Electric motors for DIY Electric Cars and Go Karts : A Simple Guide
Let’s start with Electric motors- AC vs. DC
Can I use an AC Motor? - you can but you can't use an AC motor in a go-kart or DIY Electric Car . You might think that maybe you can make it work even though some people say it's a bad idea- trust me, I'm one of those people who chases down bad ideas to see what will happen. Don't even bother.
Sure, it would be technically possible, and some electric cars use AC motors, but those are with $10k control systems. The reason is that AC is different from AC. AC stands for 'Alternating Current" and is what comes out of your wall socket. It's used because it transmits long distances better along wires (from the power plant to you) and doesn't electrocute people quite so badly. DC stands for 'Direct Current' and is what comes out of a battery. It's plain electricity, and it's what you want to use for a go kart.
To get more technical, AC is called 'alternating' because the polarity (the + and -) reverses- in the AC in your house, it happens 60 times per second. An AC motor needs this. Now, it is possible to make AC out of DC. Most people have seen inverters, which you can plug into you car's cigarette lighter and then plug in a laptop, blender, whatever. Why not just use one of those?
The answer is current, and power. For a good electric go-kart, your power demands are going to be around 1000 watts or more. 1000 watt inverters are available, but they wouldn't work- why not? Because of surge current. An electric motor is an 'inductive' load. Have you ever seen your kitchen lights dim when the refrigerator or microwave comes on? That's because those are both inductive loads, and inductive loads require a TON of power to start. Say some electric motor might need 250 watts when its running- to start under load (like a go-kart does) it might need 1000 or 1500 watts to start. Your 1000 watts kart motor starting under load might need 5000 watts. Go price a 5000 watter inverter. Yeah, you don't want to do that.
So to be clear, you can't reasonably use any AC motor in a kart unless you want to go no further than your longest extension cord. That means don't bother with any motor marked AC or which comes out of a washing machine, belt sander, or anything that plugs in to the wall. There are two exceptions to this: treadmills and really loud power tools. Most treadmills use a 90v DC motor- the treadmill contains a rectifier which converts the AC to DC. Loud power tools like angle grinders and circular saws use a motor called a 'universal motors' which can operate on either AC or DC.
I wouldn't use either a treadmill or universal motor either. Why not? They are made for 90 to 120 volts (in the US) and not very powerful. While a treadmill motor might seem like it's powerful, consider that you're going to have to carry around at least seven batteries (of car battery size) to get enough voltage and power. It's the same as with the inverter- technically possible, but as a DIY go-kart maker, it's not what you want.
OK, that's all bad news. What's the good news? Well, there are plenty of DC electric go cart motors out there perfect for go-karts. What should you look for in a DC electric go cart motors?
1. Low voltage. The lower the voltage, the fewer batteries you have to carry around. Also, if the rated voltage is lower, you can overvolt the motor, which gives you more power. Say you get a 24v motor- you could run it on 36v and get a lot more power. Could you run it on 48v... or 72v? Yes... but for a very short time. 48v is probably the limit for a 24v motor (double is the rule of thumb for the limit) Why? Well, putting that extra voltage in a electric go cart motor causes extra current to flow, which is where your power comes from. This is a problem because the more current that flows, the hotter the motor gets- and when it gets too hot, it will burn up, explode, and leave you standed.
What happens is that the insulation in the electric go cart motor is rated for a certain lifetime (say 20 years) at a low temperature. If you double that temperature, that rating may drop to say, 1 year. If you get it really, really hot, it might fry in ten seconds. Don't overheat your motors.
You could do a 12v motor at 18v or 24v. You could do a 24v motor at 36 or 48v. You could do a 36v motor at 48v. I wouldn't put more than 48v in a kart for two reasons: weight (batteries are heavy, and 4 12v batteries is about as much as you want to carry around) and safety. 48v is high voltage for DC. A person with dry fingers can touch both terminals of a 12v battery and (probably) not fry themselves. However, do it with 120v house voltage, and you'll get a nasty shock. That's because it takes a certain amount of voltage to overcome your body (especially your skin) resistance. Once there's enough voltage to overcome that resistance, you're being electrocuted. It only takes 0.025 of one amp to stop your heart, and any battery will do that easily. If you're going to make an electric go kart, you need to educate yourself on electricity safety. I won't write that book here, but go read up on it- and don't put more than 48v in a kart unless you've had technician-level training. (Note: I'm not saying 48v is 'safe', but neither are go-karts) Okay, dad safety lecture is over.
Where can you find good motors for electric go karts?
