-resistance MOSFETs in parallel for each switch in the H-bridge,
the typical combined on-resistance could be below 1 milliohm for a beefy ESC, which is why they can
handle large currents without popping instantly.
Bracksonjennecke:
Would you suggest using a different type of motor then or should I just attach a gear box to the end of the current motor?
Why don't we go back a step or two and you tell us exactly what it is that you're trying to achieve? Do they have to be 12" props? Why, what are they powering? What speed and thrust do you need? How long do they need to run for?
If I wanted to turn 12" props with a battery I wouldn't be using brushed motors and NiMH battery at all. I'd go straight to brushless motors with lipos and proper ESCs. Much more efficient use of power, and usually cheaper too these days.
