Racing Power Wheels Part 6: Motor Testing, Controller, and Sparks

So it's been a while since progress has been made on the Power Wheels Dune racer. I've been busy with school and it set me back on this project. Below is a video of some of the progress that was made last time.

      

It's not a really good video. It was mostly composed of videos that I recorded here and there and there were some parts that I didn't get to record.

But below is a picture of where I was at last time with all the electrical parts mounted on some diamond plating that was cut out to fit the back portion of the frame.

In the picture above you can see the 3 12V batteries being held down with some rubber straps. On the back side of the battery pack I have two eye bolts that are supporting the back side of the battery pack. On the front side of the battery pack, which can't be seen in the picture above, I have two L brackets supporting the front side of the battery pack.

You can see where I have mounted the motor controller. I may need to create some kind of barrier of some sort to prevent the wires from going towards the sprocket. Heading towards the front of the Power Wheels, the motor was placed in front of the controller. It took me some trial and error to decide where to place the motor to be able to install the chain.

In the end, I found that maybe I should have move the motor just a little closer to the axle sprocket. The reason for this is because when we installed the chain it was quite difficult. So I had the motor and the axle sprocket not fully bolted down to install the chain. Once installed, I bolted the axle sprocket to the sprocket hub and bolted the motor down. Once this was done, the chain was on pretty tight without any kind of slack. I believe it's too tight. So I may enlarge the mounting holes for the motor to have some play for adjustment and then bolt it down.

Here's another picture of the setup in the back without the plastic Power Wheels body. It is a much clearer picture and you better see the parts on the back.

The diamond plating was bolted down along the edge of the frame. Some testing of the motor was done with the body off but I haven't uploaded the video yet on YouTube. But I do have a video of the motor being tested when not mounted. Below is a video of the motor being tested.

         

The original motor controller that came with the kit I ordered from Monster Scooter Parts didn't work out well for me. So they sent me another controller to replace the first one. Again, I had the same issues. No voltage was being sent to the motor. I used a volt meter to see if there was a voltage across the motor connectors but there was no voltage. The hand throttle worked so it couldn't be the throttle and the batteries were fully charged. The motor kit from Monster Scooter Parts can be found at the link below.

http://www.monsterscooterparts.com/trsc/rec-categories/motor/36-volt-1000-watt-motor-controller-throttle-kit-standard.html

So I decided to order another motor controller from another place. I ended up ordering one from TNC Scooters. The link to the new controller is 

http://tncscooters.com/index.php?route=product/product&product_id=182

After doing some research and math, I think that the motor controller that was part of the motor kit from Monster Scooter was not a suitable one for the 36V 1000W motor. The current draw from this motor can be calculated by the following equation:

P = I * V

I = P / V

Where P is power in watts, I is current in amps, and V is volts. The second equation is the same equation but just solved for I. So after filling the known parameters we get the following:

I = 1000W / 36V = 27.78A

So the motor draws about 27.78 amps during use. I looked up the specs of the controller that was part of the kit from Monster Scooter and found that the controller's rating was 28 amps. The specs can be seen at the following link.

http://www.monsterscooterparts.com/36vocove1.html

The controller that I got from TNC scooters is rated at 40 amps so it was more than enough to handle the current draws of the motor. So I guess the kit from Monster Scooter Parts isn't really ideal since you would want a controller that would have some kind of room for any large current draws from the motor. Large current draws may happen under extreme loading. But this is just a possibility why the Monster Scooter controller didn't work. I followed their wiring setup and no results. But with the TNC controller I had no issue.

Another issue I came across is when I connect the motor to the controller, a large spark would form. After a few times the connectors would eventually start to display some burning damage from the sparks. Two times the connectors caught on fire. But this could be due to me improperly connecting the motor to the controller. During testing I found out that the positive connection must be made first before the ground connection. But the fires could have been caused by the spark or a short. The fires were small but still dangerous. After this, I became somewhat fearful of performing the connections. Below is a picture of the controller with some damage to the connector from the sparks.

So after doing some research about this I came across the term anti spark connectors that are designed to fix the spark problem. Below is a video of someone demonstrating the sparks that happen and the solution with the anti spark connectors.