The last thing I'll talk about with electric motors is their power ratings. There are two important things you need to know- electric motors are rated for continuous power, meaning they can make that power all day, all night, for years on end. Gas engines are rated on instantaneous power, which is how much than can produce for a moment. Secondly, electric motors produce maximum torque (the force with which it spins the wheels) at zero RPM. Have you ever ridden a two-stroke dirt-bike? All the power comes around 5000rpm, so you have to wait for the engine to get up to speed, THEN you get power. Electric motors are the opposite- you get all your acceleration at the very start, and it tapers off linearly as you speed up. This makes for very fun take-offs if your batteries, controller, and motor are up to it.
What this means is that you have to think about electric power ratings differently. A Harbor Freight 6.5HP gas motor might be fun, but a 6.5HP electric motor is nearly 5000 watts (746W = 1HP) and will rip your face off and melt your batteries. Sweet. You can use much smaller HP rated electric motor than you would a gas motor, and have the same amount of fun.
So, how do you throttle an electric motor? You have three options: on/off control (likely to fry something), progressive on/off control with multiple batteries, and a controller. On/off control is where you just have a big switch (or more likely, a big relay or contractor) and you get full power as soon as your throw the switch. I wouldn't recommend this, as the surge power phenomenon which I mention above means that you're switching on a LARGE amount of current all at once, and quite frequently what this will do is actually weld the contacts of your switch in the closed position, which now means that you're sitting on a kart which is at full throttle and won't turn off. I know a person who tried something like this on an electric motorcycle and has the scars to prove it. Unless it's small motor and big big switch, I'd avoid this.
How about progressive on/off control? Simply, this means that you are switching on your batteries one at a time. Say you're running a 24v motor, and over volting it to 36v. You'll have three 12v batteries, most likely. What you'll do is have three switches (relays). One will switch on the first 12v battery. The second will switch on both the first and second, giving you 24v. The last will switch all three batteries into the circuit, giving you full power. This is much less likely to kill you... as long as you wire everything up right. I won't draw up a diagram for you, but there are some out there to look up. I'll warn you that if you just draw one up, it's easy to wire things up such that you are dead-shorting a battery, which could weld your contacts cause the battery to explode if you are unable to break the circuit. Be careful. Be careful with this because your first battery to be switched on is going to drain much faster than your last battery. You will need to charge your batteries individually (not in series) and stop driving immediately when your performance with the first battery [/i]only[/i] starts to decline. You will permanently damage your batteries if you over-discharge them.
Lastly, you can use a controller. This is the best option, and predictably the most expensive. Your best bet is a golf cart controller. They are made for duty like this and don't require a special radio input like a brushless controller does (just a potentiometer, which is a simple electronic component). These can be had on ebay, and the brand you're likely to have luck with is Curtis. Do your research on your controller and make sure it's for a PMDC motor (Permanent Magnet, Direct Current) If it's for a series motor, that's OK (and series motors are OK to use) but you'll have to study the wiring diagram carefully and read up to hook everything up properly.
You can also find electric bike and scooter controllers, but these are likely to be too small to use for a 'fun' kart unless you're making something for your eight-year-old that weighs 60lb.
Lastly, you can get motor controllers for combat robots from the same site I linked to for the motors. These are a good option, but expensive again and require a home-made throttle, because they're meant to interface with a radio. This would be a good option if you found a cheap big DC motor and don't mind spending some $$$ to get to use it.
You may not over volt controllers. The max nominal rating is the max rating, and that's it. A controller can be instantly destroyed if its voltage rating is exceeded, even more a moment. Manufacturers build in a little bit of leeway because a 24v battery bank will be more like 28v when it's fresh off of the charger, but the rated voltage is all you can use.
The last thing I'll talk about is batteries.
Unless you're more advanced than someone who needs the info in this post, you're going to use lead-acid batteries. This is the same technology as a car battery. Don't use car batteries, though, because they're the wrong type. There are two kind of batteries here- starting batteries and deep-discharge batteries. A car battery has to supply an enormous amount of current for about three seconds when you start the car, then spends the rest of its life either being charged by the alternator, or supplying a microscopic amount of current to keep your car radio presets in memory. Car batteries are built for this duty, and if you try to use them on a kart, you'll have fun for about ten minutes, then the batteries will die- and not just being discharged, they'll be permanently damaged. Don't try this unless you want to be disappointed or will be happy with a short-lived, expensive project. If you've got a stack of car batteries you could use them for testing, but that's about it. Also, car batteries contain liquid sulfuric acid, which can spill out more easily than you think. If it gets on you, it will make you go blind, burn you, refinance your mortgage at 10%, key your car, and punch you in the gut. Don't mess with acid.
