Sunday, October 7, 2012

Parts and Build Out, Discussion of Motor, Batteries, Controllers, and Gearing

An electric go kart consists of the following:
-Go kart frame with wheels, tires, seat, brakes,& throttle
-Batteries
-Controller
-DC electric motor
-Electric throttle
-Wires and connectors
-Housing for motor and/or batteries
-Proper gearing / sprocket / chain

Before we get into choosing a motor, controller, and batteries, I would like to introduce a few concepts for those who are unfamiliar with electrical devices.

Electricity: AC and DC
First we need to talk about DC ( direct current) and AC (alternating current). Direct current means there is a constant voltage coming from the source (batteries) and going to the load (motor). Batteries are always DC and therefore, portable electronic devices are almost always DC since they are powered by batteries. AC is used when electricity is transmitted long distances, such as from the power company to your house or throughout your house to power your appliances. In AC, the voltage alternates direction (polarity + and -) 60 times per second. In the US, residential power is 120 VAC that is volts alternating current. If you watched the voltage on a display you would see the voltage go from positive 170 to negative 170 in a sine wave pattern at a frequency of 60 hz. The reason why I said the peak was 170 (approximately) is that the average positive voltage is 120 and the average negative voltage is 120 also, that means at the peak of the wave it will be higher than 120 positive and at the lowest part it will be less than 120 negative. AC voltage is expressed as an average using the root mean square (RMS) function.
The bottom line here is that you don't need to know anything about AC for the purpose of building an electric go kart. You can not use an electric motor designed for AC current in an electric go kart without significant technical modifications that are beyond my expertise. You should only look for DC motors.

Batteries: Volts and AH
The batteries provide DC current and all of the components will use DC electricity. You will see electric motors listed with their voltage and type of electricity for example 36 VDC means 36 volts direct current. Batteries are usually 6 or 12 volts. They differ in the amount of energy they hold. This is usually displayed in AH (amp hours). The more AH, the more power the battery can provide and the longer it can run without being charged. As far as getting the right voltage, you simply need to have enough batteries to add up to the voltage you need. For example if you are using a 36 volt motor you would need three 12 volt batteries or six 6 volt batteries wired in series to add up to the right voltage. The cheapest batteries that can be used for this type of project are sealed lead acid (SLA) type. You could also use marine type batteries that are rated "deep cycle", that means you can use most of the battery's energy without damaging it. An automotive battery would not be appropriate because it would be damaged by discharging it in this application.

Motors: Brushless vs. PM
Regarding electric motors, we have covered voltage and type of current (AC and DC). There are a few more concepts to understand before purchasing one. The type describes the inner workings of the motor. There are two types, permanent magnet and brushless. For a detailed description, see Wikipedia: DC Electric Motor. In summary, brushless motors are lighter, more efficient, last longer, are more expensive, and more complex. PM motors are cheaper and easier to use since the controllers are also simpler. Brushless controllers require feedback sensors to be installed on the motor to relay speed and position information. If you are going to buy a brushless motor, I would recommend you buy the motor and controller together as a package so that all of the sensors and wiring are taken care of. For example see this package at thesuperkids.com 1500 Watt BMC Brushless Motor and Controller. It is difficult to install the sensors yourself. The simplest and cheapest option is definitely PM.

Motors: Power
How much power do you need? First let's discuss how motors are rated. Horsepower (hp) is most common but you may see watt (w) or kilowatt (kw = 1,000 w) used. One hp is equivalent to 0.75 kw. The hp rating is referring to the output of the motor. You also need to know the current rating along with voltage which tells you the input to the motor. This is important because the controller has to be matched to the motor as far as current and voltage.
As far as choosing between different power levels, consider the following. The higher power level you buy, the more it will cost and the faster it will deplete your batteries, and, obviously, the faster it will drive your go kart. If this is for a child and it is a medium size kart, 500 to 1000 watts should get you to 15 to 20 mph. For an adult and for higher speeds 3 hp (2.2 kw) and up is recommended.