            

The anti spark connectors in the video above can be found on Hobby King. The link to it is below.

http://www.hobbyking.com/hobbyking/store/uh_viewitem.asp?idproduct=91005

The picture above is an image of the anti spark connectors. When ordering them from Hobby King, just make sure you select the proper warehouse from where they will ship these connectors from for the fastest shipping time. They have a warehouse for USA east and USA west as well as an international warehouse. I have already ordered 2 sets of these connectors. They should work for my setup since the voltage of the batteries in the video above amount to a much higher voltage than 36 volts.

I also came across another video that explains how these anti spark connectors work. This video can be seen below.

               

So, when making the connection between the batteries and the controller, it also connects to a capacitor within the controller. The resistance of a capacitor is very small that it is close to 0. This capacitor is in parallel with the connectors of the controller that go to the batteries. Since the resistance is very small, the current in the instant the batteries and the controller are connected the current is high and thus cause the large spark. This large current can be explained by Ohm's Law which is seen below.

V = I * R

I = V / R

So our voltage, V, is 36 volts and remains that. If our resistance, R, is very small, this in turn will create a very large current, A. So what the anti spark connectors do is reduce that current significantly by incorporating a built in resistor inside of the positive side of the connector. This resistor can be in the range of 5.6 ohms. So looking back at the equation I = V / R, with a higher resistance we could then get a lower current, I. So if we plug in our know parameters, our current will be:

I = 36V / 5.6Ohms = 6.43Amps

This is much if we say that the capacitor only has a small resistance of like 0.04 ohms. That would give us:

I = 36V / 0.04Ohms = 900Amps!

That is an insane amount of current. But this value of 0.04 ohms is not accurate, it's a just a number I chose to represent a resistance close to 0. But as you can see, without the built in resistor you would attain crazy amounts of current momentarily. This is what causes the spark. Capacitors take a short amount to charge.

So the time to charge a capacitor takes approximately five time constants. After the first time constant, the capacitor will usually be charged to about 63%. After 5 time constants it will be charged to 99%. This is just some information I found online so the validity can't be certain.

To calculate the time constant, the following equation is used: 

 tau = R * C

 where tau is the time constant, R is the resistance, and C is the capacitance of the capacitor. So five of these time constants will then fully charge the capacitor.
So without a resistor, the time constant will be 0. This is what causes the spark. With the XT90 anti spark connectors, there are two different stages of connections.

The first is when the ground is fully connected, but in the positive connector (red) there is a seat of some sort that temporarily stops you from pushing the connector all the way in. During this first stage of connection, the two positive connectors (one from the controller and the other from the battery) are not fully connected in line. At this moment the circuit is completed through the use of a built in resistor in the XT90 connector. This resistor completes the circuit between the two positive connectors and also lowers the current in the system. Without a resistor, there is practically no resistance since the resistance from a capacitor is minimal.

So with the built in resistor momentarily being in the circuit, the time constant is temporarily changed. The built in resistor in the connectors is about 5.6 ohms and the capacitance of the capacitor will be approximated at 0.22 farads. The capacitance was based on a video of a 36V motor controller for an ebike being opened up. So from the equation:

tau = RC

tau = (5.6ohms) (0.22F) = 1.232 seconds

So after 1.232 seconds, the capacitor will be approximately 63% charged. After 5 of these time constants the capacitor will be 99% charged which is its maximum charge. So total charge time:

 5*tau = 5RC = 5(5.6ohms)(0.22F) = 6.16 seconds

This is just an approximation. The charge time could be less or more and factors can include the actual capacitance within the controller that is being used. But after 1 second, the capacitor will have some charge and it is much better off than just connecting the batteries and the controller without the anti spark connectors.

So after that first stage of connecting with the anti spark connectors, the connector would be further pushed together and finally make the final connection between the two positive connectors. The resistor will still be there in the circuit but current will always follow the path with the lowest resistance. In this case, that path is simply the connection between the two positive connectors. The resistor will just be there after serving its purpose.

I'll have another blog soon about the steering of the Power Wheels and the parts for it. Thanks for reading!