Introduction: Large Motors
In this class we will be covering how to control large electric motors. For the sake of a definition, a large motor is a motor that requires 12 or more volts to operate and requires 5 or more amps. The motors we will be using in this class are suitable for building things such as scooters, go-karts, electric motorbikes, very large robots, or even, potentially, small electric cars. If you want to move something big or heavy, chances are you need to use a large motor.
Step 1: WARNING!
Before we start, I think it is important to point out a few very important things about working with large motors.
First, I expect that you have an understanding of both motors and electronics. If you don't, go read my Robots Class and Electronics Class before you even think about beginning this class.
In addition, parts of this class use an Arduino microcontroller to control the motors. It is assumed that you know how to do this already. However, if you don't, you can learn how with the Arduino Class.
Next, working with large motors is dangerous! By this I mean that WORKING WITH LARGE MOTORS IS DANGEROUS! Do you got that? Working with large motors is dangerous.
Hold up all of your fingers and count them. Now, keep in mind how many you currently have because when you work with large motors you can potentially lose some. It is good to know how many you are starting with so that you can keep track of them at all times. I don't know about you, but I like my fingers and would prefer to keep them.
In fact, if the motor can take off your finger, there is no reason that the thing the motor is attached to couldn't crush or run you over in all kinds of dangerous or potentially lethal ways. This is not even to mention the danger of high-speed crashes.
Also, the size of the batteries required to power large motors packs a lot of electrical energy. These batteries probably won't electrocute you quite in the same way sticking a fork in a wall socket would, but they should always be handled very carefully and with best practices. In other words, you never want to put yourself in a situation in which your body can potentially become a pathway between voltage and ground in the circuit.
Albeit electric shock is probably unlikely, receiving a nasty burn is very real. When a 24V battery bank accidentally shorted across the aluminum bar pictured above, it got so hot that it melted (at a temperature above 1,221°F). That is not even to mention that it was shooting flaming hot electrical sparks everywhere, which makes most everyone more likely to freeze, panic, or do something incorrectly. In short (pun intended), crossing electrical connections when dealing with this amount of current can cause fires or severe burns, so be careful of all of your electrical connections!
There is also another danger you perhaps have not yet considered. That is the threat to your bank account.
Working with large motors is extremely expensive. Every component required to interface with large heavy duty motors will also be equally large and heavy duty. These aren't cheap hobby components, but serious industrial parts designed for high electrical current applications. Costs can add up quickly! Expect to spend hundreds of dollars to work with large electric motors.
Given all of that, before you go down this road, are you sure you really want to do this? Could you solve your motor problems in another way?
It is advisable to only work with large motors if you really need to.
We are going to review controlling 3 types of large brushed DC motors in this class.
The first motor is an MY68 permanent magnet DC motor rated at 24V and 100W. This motor has a stall current listed at 5.9A. One would typically use a motor like this to power a small electric scooter or an electric-assisted bicycle. This type of motor spins fast, but doesn't necessarily have the most torque.
The next motor that we will be covering is a Drive Motor removed from an electric wheelchair, and is a slightly bigger permanent magnet geared DC motor that is rated at 24V and 250W. The stall current rating on this kind of motor can be calculated to be about 10.5A (but is likely a little bit higher, if we were to err on the side of caution).
Wheelchair motors are coupled with both a speed reduction gearbox (the silver thing on the right side). This not only decreases speed, but increases torque for moving people around. In addition to the gearbox, it also has two emergency brakes. There is an electrical emergency brake that needs to be powered up in order for the motor to spin (to the left of the silver band with the power cable), and a mechanical handbrake that needs to be released before the motor is turned on (the black rod above the gearbox).
Electric wheelchair motors are typically not sold new, and need to be found second-hand. The cheapest way to get an electric wheelchair motor is to buy a used electric wheelchair and take it apart.
The last motor is a ME0909 permanent magnet DC motor and is rated at 24-48V and 2000-4000W. Its stall current rating is listed at 98A. This is the type of motor that might be used in a (fast) go-kart, golf cart, or an electric motorbike.