Motors: Duty Cycle
 Another consideration that goes with power is duty cycle. A motor may be able to perform at a high power level for a short amount of time and a much lower level of power continuously. Look for descriptions such as continuous use or intermittent use in the motor description. If it is not specified there are some clues to look for. The most important one is - what is the intended use of the motor? Starter motors and winch motors are clearly for intermittent use. If you run them for more than 10 minutes at full power, they will likely overheat. Pump motors, fork lift motors, and golf cart motors are designed for continuous use. The size of the motor housing and the presence of cooling holes or fans also can tip you off to the ability of the motor to perform at maximum capacity for long periods of time. For example, if you have a 1 kilowatt motor that is the size of a spray paint can with no cooling holes, I guarantee it will overheat in a few minutes. There is no way that that small of a motor in a closed housing can dissipate the heat that is generated. Larger designs with more airflow will be able to sustain high power performance without overheating.

Motors: RPM
Another feature of motors is show fast they spin. This is expressed in RPM (revolutions per minute). Most motors that I have come across spin at about 3,000 rpm. Generally speaking you do not want too high of an RPM since the rotational speed has to be reduced by gearing and it is more difficult to reduce the rotational speed if you are starting with a higher number.

Controller
One more item I thought I would cover is why you need a controller. If you have an electric motor and you hook it up to a battery, it will turn on and your car will go. The problem is that you will go full speed and your only option to go slower is to turn it off. You want to have something to modulate the amount of energy (current) going to your motor so that you can have control over how fast you want to go. In a car, you use an accelerator. In a go kart we call it a throttle. The throttle signals the controller as to how much current to send to the motor. The other important reason for the controller is to protect the motor from too much current which can cause damage. If your motor is rated for 40 amps of current, you want a controller to limit the current to 40. If your motor draws 60 amps from your batteries, it will quickly overheat and burn out. Finally, many controllers will protect your batteries by shutting off when the battery gets dangerously low in charge. Batteries that are run on empty can be damaged. Some controllers have special features such as half speed reverse, and connection for brake lights. Some controllers are programmable, the more you spend, the more features you get.

Motors: Choosing from available options, DC PM 36 & 48 V
Since this is for a child, I figured I want about 1-3 horsepower motor. It seems the best motors for a go kart are either the 3 hp golf cart motors or the 10 hp Mars Etek style motors originally made by Briggs and Stratton. See below for pics.

Both of these options are quite expensive ($600-1000) and offer more power than we need. The golf cart motor also needs to be modified since it is meant for a spindle shaft from the golf cart transmission and does not have a drive shaft to attach a sprocket to. Another option is an electric scooter or bike motor.
Chosen Motor: MY1020
I found that TNC Scooters has the best selection of motors, controllers, and accessories for the power level that I am looking for. They have a 1000 watt 36 volt motor for about $100. It is called the MY1020. It is a very popular option for scooters and e-bikes and comes in different power and voltage levels.

Go Kart Frame
Before buying the motor and electronics, I had to decide on a frame. I found a really nice new option at Go Karts USA. http://gokartsusa.com/azusa3557gokart.aspx They sell the Road Rocket GoKart Kit with 5" nylon wheels for $522.00. It has a padded bucket seat, 1" live axle, 6" drum brake, 11.6" knobby tires, steering wheel and steering hardware, sprocket and hub, #35 chain, brake hardware, and throttle hardware. It comes without an engine which is perfect for an electric conversion project.
In retrospect, this might have been a better option. It looks like a really nice frame and has all new hardware.

Instead of buying a kit or new frame and components, I ended up buying a used go kart and refurbishing it with the help of my brother and his friends. Luckily he works for a lawn service company and has all the tools needed to do the job since they are used to repairing their lawnmowers and vehicles there.

My buddy Joe picked up the donor go kart for me for $150. The frame was in decent shape. The tires were dry rotted and one was flat. The steering wheel was rusted and wrapped with tape. The gas engine wouldn't start. The seat was really dirty and ugly. I lost the original picture. We brought it to the shop, sanded the rust, removed the wheels and tires, engine, and the seat upholstery and proceeded to prime and paint it. We also moved the gas and brake pedal back about 4" so that my son would be able to operate them. Here is one of the earlier pics:
It was originally blue, we decided to go with red. I bought Rust-Oleum 7830730 8-Ounce Rust Reformer from Amazon for $9.63 to remove the rust. It sorta worked but is not a substitute for sanding. I found that the wire brush attachment for the drill worked best. We used a few cans of Rust-Oleum Professional 15 oz. Aerosol Primer for $5.58 from Home Depot in addition to Dupli-Color DE1645 Ruby Red engine Spray Paint which we got from a local auto parts store for $5 per can. We originally painted the wheels silver metallic but it didn't come out good so we went with black. One tip, always buy twice as much paint as you think you might use. This project took 2 cans of primer, 3 cans of red paint, and 1 can of black paint.

