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  #16  
Old 11-28-2005, 11:34 AM
C. Beaty C. Beaty is offline
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A 500 watt scooter motor would develop about Ĺ hp. There are 746 watts/hp and small motors operate at efficiencies of around 75%, so as a general rule of thumb, 1 hp consumes 1kw. From a 12v battery, this would be 83 A.

Efficient soft start and speed control of DC motors uses an electronic circuit that switches the power on and off at some rate that does not cause objectionable buzzing; typically more than 100x per second. The ratio of off to on time controls the power to the motor. No problem for low cost solid state switches at 83 A.

However, 12v automobile starter motors can draw upwards of 300 A. with the armature stalled and the cost of solid state switches and the associated freewheeling* diodes at this current level skyrockets.

The automobile industry is in the process of changing over to a 42v standard (thatís the charging voltage, the actual battery voltage is 36v) which will make electric prerotators a practical possibility.

Iíve read that cars with 42v electrical systems are already being sold in the Japanese home market so it wonít be long before we see them here. Top of the line models at first.

*When the current flow to a device such as a motor is suddenly interrupted, an inductive kick occurs; -the same process that makes the sparks in a breaker point ignition system- and must be snubbed. Freewheeling diodes snub the inductive kick and return its energy to the motor.
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  #17  
Old 11-28-2005, 12:52 PM
Al_Hammer Al_Hammer is offline
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AllTrax makes a 12-24 VDC controller (used in golf carts) that will supply 400 amps
for 2 minutes or 200 amps for 1 hour. The throttle sensor can be
a 0-5K pot or a 0-5VDC input. The throttle acceleration curve can
be programmed via an RS232 port. AllTrax does not sell directly to
individuals but CartsZone sell the AXE-2444 for $339 at
http://www.cartszone.com/newspeed.html

The AllTrax site is at
http://www.alltraxinc.com/old/prod01.htm


AXE-2444 Series DC motor controller 12-24 VDC 400A.
Epoxy potted/Waterproof
Throttle ramp programming
Battery over/under voltage cutback
Batt Volt 12-24v
2min 400A
5min 350A
1Hr 200A


Quote:
The motor controller is designed to operate with series wound brush commutated DC motors rated for operation from 12ó72 VDC. Operation with compound and permanent magnet motors is possible.
This motor controller employs modern power MOSFETs to provide extremely low "on" resistance, in both the main switch function and freewheel diode. Synchronous freewheel rectification permits extended high power operation over similar sized controllers due to increased efficiency.
(Synchronous rectifiers are MOSFETs, driven in such a way as to perform a rectifying function. They often take the place of diodes in the output-rectification stage of switching power converters, because of their lower on-state power loss.)

Quote:
Ideally, your motor leads should be less than 4 feet long. Making a twisted pair out of the motor leads will reduce RF emissions.

Use (6AWG) battery interconnect wiring. For 400A controllers, a minimum of 4AWG wire should be used.

High current wiring to the motor controller should use 5/16" mounting hole ring terminals of tinned copper. Bolt them to the controller using 5/16" hardware.
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  #18  
Old 11-28-2005, 05:35 PM
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Quote:
Originally Posted by Al_Hammer
..........
Are starter motors appropriate for use with one of those controllers? Series wound, inductance, etc. etc. ?

If so, that seems like it would make a good alternative to the hydraulic setup I proposed in this thread, as long as you weren't looking to scale the power up too high.
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  #19  
Old 11-28-2005, 06:07 PM
Al_Hammer Al_Hammer is offline
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CypherNinja,

I think starter motors are series wound and will work with the controller, but the question is how hot will the motor get under load?

while it is true that starters can draw hundreds of amps that is generally at
stall; while running they tend to draw 50-100 amps which is probably
around 1hp or so, depending on voltage drop of the battery.

but, starter motors are not designed to run for more than a few seconds. If you run them for any length of time with a load at say 60-70 amps, they get so hot that they smoke.
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  #20  
Old 11-28-2005, 07:38 PM
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Yeah I was aware of that. Starters do indeed have a very short duty cycle. The hydraulic system I mentioned in that other thread could concievably have an unlimited one.

