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Top Speed + BottleNeck Calculations

The controller that we’re using is a kellycontroller which has a max of 40000 electric RPM. Electric RPM = Mechanical RPM * Motor poles, or half your motor poles if they’re activated in pairs. Essentially, Electric RPM is the number of times a minute that the kellycontroller can flip the current from wire to wire. We conducted a series of top speed tests to figure out if batt-mobile’s top speed was limited by the battery voltage or the controller.

We set a 50m course on the back lane of N52 and batt mobile scored 6.5s (6.49,6.51,6.51), which works out to about 7.7m/s, probably due to the really low gearing and slightly-larger-than-sprocket-wheels.

Jocose on the other hand scored 5 seconds flat, probably due to their significantly higher gearing.

Following from 7.7m/s, and given that our wheel radius was 2 inches, we can calculate the rps of the wheels: 7.7/(0.02*2.54*pi*2)=24.1 RPS = 1447.4 RPM

Given that the sprocket mounted to our wheels had 42 teeth, and the sprocket mounted to our motor had 12, we can derive that the motor RPM is 5066.

GIven that we have 7 poles, the electric RPM is 35462. Not sure if this is close enough to the kellycontroller max of 40000 electric RPM for the controller to be the bottleneck.

On the other hand, the voltage of the battery pack varies between 26-28v, and we can assume it was at 28 since we did the test after a full charge. Given that the kv of our motor was 149we should have a theoretical no load rpm max of 149*28=4172.

So….. I guess I’ll try increasing the voltage and measuring the top speed again =/

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Go Kart Reflection

Lord Charles has ordered us to do a reflection. He has given us some pointers to follow so here we are answering it!

Assessment of your machine’s performance.

1) Consider the differences between your design calculations and the actual performance.

This is an interesting one. There were many things we had to calculate at the start. Firstly, there was this sprocket to gear to wheel size ratio that is quite complicated. A small sprocket, with a large gear allows the vehicle to achieve a good top speed but poor acceleration. A small sprocket with a small gear allows the vehicle to achieve poor top speed but superb acceleration. Now for the tricky part: the wheel size also matters because everything evens out in the end. A small sprocket with a small gear but huge wheel, will have poor acceleration as the amount of torque required to move the vehicle is crazily high -.- And there are just so many permutations but I’ll just key it in some good and logical ones we thought of.

Sprocket: Small, Gear: Small, Wheel: Small (Gear about same size as wheel)

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Fast acceleration, poor top speed.

Sprocket: Small, Gear: Big, Wheel: Big (Gear about same size as wheel)

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Poor acceleration, fast top speed.

Sprocket: Small, Gear: Big, Wheel: Impossibly big

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Impossibly slow acceleration but (possibly) great top speed? If the motor can even reach that high of a torque to spin it at a high RPM.

Note that all small sprockets in this blog are 12 teeth ones. Team 7 (Ian, Karen, Shun Him) almost wanted to use 10 teeth sprockets. That would have yielded some crazy acceleration there.

So… Evidently, we chose the small sprocket small gear small wheel. And that’s the picture of our wheel.

Allow me to go on a tangent here. Initially, we wanted to build our own differential and do lots of crazy stuff but thankfully Andrew was convinced not to pursue such ambitious dreams haha. We adopted the conventional method of a 4 wheel kart.

How did the machine work?

The handling was awesome, the acceleration was awesome, the turning was awesome. Andrew made a good call of making the CG as low as possible and we came up with the towel seat. We are barely like 2-3 inches off the ground? We had a really poor top speed but we could actually reach our top speed in like 3 seconds? It was really kinda like how we expected it to be from our initial conception of the idea.

Perhaps some things that were bad would be that the wheel worn out so badly on the tarmac.

Describe what real-life factors might have influenced the machine.

uhhh. perhaps the length width ratio of a lambhorgini? We did refer to that but pfft we didn’t follow it as the kart was gonna be long.

Comparison to other machines and designs.

We worked closely with Team 7 and as such, our vehicles were almost the same. Identical controllers, motors, frame size, etc.

