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Old 01-07-2009, 01:29 AM   #131 (permalink)
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Originally Posted by Christ View Post
So if at 120v 20A the motor spins 500RPM and creates 50lbft TQ, and I switch to 240V 10A, It will create the same 50lbft, but at 1000RPM?
I don't think it works that way. I've been thinking about this all day! It has given me a headache, and here's what I think I know:

The Torque-RPM curve is approximately a straight line for a DC motor. At 0 rpm, the torque is highest, and at max rpm, the torque is 0.

At 240v, the MAX rpm is 2 times the MAX rpm at 120v. Also, at 20 amps, the MAX torque is 2 times the MAX torque at 10 amps. So, I attached a picture of the graph of the torque-rpm lines for 120v @ 20amps and 240v @ 10amps.

In the picture, w120 is the max rpm at 120 volts, and T10 is the torque at 10 amps (2*T10 is the MAX torque at 20 amps and 2*w120 is the MAX rpm at 240 volts). Their rate of rotation and torque are the same at one point, when the RPM is 2/3 of w120 (algebra! Find where 2 lines cross! hahaha!)

There are some torques that the higher voltage setting just can't match, and some speeds the higher current setting can't match.

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Old 01-07-2009, 01:38 AM   #132 (permalink)
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Quote:
Originally Posted by MPaulHolmes View Post
I don't think it works that way. I've been thinking about this all day! It has given me a headache, and here's what I think I know:

The Torque-RPM curve is approximately a straight line for a DC motor. At 0 rpm, the torque is highest, and at max rpm, the torque is 0.

At 240v, the MAX rpm is 2 times the MAX rpm at 120v. Also, at 20 amps, the MAX torque is 2 times the MAX torque at 10 amps. So, I attached a picture of the graph of the torque-rpm lines for 120v @ 20amps and 240v @ 10amps.

In the picture, w120 is the max rpm at 120 volts, and T10 is the torque at 10 amps (2*T10 is the MAX torque at 20 amps and 2*w120 is the MAX rpm at 240 volts). Their rate of rotation and torque are the same at one point, when the RPM is 2/3 of w120 (algebra! Find where 2 lines cross! hahaha!)

There are some torques that the higher voltage setting just can't match, and some speeds the higher current setting can't match.
Oops... forgot to specify - I was asking about AC motors... dunno if that makes a difference.
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Old 01-07-2009, 01:47 AM   #133 (permalink)
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Some pictures of the mosfet's gate, source, drain

The picture of the rising pulse that has the little static about a third of the way up is From gate to source. The gate has some capacitance, so it takes just a moment for the gate's capacitor to fill up before the gate's voltage can rise enough to turn on the mosfet.

The picture of the dropping pulse with the little knee on it (I think it's called the miller knee, or the people's elbow. Can you smell what the rock has cooking?) is the voltage from gate to source. As the gate voltage drops, at the knee the capacitance needs to drain away so the gate voltage can drop enough to shut off the mosfet.

Another picture is from drain to source, and other stuff that I forgot now. hehe.

If there are any mistakes in the explanations above, I'm sorry. I think it's right, but I'm just learning. I'm not an engineer, Jim, I'm a doctor. I can't do it. I don't have the power.
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Old 01-07-2009, 08:57 AM   #134 (permalink)
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Quote:
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Oops... forgot to specify - I was asking about AC motors... dunno if that makes a difference.
AC motors are a different animal.

They change speed by varying the frequency to them. AC motors don't have the same torque that a series wound DC does.
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Old 01-07-2009, 10:36 AM   #135 (permalink)
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I'm not an engineer, Jim, I'm a doctor. I can't do it. I don't have the power.
DANG IT, Bones, we only have 3 minutes and 17 seconds to repair the shuttle, heal the Ambassador from the fatal disease, and get back to the Enterprise VIII in time to apply Spock's matter-antimatter implosion to take us back in time to save the Expendable Crewwoman that I've fallen in love with - and I've just taken up the 17 seconds explaining it all!!!

The show must go on! Millions of Americans back there on Earth must have something to watch in our time slot next week!

Use your phaser, set it for reverse-inverse capacitance, tap it into Michael's SuperBeetle 500000's DC Controller, aim it directly at the Ambassador's jugular vein, and let's KILL THAT VIRUS!!!

"3 minutes until show's end, Captain..."

