ac incution motor speed control problem ..

[mike theodore]

Hi

I'm experimenting with a single phase 220V ac induction motor.
I want to regulate speed and for this i'm using a triac together with
an optocoupler to drive it.
I'm using PWM to control firing angle and hence the speed of the motor .
I see two strange things as speed is reduced
1. Motor overheats. Is this normal ?
2. I hear a buzzing sound coming from the motor .From what is it?

I want to mention that the motor has a primary and a secondary winding
The primary has the triac circuit and the seconadry is connected directly to
mains throug a capacitor.
Any help would be greatly appreciated .

Michail

 

[John Popelish]

Hi

I'm experimenting with a single phase 220V ac induction motor.
I want to regulate speed and for this i'm using a triac together with
an optocoupler to drive it.
I'm using PWM to control firing angle and hence the speed of the motor .
I see two strange things as speed is reduced
1. Motor overheats. Is this normal ?

I'm afraid it is.

2. I hear a buzzing sound coming from the motor .From what is it?

Vibration caused by the rapidly changing magnetic forces
between the rotor and stator.

I want to mention that the motor has a primary and a secondary winding
The primary has the triac circuit and the seconadry is connected directly to
mains throug a capacitor.
Any help would be greatly appreciated .

Induction motors run efficiently only near synchronous
speed. The applied electrical energy produces a magnetic
wave that rotates around the stator, while both magnetizing
and dragging around the rotor.

The only efficient way to slow an induction motor is to
lower both the applied AC frequency and the voltage. Just
chopping parts of each half cycle out of the applied
waveform does not change the applied frequency, though it
does lower the effective voltage of the fundamental, while
adding all those noisy harmonics.

There are low cost drives made to produce variable speed
operation for 3 phase induction motors.

e.g.
http://web4.automationdirect.com/adc/Shopping/Catalog/AC_Drives_-z-_Motors

 

[mike theodore]

John,
Thanks for the immediate answer

My questions are .

Why is it overhated since by chopping the cycle i insert less energy ?

If i use another method it will not be overhated ?

What is the best method to make speed control with single phase
ac induction motors without these problems ?

Should i use a rectifier and PWM with IGBT for DUTY cycle ?

Is there a low cost solution to make single phase motor speed control?

If i use PWM with "full cycle stealing" so some 50 hz cycles are ommited
will i get better results ?

The motor power is 250 Watts.

 

[John Popelish]

Thanks for the answer John

Can I ask why is it overheating since by chopping the

duty cycle I Imagine it cosnumes less energy than full cycle
operation .

The induction motor is essentially a transformer with the
secondary shorted but rotating within the primary. The
rotation keeps the short circuit from looking like a big
load. When you slow the motor by lowering the average
voltage by any means, the short circuit in the secondary
(rotor) shows up. This creates heat just like shorting the
secondary of any transformer would.

Is there a method for single phase motors to correct both

problems?

If the single phase motor normally runs with a capacitor,
then you can use one of the three phase variable frequency,
variable voltage drives, with a transformer, to convert the
three phase output to the two phases the motor would get
from the direct line connection and the series capacitor
connection to the second winding. And it will run
efficiently ad variable frequency. Any voltage control
method (at fixed frequency) will produce a lot of heat.

If I use full cycle removal (so some 50 HZ cycles are off

and some on ) instead of phase chopping will I get better
results ?

If you pass one half cycle, and then remove the next two
half cycles, you effectively reduce the frequency by a
factor of three and the motor will run slower. But the high
harmonic content will produce a lot of vibration.

In order to lower AC frequency ,voltage should I use rectification and then
IGBT pwm drive?
Is there something cheap to accomplish that ?
The motor is 250 Watts.

The link I gave you has drives for sale that are lower in
cost than the parts cost to make your own.

 

[MooseFET]

Vibration caused by the rapidly changing magnetic forces
between the rotor and stator.

Just the sudden magnetic field changes in the motor field windings can
also be causing noise. There can be magnetostriction in the iron and
a bit of jiggle in the windings. A transformer will make noise if
connected to a light dimmer.

The motor is likely not good at radiating vibrations at 60Hz into the
air. This will make th eviration at higher frequency seem bigger
because the higher frequencies do get radiated well.

 

[Steve Sousa]

If the single phase motor normally runs with a capacitor, then you can
use one of the three phase variable frequency, variable voltage
drives, with a transformer, to convert the three phase output to the
two phases the motor would get from the direct line connection and the
series capacitor connection to the second winding. And it will run
efficiently ad variable frequency. Any voltage control method (at
fixed frequency) will produce a lot of heat.

