# Controlling the speed of an AC motor?

CrzCz
For my IB Physics Extended Essay, I have just changed my topic from momentum to this and I have really little time of doing it..... (Due in Oct)

My research question would be The effect of angular velocity of circular motion of a moving sound source on the sound frequency. I am planning to place an electric motor at the bottom and put a stick or ruler on top of the motor. Then, I would put a phone with a controlled sound frequency and a counter-weight on each end of the stick/ruler, a mic will be placed somewhere near the set up. The speed of the electric motor will be changed and see how it affects the sound frequency BUT there is only an AC electric motor in my school where the speed cannot be controlled (so the speed can't be changed).

I have heard that the speed depends on the amount of voltage supplied to a DC electric motor, but is it possible for me to manipulate the speed to run this experiment?

## Answers and Replies

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berkeman
Mentor
For my IB Physics Extended Essay, I have just changed my topic from momentum to this and I have really little time of doing it..... (Due in Oct)

My research question would be The effect of angular velocity of circular motion of a moving sound source on the sound frequency. I am planning to place an electric motor at the bottom and put a stick or ruler on top of the motor. Then, I would put a phone with a controlled sound frequency and a counter-weight on each end of the stick/ruler, a mic will be placed somewhere near the set up. The speed of the electric motor will be changed and see how it affects the sound frequency BUT there is only an AC electric motor in my school where the speed cannot be controlled (so the speed can't be changed).

I have heard that the speed depends on the amount of voltage supplied to a DC electric motor, but is it possible for me to manipulate the speed to run this experiment?

Can you find or borrow a DC motor with speed control to use for this project? Is there a Robotics Club or Lab at your school? They might have something that you8 could use for the experiments.

CrzCz
Can you find or borrow a DC motor with speed control to use for this project? Is there a Robotics Club or Lab at your school? They might have something that you8 could use for the experiments.
Nope... We dont have any clubs like that... I also dont know where I can borrow one of these...

berkeman
Mentor
Can you post a picture and the datasheet/specs of the motor? Maybe I can FedEx you one for the project along with a return FedEx label.

Well, I guess I should ask, are you in the US?

CrzCz
Can you post a picture and the datasheet/specs of the motor? Maybe I can FedEx you one for the project along with a return FedEx label.

Well, I guess I should ask, are you in the US?
Unfortunately... No.... I live in China. My school only has a AC motor too.

berkeman
Mentor
My research question would be The effect of angular velocity of circular motion of a moving sound source on the sound frequency. I am planning to place an electric motor at the bottom and put a stick or ruler on top of the motor. Then, I would put a phone with a controlled sound frequency and a counter-weight on each end of the stick/ruler, a mic will be placed somewhere near the set up. The speed of the electric motor will be changed and see how it affects the sound frequency BUT there is only an AC electric motor in my school where the speed cannot be controlled (so the speed can't be changed).
Okay, we got this. Attach fan blades to your bicycle rear wheel axle, and vary the rotational s[peed manually by your pedaling speed. You should get extra credit points for innovation in your test setup.

CrzCz and Tom.G
Tom.G
Variable speed electric drill. (But I like the bicycle approach! I just don't see the purpose of the fan blades. Oh, further thought,. Then mount the sound source on another set of fan blades?)

CrzCz
Okay, we got this. Attach fan blades to your bicycle rear wheel axle, and vary the rotational s[peed manually by your pedaling speed. You should get extra credit points for innovation in your test setup.
But manually will make the experiment unreliable because there is bigger room for uncertainties.... If I can get a DC motor (I can just buy a 30RMB (Like 5USD) off somewhere), can I possible make it work[emoji24] I don't know how

Tom.G
A battery operated hobby motor of some type.. or one from an old battery operated audio tape recorder if you can find one. Just about anything with a low to medium speed motor. Drive the motor with flashlight batteries; the more batteries in series the faster it goes. And since it does not have to last for years you can get a slightly slow motor to run faster by adding more batteries.

Most of the hobby motors sold here in the US are made in China so you can probably find something to work.

CrzCz
A battery operated hobby motor of some type.. or one from an old battery operated audio tape recorder if you can find one. Just about anything with a low to medium speed motor. Drive the motor with flashlight batteries; the more batteries in series the faster it goes. And since it does not have to last for years you can get a slightly slow motor to run faster by adding more batteries.

Most of the hobby motors sold here in the US are made in China so you can probably find something to work.
I am afraid hobby motors might be a bit weak if I have to support a 1-meter long wooden ruler or some sort on top of it with a phone and a weight?

I would show you a picture of the set up I am thinking about at the moment but I don't know how to upload photos....

I found something about PMW controller? The descriptions for this is that this device will be able to control the speed of electric motors but I am not sure.

berkeman
Mentor
I would show you a picture of the set up I am thinking about at the moment but I don't know how to upload photos....
Use the UPLOAD button in the lower right of the reply window.
I found something about PMW controller? The descriptions for this is that this device will be able to control the speed of electric motors but I am not sure.
Yes, a PWM voltage source is used to control the speed of a DC motor.

CrzCz
Use the UPLOAD button in the lower right of the reply window.
Ooo, thanks. So here is the setup @Tom.G

Yes, a PWM voltage source is used to control the speed of a DC motor.
So by using a PWM I will be able to manipulate the speed of the motor easily?!

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Tom.G
Just a thought.
Have you worked out the numbers for how fast the motor needs to spin for a given change in sound frequency? That will be importatnt for selecting the motor.

Two Ideas...

1.) Use gears as a transmission. A multi-speed bicycle can be used for this.

2.) Use a friction brake on the rotating device while keeping the voltage and current to the AC motor constant. Changing the friction work load on the regulated AC motor will vary the speed. ( Remember to use a slip-clutch between the AC motor and the rotating device, or you will overheat and burnout the motor. )

CrzCz
Just a thought.
Have you worked out the numbers for how fast the motor needs to spin for a given change in sound frequency? That will be importatnt for selecting the motor.
No... I guess I will have to do it today. I am not sure how the motors are measured, I think they use frequency too?

Tom.G