Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Variable speed DC motors

Learn more physics!

Most recent answer: 02/16/2008
Q:
How does variable speed DC motors work? What conditions maximize torque at a given speed?
- Anonymous
A:
You can find out lots and lots about electric motors by visiting , complete with diagrams and explanations. The very simplest DC electric motors work by placing an electromagnet in a magnetic field made by a permanent magnet. The electromagnet is a piece of iron or steel with wire wound around it in one direction, and it turns the drive shaft which can spin freely along one axis. To supply electricity to the wires from outside, where the outside wires do not spin with the electromagnet, moving contacts have to be made so the wires from outside the motor do not have to wind around the driveshaft. These moving contacts are called "brushes" because the stationary electrical contacts rub or brush against the moving ones.

But why does this all work? The electromagnet feels the magnetic field of the permanent magnet around it and wants to line up with it, like a compass needle pointing north. The main gimmick is, at this point, to reverse the direction of electrical current in the electromagnet when this happens so that the electromagnet wants to turn around to line up the other way. This is done by aligning the brushes so that they rub on one contact when the electromagnet is pointed in one direction and rub on the opposite contact when the electromagnet is pointing the other way. It is arranged so that whenever the electromagnet is pointed towards one of the directions of the permananet magnet's field, that is just when it suddenly changes current direction to want to line up in the other direction, and thus always wants to spin around because it is never quite happy where it is.

These simple motors can spin in either direction -- they sometimes need a little push to get them going (more complicated motors have a starting-up mechanism or multiple poles instead of two to get them going in only one direction - see the above website on that).

Their speed is determined only by how fast the current can change direction in the electromagnet, how strongly the electromagnet is attracted to the opposite pole of the permanent magnet, and how much load there is on the driveshaft. The applied DC current does not have any time structure to it, and so does not set the turning rate of the motor. The rate at which the current can switch directions in the electromagnet and the force of attraction to the permanent poles are directly related to the applied DC voltage. The more volts you apply, the faster the motor will run. You can do an experiment with a motor -- what is the relationship between the speed of the motor and the applied voltage?

To get more torque, it is good to make a bigger motor or apply more DC volts to it. A larger electromagnet can have more wires wrapped around it for a stronger field, and also can apply more torque to the driveshaft because longer levers have more mechanical advantage. It will take more electrical power, and some may be wasted. If your motor is bigger than needed for the job, not only will it cost more to buy or build, but it will take more electrical energy than is needed for the job.

It is hard to control the speed of a very simple motor like the one above, another reason why they aren't very popular for applications. Many household appliance motors work on the AC power which comes from the power company. AC power is also easier to distribute and change the voltage with transformers. The frequency of the AC power is a natural timing base for these motors' turning speed.

Tom

(published on 10/22/2007)

Follow-Up #1: Variable speed DC motors

Q:
Why are some small dc motors NOT variable speed and how can you identify which ones are variable speed and which ones are not before you purchase them? Thank You
- Tom Lee--61
Cullman Al. USA
A:
In general if you don't do anything special to it, a DC motor will have variable speed.  The main factors are the DC voltage applied to the armature coil and the amount of torque load you are trying to drive.  In order to make it into a constant speed DC motor you have to have some sort of electronic feedback.   There are a number of ways to do this but they all have in common two things: a method to determine the speed of the motor and a method to control the amount of current supplied to the coils. 

Now to your question as to how to identify them:   you have to read the label on the motor and you have to ask questions.  If there is no label, it's probably variable speed.  Typically the constant speed motors will be more expensive due to the cost of the control mechanism.  

The web site: contains a wealth of information on both AC and DC motors and their speed controls.

LeeH

(published on 02/16/2008)

Follow-up on this answer.