This motor is heavy-duty and serious business, and is rated at 4HP (horsepower) continuous and 12.8HP peak operation. I once built a robotic platform powered by two of these and had 12 people sitting atop it without a noticeable reduction in speed. Including the weight of the robot itself, these motors were moving over 3,000 pounds (1.5 tons) with little problem.
Horsepower is a measurement of the rate at which work is done. Put into electrical terms, mechanical horsepower is equivalent to 745.7 Watts of power. Large motors should be thought of in these terms.
Without getting too deep into the math, it is best to think about horsepower as literally the amount of constant energy a horse would be able to provide to your project. In other words, if you were to replace a 5HP motor with horses, you would need 5 horses to provide the equivalent amount of power as the motor.
For instance, the wheelchair motor we are using is 1/3 horsepower. If we think of it in terms of horses, it would look something like this. Alternately, should 1/3 of a horse not be useful to your imagination, you can imagine one really lazy horse instead.
While it is silly to think about 1/3 of a horse, the ME0909 motor on the other hand would seem less silly requiring 4 full horses to replace it during standard operation.
When you attach this motor to your project, it is helpful to consider how much larger a horse is than yourself. Then, imagine strapping four horses to your project and letting it loose. That should at least give you a moment of pause, and some more respect for the power of the motor.
As we have learned in the Motors and Motion Lesson of my Robots Class, when dealing with motors there is a relationship between voltage and speed, as well as current and torque. Nothing much has changed in this regard except the magnitude of values we are dealing with. Larger motors require more voltage and current, and thus (typically) provide greater speeds and torque.
To review, the more voltage that is applied to the motor, the faster the speed. And the more torque that is applied, the more current the motor draws.
When dealing with brushed DC motors, there is a maximum operating voltage and by extension a top motor speed that it will never exceed. Additionally, the motor also has a maximum amount of torque which can be applied up until the point that the motor stalls. When the motor stalls, the amount of current it is drawing is the absolute maximum it ever can, and this is called the stall current.
When I discuss a motor having a top "speed", I am actually referring to the rotations per minute (RPM) of the motor. This is the rate at which the shaft spins in a minute.
Let's say we want to measure the speed of the entire mechanical system in terms of the distance it will travel over a period time. In other words, let's say we are building a go kart and want to get a measurement in miles per hour (MPH). To figure out what the actual theoretical top speed of the system we are building, it is important to understand the diameter of the drive wheels attached to the motor's shaft (or the output shaft of a geared system). Let us imagine that we are trying to figure out how fast our ME0909 motor would be driving a go-kart with a 12" wheel at the speed of 2,000 RPM.
First thing, you would need to calculate the 12" wheel's circumference. Using the above formula, we can calculate that it has a circumference of 37.6991 inches. What this means is that every time the wheel does a full rotation, it will theoretically move 37.6991 inches along the ground.
Now that we know the distance traveled per rotation, we need to multiply this distance out by its speed (2,000 RPM). To do this we simply multiply 2,000 with 37.6991. The value we get is 75,398.2 inches per minute. Of course, inches per minute is not exactly a useful unit of measurement.
What we really want to know is how many miles per hour our motor is going to go. To do this, first we need to multiply 75,398.2 inches per minute by 60 because there are 60 minutes in an hour. This gives us a value of 4,523,892 inches per hour. However, that number is ridiculous and inches is still not a useful a measurement at this scale. We need to convert this to miles by dividing the total value by 63,360 (the number of inches in a mile). This gives us a total top speed of approximately 71.4 MPH.
This speed is a theoretical ballpark estimate. The actual speed is influenced by all kinds of factors including (but not limited to) the weight of the machine, friction within the system, the traction of the wheels, gravity's pull, the charge of the batteries, and wind resistance (to name a few). Nevertheless, it is important to understand what the theoretical top speed is so that you are not caught by surprise by a machine that is faster (or slower) than anticipated, and can plan accordingly.
Of course, this speed is likely faster than we might want the machine that we are building to travel. We are going to discuss controlling a motor's speed in the third lesson. However, before we can get to that, we need to take a look at battery power supplies. We will do this in the next lesson.