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Types
of Motors How to Choose Motors
Finding the Information RPM
How Fast Over
Volting
Types
of Motors
PMDC or permanent magnet direct current motors are by far the most common motor used in combat robots and are usually referred to simply as DC motors. This type of motor is commonly found in toys and in automotive applications such as window crank motors, and windshield wiper motors. Other sources for DC motors are items such as cordless drills and
cordless saws. DC motors can also be found in electric wheel chairs, golf carts and power
scooters and are commonly used in combat robots. DC motors come in a incredibly wide variety and can readily be found at salvage yards and in liquidation centers.
Brushless motors, stepper motors and series wound motors all require special components to work with radio control equipment. They can also be expensive and difficult to obtain or replace. Alternating current motors, commonly called AC motors, that you find in applications such as household fan motors also need special power inverters which makes them difficult and expensive to interface with radio control equipment.
The basic DC motor is easiest to work with, usually requiring only a couple of wires to be connected to make it work. DC motors are usually reversible simply by reversing the wires.
Of course an important factor for new builders, they can be quite
inexpensive.
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How to Choose Motors
In choosing the motors that are right for your robot or whether the motors you are considering will work you will need to consider a few things.
First you should figure out the size of the wheels you intend to use to drive your robot. You will need to decide how fast your bot should go. As a guideline the average combat robot moves about 10 feet per second in the arena. Combat robots that move at speeds closer to
16 – 18 feet per second become difficult to drive and require lots of practice to drive effectively. The drive system and design of your robot will also affect its drivability. The type of robot you are building should also be considered when trying to establish how fast it should move. For example, if you are planning on building a ramming style bot, you should look towards having lots of speed to make your robot effective on the other hand, a powerful shell spinner combat robot can be effective operating at
5 – 7 feet per second or even slower.
You should try to establish how much torque it will take to move your combat robot. A general rule of thumb is to have 2 lbs of torque on the floor for every pound of bot. We will discuss torque a little later on.
And you should have an idea about what voltage you will run your motors on based on the type of speed controller you intend to use. The problem here is that you need a good idea of the current or amperage your motors will use to be able to choose an appropriate speed controller.
Fortunately there a wide variety of DC motors that exist, potentially you could choose one of the speed controllers that are commonly used by
other combat robot builders and then choose motors that would work.
If you are a first time builder, try to make it easy for yourself and see if you can locate motors with a gear reduction system already built in. Typically called “gear head” motors, these can commonly be found in cordless drills, windshield wiper motors and power or wheel chair motors.
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Finding
the Information
Most motors have a manufacturers plate that provides information such as torque which is usually expressed in inch pounds, foot pounds or inch ounces of torque. Also on the plate you can find the operating voltage of the motor, the current draw measured in amperage and the no load RPM. If these specifications are not on the motor, try contacting the manufacturer. Unfortunately you will also need to know the “stall current” of the motor, which is not commonly found on the plate. If any of the information you need to know about your motors is missing we have a few ways for you to be able to get the information you’ll need.
To find the information you are missing you will have to perform a few measurements and find out a few things about
your motors.
The first thing you need to find is the electrical resistance of your motors. To do this
you will need a small voltage source such as a 1.5v D cell battery. Stall the motor by locking the shaft with a pair of vice grips,
being careful not to gouge or damage the shaft. Use an amp meter to measure the current.
This is more accurate than measuring the resistance of the motor.
Now just plug numbers into a simple formula known as Ohms Law
Resistance = voltage/amperage
Now you know the resistance of the motor we can calculate the stall current for the voltage that the motor will be running on.
Use ohms law again to calculate the how many amps the motor will draw when stalled.
This time take the value that
you calculated for resistance and the voltage you plan to use to calculate the amperage.
Amps (stall current)=voltage (that you plan to use) / resistance (calculated)
Now that you know the stall current of our motors you have the information you need to select
the electronic speed controllers and batteries. A good general rule is to select speed controllers and batteries with stall current in mind.
Knowing the stall current will also allow you to select the correct wire sizes needed to provide power to the motors from the batteries.
The wire size appropriated for what is needed can easily be determined from a chart of wire sizes. Try to keep your wires lengths short to reduce the amount of power loss.
