I just had a gadget arrive to use for sharpening tungsten tipped saw blades and while all of the bits are there, the electric motor runs the wrong direction. If it was a DC brush motor it would be a simple swap the wires around but its a mains 240 volt 50 cycle motor that has permanent magnets in it so I guess its an AC brush motor. It has been so long since I touched one of them that I forget how to reverse the running direction. After the switch is a .47 mf capacitor on each side, a couple of diodes on a circuit board and two wires going to the motor.
What I forget is whether you can simply reverse the direction by swapping the 2 wires going to the motor or whether you have to swap wires in the guts of the motor. Anyone remember how these small AC brush motors actually work ?
diodes ?
it may be a DC motor
can you draw a schematic ?
it doesn't have to be to scale - lol
on second thought....
you may not need to draw it
this is one of the many rectifier topologies
(http://www.osha.gov/SLTC/etools/electric_power/images/full_wave_rectifier_circuit.gif)
if there are only 2 wires going to the motor, it is likely a DC motor
an AC motor with a starting capacitor will typically have field and armature windings, so there will be more than 2 wires
(http://www.electrical-contractor.net/theory/mspcs_12.gif)
Geez Dave you really need a capacitor on that bridge to get rid of the ripple ! I've been studying power supplies as part of my new hobby in electronics (specifically microcontrollers), I'm going to be trying to build a 5v switching supply this weekend :)
switchers are a lot of fun
be sure to wear safety glasses :bg
truthfully, even a simple shunt regulator circuit isn't as simple as it sounds
to make it work correctly over full line and load conditions requires a lot of calculations
(http://www.masm32.com/private/motorend.jpg)
This is the small board inside the motor end cover, it may just be a crappy little half wave rectifier running at mains voltage and the motor may just be a DC motor. Never seen stuff like this before.
Friend says its single phase. Isn't sure if you could change the wires after the diode. What model is it?
well - it probably wouldn't hurt to reverse a couple wires - lol
just don't run it long if it starts growling at you
the rectifier circuit may be for the relay
i can't tell where some of the wires are going, and i can't see the circuit board "runs", either
the capacitor hardly looks big enough to be a starter
don't suppose you have a meter ?
i think it's a DC motor
John brings up a good point....
have you looked everywhere for the reversal switch ? :bg
the red and blue wires off the right are obviously DC (marked + and -)
notice that the diodes are different
maybe - one for the motor and one for the relay ?
hmmm - just noticed the third one
QuoteIt's a single phase motor, the capacitors are for starting it, they shut off once the motor is running. Without seeing the nameplate, I have no idea how to change rotation, other than it is done in the motor housing.
This is from a electrician friend.
make and model ?
we might find info online...
http://www.eng-tips.com/viewthread.cfm?qid=254253
They are talking about several motors. Might help.
Done, just reversed the wires after all of the switch gear and capacitors and it runs the right way now. It appears to have been an error in assembly as there was a color code swap with the wiring.
I have not seen high voltage DC motors in this type of context before, you see plenty of AC brush motors in ordinary power tools.
(https://lh3.googleusercontent.com/-ZD_gqPV2VYs/T1Wdu5QipXI/AAAAAAAABvo/ECRfjmBeMCU/w353-h264-k/HPIM1399.JPG)
Wonder how these worked Hutch?
yah - hand drills and saws don't have starter windings because they are wound on magnetic cores
in those types of AC motors, the magnetic poles determine the rotation direction
also - starter capacitors are non-polar - so you kinda get the impression that one is for filtering DC
Can't really tell if the circuit is a rectifier unless you can see the traces on the board but the electrolytic (10uF) capacitor would blow pretty quick in an AC circuit so I guess it must be. However, the other problem is that at 240v there is no transformer and the diodes wouldn't last long (there's no resistor to drop the voltage across), there would have to be some sort of step down transformer and usually at least a zener and resistor to limit the voltage.
well - you can get diodes that handle the voltage (PRV)
but, i would normally expect a full-wave bridge in that application (shown above in the drawing)
i dunno - maybe electricity in Oz is "split" like our 220 is
in which case, it could be a full-wave "center-tapped" rectifier
(http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/ietron/rectct.gif)
last choice is a half-wave rectifier - hardly see them in use because they are inefficient and harder to filter
Quote from: hutch-- on March 06, 2012, 03:30:44 AM
. . . but its a mains 240 volt 50 cycle motor. . .
I've done a good bit of home and business wiring in my day Steve and there's nothing silly about a question when it involves 240 volts!
(http://www.quickersoft.com/pictures/shock.jpg)
that's fun stuff :P
the few times i worked with 440, i used the lock-out/tag-out protocol, though :U
that stuff will kill you dead
i am always surprised that countries like Australia and England have such high line voltages at the standard wall-plate
and 50 Hz - you can almost count the cycles as your teeth chatter - lol
True confessions while wiring machinery. In OZ the industrial standard is 3 phase 415 volt AC and one of the machines I set up and used was a star - delta 10 hp induction motor that had a manual starter switch on it. After I first put it together it ran the wrong way so I went to the main switch for the factory busbar and pulled the lever down until it went clunk. Went back to the machine, took off the front cover for the switch block then touched the first terminal and WHACK, grabbed a live singe phase. After a few expletives I went back to the busbar switch and found out that it was a "clunk - CLUNK" two step switch. Been using neon tester screwdrivers ever since. :bg
The factory has a 450 amp 3 phase 415 volt power supply that could run almost anything, had more than enough grunt to run the Lincoln SAE welder.
Had a uninvited kick from 400 before, where some dickhead switched the mains back on, after I'd explicitly solidly taped the switches in the off position (standard procedure).
Half my right hand was numb for 30 minutes, and I ran around that hospital floor shouting and cursing for the idiot to own up.
Nope, I don't stand back when some moron tries to end my life.. I just let them have it.. idiots!!!!
:wink
(http://l.yimg.com/us.yimg.com/i/mesg/emoticons7/24.gif)
"taped in the off position" is not standard procedure :bg
lock-out/tag-out protocol
http://www.northernsafety.com/news/Article/800569205/proper-lockout-tagout-protocols-protect-workers
I know I know, but in darkest africa it is :bg
There no other alternative as our dopey electricians and building engineers do not think to seperate the different power systems and throw them all into one DB panel.
So along comes another dopehead and starts throwing switches.. even ones that have tons of tape on them.... If you can figure this out you'll understand why africa is what it is... :bg
The safety thing is not big here, but it's slowly creeping in with more modern companies... the ones that can survive the AA/BEE onslaught, or at least reject that form of racism. :tdown
it depends on the switchboard but the safe way is to remove the fuse if its an old one or the circuit breaker if its a modern version. Then put a sign on the switchboard that you will KILL anyone who changes anything while you are working on the wiring. Then use a neon screwdriver indicator just in case and keep track of the switchboard.