Power AC Electric motor from 12v DC

[mikeinfodoc]

I want to build an electric go-kart for my son.  Just for fun (low power, low speed). 

I hope to design/build the circuit to run a cheap AC electric motor.

http://www.harborfreight.com/2-hp-compressor-duty-motor-67842.html

So a circuit which could convert a 12v DC lead-acid battery, to a 110V AC capable of a 20 Amp load.

I have found a few DC to AC circuits, but most do not handle 20 amp's. 

Also, would a simple square wave work for such a motor?  (aka: would I need to smooth output to a nice sine wave?)

I welcome thoughts, schematics, ideas.


ps: I have used 555's many times in the past in small squarewave circuits (as an FYI).

[iainwhite]

You could use an inverter, like RV campers use to get 120v AC.

However, have you thought of getting some 12v DC motors intended for an electric wheelchair/buggy?  You can pick a pair of them up for about $150 on ebay

[rstofer]

I'm not sure your proposal is practical.  Remember, the inrush current starting the motor will be about 6 times full load current and unless you implement a complete variable frequency drive, you can't vary RPM of an AC induction motor.

Why not look around and buy a used battery powered car or bicycle?  Then the motor, drive and battery will all match.  Craig's List or eBay...
http://www.ebay.com/itm/150-W-24-V-12-V-DC-electric-motor-zy6812-f-scooter-bike-go-kart-or-minibike-New-/161580767866

There are many sources for DC motors of various ratings.  Used golf cart motors come to mind but they are probably too powerful.

Even new, battery toys aren't terribly expensive:
http://www.ebay.com/itm/Kids-Ride-On-ATV-Quad-4-Wheeler-12V-Battery-Power-Electric-Led-Lights-and-Music-/361487281839

[Benta]

Single phase AC motors are not really suited for variable speed control. My vote goes to a DC motor as well.

[KD0CAC John]

A number of issues here , 1st off unless you know something nobody else knows , follow what is being done in the field - battery powered motion , unless its for the purpose of learning ?
Already touch on , efficiency - using battery / DC & converting is not good for multiple reasons , adding cost - in this case also wastes energy [ the conversion ] , add weight [ efficiency again ] and loss's .
Then when I looked into using remote power / motors , the number / types of motors out there & some do , or do not work with a variety of electrical types , AC , DC etc. , and applications , also mentioned earlier . 

[bills]

How about a long extension cord

[james_s]

Don't forget that to turn 12VDC into 120VAC you're going to need 10 times the current from the battery as you deliver to the load, and that's ignoring losses. So 200A from a 12V battery, that's not going to run for very long.

If you really want to use an AC motor then a 3 phase motor with a VFD is what you want. Otherwise look at BLDC motors, there are some really powerful ones available for RC airplanes these days that are not even all that expensive. I've seen them up to about 10kW, much smaller and lighter than AC induction motors, easy to control the speed. The LiPo and LiFe batteries used for RC cars/boats/planes have far higher power density than lead acid too.

[Seekonk]

Plenty of 90V permanent magnet DC motors around. Maybe add a second battery or a voltage doubler.

[mikeinfodoc]

A number of issues here , 1st off unless you know something nobody else knows , follow what is being done in the field - battery powered motion , unless its for the purpose of learning ?
Already touch on , efficiency - using battery / DC & converting is not good for multiple reasons , adding cost - in this case also wastes energy [ the conversion ] , add weight [ efficiency again ] and loss's .
Then when I looked into using remote power / motors , the number / types of motors out there & some do , or do not work with a variety of electrical types , AC , DC etc. , and applications , also mentioned earlier .

Yes, this was mainly for learning, and stretching my limited skills.  However, based on all the feedback so far, it would seem this DC2AC idea (for a cheap single phase ac motor) is really not worth it, nor practical. 

[mikeinfodoc]

Plenty of 90V permanent magnet DC motors around. Maybe add a second battery or a voltage doubler.

Thank you for the tip.  What would a decent HP rating be, for a small go-kart?     (which we would ride around the yard)

[mikeinfodoc]

Don't forget that to turn 12VDC into 120VAC you're going to need 10 times the current from the battery as you deliver to the load, and that's ignoring losses. So 200A from a 12V battery, that's not going to run for very long.

