Re: Fuse for an AC motor

Uriah wrote:
On Feb 1, 1:37 am, "Homer J Simpson" <nob...@xxxxxxxxxxx> wrote:

"Uriah" <uriah...@xxxxxxxxxxx> wrote in message

news:1170241636.521519.289640@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx




Ok, so the bottom line is no a fuse won't work? But it looks like I
have a number of good ideas. Thank you to all. I will start checking
into all of them. They all sound very workable.

You CAN protect them but a regular fuse is unlikely to help. What sort of
HP/amps are they?


Here is what I found out. When I wire them up the draw .37 amps.
This is with them not being installed in the machine. When I stop the
motor with a pair of vise-grips it draws .49 amps. So I have about .
10 of an amp to deal with. But if I put them in the machine the are
probably going to drag it down a little. So that might cut my margin
to less then .10 amps. Maybe .07 Amps. I think that this is too
little of a difference to be able to catch with a fuse? I would have
to find a really acurate fuse that blows at exactly .45 amps. I think
it will have to be a Rockwell CB fuse but will it work with something
with this small of a margin? As far as the thermal I guess I will
have to let the motor run in the machine check the temp and then add 5
degrees? Is that the right idea?


You can use a 6 ohm 5 watt resistor in series with the
motor, to create a 2.22 volt drop under the normal
condition, and a 2.94 volt drop when jammed. Rectify,
filter and feed that into a 4N35 opto coupler circuit
that disconnects power to the DPDT motor relay until
manually reset. Something like this for the opto sensing
portion:

|> | D1
AC ---o o---[Motor]---+--->|---+---+
RLY1-2 | | | Pot = 500 ohms
| | P
[6R] [47uF] O<---[LED]---+
| | T |
| D2 | | |
AC --------------------+---|<---+---+---[33R]----+

The LED is the input side of the opto coupler.
Adjust the 500 ohm pot so the relay drops out when the
motor slows way down but is not yet stalled.

For the relay portion:

+ 12 ----+--------+-----+----+
| | | |
[1K] [10K] [RLY] [D5]
| | | |a
| | +----+
| | |
| D3 | /c
+---|<---+----| Q1 NPN
| \e
| |
| +-----+
/c | |
opto o-- o
\e RLY1-1 o-^ |= PB
| | o
| D4 | |
Gnd ----+-----|<--------+-----+

When you press the pushbutton, current flows
through Q1, which is biased on by the 10K resistor.
This energizes the relay and transfers the contacts,
which starts the motor through RLY1-2. The RLY1-1
contact maintains the Q1 emitter path to ground,
keeping the relay energized when you release the
button.

When the motor stalls, sufficient voltage is
developed across the 6 ohm resistor to charge
the capacitor to ~2.5 volts. That lights the
LED portion of the optocoupler, which causes
the opto transistor to conduct through the 1K
resistor. That biases Q1 off, and the relay
drops out, stopping the motor. Since that opens
the emitter path to Q1, it cannot energize the
relay again until the pushbutton is pressed.

With the low current requirement, most any
DPDT (Double Pole Double Throw) relay rated
for a 12 volt coil and 120 VAC contacts will
work. You can use 1N4004 diodes and any NPN
transistor. The 6 ohm resistor is made with
a 1 ohm and a 5 ohm resistor in series.
6 ohms resistance will disspiate about 1.5
watts, so use 2 watt or higher resistors.

Ed
.

Loading