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A Discriminating Circuit

R

Randy Gross

Jan 1, 1970
0
Greetings,

I am searching for a circuit that either senses the rectified feed to a
12vdc motor, or its speed, and, if the voltage, or speed, falls below a
preset, then it either trips a breaker else energizes a relay that will
supply an alternative (a battery to boost rpms).

Here's the catch. That same circuit would have to sense that the motor feed
has regained the correct voltage and disengage the booster.

A good analogy here would be a motor driving an alternator. When the motors
speed drops below that necessary to maintain the desired output, because
the primary (rectified 12vdc) feed has been interrupted, the feed is
disengaged and an alternative feed (battery) is engaged until the primary
feed has regained, at which time the secondary feed is disengaged and the
primary resumes.

This is an experiment and the process is trail and error. I know very
little about digital circuitry and this system needs that kind of
precision.

an Inquiring mind,
Randy Gross
 
R

Robert Monsen

Jan 1, 1970
0
Randy Gross said:
Greetings,

I am searching for a circuit that either senses the rectified feed to a
12vdc motor, or its speed, and, if the voltage, or speed, falls below a
preset, then it either trips a breaker else energizes a relay that will
supply an alternative (a battery to boost rpms).

Here's the catch. That same circuit would have to sense that the motor feed
has regained the correct voltage and disengage the booster.

Just for clarification, are you worried about voltage dropout of your
primary source, or overcoming a stall?

Regards,
Bob Monsen
 
T

Tim Jackson

Jan 1, 1970
0
Randy Gross said:
Greetings,

I am searching for a circuit that either senses the rectified feed to a
12vdc motor, or its speed, and, if the voltage, or speed, falls below a
preset, then it either trips a breaker else energizes a relay that will
supply an alternative (a battery to boost rpms).

Here's the catch. That same circuit would have to sense that the motor feed
has regained the correct voltage and disengage the booster.

A good analogy here would be a motor driving an alternator. When the motors
speed drops below that necessary to maintain the desired output, because
the primary (rectified 12vdc) feed has been interrupted, the feed is
disengaged and an alternative feed (battery) is engaged until the primary
feed has regained, at which time the secondary feed is disengaged and the
primary resumes.

This is an experiment and the process is trail and error. I know very
little about digital circuitry and this system needs that kind of
precision.
The really simple solution is simply to use a changeover relay with its coil
connected across the primary supply. Apart from not being able to adjust
the trip voltage, this also suffers from a tendency to chatter when the
voltage is close to threshold. To overcome this you need to add hysteresis,
which is most easily done by putting a resistor in series with the coil and
using an extra normally-open relay to bypass the resistor when the relay
activates.

To add an adjustable trip voltage you need some sort of amplifier, and that
needs to be powered. If we are talking about a float-charged battery as the
secondary supply then you use the battery voltage as its power supply, but
if it is primary cells, then you don't really want the amplifier draining
the battery when it is not in use, so you'll have to be a bit cleverer, the
simplest solution being to use a pair of diodes to feed it from the higher
of the supply voltages. You will also probably need a smoothing capacitor
in this case.

As for what amplifier to use, there is a wide variety, most comparators (or
even op-amps) would do the job provided the output is man enough to drive
the chosen relay. One cute option would be to use an NE555 "timer" chip to
independently set turn on and turn off voltages. Drive the relay from pin 3
and connect potential dividers (pot and resistors) from the primary supply
to pins 2 and 6 to set the trip and reset voltages. The output goes low
when pin 6 exceeds the pin 5 voltage (2/3 of supply if 5 unconnected) and
high when pin 2 goes below half of that, and remains latched when neither
condition is met.


Tim Jackson
 
K

Kevin McMurtrie

Jan 1, 1970
0
Randy Gross said:
Greetings,

I am searching for a circuit that either senses the rectified feed to a
12vdc motor, or its speed, and, if the voltage, or speed, falls below a
preset, then it either trips a breaker else energizes a relay that will
supply an alternative (a battery to boost rpms).

Here's the catch. That same circuit would have to sense that the motor feed
has regained the correct voltage and disengage the booster.

