I like it! You even used the rectifier block as a protection device. Sweet.
That 1k resistor on the P-channel IRF1104 gate should pull to ground instead of back to source.
Alternator as a stepper motor
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30th April 2012 11:09 AM #1ClydeCrashKop
Alternator as a stepper motor
I did this because I wanted to computerize my milling machine and lathe, and to prove that I could. The strong stepper motors and drivers are expensive and are rated in HOLDING torque so you don't know what you are getting. If I wanted to hold something I would use disc or drum brakes, not a motor. This Alternator, wired as a stepper motor has 30 inch pounds of real torque. With a string on a 1" radius pulley, it lifted 30 pounds.
It uses a PIC 16F628A with step pulses input on RB0. I just used a 555 timer for adjustable frequency. RB7 can be used for direction eventually.
RB1 thru RB6 are outputs to turn on the appropriate FETs in 3 phase sequence. For each coil, it turns on the low side FET and then pulses the high side, longer at first then diminishing to very short positive pulses. That is so if you are turning it slow, it gets power to step but doesn't get hot waiting for the next step. It uses bootstrap circuits to turn on the high side FETs. You need fast diodes and caps for that. If they make real P channel FETs now that are as easy as the low side, use them instead.
You will want large heat sinks on the FETs. When making your first board, make it easy to replace the FETs until you get it perfected. I let the magic smoke out of a few of them.
Some of the PIC program is thanks to Nigel's tutorials, check them for the PIC part of the circuit. Direction input and turning right are in there from notes my but commented out. I had not used them yet.
I did this in 2007. I may have a basic version for LPT1 from 2005. I'm sure you guys can make lots of improvements.
StepinatorChanged.jpg
The 1k pull down resistors are going to ground now, thanks to Duffy.Last edited by ClydeCrashKop; 1st May 2012 at 10:07 PM. Reason: Improvements
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1st May 2012 04:27 PM #2duffy
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Last edited by duffy; 1st May 2012 at 04:30 PM.
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1st May 2012 04:57 PM Thread Starter #3ClydeCrashKop
Thanks Duffy
I think you are right about the 1k pull down resistors. If none of the low side FETs are on, the pull down resistors are not grounded. I did find out that if the gate floats, it can turn on and cook things.0C:\WHUT ?
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1st May 2012 07:02 PM #4duffy
More importantly, if they are connected to the source terminal you have to add the absolute value of the turn-on gate voltage to the total Drain-Source voltage drop. Then THAT gets multiplied by the Drain-Source current to show the total wattage being burned off in the transistor as heat. So, because the MOSFET isn't as "on" as it can get, it runs hotter than it needs to.
When I'm using steppers, I always use a higher voltage than the rating to improve acceleration and max RPM - the inductive reactance at high stepping speeds really starts to limits the power a stepper can handle. Have you tried driving this at higher voltages?0Please respond to items in forum, not in private messages.
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1st May 2012 10:13 PM #5Sceadwian
duffy, there better be some way to deal heavy loads or stalls from a rotor lock or whole thing will go up in flames fast. Not bad in many situations but it's very application dependant on weather or not that it's safe.
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1st May 2012 10:54 PM Thread Starter #6ClydeCrashKop
I changed the schematic with the pull down resistors going to ground now.
I probably put 24 volts on the rotor but I don't think it helped much.
I thought I was already abusing the poor MOSFETs so I didn't try more voltage on them.0C:\WHUT ?
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2nd May 2012 12:01 AM Thread Starter #7ClydeCrashKop
Hi Sceadwian
The positive pulses shorten quickly for each step, until it is only on about 2% of the time.
I don't know what the holding torque would be but you can stop it or go 1 step at a time.0Last edited by ClydeCrashKop; 2nd May 2012 at 10:53 AM.
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2nd May 2012 03:07 PM #8shortbus=
I've got a dumb question about using this for CNC. How are you getting around the giant steps? Most alternators that I've taken apart only have 12 rotor poles in them. 360° divided by 12 rotor poles is 30° per step, divided by 3 phases = 10°per step. Most steppers used in a CNC are 1.8° (200 steps per revolution). Or am I missing something?
0Everyone is entitled to their own opinion but not their own facts
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2nd May 2012 03:17 PM #9duffy
Open-source mill driver programs have provisions to change the steps/inch settings.
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2nd May 2012 05:01 PM Thread Starter #10ClydeCrashKop
You are right. That is pretty coarse. I planned on gear reduction or cogged timing belts to get at most .001" per step. That would raise the torque quite a bit too.
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