Author Options:

Got a 1/2 HP motor Answered

I just got a Got a 1/2 HP motor for free from my school (there were 2 old vacuum pumps that were covered in dirt and have been in a shed for like a year. One of my friend got one and is turning it into a lathe. They are really nice and quiet GE motors. Their bearing just needed some lube and then they are all fixed!). All it needs is a capacitor (can get from ebay for like 15 bucks including shipping) What should I make with it? So far I'm thinking rotary spark gap for my tesla coil (take THAT plamana!) but i'd like to utilize the power more (don't really need 1/2 HP for a spark gap!). Any other ideas? BTW I'm not making a gokart. lol. But it would be cool.... BUT NO


Don't waste a 1/2 HP motor on a rotary spark gap! You can just use a much weaker one and it will still work. If I had the tools you have, I would be making an electric scooter right now. I have all of the parts, but seeing as I can't weld, or do any metalwork, I can't do anything with them.


9 years ago

So you have a motor as strong as this.


Quick question: using PWM can I make the motor go slower? Or will this not work because this motor requires a start cap?

Yes you can, and you can also brake (( Stop it )) electrically and reverse it.... and make it so you don't need the cap ...

if you can create a phase shifted 'copy' of the pwm then maybe you can do without a cap at all some motors can work slower on a standard light dimmer (idk if its good to them though) and some cant (motor / dimmer overheat etc)

electrical raft on wheels power it with car battery and inverter

Exactly 746 watts of electrical power will produce 1 HP if a motor could operate at 100% efficiency, but of course no motor is 100% efficient. A 1 HP motor operating at 84% efficiency will have a total watt consumption of 888 watts. This amounts to 746 watts of usable power and 142 watts loss due to heat, friction, etc. (888 x .84 = 746 = 1 HP).

So your looking at 444 watts for a 1/2 hp and thats not looking at the current required to start the motor... The power grid is easily able to deliver that current how ever an inverter isn't able to, but most new inverters will try and soft start the motor but this can can take several attempts and items like air compressors won't start on a 1000 Watt inverter unless you drain the tank and allow it start under a no load condition... (( and you need more then one battery ))

then is it possible to soft start with resistor and then short it out ?

if you add a resistor all your doing is increasing the load on the inverter your just switching the current going to the motor, which it requires to start it, into a resistor which would have to be huge to get rid of the heat, you could use a electronic soft start / motor speed controllers but they are not the cheapest of items ((( I just played with a Siemans 7.5 hp controller nice web based interface but it cost $9500 USD and it controls only one motor, I don't know what a smaller one sells for but I know a 1000 Watt inverter is only $89 and 3000 Watt is only $259 so I'm guessing a larger inverter is cheaper and in the long run a better investment because it has more uses then a motor speed controller ))) And I often find old UPS's at the recycle center, normally in the 300 to 750 VA range but every once and while you'll run into a 3 or 5 kVA one and I suppose you could use this to power a motor, but they normally require a higher voltage battery pack (( 24-48-60-72-84 or more volts DC ))

how can series resistor increase the load ? you can use electrical room heater and toaster etc as resistors

Yes more heat, more power being consumed, be placing the resistor inline with the motor your increasing the time for it to start up and this is when it consumes the most power, Locked Rotor Amps for a 1/2 HP compressor can be as high or higher then 47 AMPS (( 110 vac ))

its the same as if the motor just does not start the resistor limits tho load to max what it'd be as load on its own the only thing i'd care about is not overheating the motor (higher current + there is fan inside that spins with the motor but is stuck now)

Ok if you have to start a motor with only an inverter (( I just opened my 1000 watt inverter to see it's basic design )) here's what you need to do... The first stage of an inverter is taking the 12 VDC in and switching it at a very high frequency and after going threw the transformer and diodes it is stored in a few small capacitors as about 160 VDC this Capacitor is only a few hundred uF and it needs to be increased to a few thousand uF... This DC voltage is then switched into the AC output used to drive your motor... The increased Capacitor storage will allow you to start the motor with ease, however you may need to address the primary switching transistors (( I actually think they are FET's or IGBT's but I haven't run the numbers yet )) and make sure they can handle the added power requirements needed to charge the caps... Short of a few case mods to allow the caps to fit it is entirely do-able and more efficient then adding resistors.... Or if you wanted nice control of your motor you could use the high voltage DC to run your own motor speed controller and this would allow you to control the torque, RPM, Direction, duty cycle... and you could even run three phase motors with a few more IGBT's

is the overload on the transistors is momentary ? then a larger heatsink may be enough

It is momentary, and heatsink may be all that is required, however saying that, and knowing how these things are built (( with the cheapest components that will get it to work, and while mine is the same make and model as my friends they both have different transistors in them so it's what ever is cheapest that week but thats what you get for a $89CDN 1000 watt inverter )) While power transistors may be able to get rid of large amounts of heat, a very fast very large load going through it for a short time can heat the transistor die to a point where it fails... so I would check the data sheet and see if can handle a larger load.... (( It's better safe then sorry and since you have the case open it's as simple as typing the part number in Google and taking a look )) And who knows you might be able to hook it up and not alter them ... Another thing to look at is the PCB traces and see if they get warm, some times they need a large gauge wire soldered to help them carry large loads, even a good coating of solder will help,.keep them running cool...

We had that problem with an electric air compressor firing up off the generator however the tank was pretty big so it was a time consuming process to let it get up to full pressure again, simply making a little idler pulley on an arm to tension the belt drive all the time unless we hadn't mains running, then let it start free and put the tension back on... Poor generator couldn't keep up...

It's just a simple pulley clutch, some centrifugal clutches are made like this for belts, except they just get bigger themselves... I suppose it's probably the easiest way to transfer power on without stalling on a go kart with a small engine too, you could rig a pedal to that any day.

Is that all?! You've got me feeling optimistic. I've got several electric motors I'd written off as non-viable for vehicles. I can't afford a giant inverter, but a 500W isn't out of the question.

500 watts won't start it (( sorry for being such a wet blanket )) and if it did the motor would have to be under no load... and it might be cheaper finding a 24 volt or 36 volt DC motor, like the ones used in commercial floor cleaning equipment, I've seen some selling for $20 to $30 and they are 1/2 to 3/4 HP((( and yes I'm still kicking my self for not buying them but maybe the guy will get a few more ... )) , and then building yourself a PWM type power supply to give you speed / power / directional control all at the touch of button / rheostat... While this might sound like more work, in the long run you'll go further and have more control plus reverse is a button away,....

something worthless a blanket keeps you warm a wet blanket doesnt kind of being worthless, he should have said rain on your parade, lol

No a wet blanket does mean the same thing... The basis of the terminology is a bit broken though...

there is some issue (found now on inet) 370 W is output input is more 500 W inverter may be on the border measure the thing with KillAWatt first

It's extremely light, but with the right gearing, anything is possible.

is it AC? you can connect it to a lawnmower, when t3h mower runs, it will spin the motor, generating electricity

you know, I might make a gokart in the summer... But i think it's kinda worthless cause I can drive in like a year

A go-cart with a .5 hp engine will only waste time, there isn't enough 'Uhmpff' to get you around very quickly...

Unless, you did some crazy modifications to your gearing! :D