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Can a 3/4 HP electric motor ( from an old garage door opener ) have enough power to be put in a bike and get at least 10 MPH out of it. Also what batteries should i use, i think i have an 18v or 16v battery somewhere..

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. Probably. You may have to help it get moving, but it doesn't take much HP to maintain 10 mph. You'll probably need an inverter to supply 120 VAC (or whatever mains voltage is where you got the motor) to the motor.

so i need a 120 V battery? oh and how much weight will 3/4 hp will able to carry at 10mph. thanks for the help

No, an inverter would convert the 12v DC to 120v AC. It's for when you have batteries, but you need to power something that normally plugs into the wall.

How much does the inverter cost

I'm not sure. One heavy enough to run a 3/4hp motor could easily be \$100-\$200, maybe more. I only have a 400w one and it cost like \$40.

hmm then can I go for a 400w instead , maybe higher?

400w will be way too low. I really think 1000w-1500w would be a bare minimum.

but i saw someone using 400w on youtube yesterday

Maybe so. All I know is that from my experience, 400w won't run a power-drill. It might be enough to maintain your motors speed, but I think it would overload at start-up.

hm then whats the minimum W

. Look on the data plate for Full Load Amps (FLA) or similar wording. . Multiply by Volts (120?). . Multiply that by ~3. That should give you a ballpark figure for how many Watts you need. . If you are not going to use the motor for acceleration (just to maintain speed once you get there), you don't need to multiply by 3.

You're so smart! I knew there had to be a rule to that, but the "X3" thing really makes life easier for me too!

. One of those rule-of-thumb/get-ya-in-the-ballpark things. I've seen x10 used in some applications.

When we were living in New Guinea (without power), I learned a lot about the limitations of inverters. Electrical work has always been my short-suit, so I'm learning about it slowly but painfully.

You'll really have to check your motor. If it doesn't say on the plate, you'll have to use a guage. I can't imagine a start-up wattage of less that 750w or more.

1000-1500 watt ones go for right around a hundred. My dad's 400 was \$15 and my 200 (in a set with spotlight, seatbelt cutter, air pump, battery pack) was around \$18

Oh, THat's good! I haven't priced them lately. Mine's like 12 years old. At least the price of som e things go down!

. The ones for powering laptops, DVD players, &c (50-300W) have gotten dirt cheap as of late. Just about anything under 2.5KW is cheap enough for an Arkie.

They have gone down, but then again these were clearance or rebate items.