Our machine stood out because of the wheels. Team 7 chose the small sprocket big gear big wheel combination and as such had a poor acceleration. We were lucky that the track had relatively short straights and as such, we were able to take the bend fast and go super fast on the straights.

If the straights were longer, team 7 would totally have destroyed us and won the race!

Another machine that really stood out was GoldKarts (Edward, Jin Kai, Olivia). They had only 1 149kv motor but they were going as fast as our 2 149kv motor kart! I guess the single controller could give such a high current that totally offset the 2 motors! Impressive stuff 🙂

Single best idea would be Team 7 I guess. They were generally really good. Comfortable seat, great wheels, could go over rough terrain. Batt Mobile was generally very rough during the ride as the wheels were small and susceptible to damage quickly as we were so close to the ground! We would have owned indoors but outdoors was bad.

Gauge your own learning

I can’t speak for Andrew, but I’m sure he is super proficient in building a mini go kart.

These are the pointers that Charles gave:

  • Mechanical power transmission design using CAD software, analyzing mechanical systems for torque, speed, power, etc. with first-order math
  • Understanding, designing, and fabricating low-voltage electrical systems
  • Ability to read product specifications and gauge their usefulness to your design
  • Correlating design calculations and specifications to existing parts in a catalog and making design compromises based on that.

We are proficient in all of them.

Many thanks to Jon and Kate, Charles, Justin, Nancy, Farhan, SUTD, and many others for this learning experience. It was great.

 

 

 

 

 

BATT MOBILE COMPLETE

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Okay so we fired up our vehicles today but it didn’t start. Andrew was wondering why didn’t it start. And then he realised that we didn’t power the controller. My mistake (Samuel). I totally overlooked the manual. I forgot to connect the 5V wire and the Ground to the controller. OOPS.

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So yes we fixed it and the controller starts! Time for the really messy part. We didn’t really take photos of that because it was basically just trying all permutations of the UVW and ABC wires. Only one combinations allows it to turn forward.

So we fiddled around with it for a little and we managed to get the left motor spinning at a really good RPM. However, the right motor was just being retarded and it didn’t wanna spin as fast. We had to troubleshoot the problem.

We realised that the problem is due to the hall effect sensor. One of the wires wasn’t soldered properly.

***LONG TECHNICAL POST COMING UP*** (Skip if you hate nitty gritty)

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Picture of the hall sensor. Not really close up but I think we have a close up pic in our previous post.

Okay so there are actually 3 hall sensors. Brushless DC motors (BLDC in short) actually “fire” in sequence to keep increasing the torque on the motor. They have to be fired at the precise moment. If they are not fired precisely at the right moment, the “firing” can actually slow down the motor. Think of it as pushing a child on the swing. If you don’t push the child on the highest point of the swing, you’re never gonna get him to go higher.

So our motor was actually not spinning at fast. We thought it was due to the firing sequence. As such, we moved the hall sensor to the left/right to find the optimal. However, we could still not achieve the same RPM as the right motor. As such, we decided to test if the hall sensors were soldered properly. It was definitely not the UVW ABC problem as it was spinning forward.

True enough, it was the hall sensor soldering problem. We soldered the B sensor. Well it wasn’t actually working, so the motor was only firing twice instead of thrice, which doesn’t make it as fast. (Naturally of course)

***LONG POST ENDS***

After soldering it, it works!

 

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Samuel in the car.

Next post: VIDEO OF BATT MOBILE MOVING

Solder, solder, solder, solder.

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Our controller came with the ring terminals. However, we decided that they wouldnt be too glam when we put it our our kart. Therefore, we decided to swap all of them out into the bullet connectors. They look awesome.

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Well, here’s a picture of the ring terminals before soldering.

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After soldering millions of parts, we had to painstakingly connect all the wiring. It’s really quite messy.

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Our 2 cool switches in the middle of our kart.

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Just look at the mess of wires!

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The Batt Mobile. Complete with all electronics. We can’t wait till we connect the battery and fire it up tomorrow!

Till then.