Large Motors Class
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Lesson 5: Further Considerations
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Introduction: Further Considerations

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Once you have figured out how to build a circuit to control a large motor, the next step is to build something with it. Learning how to do that is far beyond the bounds of this class. However, I would be remiss to end this class without discussing some important safety and construction considerations.


Safety Switches

The most important addition that you will want to add to your circuit are safety and power switches. Follows are the three most common types of safety switches that you should include.

The first type of switch that you should always consider incorporating into your circuit is a high current kill switch. A kill switch is a red switch that cuts the connection to the battery when it is pressed down. This switch should be wired in series to the positive terminal of the battery bank before any of the other components. When the switch is pressed it will immediately kill all power. Keep in mind this is an emergency switch and will cause your motor to stop short, so should only be pressed if necessary.

Another type of safety switch you might want is a high current power switch. This one in particular may look similar to a kill switch, but is not to be confused with one. First off, a kill switch should turn the power off when pressed down, and not pulled up because that is most people's first instinct. Secondly, this switch is yellow, and not red (the color of danger).

This switch should be wired in series between the kill switch and the first components in the circuit. The reason you would want such a switch is because of the precharge resistor. If you remember, the precharge resistor bypasses the terminals on the main contactor to keep the capacitors in the Alltrax motor controller charged. Over a (very long) period of time, this will drain the battery bank. By having a switch which cuts all power, you make sure that you are not leaking any to other parts of the circuit.

While a kill switch could serve this purpose as well, it is good practice to have a separate safety and power switch.

A key lock switch would make a good KSI switch, and, unsurprisingly, is what they had in mind when they named it a Key Switch Input. Having an actual key switch to activate the motor controller ensures that not just anyone can come along and turn on your device and ride off with your device. Having a unique key is obviously a solid safety feature.


Speed Limiter

Another great safety feature is the FN1 Control Box. This interface box provides speed and acceleration control to your Alltrax Motor controller. Put simply, when the dial is turned to the left, the speed and acceleration can be set to go real slow, and as you turn it to the right, the speed and acceleration can increase.

A real world scenario where this might come in handy is if you are building a go-kart and want a slow mode for children, and a fast mode for adults. Another such scenario is if you are testing something that is actively being built. You can start with your system going very slowly, and gradually increase the top speed and acceleration as you grow confident the mechanism is working.

In order to use the FN1 Controller, two new connections need to be made with the Alltrax motor controller.

The red wire from the controller needs to plug into the KSI input. Of course, the KSI switch also still needs to plug into that input. To solve this, a tab terminal splitter is used to be able to plug both in at once.

The black wire from the controller needs to be plugged into the green 'User' ("GRN-USER") input terminal on the controller.

No additional changes are needed, and all of the remaining connections in your circuit should stay the same.

Once the FN1 Controller is plugged in, it needs to be configured to work with the Alltrax Toolkit software. Launch the program and go the "Controller Settings" tab. Click the "FN1" button where it says "User 1 Input." This should be pretty apparent as there will be a picture of a speed dial.

You will notice that the "Max Forward Motor Speed" and the "Throttle Rate" now have very wide sliders. These sliders indicate the range the dial can adjust between. Put another way, when the dial is set to the far left, it has a max speed of 20% and a throttle of 4%, and when it is to the far right it has a top speed of 80% and a throttle of 14%.

By adjusting the size of these slider bars, the speed and acceleration response range can be configured. This allows for a highly programmable experience that can be adjusted with the speed dial.

To adjust the speed and throttle, it is just a matter of turning the knob. Once again, when it is being turned clockwise (to the right) speed and acceleration increases, and when it is turned counter-clockwise (to the left) it decreases. This simple knob provides an easy way to limit the speed of your system.


Gearboxes

While an electronic speed limiter is a useful safety tool, a speed reduction gearbox is the safest way to ensure your motorized contraption has an absolute maximum speed that it can never exceed.

Using a gearbox to reduce speed is a much more beneficial than using a motor controller for two reasons. First, if something were to go wrong with your control system and the motor were to go on at full speed, it can never exceed the top reduced speed. The other reason is that a large motor's lowest default speed that it will run at (even when using a motor controller) might still be faster than you want it to be. Having the proper gearbox ensures that the motor always kicks in at a very slow speed, and you don't need to worry about if you are going to be able to provide enough voltage to make it turn on.

The other added bonus of using a speed reducer is it increases torque. For instance, the reduction gearbox on a wheelchair motor assembly not only slows the speed down for safety reasons, but it increases torque. This makes it suited for moving around adult humans at relatively slow speeds over many kinds of terrain.


Timing Pulleys

There are many ways to connect a motor's output shaft to gears, pulleys, sprockets, and chains. In fact, there are so many to go over, it is beyond the scope of this class.

However, I'd like to just mention timing belts, which is one method that I am particularly fond of. Timing belts have teeth like gears, which are caught by the cogs that pull them around. This configuration provides a nice degree of reliability.

However, they are soft like pulleys. So, if the system seizes, or something goes wrong, the belts will slip (and continue working later) or break. This makes it much easier and cheaper to repair than a broken gear system.

The other nice thing is that it is very quiet compared to gears or sprockets and chains. Considering this is an electrical system which is near silent to begin with, if you use timing belts, you will barely hear anything moving. It will sounds just like a quiet low whirring noise while it is operating.


Construction

The last thing I would like to bring up is that large motors apply large forces upon whatever it is attached to. Therefore, it is important to build things out of solid materials that can both handle the amount of force being applied by the motor, and also withstand the forces of a modest collision.

More often than not this means building things out of metal. While metalworking and machining are way outside the scope of this class, fortunately there are a few classes that cover these subjects. The Metalworking Class is a good source of information if you have never done any work with metal before. For those looking for something a little bit more in-depth, the Welding Class is a great place to start.


Hopefully this class has gotten your gears turning, and will help bring your large motor project to life!

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