Re: [TCML] How To Turn A Vacuum Cleaner Motor Into A Synchronous Motor

[Futuret <futuret@xxxxxxx>]

 Hi,

I did more experiments in modifying the same 120VAC universal motor.

1)  I added a 6A, 1000V rectifier diode across the 2 opposed sections of 
shorted commutator segments.  There were 4 adjacent segments shorted
together on one side of the commutator, and 4 adjacent segments shorted
on the opposing side of the commutator.  These opposing areas were then
connected by the diode.  I used a single diode and didn't worry about
weight balance because the diode is lightweight, and the speed (3600rpm)
is not that fast.  I didn't use any glue to hold the diode in place.  The
addition of the diode seemed to cause the motor to self start from any
spot (?), but sync operation was obtained only from 20V to 22V, and the
sync lock was not very stable.  There was constant hunting (rocking
back and forth of the visual pattern while sync-locked).  The presence of
the diode caused arcing at the brushes.

2)  I removed two shorts so that only 3 segments were shorted on
opposing sides of the commutator.  The diode was left in place.
Now the motor didn't self start from one particular spot.  There was
still arcing at the brushes.  Sync lock was obtained between 21V and
29 V.  Hunting occurred at the lower end of that voltage range, but
as the voltage was increased, the hunting died down, and the
sync lock was stable (no rocking of the pattern).  

3)  I removed two more shorts so that only 2 segments were shorted
on opposing sides of the commutator.  The diode was left in place.
Now the motor didn't self start from two spots.  There was still
arcing at the brushes.  Sync lock was only obtained between
16V and 18V.  However the motor did not hunt as in (1) above,
it was more stable, but didn't give much torque while in sync.

It seems to me from the tests above (and below), that adding
the diode improves self starting, but weakens the sync-lock
and can promote hunting.  I expected this, because the addition
of the diode causes the motor to be shorted across the opposing
segments during 1/2 of the A/C cycle, and I found that completely
shorting across the opposing segments kills sync-lock completely.
I assume that shorting for 1/2 the A/C cycle "kills" the sync-lock
only partially, and allows for some sync-locking to occur.  I didn't 
try connecting the diode to only one segment on each side of
the commutator, but my guess is this would work poorly since
the operation improved when I went from 2 segments to 3 segments.
Also Stefan's motor improved when he used more shorted segments.
Using 2 segments shorted on each side gave me bad hunting, which
Stefan also saw until he shorted more segments.  It is interesting that
sync lock occurred around 20 volts or so which happens to be 1/2
the voltage that Stefan and Clive used.  This makes sense because
they were using 240V motors, but I'm using a 120V motor.  

None of my tests gave sync-lock with self-starting in all starting
positions.

I did not try using a heavier rotor on the motor, to simulate use in
an SRSG.  

In any case the sync lock was much better without the diode, although
the self-starting was worse.  It's possible that larger more powerful
universal motors such as Stefan's washing machine motor are more
suitable for this modification.  It's also possible that 240V universal
motors are better suited for this modification since Clive had good
results using his 240 volt vacuum cleaner motor.  

Cheers,
John

 


 

 

-----Original Message-----
From: Futuret <futuret@xxxxxxx>
To: tesla <tesla@xxxxxxxxxx>
Sent: Fri, Dec 17, 2010 8:11 pm
Subject: Re: [TCML] How To Turn A Vacuum Cleaner Motor Into A Synchronous Motor


Hi,

I did some experiments modifying a 120VAC universal motor (series wound)
from a vacuum cleaner.  The motor current and HP ratings are unknown.  
The commutator has 22 segments in total.  

First I shorted 4 adjacent segments, and connected these to 4 more adjacent
segments on the opposite side of the commutator.  So the 4 segments on 
one side were shorted to the other 4 segments on the other side of the commutator
of the armature.  I didn't use any diodes.  With this setup, it was impossible to 
obtain synchronous operation.  I forget if the motor was always self-starting 
with this arrangement.  

Next I left the 4 segments shorted on opposite sides of the commutator, but
I cut out the short between the 2 groups of 4 segments.  So 4 segments were
still shorted together on each side, but were no longer connected to the
segments on the opposite side.  Again no diodes were used.  This arrangement
gave synchronous operation between 24V at 4.5A, and 42V at 10.5A.  
A reasonable amount of phase shift was obtained as the voltage was varied
between 24V and 42 volts.  It was much easier to pull the motor out of sync
by loading it at the lower voltage than at the higher voltage as would be
expected.  The sync-torque was good overall, but it didn't seem that 
different than an induction sync-modified motor.  I don't know if using 
diodes helps the torque.  The motor self-started at times, but not at other 
times, depending on the armature rotational position at start-up.  The higher 
the voltage, the more likely the motor was to self-start.  At 40 volts, the 
motor always self-started.  If the voltage was over 35 volts, any sudden 
loading of the motor would cause the motor to speed up and lose sync.  
Sudden loads would not tend to occur however in normal SRSG operation.  

I confirmed sync operation, and phase shifting by observing a black line 
drawn on a cardboard disc, attached to the motor shaft, under ballast-type
fluorescent lighting.  I'm experienced at doing this and I have no trouble
telling if a motor is in sync or not.  

These results show that sync operation can be obtained without using
diodes, however the diodes may perhaps improve the self-starting, and 
reduce the current draw some?  I didn't try shorting fewer than 4 segments
on each side.  I have no photos or videos available for these tests.  
In no cases did I encounter the "crazy mode" of operation.  However 
there were some hunting sounds just before the voltage was raised
enough to sync-lock the motor.  In no cases did I see any arcing
at the brushes.  I assume the motor was running at 3600 RPM (not
1800 RPM) because the motor has two poles, but I had no easy way 
to verify that.  Actually, by looking at the light pattern on the disc, it
looked like a 3600 RPM pattern, not an 1800 RPM pattern.   

Cheers,
John 




 
 
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