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RPM
If the identification plate on the motor does not provide you with the speed of the motor measured in RPM, you will have to either purchase a tachometer. You can get a tachometer from a hobby store that works well for this purpose. They cost around $40 - $50 and are typically used to measure the propeller speed on radio control airplanes. If on the other hand you want to save a little money, you can use this less accurate but acceptable method. You are going to need a stop watch and about 15 feet of nylon fishing line or dental floss will work very well.
Armed with the following formula you may already have an idea of what it is you need to do.
RPM =(length of string / (3.14 x Diameter of shaft in inches /12 )) / (time /
60)
Attach the string to the shaft of the motor. Have one person hold the string to feed it neatly onto the shaft. Wear a pair of leather gloves to protect your hands when doing this. Make sure the motor is securely mounted or locked into a bench vice. While one person holds the string the other person turns on the motor and starts the stop watch. Time how long it takes to reach the end of the string. While the string is wrapping around the shaft, try to be as neat and even as you can. Allowing the string to build up in one spot will cause errors in your measurements. You may want to do this a few times to get the best results.
Example 15ft string, ½” shaft, 21 seconds
RPM = (15 / (3.14 x 0.5 / 12 )) / (21 / 60)
= (15 / (3.14 x .41666)) / ( 0.28333)
= (15 / .13083) / ( 0.28333)
= 114.6 / .28333
= 327
You now know the RPM and the torque of the motor. The question still remains. Will these motors work in
the robot?
So…..for example
Say your motors provide 100 inch pounds of torque and run at 327 rpm. You are planning on building a light weight robot weighing about 60 lbs.
Keeping in mind that you should have about 2 pounds of torque for every pound of robot. At first it would appear that the motors will easily do the job, but
you still need to consider the size of the wheels.
If you measure the radius of the wheel (from the center to the outside) and then divide the torque by the radius you will now know the amount of torque
you are applying to the ground.
If you use a 4” wheel, this means we have a radius of 2”. And so quite simply…
Torque = 100 inch pounds / 2”
= 50 in / lb
Assuming you are running two motors….this is equal to about 100 inch pounds of torque. In a 60 pound combat robot this is just a little bit week but it
should work. However you do have a few options if you want a little bit more torque. You can use a smaller wheel…..say a 3 ½” or you can use a different gear ratio in your drive system.
However changing the gear ratio or the wheel size will not only change the amount of weight you can push but also affects the speed of your bot.
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How
Fast
What you need to now do is figure out how fast your robot will move.
The first thing to find out is the circumference or distance around the wheel. You can use a piece of string and wrap it around the wheel but doing the math is much simpler. Simply measure the diameter of the wheel and then multiply the diameter by 3.14.
So with a 4 in wheel you get……
Circumference = 3.14 x 4
= 12.56 inches
Since the motors run at 300 rpm the top speed of the bot will be……
Top speed = 12.56 inches x 300 rpm
= 4107 inches per minute
If you divide by 12 (inches in a foot) you get.....
feet per minute = 3768 / 12
= 342 feet per minute
You can divide this by 60 (seconds in a minute) to get feet per second......
Feet per second = 342 / 60
= 5.70
For most combat robot designs this is probably a bit to slow and a the torque is a bit weak.
As mentioned before the average combat robot moves at a speed of about 10 feet per second. Wedges and Ram bots usually run
faster where as shell spinners don’t need that sort of speed to still be effective.
You can decide to keep looking for motors that are more suited to your design or you can try another option.
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Over
Volting
Over volting is option that many builders will go with. Over volting a motor means to run the motor on a higher voltage than what they are rated for. If you double the voltage that you apply to a
DC motor it will double the rpm and it will double the torque. Of course some things do limit the actual increases. The motor may not physically be able to handle the increase in speed or current. Wires could burn, magnets become demagnetized or
extreme heat could melt parts of the motor. In short….you could destroy the motor. On the other hand builders have found that they can apply 1 ½ to 2 times as much voltage to a motor and not have a problem.
If you decide to over volt your motors remember that you will need to recalculate your battery requirements and check your speed controllers to ensure that they can handle the increased voltage and amperage.
But lets not overlook the obvious…..increase the number of motors. You can hook your motors up to the speed controller by hooking the motors up in “parallel”.
Of course you will be drawing twice as much power from the batteries through the speed controller. So you will need to re-evaluate your battery capacity and the capabilities of the speed controller.
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