nedfunnell
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Howdy everybody,
I stumbled across this forum today and looked in here on the electrics section. I saw some repeated and easily-answerable questions so I thought I'd write up a quick guide to electric power systems for those who want to make an electric cart. I'll explain the basics of electric motors and batteries, and also the best way to control them.
To start: Electric motors- AC vs. DC
I'll just put this plainly- you can't use an AC motor in a go-kart. 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. 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.
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 motors out there perfect for go-karts. What should you look for in a DC motor?
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 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 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, safety lecture over. Where can you find good motors for electric go karts? Think of things with big batteries. Golf cars motors make GREAT go-kart motors. So do forklift motors (you'll probably end up using the auxillary motor from the hydraulic pump, or steering). Car starters can work for a small, light go-kart, but don't overvolt and make sure it is kept cool (a small radiator fan is a good idea). Give it a break to cool off every so often. If the design lets you, you could open it up for airflow since it doesn't have to be waterproof anymore. If you can get one, the starter from a large semi truck is super- most of them run on 24v and are big enough that the load of a go-kart is not going to be a problem from a continuous-operation point of view.
What else? Motors from electric scooters are okay, but not as good as the above motors- you'll probably have to overvolt to get 'fun' amounts of power and that will cook the motor sooner or later.
You can also find random industrial motors that for some unknown reason are DC. I have one of these, I have no idea what it came out of, but when I tried to put it on a bike it kept on breaking chains because of RAW POWER. Sweet. They're hard to find, but watch ebay. You can also check out the motors on http://www.robotmarketplace.com, which are meant for combat robots (think Battlebots) and could be a great option, if expensive.
If you want to get technical, LARGE brushless motors from RC airplanes are great, but they are expensive and require an expensive controller as well. You can't just hook them up and go. If you're going to use one of these, you probably need technician-level training as well, because you're not operating the controller via radio and will have to hook up a homemade throttle. For these, check out http://www.hobbyking.com .
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 continous 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 dirtbike? 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 acceration 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 contactor) 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.
(Continued on next post)
I stumbled across this forum today and looked in here on the electrics section. I saw some repeated and easily-answerable questions so I thought I'd write up a quick guide to electric power systems for those who want to make an electric cart. I'll explain the basics of electric motors and batteries, and also the best way to control them.
To start: Electric motors- AC vs. DC
I'll just put this plainly- you can't use an AC motor in a go-kart. 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. 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.
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 motors out there perfect for go-karts. What should you look for in a DC motor?
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 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 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, safety lecture over. Where can you find good motors for electric go karts? Think of things with big batteries. Golf cars motors make GREAT go-kart motors. So do forklift motors (you'll probably end up using the auxillary motor from the hydraulic pump, or steering). Car starters can work for a small, light go-kart, but don't overvolt and make sure it is kept cool (a small radiator fan is a good idea). Give it a break to cool off every so often. If the design lets you, you could open it up for airflow since it doesn't have to be waterproof anymore. If you can get one, the starter from a large semi truck is super- most of them run on 24v and are big enough that the load of a go-kart is not going to be a problem from a continuous-operation point of view.
What else? Motors from electric scooters are okay, but not as good as the above motors- you'll probably have to overvolt to get 'fun' amounts of power and that will cook the motor sooner or later.
You can also find random industrial motors that for some unknown reason are DC. I have one of these, I have no idea what it came out of, but when I tried to put it on a bike it kept on breaking chains because of RAW POWER. Sweet. They're hard to find, but watch ebay. You can also check out the motors on http://www.robotmarketplace.com, which are meant for combat robots (think Battlebots) and could be a great option, if expensive.
If you want to get technical, LARGE brushless motors from RC airplanes are great, but they are expensive and require an expensive controller as well. You can't just hook them up and go. If you're going to use one of these, you probably need technician-level training as well, because you're not operating the controller via radio and will have to hook up a homemade throttle. For these, check out http://www.hobbyking.com .
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 continous 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 dirtbike? 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 acceration 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 contactor) 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.
(Continued on next post)
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now you've got me thinking!!! very comprehensive btw 
