Motor Speed Control


  #1  
Old 07-29-04, 10:21 AM
jakebrown5
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Lightbulb Motor Speed Control

I am currently building a motorized attachment for my scooter. I have an old garage door opener motor, which is 1/3 HP. It runs at 120VAC 60Hz 4.5Amps, which I believe is an equivalent to 540 Watts. I am hoping to hook up a car battery to a 600 Watt inverter, which should take care of my "mobile electricity" problem. Anways, garage door opener motors only run at one speed. DUH! It runs at 1600 RPM, so I'm pretty sure my scooter would literally "burn out". Considering this, I believe I need some sort of speed control device that can accurately control the RPM of the motor. I'm thinking of using a regular lighting dimmer switch. Same principle, right? Would that work? I'm afraid if I use something like that it will just overheat and short out. What do you think?
 
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Old 07-29-04, 10:41 AM
R
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I'm not absolutely certain, but I believe a garage door opener uses a synchronous motor. The speed is determined by the input power frequency...60Hz. You'll need to devise some type of mechanical speed control, I believe. I'm sure one of the resident experts can add to this.
 
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Old 07-29-04, 10:51 AM
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I don't know what kind of garage door opener you have but I wouldn't expect that you could change the speed of the motor much. It's most likely a single phase induction motor and will only run at one speed. Your best bet is to come up with a DC motor. You can easily vary the speed of that. Besides going through a DC to AC inverter will cause an efficiency loss as well as much shorter battery life.
 
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Old 07-29-04, 11:55 AM
jakebrown5
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Yes, jughead, you were correct. The motor is a single-phase induction motor. Are you sure there is absolutely no way to control its speed? I'm kind of stuck here, because I am not willing to pay $100+ for a DC motor. Do you have any suggestions? I guess if I have to I can push the scooter off and get going really fast and then start it up and hold on for dear life! Not my preferred "method of transportation" though. haha
 
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Old 07-29-04, 01:57 PM
G
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First, why go direct drive? You can reduce the speed thru pulleys. Second, have you considered a used starter motor? It's 12 v and could probably run under light load without overheating. I would look at some old electric golf carts and see how they work.
 
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Old 07-29-04, 04:06 PM
jakebrown5
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No, goldstar, I haven't considered that. But now that you bring it up, it does sound like a very cheap alternative. Two problems though:

1. According to this webpage here, it looks like a lot of confusing disassembly would be involved. I am not exactly an auto mechanic.

2. I would still need a speed controller to change the speed of the starter. This website here kind of explains how DC speed control works.

So do you guys think that disassembling a starter would be that difficult? What about the speed controller? Could I just use a simple lamp dimmer switch there?
 
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Old 07-29-04, 05:14 PM
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I'm afraid you need to do quite a bit of basic electrical research.

jakebrown5,

A light dimmer is an ac device.
It takes ac power and then runs it through a simple diode to convert the ac line voltage to dc. The potentiometer then varies the dc voltage to the light.
This is why you can only operate lights and shaded pole motors with heavier windings. Not near enough amperage capability for what you want.

This is a dooable project but extensive mechanical and electrical skills will be required to pull something like this off, not to mention the $100.00 you don't wish to pay is a pittance to what "inexpensive projects" wind up costing in the end.
Just ask me, I know!

Why don't you try to dream up a way to power your scooter with gas.
Gasoline offers very high efficiency when considering the amount of power in a gallon of gas and the weight of the powerplant.
 
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Old 07-29-04, 06:03 PM
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My understanding is that dimmers today use neither a diode nor a potentiometer, but a triac. The triac chops up the wave form. It's still AC but it's not a nice sine wave any more.
 
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Old 07-29-04, 08:10 PM
CSelectric
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John, you are precisely correct, modern dimmers are triac based (generally a dual triac configuration.) Essentially, they operate by cutting power mid cycle. Think of it as a light switch turning on and off very rapidly.