A good analogy here would be a motor driving an alternator. When the motors
speed drops below that necessary to maintain the desired output, because
the primary (rectified 12vdc) feed has been interrupted, the feed is
disengaged and an alternative feed (battery) is engaged until the primary
feed has regained, at which time the secondary feed is disengaged and the
primary resumes.

This is an experiment and the process is trail and error. I know very
little about digital circuitry and this system needs that kind of
precision.

an Inquiring mind,
Randy Gross

I think what you want is a voltage regulator. Put a regulator on the
secondary and set its output near the minimum allowable voltage. You'll
only draw power from the secondary if the primary is overpowered or
failed.
 
J

John Crighton

Jan 1, 1970
0
Greetings,

I am searching for a circuit that either senses the rectified feed to a
12vdc motor, or its speed, and, if the voltage, or speed, falls below a
preset, then it either trips a breaker else energizes a relay that will
supply an alternative (a battery to boost rpms).

OK Randy,
you have got me interested. What is the motor for?
Regards,
John Crighton
Sydney
 
R

Randy Gross

Jan 1, 1970
0
<_6Vxb.249925$275.921964@attbi_s53>...

<Snip>

Voltage dropout and maintaining frequency, 900rpm is the preset.

: Just for clarification, are you worried about voltage dropout of your
: primary source, or overcoming a stall?
:
: Regards,
: Bob Monsen
:
:
:
 
R

Randy Gross

Jan 1, 1970
0
<[email protected]>...
: On Sat, 29 Nov 2003 02:18:50 +0000 (UTC), "Randy Gross"
:
: >Greetings,
: >
: >I am searching for a circuit that either senses the rectified feed to a
: >12vdc motor, or its speed, and, if the voltage, or speed, falls below a
: >preset, then it either trips a breaker else energizes a relay that will
: >supply an alternative (a battery to boost rpms).
: >
:
: OK Randy,
: you have got me interested. What is the motor for?
: Regards,
: John Crighton
: Sydney
:
This motor (12vdc, 10amp, C.D.) will drive a generator. I chose DC because
of simplicity and options. I'm concerned about maintaining the frequency at
a constant 60HZ, which in this case is 900rpm. The primary source will be
rectified AC, the secondary will be a 12vdc battery backup. I need a
controller monitoring the primary and ready to engage the secondary should
the primary drop, fail, etc.

Randy
 
T

Tim Jackson

Jan 1, 1970
0
Randy Gross said:
This motor (12vdc, 10amp, C.D.) will drive a generator. I chose DC because
of simplicity and options. I'm concerned about maintaining the frequency at
a constant 60HZ, which in this case is 900rpm. The primary source will be
rectified AC, the secondary will be a 12vdc battery backup. I need a
controller monitoring the primary and ready to engage the secondary should
the primary drop, fail, etc.

Randy
Why not just connect the battery through a diode then? Or even run the
motor off the battery and use the rectifier as a float charger, the same way
a car battery is used.


Tim Jackson
 
R

Randy Gross

Jan 1, 1970
0
<[email protected]>...
:
: : > This motor (12vdc, 10amp, C.D.) will drive a generator. I chose DC
because
: > of simplicity and options. I'm concerned about maintaining the
frequency
: at
: > a constant 60HZ, which in this case is 900rpm. The primary source will
be
: > rectified AC, the secondary will be a 12vdc battery backup. I need a
: > controller monitoring the primary and ready to engage the secondary
should
: > the primary drop, fail, etc.
: >
: > Randy
: >
: Why not just connect the battery through a diode then? Or even run the
: motor off the battery and use the rectifier as a float charger, the same
way
: a car battery is used.
:
:
: Tim Jackson

We're on the same page Tim, a float charger will be used to recharge the
battery but, how will the battery be recharged while the motor is making
demands from it?

Randy
 
T

Tim Jackson

Jan 1, 1970
0
Randy Gross said:
Tim Jackson <[email protected]> wrote in article
: Why not just connect the battery through a diode then? Or even run the
: motor off the battery and use the rectifier as a float charger, the same
way
: a car battery is used.
:
:
: Tim Jackson

We're on the same page Tim, a float charger will be used to recharge the
battery but, how will the battery be recharged while the motor is making
demands from it?
Why not? The rectifier supplies the motor and any charging current
required. If the supply fails then the battery supplies the load. This is
what a car does - the lights stay on when the alternator stops. The charger
has to be able to supply the motor and a bit more for battery charging.