If you really want to use an AC motor then a 3 phase motor with a VFD is what you want. Otherwise look at BLDC motors, there are some really powerful ones available for RC airplanes these days that are not even all that expensive. I've seen them up to about 10kW, much smaller and lighter than AC induction motors, easy to control the speed. The LiPo and LiFe batteries used for RC cars/boats/planes have far higher power density than lead acid too.

Thank you.  So I suspect I would need something like this to control a 3phase VFD motor, correct? 
https://www.amazon.com/Ten-high-3000w-Variable-Frequency-Inverter/dp/B00BKWKR50/ref=pd_lpo_200_tr_t_3?_encoding=UTF8&psc=1&refRID=QD6Y4KDQPWS85KT1F57Q

Also, do you have any links for a decent power BLDC motor? (I was not having much luck finding...)

[BrianHG]

Find an old/used/refurb treadmill motor.  They usually come in 2 flavors, 12vdc and 120vdc.  You can find these for 25$ with a little effort.  For your throttle, you can use a PWM with IGBTs & heavy duty inductor for the 12v motor, if you are using a higher DC voltage source, say above 60v, use the 120v motor and a Mosfet switching circuit.  Careful about rating HP.  Electric motors fed to the wheels is not like a gaz motor, you will get really fast really quick, even feeding 60v into a 120v motor.  Unless you are very heavy, feeding 12v into a 12v treadmill motor may peel rubber if geared accordingly.  You will want high current wiring as well not to loose power, we are talking in the neighborhood of 4 gauge wiring with huge lugs at all the connections.  This is the advantage of high voltage, you wiring thickness goes down, but if you are using an inverter, you will want even thicker wiring between the battery and inverter, like 2 gauge since this is where all your losses will be.  This is the el-cheapo method.

Your other solution is getting a BLDC hub bicycle/scooter motor.  These motors are the wheel and use neodymium superconducting magnets, brushless 3 phase.  They are as efficient as you will get.  There are many controllers for these designed to run off of Lipo battery packs at 48v & 72v and they have built in throttle, battery charge display & regenerative braking.  Many are available on Alibaba & you can hit speeds over 30mph with even the 48v ones.  However, expect to dish out in the neighborhood of 500$ for a semi decent unit not including the batteries.

For the DC 12 treadmill motor, you can use a 555 to pwm it, but, I would recommend a MCU so with your throttle control, you can limit current, limit acceleration, limit overheating of IGBT and monitor battery voltage as well as some other simple useful functions.

A good treadmill motor should be rated at least at 2hp continuous.  I know my 120v treadmill motor is rated at 2.5hp continuous, and was designed to be driven with a square wave PWM as stated on the motor.

For a cheaper 1hp treadmill motor, if your go-cart is designed well, it will achieve 30mph.  (For this guy, you can use 8 gauge cabling and still get good response.  This will operate max in the 1kw range)

[CCitizenTO]

Oh so PWM is how you control the speed? Is it typically linear? IE 10% duty cycle = 10% of maximum speed? Also gives a good means to limit the maximum speed as well I suppose if you set it up so the maximum is a 50% duty cycle then the go-kart would only go 15MPH instead of 30MPH? In the event you want some sort of 'kids mode' on it.

[mikeinfodoc]

I have created robots which use small motors controlled via an h-bridge (LM293 if I recall).   Those small inexpensive chips were very limited in amps, but they could handle controlling 2 motors.

I simply sent pwm to the chip to control the speed.  In fact, I have a bluetooth module on the robot which communicated to a small android app I wrote, which allowed for basic steering and speed control.

The higher the speed the more the duty cycle.  It was pretty simple actually, as the motor just goes when it is told to go, and its like flipping a light switch on and off very rapidly.  if you do it fast enough, it appears that the light stays on (but dimmer), etc...

if you want to know or see more about the pwm coding, let me know.

On that note, I am curious what the electronic experts on this forum have used for higher amp motor controllers?  (aka: a small go-kart electric motor)

[james_s]

I actually have a used treadmill motor you can have for free if you want, just pay shipping. It's heavy but I think it will fit into a large USPS flat rate box. Forgot I even had it until I was digging around in my garage.

[mikeinfodoc]

Wow thank you for the offer.

I will take you up on that (as I hope to start the build sometime this summer). 

My email is mikeinfodoc@gmail.com