Low Speed Motor Performance Amps vs. Volts

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brightwhite
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Low Speed Motor Performance Amps vs. Volts

I just built a solar powered R/C car as seen here and I would like to make the best of the motor and solar panel
http://www.instructables.com/id/Building_a_Solar_Powered_RC_car

I recently heard of Maximum Power Point Tracking devices used to keep the load seen by the solar panel at or below the "Maximum" so that the voltage will not drop and the total wattage is less.

I'll let some of the better gifted writters explain MPPT.
http://www.mrsolar.com/faq/faq27.php
http://www.solarfreaks.com/download/file.php?id=148
http://dreamtec-rc.com/product_info.php/cPath/24/products_id/48

I do understand that if I exceed the Maximum Power Point load the total power is reduced. On my car if I give the motor hard throttle, sometimes the voltage drop is so great, the radio system and motor controller will shut down. To avoid this, I just geared the motor lower which affect top speed.

What I do not understand is do motors act differently with lower voltage and higher amps vs. higher voltage and lower amps and the total wattage is the same.

I will provide some information about my solar car:
Panel is 9.9V open circut, 3.6A dead short. My caluclated Maximum power point is 8.4V at 3amps or about 25watts. At dead short the voltage is 3V so the power is about 10W. The motor has 0.333ohm of resistance per phase and is rated for 5A continous current.

I just purchased the EasyMax for my solar car and when the device senses a higher load, it will convert my power into a lower voltage current with greater load. As the load lessens, the voltage is returned to normal. While I am waiting for the post man to come with the device I wanted to know if the device will really improve accelleration by providing more amps at a lower voltage compared to a higher voltage at lower amps. This situation really only occurs on standing starts or comming onto the throttle after a corner. If the device just prevents my motor from exceeding the MPP it is still worth it to me as I want to make the most of my panel.

LinkOfHyrule
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Re: Low Speed Motor Performance Amps vs. Volts

Ha ha ha I did the same thing with a cheap $15 car from Radio Shack and about 5W worth of cells for a science project myself.

From what I can tell, the MPPT thing is just a fancy voltage converter that takes the total output power of the panels and converts it to whatever voltage and current it needed.

Another possible solution to your problem is a small battery in parallel with the cells. It would take the load during acceleration and keep things from cutting out. Though, in an application this small, you might try a supercapacitor, too. I experimented with a 1.5V 50F cap on the same Radio Shack car with positive results.

The author of this post isn't responsible for any injury, disability or dismemberment, death, financial loss, illness, addiction, hereditary disease, or any other undesirable consequence or general misfortune resulting from use of the "information" contai

brightwhite
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Re: Low Speed Motor Performance Amps vs. Volts

the car is to be completely solar powered by the rules provided by Solar R/C Cup
The rules allow for up 10,000uf capacitior which I have installed.

Thank you for your honest opinion. I was thinking the MPPT for my application is a fancy DC-DC convertor with a current limiting capibilty so not to exceed the solar panel's sweet spot.

For battery charging, there is proof that MPPT's can extract up to 30% more power than just a panel and a charge controller.

So does anyone know if motors at low speed are more efficient with lower volts and higher amps or it does not matter as long as the voltage and amps are within the range of the motor.

LinkOfHyrule
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Re: Low Speed Motor Performance Amps vs. Volts

In theory, it doesn't matter. In reality, higher volts and less amps is usually more efficient (to a point). What's really important to efficiency is how close the motor is to its no-load speed for a given voltage. In other words, volt up, gear down, is the general rule...

You're running a speed controller, right? The motor effectively sees a voltage directly proportional to the duty cycle of the PWM signal coming out of the controller. So as long as the motor's not bogged down much (accelerating, hill climbing), it should be about as efficient as anywhere else in its RPM range.

The author of this post isn't responsible for any injury, disability or dismemberment, death, financial loss, illness, addiction, hereditary disease, or any other undesirable consequence or general misfortune resulting from use of the "information" contai

brightwhite
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Re: Low Speed Motor Performance Amps vs. Volts

For the motor it has a IO of 0.3A or 10% of my max current of 3A. My first motor had an IO of 0.85A and yes the solar panels would get the car to roughly the same top speed but the car had issues with starting from a stand still.

You are correct with the motor controller uses PWM so the if I could set the controller to keep the amps at 3 or less I wouldn't need a MPPT to keep the car from stalling and powering off from low voltage.

chas_stevenson
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Re: Low Speed Motor Performance Amps vs. Volts

I am an old RC racer a 7 time NASCAR Champ and a 1 time Sprint Car Champ. The problem you are having is not new to RC cars. During start up the motor wants to draw all the power the battery or power source can deliver. I am guessing you have the 10,000 uf cap across the solar cell which is not bad for the motor but the real problem is the receiver is being starved for power. We used a cap across the power wires for the receiver and a diode so the current could not flow back to the battery/power source so when the PWM speed controller pulled the power for the motor the receiver could run from the cap for a very short time keeping it from glitching. Remember the diode must be between the cap and the power source or the cap will just discharge back into the power source and you gain nothing.

Good Luck and let me know if this helps.

Grandpa Chas S.

brightwhite
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Re: Low Speed Motor Performance Amps vs. Volts

The power to the RX is porvided by the Motor Controller (ESC). The ESC has a 5V regulator that feeds the RX power. I assume power can not flow backwards in that circut so I do not need a Diode. I can install a diode if suspect I do have a problem.

The RX is a 3 channel Spekturm 3500 micro rated for as low as 3V use, 6V nominal. It has 4 slots for plugging in servos/esc's. The 4th slot is a battery slot. I took a 3300uf 6.6V capacitor and crimped on a servo plug and inserted directly into the 4th slot. The ESC also has a on board 66uf capacitor bank. For the brief moment a voltage drop occurs on my panel, the RX does not shut off if I release the throttle or the solar car has accellerated beyond the high load point on the motor.

Chas, 7 time NASCAR Champ? Do you think you can get a a race organized for solar r/c cars for this summer?

chas_stevenson
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Re: Low Speed Motor Performance Amps vs. Volts

Yes, 7 time NASCAR Champ. We ran a NASCAR event in our city for many years. Everyone ran the same motor, supplied by the race director, and a 6-cell battery pack. A season was 12 weeks long where we raced 400 to 500 lap races. Yes this required a pit stop, 2 in fact, to go that far. My son was my pit crew and could change the battery pack in about 4 seconds. He also held the track record for pit stops. We made a great team but that was 15 years ago, so organizing a race is not something I have done for a long time.

The glitch problem may also be caused by the motor on the car. During heavy acceleration the brushes may cause noise which can interfere with the RX. We used a very small .22uf cap across the motor to stop this problem also.

good luck,
Grandpa Chas S.

brightwhite
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Re: Low Speed Motor Performance Amps vs. Volts

My setup is brushless so no need for film caps to catch noise. Startups are in issue. The feedback noise is hard for the controller to pickup when the RPM and amps are low and the controller is sometimes lost when to start teh next phase.

Two weeks ago I ran a 500 lap run till it stops oval enduro with a 4-cell NiMh pack. My battery pack made it 415 laps and the leader 427. It took over 90 mins to get that far. I could have gone farther but some early tangles robbed some capcity and my motor controller was set with high punch control so towards the end the controller was limiting the current to the motor and that is not what I needed when trying to get every second off the battery.

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