Jake, I don't see this as working for your project. The rapid cycling of power would tend to drastically reduce the life of the motor. Furthermore, the "coast" effect of the motor would likely negate any effect the dimmer may have at modest levels. If you were to "dim" the circuit far enough, you could effectively reduce RPM I would suppose, but it is likely you would overheat the motor. standard induction motors are self cooled, normally by a shaft mounted fan. The fan system is engineered to function at the rated RPM of the motor. Any significant reduction in shaft RPM will lead to overheating. Said overheating would likely be more pronounced in your application because a garage door opener motor is not designed for continuous duty.

It is possible to vary drive speed of induction motors. It is typically accomplished by way of a variable frequency drive control. This is only functional when used in conjunction with a NEMA type B, or similar motor that is insulated and designed to prevent low speed overheating (a garage door opener doesn't meet that criteria.) What's more, a VFD unit will cost you more than the $100 you don't wish to spend on a DC motor.
 
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Old 07-29-04, 09:46 PM
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For your budget, forget about using electronics for speed control. The auto starter is the best way to go but you値l need to use a mechanical method for speed control. What I値l describe is actually better for the starter motor because it is only intended to run for short periods of time. I知 guessing a starter motor for a V8 is about 2-3 hp, which should be plenty for your purpose, if you want more, get one used on a diesel engine. You値l want 3 pulleys, one on the motor, one on the driven wheel axle, and one idler pulley. I think a starter will run somewhere around 2000rpm, so calculate pulley sizes accordingly. The idler pulley is free to move and should be mechanically linked to a hand or foot control to serve as a clutch. A spring should be incorporated acting in opposition to the hand or foot control. View the underside of a belt driven garden tractor to get ideas. The same linkage will be used to activate a switch that you値l have to fabricate from large brass or copper bolts. Go for as much contact area as you can. You could use a Ford starter relay instead. In brief you will operate the motor 5 30 seconds, followed by coasting to control speed.
 
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Old 07-30-04, 09:29 AM
jakebrown5
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1Geniere, what I understand is you're basically telling me to use a starter motor for a couple of seconds when I need the boost and then turn it off when I don't. If that's the case, why not just use a simple ON/OFF switch? I think I'm still gonna use my garage door opener motor though because starter motors are not meant to run more than a few seconds before they overheat. Also they seem to use a lot of battery juice. I guess if my garage door opener motor doesn't work, which I'm sure it won't, I'll try a starter, and if that doesn't work, I'll use GregH's idea and use gas power. I was thinking a weed trimmer motor.
 
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Old 07-30-04, 10:39 AM
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jake,

I too enjoy fiddling with things.
I have been looking for a bargain 12 volt DC hydraulic power pack for a tilting trailer I plan to build. For a year I've been trying to find one that fits my buget and have recently managed to aquire four for almost free!
The motor on these draw a great deal of current and would require a fairly large battery.
Any 12 volt motor could be switched through a relay which would allow the use of even a small microswitch that is DC rated.

I still think a gas engine would be the simplest.
A chain saw would be the easiest gas engine to try.
The shaft is horizontal and already has a chain sprocket of sorts.
Adapting a weed trimmer powerhead would be a bear. The clutch is extremely light and the little square opening in the shaft where the cable fits in would also not be strong enough.
 
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Old 07-30-04, 11:13 AM
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If you can find an old tread mill they usually have DC motors and speed controls in them.
 
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Old 07-30-04, 07:02 PM
scrapiron
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A while back a friend adapted a small chainsaw engine to his bicycle. A chain from the clutch pulled a drive wheel that pressed against the bicycle tire.
 
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Old 07-30-04, 10:21 PM
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For a starter motor you can稚 use simple switch as the motor could draw 300 amperes or more from a standstill then maybe 75-100 on level ground. A simple switch to activate the starter relay I mentioned would be ok. I seriously doubt that the garage door opener has sufficient power; I知 guessing a good one may be ス HP. It may be all right if you set up a big gear or pulley ratio and settle for 5mph on level ground. For the ス HP door motor, you値l want to have reserve in the DC/AC inverter so I壇 use a 600+ watt device. There is no inexpensive way to control the RPM of the induction motor, so you池e still left with that problem. If you use the power coast method with the door opener, switch the AC side as the current will be about 1/10th the DC side.

