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How to use DC to stop AC motor?

E

Eric R Snow

Jan 1, 1970
0
Greetings All,
I have a grinder with a 1/4 HP reversable induction motor in it. It is
not a capacitor start or run motor. It has a centrifigul start switch.
When grinding tools it is often necessary to reverse rotation several
times to grind different features on the same tool. This entails lots
of waiting for the motor to slow down enough to throw the reversing
switch. Though I could replace the AC motor with a DC motor and PWM
drive (which I already have), I would rather put in some kind of DC
injection and a timer to stop the motor faster. Is this something a
person with limited understanding of electronics can build? If so, can
anyone here provide me with a schematic or point me to a site or book
which would?
Thank You,
Eric R Snow,
E T Precision Machine
 
J

John Popelish

Jan 1, 1970
0
Eric said:
Greetings All,
I have a grinder with a 1/4 HP reversable induction motor in it. It is
not a capacitor start or run motor. It has a centrifigul start switch.
When grinding tools it is often necessary to reverse rotation several
times to grind different features on the same tool. This entails lots
of waiting for the motor to slow down enough to throw the reversing
switch. Though I could replace the AC motor with a DC motor and PWM
drive (which I already have), I would rather put in some kind of DC
injection and a timer to stop the motor faster. Is this something a
person with limited understanding of electronics can build? If so, can
anyone here provide me with a schematic or point me to a site or book
which would?
Thank You,
Eric R Snow,
E T Precision Machine

I don't have a schematic for you, but all it takes is a single dry
cell connected across the motor for a few seconds. If you can figure
a way to interlock a relay to your forward, reverse control, so that
the DC cannot be applied while the motor is powered, you can just
connect the DC through that interlock with a momentary push button.
You should interlock both sides of the Dc circuit, so you cannot get a
shock off the battery. A D cell should power a hundred or so stops.
 
J

John Larkin

Jan 1, 1970
0
Greetings All,
I have a grinder with a 1/4 HP reversable induction motor in it. It is
not a capacitor start or run motor. It has a centrifigul start switch.
When grinding tools it is often necessary to reverse rotation several
times to grind different features on the same tool. This entails lots
of waiting for the motor to slow down enough to throw the reversing
switch. Though I could replace the AC motor with a DC motor and PWM
drive (which I already have), I would rather put in some kind of DC
injection and a timer to stop the motor faster. Is this something a
person with limited understanding of electronics can build? If so, can
anyone here provide me with a schematic or point me to a site or book
which would?
Thank You,
Eric R Snow,
E T Precision Machine


If the main motor winding were driven from 120 VAC through a diode and
a power resistor (in series), you should get pretty good braking. We
used to do this on tape deck reel motors, and it didn't take a lot of
DC current to give nice drag.

I'd suggest a 1N4005-type diode and maybe a 300 ohm, 50 watt resistor
as things to try. There's probably a place in the reversing switch
setup where you could arrange for the small DC to flow whenever the
motor wasn't supposed to be running (no timer!) but that would depend
on how things are wired.

What sort of machining do you do?

John
 
E

Eric R Snow

Jan 1, 1970
0
If the main motor winding were driven from 120 VAC through a diode and
a power resistor (in series), you should get pretty good braking. We
used to do this on tape deck reel motors, and it didn't take a lot of
DC current to give nice drag.

I'd suggest a 1N4005-type diode and maybe a 300 ohm, 50 watt resistor
as things to try. There's probably a place in the reversing switch
setup where you could arrange for the small DC to flow whenever the
motor wasn't supposed to be running (no timer!) but that would depend
on how things are wired.

What sort of machining do you do?

John
Greetings John,
It appears that I never replied to your message here. Sorry about
that:( Anyway, the machining I do is general job shop stuff. So,
almost anything except automotive and aircraft. Plastics, metals,
ceramics, and composites all run through my shop. Mostly, these days,
it's aluminum, brass, stainless, and acetron plastic getting done.
Part descriptions range from door hinge finials to baseplates for
fingerprint reading machines. I modify stepper motors and ballscrews.
So, pretty varied. Also, some welding gets done. Some retired guy
will need the handle for his lawnmower welded up and things like that.
They get the "Good Neighbor" rate. And some of my weld jobs require
tig welding of stainless parts and aluminum parts.
Cheers,
Eric
 
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