# How do you control the speed of an AC BRUSHLESS motor? (Used on a scroll saw)?

I have a small scroll saw i would like to build a cheap speed controler for
It uses an AC motor which appears to be brushless when i dismantled it

Could anyone tell me how brushless AC motors work, and how their speeed controlers work (i assume you need to change the phase of the ac current)

Many thanks
Chris

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iceng5 years ago
A cheep universal motor could be running the scroll saw but you said there were no brushes and
Wow you are a Second Prize Winner in a robot contest,
you should have been Featured.

Steve is correct about the Induction motor.  Let me add,   it is the Slip
( the difference between the Rotating Magnetic Field and the Rotor RPM )
that develops the motor torque.

As you load the motor the Slip will increase to provide more torque.
The rotor acts as the secondary of a transformer.  Usually copper or Al
bars in a magnetic frame resembling a squirrel-cage see pics

Slip causes a low frequency magnetic field inducing current in the
conducting bars of the cage.  As load increases slipage does too.
That means the frequency increases and more energy is induced into
the rotor which produces a stronger reaction field to be dragged around
by the primary rotating field.

The rotor cage is scued sideways to avoid a cogging effect.

A
5 years ago
I DID mention Slip.....

Also, there comes a point when the motor "drops out", and the slip ceases to cause rotation, the rotor just heats up (fairly quickly)

Ah, we old timers that actually studied electric machines at University......

Steve
ooda55 (author)  steveastrouk5 years ago
Thanks for the replies guys!
D'oh, i was thinking about frequency and talking about phase in the question! (Not too good for a physics student)
I will have a play with it when I have a chance and see if changing the voltage has any effect
Is there anywhere I can find some circuit diagrams for a suitable control circuit?
(PS. i was very surprised about the robits contest win, there were some other fantastic robots in the competition, but i was aiming to build a very simple platform and some nice source code for interfacing with the K8055 in VB.net)
5 years ago
Here is a Triac phase control circuit that does not change frequency but
does reduce the average power to the motor increasing slip thereby

You should know a phase control generates an increased RMS pulsed
current that is responsible for increased waste heating in the motor.

As Steve pointed out, do not expect a wide speed range from your
induction motor.

A
steveastrouk5 years ago
An AC induction motor can be speed controlled by changing the frequency of the supply, and also, depending on type, to a limited extent, by changing the motor supply volts.

AC induction motors work by the field coils creating a rotating magnetic field. The rotating field induces currents in the rotor which cause the rotor to spin and accelerate. As it accelerates, the currents induced reduce, until there is just enough current flowing to overcome losses. Applying a load, causes the rotor to slow, and the induced currents to increase.

So a normal AC motor CANNOT spin at the same frequency as the supply, there is ALWAYS a "slip" - so a 60Hz motor, which one might think would spin at 3200 RPM will actually spin nearer 3100, like wise a 50Hz motor, 2900 RPM or so,

Steve