It just occurred to me, I知 being stupid. If you go with a starter motor that has a 釘endix unit on it, you will not need the idler pulley or a heavy-duty switch, as the solenoid, heavy-duty contacts, and gear disengagement are built into the starter. You could control the speed in bursts with a simple push button switch with a 15-amp rating.
 
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Old 07-31-04, 10:13 AM
jakebrown5
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Correct 1Geniere, what you said was pretty much exactly what I'm going to do. I'll have the garage door opener hooked up to a 600-watt continuous AC to DC power inverter. I calculated that if the motor runs at 540 continuous watts, a 95 amp-hour 12-volt battery will last more than 1.5 hours. I have a 2:1 gear ratio so that I get more torque over speed. I have a simple light switch hooked up to the motor and I simply flip that on when I need a little boost. Sound reasonable?
 
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Old 07-31-04, 03:12 PM
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Good luck and remember the 120vac out put from the inverter can be hazardous. I知 trying to decide if it would be better to connect the frame of the inverter to the chassis of the bike or to insulate one from the other. I can稚 offhand see how all the 120vac components could be isolated from the chassis without great difficulty. I壇 electrically tie all components together so there痴 no voltage difference from one metal surface to another.
 
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Old 08-01-04, 07:08 AM
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You'd be much better off using a old junkyard starter motor for a car. I've seen them used in go carts before. They don't run very fast, but they do run. Your idea to use an old garage door motor wouldn't be my first choice. The big disadvantage there is the inefficiency of the power conversion process. Anytime you change the power from DC to AC you will loose some of that power in the conversion process itself. That's power unavailable to power the motor. Using a starter motor does't involve any conversion so your batter should last a whole lot longer or you can go a whole lot faster.

The comments about light dimmers using triacs are correct.

Garage door motors are usually simple single phase induction motors and don't have a lot of starting torque. That's another disadvantage. AC motors typically found in variable speed electric drills are usually what's called universal motors and have brushes. They are usually just a DC motor run on AC. In that case you can get away with using a speed control on them, but you would still have the power conversion inefficiency problem.

A DC starter motor is typically a series field motor and has excellent starting torque and would be a much better choice for your project. You might have to push start a cart built with a single phase induction motor or at least always start going down hill. Good luck.
 
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Old 08-01-04, 09:21 AM
jakebrown5
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Yes, jughead, but the reason that steered me away from the starter idea was heat. Everyone I ask tells me that starters aren't meant for being run for more than 10 seconds, and will easily overheat. I guess that's why I chose to leave that idea alone.
 
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Old 08-01-04, 10:02 AM
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I've seen it done and heat wasn't a big problem. Starter motors are designed to crank a cold engine in the winter time. Under those conditons you probably shouldn't run the motor for a long period of time because it will get hot. In a scooter application the maximum load will only occur upon intitial starting and acceleration. Once the scooter is up to speed the load is minimal. If the gearing is correct the motor will run at a higher speed and the back EMF from the motor will limit current draw and heat build up. The disadvantages will be speed control and the fact that series field DC motors will self destruct if you try to run them without a load. Starter motors typically have series fields. The advantage will be it's about the cheapest thing out there. In other words, you get what you pay for. For cheap & dirty an automobile starter motor is your best bet. If you wish to have good speed control you have to pay some $$ to obtain a motor designed for that purpose and spend some more $$ to get a speed controller. Everything you need is out there. All it takes is plenty of $$. If you aren't willing to spend your $$ be prepaired to make plenty of compromises. Trying to use a single phase AC induction motor with an inverter is about the worst choice you can make. A small gas engine would be a lot better and maybe even better than a DC starter motor depending on a few other factors.
 
 

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