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Wasn't your deceased h-bridge chip rated at only 2A?
Either the motor is shorted or it is a whopping big one! A big winch? An automobile starter motor?
Assuming 70AMps is correct for these motors, do you think these FET's would work? The Gate threshold voltage is 3-5V, this is the voltage to make it switch on right? They are $6 a piece, but if they work, great!
Then, if the new FETs work better, what about the batteries? I am using 20x 1.2 V Ni-MH cells in series with a 12V voltage regulator (7812). Will they be able to handle the kind of current I will need?
~Mike
Hi Mike,
I looked at Traxxas on the web but they don't have any specs for your motor. You need to know what its maximum current is. Its max continuous operating current is probably 70A but its max momentary current might be 140A!
If you switch the motor with a Mosfet, you better turn it on hard with a gate voltage of at least 10V. At the gate threshold voltage a Mosfet is just barely turned-on, and with a little more gate voltage it will smoke trying to power your motor. After all, if it is turned-on just enough to pass 20A, driving your motor it will have about 10V across it, that's 120W of heat!
Why use 20 cells? That's 24V. You can't drop it to 12V with a 7812 regulator for your motor controller because the 7812's output current is only about 1.5A max. 10 cells will provide 12V but it will drop a lot at 70A unless they are very big cells.
The thinking behind using 20 cells and dropping the voltage down to 12 volts for the motors and 5 volts for the electronics was for longer battery life.
Hopefully I can find some motors that are under 2 amps but still provide enough torque to turn the wheels fast enough.
The thinking behind using 20 cells and dropping the voltage down to 12 volts for the motors and 5 volts for the electronics was for longer battery life.
Hi Mike,
Your thinking would have caused 1/2 the power of the battery to be thrown away as heat. 12V across a linear regulator or even just a resistor and 70A through it creates 840W of wasted heat.
I know what you mean. An electric motor with a higher voltage rating draws less current than a similar motor with a lower voltage rating. Their power is still the same at their rated voltages but the reduced current results in less power wasted heating the motor, transistor switches, wiring and battery.
Of course, you have to purchase more cells for the higher-voltage motor and haul them around but they can be smaller, still produce the same power and have the same run time. With less power wasted, maybe they produce more power and run longer.
Don't give-up on your motors. Model car, airplane and boat manufacturers use them with Mosfet switching. You won't get anywhere near the same torque from a 2A motor, unless its voltage rating allows it to produce the same power. A 12V motor drawing 70A is 840W which is more than 1HP. To get 840W with only 2A requires 420V! The electric motor in a new-version Toyota Prius runs on more than 500V.
I'll have to look up Mosfet switching. So since these motors are originally from a radio control monster truck, they should work for me as long as I choose the right way of getting power to them.
I'll have to look up Mosfet switching. So since these motors are originally from a radio control monster truck, they should work for me as long as I choose the right way of getting power to them.
These are for the Balancing bot. I need them to be able to move quickly in small bursts to keep balance while still being able to move quickly durring forward motion. I will look into the RC motor controllers though.
Thats what I forgot...WIngs! hehe
I guess I did get motors that are a bit overpowered.
I think its because another site with a balancing bot mentioned that I needed a lot of torque, but I guess a lot isn't really a lot.
~Mike
Those RC motors might not be a good fit for your application. High power motors tend to have a lot of "cogging". Cogging is when the motor shaft doesn't turn smoothly - when you twist the shaft it will feel like the resistance to turning is varying widely depending on the angle of rotation. This isn't a problem when you are just trying to spin wheels fast but it's a big problem for position control - like in your balancing bot. For position control you want a motor that spins very smoothly when you twist the shaft - that smooth rotation means that when power is applied the shaft will turn smoothly.
Jameco.com and allelectronics.com both have a decent selection of motors. Unless the motor is fairly high quality (expensive) it probably won't have a spec for cogging. Stay away from motors made from cheap stamped sheet-metal; odds are if it has nice machined parts it is designed for more precision apps. You might try your local electronics shop or hobby store.
I will hopefully return the current motors and get my $50 back. Maybe the shop will have better motors. Before I got the big ones, I got a couple from digikey but they are only 30RPM at 12 volts. They do have a LOT of torque being geared down so much so if I had to, I suppose I could gear them back up slightly to get a faster RPm. But I think I will look for different motors before I try to use the 30RPM's
~Mike
The wheels on my bot are 10.5 inches around the tire and the gears I am using gear down the motor by 3.428 times. Figuring this out, if I want it to be about walking speed I'll need a motor about 1000rpm. at 5mph, I would need a motor of about 1700rpm.
I found this motor at Electronics Goldmine https://www.goldmine-elec-products.com/prodinfo.asp?number=G13367&variation=&aitem=7&mitem=60
Looks like it can do anywhere from 3kRPM at 36v down to 1kRPM at 12v. Looks to be about the same size for the mouting holes and shaft size, but is much longer. It will still fit in my frame and even comes with a built in encoder. If I decide to make it autonomous, it will be able to tell how far it has gone with the encoder.
at 12v, it runs 200mA so I see no reason why this motor wouldnt' work for me.
~Mike
Hi Mike,
Watch out with that old motor. The internet is full of bad stories about it no longer being in Electro-Craft's old catalog and nobody says for sure what its steps resolution is (it's a jerky stepper motor). Making a $500.00 driver circuit for a stepper motor is said to be a can of worms.
One guy says the motor is designed for 60V and might not have much torque at 30V. It will be much less at only 12V. One guy says not to buy a stepper motor unless you have all its spec's and the owner's manual, etc. He says he knows because he has a basement full of motors that he bought but he can't use
Read about it here: https://www.cnczone.com/forums/showthread.php?t=6685
Your info says its no-load current is 200mA and if you apply the current increase with load that it says with 24V (5 times), then you'll have 1A or more at 12V when loaded.
Hi Mike,
I suppose you could call that a DC stepper motor where DC pulses are applied to its coils.
DC linear motors don't have encoders and don't need so may wires for all those stepper coils and encoder sensors.
How many wires are on your powerful DC linear motors: 2.
How many wires are in the ribbon cable for the stepper: 8 to 12.
You might not get your money back for these used old motors.
He, he. I can picture your 'bot wobbling beween steps of the stepper motors! Oops, too far forward. Oops, overshoot again. :lol:
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