What it means is that when the supply is on you get about 14V, when it's off
you get about 11 or 12V, so your speed/frequency control system has to cope
with a range of supplies from say 10 to 15 volts. The battery takes charge
from what is essentially a constant voltage supply, so you have to use a
battery of the lead-acid class, not the Ni-Cd class which can't cope with
constant voltage charging.

Tim
 
R

Randy Gross

Jan 1, 1970
0
<[email protected]>...
:
<Snip>

how will the battery be recharged while the motor is making
: > demands from it?
: >
: Why not? The rectifier supplies the motor and any charging current
: required. If the supply fails then the battery supplies the load. This
is
: what a car does - the lights stay on when the alternator stops. The
charger
: has to be able to supply the motor and a bit more for battery charging.
:
: What it means is that when the supply is on you get about 14V, when it's
off
: you get about 11 or 12V, so your speed/frequency control system has to
cope
: with a range of supplies from say 10 to 15 volts. The battery takes
charge
: from what is essentially a constant voltage supply, so you have to use a
: battery of the lead-acid class, not the Ni-Cd class which can't cope with
: constant voltage charging.
:
: Tim
:
Can it be that simple Tim? For years I have removed batteries from cars,
with the engine running, to either check the alternator or to jump another
vehicle but, I never once thought to check for a voltage across the clamps.
I always assumed that the battery was isolated from the circuit but then,
how would it recharge? Talk about DUH! There's no shortage on stupidity, is
there?

O.K., I can now see that the only addition this circuit needs is a
regulator and maybe speed control circuitry, depending on how close the
gear reduction gets the rpms' from 2800 to 900rpm and, whether or not I can
find the 3.3 to 1 ratio.

After I finish this post, I'm going to kick my a__ around this room until I
either get tired, or develop a cramp;-)

Randy Gross
 
R

Randy Gross

Jan 1, 1970
0
<[email protected]>...
: :
: > : Why not just connect the battery through a diode then? Or even run
the
: > : motor off the battery and use the rectifier as a float charger, the
same
: > way
: > : a car battery is used.
: > :
: > :
: > : Tim Jackson
: >
: > We're on the same page Tim, a float charger will be used to recharge
the
: > battery but, how will the battery be recharged while the motor is
making
: > demands from it?
: >
: Why not? The rectifier supplies the motor and any charging current
: required. If the supply fails then the battery supplies the load. This
is
: what a car does - the lights stay on when the alternator stops. The
charger
: has to be able to supply the motor and a bit more for battery charging.
:
: What it means is that when the supply is on you get about 14V, when it's
off
: you get about 11 or 12V, so your speed/frequency control system has to
cope
: with a range of supplies from say 10 to 15 volts. The battery takes
charge
: from what is essentially a constant voltage supply, so you have to use a
: battery of the lead-acid class, not the Ni-Cd class which can't cope with
: constant voltage charging.
:
: Tim
:
P.S.

My original design for the supply was to have it deliver the voltage and
current necessary to power the motor: 12vdc @ 10amps with a 5amp cushion
for initial surge. I find no problem with increasing the voltage to 14vdc.
My concern is with the amperage.

Randy
 
R

Robert Monsen

Jan 1, 1970
0
Randy Gross said:
<_6Vxb.249925$275.921964@attbi_s53>...

<Snip>

Voltage dropout and maintaining frequency, 900rpm is the preset.

: Just for clarification, are you worried about voltage dropout of your
: primary source, or overcoming a stall?
:
: Regards,
: Bob Monsen
:
:
:

Does the motor have an encoder? How will you sense the RPM?

Regards,
Bob Monsen
 
T

Tim Jackson

Jan 1, 1970
0
Randy Gross said:
<[email protected]>...
: :
: The rectifier supplies the motor and any charging current
: required. If the supply fails then the battery supplies the load. This
is
: what a car does - the lights stay on when the alternator stops. The
charger
: has to be able to supply the motor and a bit more for battery charging.
:
: What it means is that when the supply is on you get about 14V, when it's
off
: you get about 11 or 12V, so your speed/frequency control system has to
cope
: with a range of supplies from say 10 to 15 volts.

