Dual load automatc switching

Thread Starter

mtr79821

Joined Aug 25, 2022
19
I have 2 240v 20A motors powered by 1 power source. I need help wiring relays and/or transformer that will only allow 1 of these to work at any given time.
EXAMPLE
If motor 1 is running I need motor 2 to be isolated.

And the reverse.
It matters not if one has priority or they are equal.
 

LowQCab

Joined Nov 6, 2012
2,510
Are You doing this to prevent Tripping a Circuit-Breaker or blowing a Fuse ?
Why do You want to do this ?
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Thread Starter

mtr79821

Joined Aug 25, 2022
19
Are You doing this to prevent Tripping a Circuit-Breaker or blowing a Fuse ?
Why do You want to do this ?
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Neither, it is is being done to prevent both from running at the same time due to emptying a building of conditioned air all at once. Smart controls are NOT in the budget right now.
I guess ideally thy would alternate in 15 minute intervals.
 

djsfantasi

Joined Apr 11, 2010
8,579
use two relays/contractors whose coil is also rated for 240VAC and can switch 20A.

Wire each motor through the NC contacts of a relay. When switching on Motor A, also activate Relay B, thus isolating Motor B. And vice versa.
 

LowQCab

Joined Nov 6, 2012
2,510
Why do You want to do this ?
If You keep us in the dark, you're not likely to get any useful answers.

It sounds like ??? 2 Exhaust Fans maybe ???

What turns on the Exhaust Fans ?
Why are there 2 Exhaust Fans and not one ?
Does this have anything to do with an Exhaust-Hood in a Commercial-Kitchen ?
Is there any situation where You might want to run both at the same time ?
Are they on thermostats ?
Are they on Timers ?
Are they normally manually controlled with Switches ?
Is there a "minimum-run-time" or "off time" required ?
Why do they get turned-off in the first place ?
Do the Motors use 3-phase-Power, or Single-Phase-Power ?

All of these questions must be explained in detail to formulate a solution for You.
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Thread Starter

mtr79821

Joined Aug 25, 2022
19
Why do You want to do this ?
If You keep us in the dark, you're not likely to get any useful answers.

It sounds like ??? 2 Exhaust Fans maybe ???

What turns on the Exhaust Fans ?
Why are there 2 Exhaust Fans and not one ?
Does this have anything to do with an Exhaust-Hood in a Commercial-Kitchen ?
Is there any situation where You might want to run both at the same time ?
Are they on thermostats ?
Are they on Timers ?
Are they normally manually controlled with Switches ?
Is there a "minimum-run-time" or "off time" required ?
Why do they get turned-off in the first place ?
Do the Motors use 3-phase-Power, or Single-Phase-Power ?

All of these questions must be explained in detail to formulate a solution for You.
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.
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I'm sorry I was trying to be brief. I was trying not to write a book.
Single phase
Yes 2 old huge exhaust fans
Located at opposite ends of building
Presently I use timers but they aren't accurate, over a week the timers lag enough to overlap So we manually switch them on and off.
 

MrSalts

Joined Apr 2, 2020
2,378
Are there three possible states (both off | Fan1 on & fan2 off | Fan1 off & Fan2 on). Or is one fan always running (only two states)?
 

LowQCab

Joined Nov 6, 2012
2,510
You "NEED" to "write a book".
Details make all the difference.

Ok, so you're in a warehouse that has 2 Exhaust-Fans .........

Why do you want to alternate their operation ?

What determines which Fan should be running ?

Why can't both run at the same time ?

You mentioned Air-Conditioning .........
Is the whole space Air-Conditioned ?

Are the Fans for Fume-Extraction, and maybe they're just too big for that purpose ?

Are the Fans for removing heat that collects near the ceiling ?
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Reloadron

Joined Jan 15, 2015
6,985
OK two exhaust fans. Only one can run at a time because running both would "due to emptying a building of conditioned air all at once". How would I go about it? You don't mention single or poly phase but matters not. I would use two motor starter contactors rated for the HP of your motors or as you mention 20 Amps but allowing overhead for current. I would use motor starters with auxiliary Com, N/O and N/C contacts. You wire the aux contacts so neither motor starter coil can get coil power when the other is energized. I would also use motor starters which include thermal overload so if any motor goes over current (locked rotor) it shuts down. That is how I would likely go about it in an industrial application.

Ron
 

Thread Starter

mtr79821

Joined Aug 25, 2022
19
You "NEED" to "write a book".
Details make all the difference.

Ok, so you're in a warehouse that has 2 Exhaust-Fans .........

Why do you want to alternate their operation ?

What determines which Fan should be running ?

Why can't both run at the same time ?

You mentioned Air-Conditioning .........
Is the whole space Air-Conditioned ?

Are the Fans for Fume-Extraction, and maybe they're just too big for that purpose ?

Are the Fans for removing heat that collects near the ceiling ?
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.
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I simply need to keep them both from running at the same time. Conditioned air does not mean air conditioning. It is low humidity in the summer and I need fans to alternate or it will fill the entire space with muggy air.

So say 15 min running for fan A and 15 minutes running for fan b. And continue on like that.
 

Reloadron

Joined Jan 15, 2015
6,985
So say 15 min running for fan A and 15 minutes running for fan b. And continue on like that.
Alrighty so to my suggestion above Post #13 I would just add a Repeat timer function. Both fand are on a single switched line On / Off which is controlled by what is called a Repeat Cycle Timer. The link is merely an example, I am not suggesting you run out and buy a $150 USD timer, there are plenty of inexpensive timers which do a repeat cycle. You preset the time periods so fan one runs for your preset time followed by fan two runs for your preset time. You are merely alternating the coil power to the motor starters I mentioned earlier. You can still include wiring the aux contacts to ensure both fans will never run at the same time but it won't hurt to eliminate that feature. I used this method once for large pumps where only one or the other would run unless a level in a tank exceeded a set level and both pumps would run. Anyway I would just use a repeat cycle timer and set the intervals to your choosing.

Ron
 

Reloadron

Joined Jan 15, 2015
6,985
The perfect Electrical solution ...............
and the problem still isn't solved.
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What is the problem which I apparently missed?
I have 2 240v 20A motors powered by 1 power source. I need help wiring relays and/or transformer that will only allow 1 of these to work at any given time.
EXAMPLE
If motor 1 is running I need motor 2 to be isolated.

And the reverse.
It matters not if one has priority or they are equal.
I figured they want motor A or motor B but never A and B at the same time? What am I missing here? :)

Ron
 

crutschow

Joined Mar 14, 2008
30,821
You can also use two SPDT relays (as below):
One timer controls whether either fan is ON, and the other timer controls which fan will be on.

1665529432675.png
 
Last edited:

LowQCab

Joined Nov 6, 2012
2,510
What is the problem which I apparently missed?

I figured they want motor A or motor B but never A and B at the same time? What am I missing here? :)

Ron
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Electrically, nothing.
This is exactly what the TS asked for.
Take his Money.
But any actual improvement in Air-Comfort is highly questionable.
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