We used a metal plate which was placed over the original motor mount and secured into place with u bolts and other bolts. This was designed to be a platform for the batteries and the motor.

Chosen Batteries: 12V 18 AH SLA
As far as the voltage, there are a few options. The 1000 watt motor can be ordered in 36 and 48 volt form. I figured I would rather use three batteries than four for simplicity so that is what we decided. As far as batteries, I ordered 3 Rhino SLA 12V 18 Ah Battery from Amazon for $37.99 each as of 10/7/12, price may have dropped since then. I figured that I always could add another battery to make it a 48 volt system in the future. In that case I could over-volt the 36 V motor and change the controller.

Seat Refurbish
The seat was stripped and the frame repainted. I bought upholstery stuffing from Michaels for about $20 and some vinyl covering for another $20 from a local fabric store. I used a stapler to attach the vinyl to the wood pieces which was then screwed back onto the frame which I painted black.

Itemized order from TNC Scooters:
ProductModelPriceQuantityTotal
1000W Motor - 36 Volts with Mounting Bracket (Style: MY1020-B)MOT-106165$92.001$92.00
Metal Foot Pedal Throttle Cable (Hall Effect)THR-101124$18.501$18.50
36 Volt Controller (Model: CT-660B9)CTL-101205$30.001$30.00
40 Amp DC Circuit BreakerCBK-101266$7.001$7.00
36 Volt - 3.0 Amp XLR Battery Charger FAN COOLEDCHG-101142$40.001$40.00
XLR Charging Port (3 Pin)CHG-101245$3.501$3.50
2 Pin Modular ConnectorCON-102300$0.506$3.00
6 Pin Modular ConnectorCON-102306$0.501$0.50
Red On/Off Rocker Switch with Indicator LightSWT-102050$4.001$4.00
Battery / Motor Connector (2 - Pin)CON-102310$0.502$1.00

The total was around $200 before tax. I was really happy with TNC Scooters. I called them after I had some problems later on and they were very helpful. I strongly recommend the DC circuit breaker, it protects the batteries and is easier to reset than if you use a fuse. I wired everything with 10 gauge wire from Home Depot. Since I am using 40 amps, I could have gone with 12 gauge but I decided to go a little bigger than necessary for minimal added cost since I may upgrade the motor one day and do not want to have to rewire everything. The 36 volt 3 amp charger works very nicely, it is fan cooled so it doesn't get too hot. It charges the batteries in about 8 hours and then goes to battery maintenance mode.

Choosing a Controller
As far as controllers, there are many options. I think the best controller for this job would have been from Kelly or Alltrax which you could buy from Cloud Electric but they are pricey. Some are programmable which is really neat since you can limit the current or change the throttle response. They also can handle very high amperage. The controller I got from TNC was rated at 40 amps, 1000 watts, and 36 volts and only cost $30. You can't beat that. I figured if it burned out I would replace it with a more expensive model but that hasn't happened yet. One drawback of the cheap controller I bought is that it lacks a reverse feature. There is a way to hard wire reverse with a switch to work around the lack of the controller feature.

Tires

I ordered 4 new tires from Dennis Kirk Cheng Shin Front or Rear C829 145/70-6 Tire to spruce up the look of the go kart, I could have salvaged some of the originals but I decided to splurge. They were $17.54 each. FYI, Go Karts USA has a really great table that lists all of the common go kart tire sizes with their dimensions. This is important for projecting the maximum speed of your go kart and selecting the proper gear ratios. Here is a link to the Knobby Tires Table.

Changing Tires (Problem)
It is very difficult to change tires on a go kart. I first tried to separate the bead with a screwdriver and by standing on the deflated tire. This accomplished nothing. I then took it to Pat's garage. We used the auto tire machine and tried to compress the tire to release the bead and it slipped out. We then placed the tire under the lift and dropped the lift on it - nothing happened. These damn things were really corroded in place. A guy named Doc (as in the lawn mower doctor) showed me how to do it. It took about 15 minutes per tire and it was ridiculously hard to do. We put the tire in a vise grip and sprayed the gap where the bead meets the rim with lubricant. Then using various pry bars and working it around little-by-little, the bead slowly released. These are two part rims, once the bead comes off they separate easily.
The original wheels were rusted and had some sharp edges after using the pry bars. They were sanded, primed and painted before the new tubes and tires were mounted. No other difficulties were encountered.