My original design for the supply was to have it deliver the voltage and
current necessary to power the motor: 12vdc @ 10amps with a 5amp cushion
for initial surge. I find no problem with increasing the voltage to 14vdc.
My concern is with the amperage.
A rectified nominal 12V AC peaks at about 17V, so there is no problem with
producing enough voltage. As far as current is concerned the system is
usually self-regulating if the component sizes are no too disparate. If the
battery is full then it happily goes up to 14 or 15V and the charging
current comes down to a trivial amount. Yes is still charges during the
peaks of the AC cycle, but promptly discharges again during the rest of the
cycle so does not overcharge. When the battery is flat, the current gets
limited by the transformer+rectifier resistance, which means the transformer
gets hot for a while but after a short time the battery voltage comes up,
the current falls and things cool off. This way a 4A battery charger can
happily charge a car battery without frying itself. and without any current
limiting circuitry. When God designed the lead-acid system He put in all
the characteristics you need to make it really easy to use, unlike more
recent designs.

Your hard work comes in keeping a constant rpm over quite a widely changing
supply voltage. Car designs have to consider this too. Light bulbs have a
relatively voltage-independent characteristic: the resistance increases as
they get hotter. Motors rev proportionately higher as the voltage rises
which is mostly harmless in a car, but the supply has to be regulated for
things like fuel gauges. You are going to need some sort of regulator to
drop the armature voltage to around 10V, and it needs to be pretty
low-dropout like a switch-mode FET regulator. Its only a handful of
components but I don't think you'll get one off the shelf. If you've got a
motor with a field coil you can regulate the field voltage, which is easier.
(That's how the car alternator regulates.)

Tim
 
R

Randy Gross

Jan 1, 1970
0
Thanks Tim for your time and your time saving advice. I chose a 12v system
for one reason, and it turned out to my benefit in another.

Randy Gross


<[email protected]>...
:
: : > <[email protected]>...
: > : : > :
: > : The rectifier supplies the motor and any charging current
: > : required. If the supply fails then the battery supplies the load.
This
: > is
: > : what a car does - the lights stay on when the alternator stops. The
: > charger
: > : has to be able to supply the motor and a bit more for battery
charging.
: > :
: > : What it means is that when the supply is on you get about 14V, when
it's
: > off
: > : you get about 11 or 12V, so your speed/frequency control system has
to
: > cope
: > : with a range of supplies from say 10 to 15 volts.
: >
: > My original design for the supply was to have it deliver the voltage
and
: > current necessary to power the motor: 12vdc @ 10amps with a 5amp
cushion
: > for initial surge. I find no problem with increasing the voltage to
14vdc.
: > My concern is with the amperage.
: >
: A rectified nominal 12V AC peaks at about 17V, so there is no problem
with
: producing enough voltage. As far as current is concerned the system is
: usually self-regulating if the component sizes are no too disparate. If
the
: battery is full then it happily goes up to 14 or 15V and the charging
: current comes down to a trivial amount. Yes is still charges during the
: peaks of the AC cycle, but promptly discharges again during the rest of
the
: cycle so does not overcharge. When the battery is flat, the current gets
: limited by the transformer+rectifier resistance, which means the
transformer
: gets hot for a while but after a short time the battery voltage comes
up,
: the current falls and things cool off. This way a 4A battery charger can
: happily charge a car battery without frying itself. and without any
current
: limiting circuitry. When God designed the lead-acid system He put in all
: the characteristics you need to make it really easy to use, unlike more
: recent designs.
:
: Your hard work comes in keeping a constant rpm over quite a widely
changing
: supply voltage. Car designs have to consider this too. Light bulbs have
a
: relatively voltage-independent characteristic: the resistance increases
as
: they get hotter. Motors rev proportionately higher as the voltage rises
: which is mostly harmless in a car, but the supply has to be regulated for
: things like fuel gauges. You are going to need some sort of regulator to
: drop the armature voltage to around 10V, and it needs to be pretty
: low-dropout like a switch-mode FET regulator. Its only a handful of
: components but I don't think you'll get one off the shelf. If you've got
a
: motor with a field coil you can regulate the field voltage, which is
easier.
: (That's how the car alternator regulates.)
:
: Tim
:
:
:
 
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