A diagram please?

Thank you

Steve Sousa

 

[Jamie]

Hi

I'm experimenting with a single phase 220V ac induction motor.
I want to regulate speed and for this i'm using a triac together with
an optocoupler to drive it.
I'm using PWM to control firing angle and hence the speed of the motor .
I see two strange things as speed is reduced
1. Motor overheats. Is this normal ?
2. I hear a buzzing sound coming from the motor .From what is it?

I want to mention that the motor has a primary and a secondary winding
The primary has the triac circuit and the seconadry is connected directly to
mains throug a capacitor.
Any help would be greatly appreciated .

Michail

that really isn't the proper way to do that how ever, induction motors
have been mode to deal with heating stators with what looks like an open
frame chassis to allow for air circulation around the stator. These type
of motors are normally referred to as tension/stall motors where they can
be used as pull and hold drive motors.

If you want to vary the speed, then you should be varying the frequency
going to the motor.
A Vector type feed back system (encoder on the shaft) can precisely
get proper frequency at lowest voltage possible for more energy
efficiency.. this also allows for the motor to run as slower speeds with
torque with out losing it. This is because the frequency being generated
matches the rotation speed desired with the number of poles in the
factor. from there, all that has to be done is selecting only the amount
of voltage required to the stator to achieve this rotation speed at
matching frequency.
The rotation RMP is 95% of frequency *NumberOfPoles, this is
considered to be in Asynchronous. You can not achieve 100% rotation speed
with the supply frequency with this type of motor..
That's where Synchronous motors come in
etc...

something to think about.

 

[The Real Andy]

I'm afraid it is.


Vibration caused by the rapidly changing magnetic forces
between the rotor and stator.


Induction motors run efficiently only near synchronous
speed. The applied electrical energy produces a magnetic
wave that rotates around the stator, while both magnetizing
and dragging around the rotor.

The only efficient way to slow an induction motor is to
lower both the applied AC frequency and the voltage. Just
chopping parts of each half cycle out of the applied
waveform does not change the applied frequency, though it
does lower the effective voltage of the fundamental, while
adding all those noisy harmonics.

There are low cost drives made to produce variable speed
operation for 3 phase induction motors.

e.g.
http://web4.automationdirect.com/adc/Shopping/Catalog/AC_Drives_-z-_Motors

Damn they are cheap. About 18 months ago i blew about a $1000 to
design and build a half dozen of those for my father. I feel robbed.

 

[MooseFET]

On Jun 6, 8:06 pm, Jamie

[.....]

The rotation RMP is 95% of frequency *NumberOfPoles, this is

Make that (2 * Frequency) / NumberOfPoles

 

[John Popelish]

A diagram please?

Thank you

The exact design depends on the winding ratios in the motor.
Capacitor run motors often see more voltage across the
capacitor driven winding than the full line voltage applied
to the main winding, because of resonant boost effect. But
you have to run the motor unloaded, and with the removed
(starting the motor with a spin) and measure the voltage
across that winding, to know what the optimum relative
winding voltages are.

The relative phase of the main and capacitor windings is
generally 90 degrees (but that can be measured with the
unloaded test, also). So the trick to run them with a 3
phase drive is to extract the two required voltage ratios at
the required 90 degree phase shift from the 3 120 degree
shifted outputs from the drive. Here is a phasor diagram
(where the length of the windings represent voltage and the
orientation represents relative phase shift) of how to use
an auto transformer to get 2 phases from 3.

Cs represent capacitor driven winding, Ms represents main
winding, Ws represent autotransformer winding:

* @ phase A
* C
* C
* C
* C
* C
* phase B +MMMMMMMM\
* @ | >@ phase C
* \WWWWWWW+WWWWWWWW/

When driven from a variable speed drive, the capacitor is
not needed, since the drive provides the 90 degree phase
shift between the two windings.

There are lots of other possible ways to derive arbitrary
ratios voltages with a relative phase difference of 90
degrees from a 3 phase source. This one is just the easiest
for me to draw with characters. Doodle a little and see
what you can come up with.

 

[Jamie]

On Jun 6, 8:06 pm, Jamie

[.....]

The rotation RMP is 95% of frequency *NumberOfPoles, this is

Make that (2 * Frequency) / NumberOfPoles

Yup.. Thanks for the over sight, i must of been tired when i did that one./

So that only means that some do read my post.