Tubes
 I also ordered 2 tubes and a steering wheel from Go Karts USA.
7190 Innertube - $12.90 each

Steering Wheel & Cap

Also from Go Karts USA
Steering Wheel: 1801 Steering Wheel, Neoprene 10" Diameter - $43.85
Steering Wheel Cap: add 1877 Steering Wheel Cap

The steering wheel was another splurge and I didn't end up using the cap.

Engine Housing
For the engine housing, I ended up customizing a mailbox that I bought at Home Depot, also found at Amazon
I am pretty happy with the idea to use a mailbox. It allows me to hide the electronic components and mount the charger port and switches. It is durable and cheap. I strongly recommend it.

Here are some more project photos:
Frame
You can see the rear of the go kart with the metal platform. The motor has been placed next to the axle sprocket.

Mounting the mailbox over the motor. Batteries placed in the rear and held with a tie-down strap. Wheels and tires are put back on.


Close up of the throttle. We drilled a hole in the throttle from TNC and attached the original cable. I searched for a long time for a cable actuated electric throttle but couldn't find one. This was a nice solution. 

Throttle: Hall vs Pot
When purchasing a throttle, you have to make sure that the throttle is compatible with your controller. The most common type of throttle is a "0-5 volt hall effect". The other is a resistance type throttle which has a variable resistor or potentiometer (sometimes called pot). The resistance is usually given as 0-5K ohms.

Misc Parts
I also bought an on/off switch and power indicator light from amazon. 
GB Electrical GSW-11 Heavy Duty Toggle Switch $6.40 as of 10/7/12, really nice item. solid feel.
Dorman 84926 Red 9/16" LED Indicator Light $7.72 as of 10/7/12, nice item, but this is where I encountered one of my first problems.

Problem with Sprocket
Actually the first problem was the gearing. It turns out that I have a #40 chain on my donor go kart. This can not be changed since the drive sprocket is completely rusted and seized on the rear axle. The motor came with a 11 tooth Dual-D-bore sprocket for a #25 chain. There is no such thing as a Dual-D-bore sprocket for a #40 chain that would fit on my 10 mm motor shaft. I spent hours looking for it.It does not exist.
Sprocket mounted to motor with chain attached. You can see the spot welds from the inner sprocket which was placed inside the larger sprocket.

I ended up having my friend Pat who is a mechanic weld the dual-d-bore sprocket to the original 10 tooth #40 sprocket from the gas motor that came with the go kart. I had to first grind down the dual-d-bore sprocket so that it would slide inside the sprocket that came with the go-kart. This actually worked beautifully, thanks Pat!

Gearing
It is important to understand how the speed of the motor translates to ground speed of your go kart. Your motor will have a shaft and a drive sprocket will be attached to the shaft. The number of teeth on the drive sprocket needs to be specified (or counted), it is usually somewhere between 9 and 12. The chain connects the drive sprocket to the bigger sprocket which is on the axle that turns the rear wheel. You also need to know how many teeth are on the axle sprocket, 60 is a common number. You also need to know your rear wheel diameter and motor RPM. With this information, you can go to a calculator and figure out the theoretical maximum speed. This site has a Go Kart Speed Calculator
For my go kart, the engine RPM is 3000, the drive sprocket is 10t, the axle sprocket is 60t, and the wheel diameter is 14.2". Here is a screenshot:
As you can see, the max theoretical speed for my go kart is 21 mph. This is pretty reasonable for the street. If the kart was meant for grass and mud, a lower ratio (less teeth on the drive sprocket) would translate into more torque so that it would have an easier time with resistance such as climbing up a hill. The downside of a lower ratio is less top speed. If you have a low power motor, you will never get a top speed over 20 mph so you need to adjust your gear ratio if the speed calculator shows a high top speed. For example, with a 750 watt motor and a predicted top speed of 40 mph, what would likely happen is the kart would barely move at all. The motor wouldn't be able to generate enough torque, it would overheat and the go kart wouldn't be able to handle any resistance such as a heavy rider or drive on grass or go uphill. Proper gearing is important to match the speed and power of the motor to the requirements of the go kart. You may have to change the sprockets on the axle and drive shaft to get the optimal ratio.






4 comments:

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