(2 minutes and 59.9 seconds later...) >
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Old 01-07-2009, 10:48 AM   #136 (permalink)
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Thanks for that explaination. I knew how they operated, but did not know that the transient would generate so much heat. So the frequency of switching is a trade off between efficiency (heat generated on the board) and smoothness/sound. ie, you COULD switch it at 1Hz and generate practically no heat, but shake your car to death, 1kHz and hear an annoying buzz with very little heat, or 4kHz like you and generate some heat that can be dissapated and there's some vibration/dog whistle or 1Mhz with huge heat loss and perfect smoothness....

I guess you'll have to do some tuning

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Originally Posted by MPaulHolmes View Post
Great question! There's 3 pins. Gate, Source, and Drain. Source is connected to ground. When gate is near 0 volts (compared to source, which is ground), the resistance from Drain to Source is around 1 million ohms or something ridiculously big, so basically no current flows from Drain to Source. Once gate gets over like 4 volts, the resistance from Drain to source suddenly plummets to like 0.004 Ohms. That's when huge current (they can handle like 100 amps or even more!) goes from drain to source.

So, there is almost no heat loss in the mosfet when the gate is 0v (no current flowing from drain to source), and almost no heat loss when the gate is over like 4 volts, but when the mosfet is transitioning from off to on, or on to off, there is a huge heat loss. So, you want the switch to happen as fast as possible, to minimize "switching losses".

Usually, to turn on the mosfet, people set the gate to at least 12v (but less than like 18v), because the gate behaves like a small capacitor. You want to fill it up as fast as possible so the voltage from gate to source gets above the turn on point as fast as possible.

So, a mosfet is an on/off switch that HATES to be in the transition state between off and on.
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Old 01-07-2009, 12:13 PM   #137 (permalink)
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Ben - You're killin me... LOL. I was hoping to get an answer closer to "yeah, something like that..." I guess it's back to the webpages for me!

Matt - 1hz would be annoying, yes. But, I doubt it would "shake your car to death", simply b/c the controller just couldn't generate a frequency that low strong enough to shake much of anything, especially w/o destroying itself first.

If you could manage to get it to work that low - without killing itself - you could just rubber mount the controller and take care of that vibration. You can't hear (on average) below 20hz anyway, so you'd be in quiet while you were driving, and 1hz has a backfrequency most of the time that blocks resonance in the air as well.

Get a sound program and play 1hz on your speakers in a quiet room as loud as you dare... you'll hear a "whoosh" noise every second. If you do this in a room that has normal ambient noise, you'll lose the "whoosh" noise along with some other sound you should probably normally hear.
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Old 01-07-2009, 01:13 PM   #138 (permalink)
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Quote:
Originally Posted by MazdaMatt View Post
Thanks for that explaination. I knew how they operated, but did not know that the transient would generate so much heat. So the frequency of switching is a trade off between efficiency (heat generated on the board) and smoothness/sound. ie, you COULD switch it at 1Hz and generate practically no heat, but shake your car to death, 1kHz and hear an annoying buzz with very little heat, or 4kHz like you and generate some heat that can be dissapated and there's some vibration/dog whistle or 1Mhz with huge heat loss and perfect smoothness....

I guess you'll have to do some tuning
Ya, I'm thinking of maybe just biting the bullet and running it at 16 kHz like everyone else. I don't know. I found out that lower switching frequencies causes heating of the capacitors (more voltage ripple). However, I'll be using quite a few of them, but still, maybe it would be best to raise the frequency a bit. It's easy to do. Like 1 like of code or something. It will change the number of possible throttle positions to 512 if I run it at 8 kHz, and 256 possible positions if it's run at 16 kHz. I think 256 different settings would be plenty smooth.
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Old 01-07-2009, 01:33 PM   #139 (permalink)
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Seeing as how Citicars only have 3 speeds, 256 would be plenty!

You might also want to check how you do a "speed of response" to the throttle.

The Alltrax controller on my motorcycle has a slider control in the programming interface to make the controller LESS responsive to the throttle.

I used this feature to make the throttle slightly less responsive, because it was otherwise hard to hold a steady speed in town. I would hit a bump, my hand would twist the throttle the tiniest bit, which would make the motorcycles speed jerk up and down quickly, which, would further shake my hand.

Kinda the same feeling as when you let out the clutch in 1st gear in a car wrong and it does that front/back lurch.

This shouldn't be nearly as big of a deal in a car, as you are still using a transmission, but on my motorcycle - directly driven by a chain, it could have been an issue without that adjustment.
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Old 01-07-2009, 01:47 PM   #140 (permalink)
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Our internet bake sale has raised enough money for 3 Mosfets!



Thanks to everyone who has given so far!

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