By contrast, you want to be able to ride your cart for, say, 30-60 minutes drawing a moderate amount of current the whole time. For this, you need a deep-discharge battery. The only car batteries that are good for this are Optima Yellowtop or Bluetop batteries, or similar. They don't have liquid acid inside and are made for deep discharge. These are great batteries to use if you can afford them. You can find other lead-acid batteries called AGM, or Absorbed Glass Mat. These are like sealed lead acid (see below) except they electrolyte (acid) is absorbed up in fiberglass mats inside the battery, making them shock resistant. AGMs are typically high quality and high cost.
Also, you can use a sealed lead acid battery (SLA). These are great, and probably what I'd use. You'll be tempted to buy the small ones- they come in tiny, affordable sizes that are complete crap for kart use. You want the big ones. At minimum, 12Ah for a small scooter-motor kart ridden by your eight year old, and 18-30Ah or more for bigger karts. More battery is better 99% of the time. Until you get to the point that your kart has so much battery that it weighs the same as a brontosaurus, more batteries are going to help.
Why? Because of current, again. Karts require a lot of current. Small batteries put out a small amount of current happily, or a large amount of current, and then die immediately. If you don't want to be limited in performance and killing your batteries dead, use big batteries. The same way that over discharging your batteries by running them completely flat will kill them dead, over discharging by asking for too much current at once will quickly kill them.
On top of that, asking for lots of current will reduce how long you can ride- because of something called the Peukert effect, drawing a lot of power from a battery effectively reduces how long it will last. An SLA battery is generally rated at a 20-hour discharge rate. So it may have 18Ah of juice in it... but only if you ask for it slowly over 20 hours. If you ask for all of its juice in 30 minutes, you may only really get 10Ah out of it. (I pulled that number out of thin air). It's a pretty significant effect, though.
Okay, what are Ah? Ah stands for Amp-hours. If a battery is rated at 18Ah, it can put out one amp for 18 hours, or if you ignore the Peukert effect I just explained, 18 amps for one hour. Or 9 amps for 2 hours. Get it? You might also see batteries rated by 'RC' or Reserve Capacity. This is how many minutes the batteries will last at 25A discharge (that is, if your alternator gives out and you need your headlights and engine control unit). You can convert RC to Ah with simple math- if you're embarking on an electric kart build, figuring that one out should be something you can do.
What about CCA and CA? These are not ratings of how long a battery will last, or ratings that you will see on batteries that you want to use in a kart. Note above where I talked about starting batteries vs. deep discharge batteries. CCA stands for 'Cold Cranking Amps' and is a measure of how much current a battery can put put for just an instant when it's cold. (CA is the same thing, but not as cold- cold affect batteries) Generally, only starting batteries are rated for CCA or CA. There are some dual-use batteries that might be rated for CCA and still be deep-discharge, but these are more expensive and you can do better with a properly-sized deep-discharge battery. Big wheelchair batteries are super for most karts.
How do you charge your batteries? I'd recommend getting several normal 12v car battery chargers and charging each battery that way, or using one and doing each battery after the last. (That takes forever) If you can find one or afford one, a golf cart battery charger that matches your voltage is the best thing.
Some basic stuff to round it out:
Wiring something in parallel means + to +, - to -. You'll get the same voltage, but more current and capacity.
Wiring something in series mean + to -, and then you take your power off of the other + and -. You get more voltage (it adds) but no extra current or capacity.
Current measure how much electricity is flowing, like the rate of water through a pipe. Current is measured in amps.
Voltage measures how much electrical 'force' there is, like the pressure of water in a pipe. Voltage is measured in volts.
Power is a combination of the two, and is like measuring both- how much water is flowing through the pipe and with how much force. Power is measured in watts, and volts times amps equals watts. You can also go backwards- a 500 watt motor at 24v will need 500W divided by 24V = 20.8A theoretically but in practice will need more, due to efficiency losses. 70% is a fair estimate for motor efficiency, so really it'd be around 20.8 divided 70% (0.70) = 29.7A.
Okay, that's a good starting point for what you need to know for electric power systems on karts.
So you want to make a fun electric go-kart? Inclined to do overkill? Here's your recipe, pre-overkilled:
Get an electric golf cart motor, it will probably be 36v rated and lots of power. Get a 48v-rated golf cart controller and four Optima Yellowtop batteries. Strap this all to a frame of your preferred format and go have fun.
Want to make a fun little electric kart for your kid that's outgrown his powerwheels? (Or are you skinny?) Get one of those 900W scooter motors and run it at 36V instead of 24V, and use the 18Ah SLAs that are common for wheelchairs.
There you go. Go for it and make some cool electric karts.
Some Solutions
Briggs & Stratton Etek motor
Motenergy ME1305 PMAC Motor, 24-48V, 6 hp cont, 15 hp pk (replaces ME0907)
Alltrax esc SR48300
48V 100ah lithium batteries