Asynchonous to synchronous

[Doodlebug]

On another forum (for tesla coils) some one wanted a mains synchronous motor.

So he took a mains brushed asynchronous motor from a hoover and soldered a
large diode on the commutator at 180 degree segment spacing.
Hey presto, it run synchronously.
How come?

[ppppenguin]

That sounds very odd. Brush motors are not even vaguely synchronous. The one from a hoover would have been a universal series wound motor where the speed is very dependent on load. At zero load the speed tends to infinity.

https://en.wikipedia.org/wiki/Universal_motor

[Doodlebug]

Apparently it is run at reduced volts as it gets hot at 240v.  Here's pic of diode in place.
Another diode  is added on opposite side for rotational balance.    

[ex 2 Base]

Hello Doodlebug, can you tell me which forum you found the information on. Ted

[boater sam]

I'm sceptical but hope to be convinced.

I can't understand the theory or reason behind this and suspect that the diodes would be flung off when it ran.

Amaze me please.

[Doodlebug]

First mentioned here, some years ago where some tried it and were impressed !
Read all the thread to get the full picture.
No report of diodes being flung off........
.........I suppose it depends on one's soldering skills . :-)

http://4hv.org/e107_plugins/forum/forum_...php?102515

[Nick]

Clive is one of the UK Tesla lot - he used to go to the Cambridge Teslathons years ago - don;t know if he's still active as I dropped out of the 4HV/UK Tesla world a few years ago too. Clive was into huge SRSG (Synchronous Rotating Spark Gap, i.e. traditional) Tesla coils and I was very much smaller DRSSTCs (Doubly Resonant Solid State Tesla Coils) which could play music etc.

You could mail/message him on 4HV and see if he responds? He may also still be on the TCBOUK (Tesla Coil Builder Of the UK) - his contact details are on the TCBOUK web site (http://www.tcbouk.org.uk/)

Derek Woodroffe who organizes the UK Teslathons owns the Extreme Electronics site and he'll know for sure.

[boater sam]

He glued the diodes on.
What is not mentioned is the polarity of the diodes, are they in series or parallel?
Must try this sometime.
There is a note that the current was seen to reduce with an applied load, this is normal with series wound universal motors up to a point and with squirrel cage motors. We had fan units that needed a restriction in the ducting to reduce the running current, perverse but true.

[Pat Pending]

He glued the diodes on.
What is not mentioned is the polarity of the diodes, are they in series or parallel?
Must try this sometime.
There is a note that the current was seen to reduce with an applied load, this is normal with series wound universal motors up to a point and with squirrel cage motors. We had fan units that needed a restriction in the ducting to reduce the running current, perverse but true.

Not really perverse Sam, the load for a fan is the air it has to move, restrict the air flowing into the fan and you reduce the load and therefore the current.

People often misinterpret the increased revs on a vacuum cleaner motor when the pipe becomes blocked as a sign of increased load, however it is the decreased load that in fact is causing the motor to over speed, this could easily be proven by a check on the current being drawn actually dropping, just as would be the case with a synchronous motor in your example.

The reason that a vacuum cleaner motor is likely to be destroyed quite quickly when abused like this is because not only will the revs cause unsafe levels of centrifugal force, but also the normal vac motor relies upon the gale of cooling air to allow it to produce its insane power levels.

Andy.

[ppppenguin]

The motors on "wet and dry" vacs have a separate cooling fan. Precisely because the airflow can be blocked in normal use.