But if your only goal is short bursts of prerotation just prior to take-off, the electric system being discussed here may be a simpler solution than a hydraulic system with an electric clutch, variable flow valve, clutch bearing, etc.

That being said, my opinion is that if your looking to turn some large rotors for anything more than 10 seconds or so (or maybe even partially powered rotors), hydraulic will probably easily win out as you move up the power range.
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  #21  
Old 11-28-2005, 08:15 PM
danmcallister danmcallister is offline
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Bob, I am interested in a soft start system for a automotive starter, I want to use it for reverse on my three wheel motorcycle.
Thanks Dan
danmcallister@cox.net
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  #22  
Old 12-18-2005, 12:33 AM
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M. Pearce M. Pearce is offline
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Hello,
How about just buying an electric 12V trolling motor. The controller is already there for soft starts.

Mark
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  #23  
Old 12-18-2005, 05:42 AM
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Quote:
Originally Posted by CLS447
You guys need to subscribe to Homebuilt Rotorcraft magazine!
Hi,
I've heard about this fabled publication, but so far its been a case of rocking-horse droppings. Any idea where I can find more info? Sounds like a publication I would really enjoy.

Regards,
Duncan
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  #24  
Old 12-18-2005, 09:22 AM
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I would also like some info on this awesome sounding mag.
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  #25  
Old 12-18-2005, 10:19 AM
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Alan Coats Alan Coats is offline
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Homebuilt Rotorcraft
RFI Publishing Group
5555 Zuni S.E., Suite 281
Albuquerque, NM 87108

(505) 323-8455
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  #26  
Old 12-18-2005, 05:00 PM
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What is up with the trolling motor idea, that sounds like a good idea!
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  #27  
Old 12-18-2005, 05:18 PM
Al_Hammer Al_Hammer is offline
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Quote:
Originally Posted by M. Pearce
Hello,
How about just buying an electric 12V trolling motor. The controller is already there for soft starts.

Mark
Yes, it sounds good, but if you run the numbers a trolling motor doesn't have enough power. I looked up some specs.
A Minn-Kota 12V model that draws 50 amps max will deliver about 0.6 hp, assuming 80% efficiency. The current falls off as the rpm rises, so the actual power is even less. The trolling motors have nowhere near enough power.
To get a decent prerotation speed you'd need 3 or 4 of these things at $459.00 each.

On the other hand, if you went to 48 volts you could get as much as 4.5 hp out of a larger type trolling motor from Ray Electric for a mere $3800.00
That's 4 car batteries (and a lot of lead) to lug into the air, plus the motor itself looks too large to hang on the top of the mast.
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  #28  
Old 12-18-2005, 08:05 PM
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Quote:
Originally Posted by Alan Coats
Homebuilt Rotorcraft
RFI Publishing Group
5555 Zuni S.E., Suite 281
Albuquerque, NM 87108

(505) 323-8455
Homebuilt Rotorcraft Magazine moved to Texas some time ago. Don Parham left it in the capable hands of Forum inhibitant Bob Stark.

Homebuilt Rotorcraft
PO Box 428
Olney, Texas 76374

(940) 564-2938
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Last edited by gyroplanes; 12-18-2005 at 08:11 PM.
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  #29  
Old 12-20-2005, 04:33 PM
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you may want to check this out.

http://www.rotordyne.com/parts.htm
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Last edited by gyro; 07-10-2007 at 07:22 AM.
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  #30  
Old 12-20-2005, 06:07 PM
Al_Hammer Al_Hammer is offline
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Jan/Ken Rehler's electric pre-rotator design looks good.
www.rvk-architects.com/Ken/Gyro/Jan.html

I think Ken sent out some drawings of it to a few people on the forum.

If you only want 100 rpm's or so, an electric motor is OK.
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Last edited by Al_Hammer; 12-21-2005 at 06:02 AM.
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