Alternating operation of 2 motors daily

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Tainted

Tainted

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New York
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Engineer (PE)
There are two 208V 3-phase 3HP motors that I would like to alternate power daily. Let's call them motor 'A' and motor 'B'

For example,

motor 'A' will run on day 1, 3, 5, 7 and so on...
motor 'B' will run on day 2, 4, 6, 8 and so on...

There are multiple methods of doing this but I want an efficient and safe method. These motors will be fed by the same circuit and will be non-coincident load. For this reason I want to ensure the motors do not run at the same time.

I was thinking of using two VFDs (1 for each motor) and interlocking it with an external digital time clock so that it cycles the motors. I am not sure if this is a good method because I am afraid that someone will tamper with the VFD and time clock and cause both of the motors to be running at the same time which is what I'm trying to avoid.

Is using VFDs with external digital time clock a good idea for what I'm trying to do? Let me know if you have any recommendations.
 
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A cheap and easy way would be to use an alternating relay. Used a lot for alternating sewage lift station pumps. Use a 24 hour timer to trigger the alternating.
 
Whatever scheme you come up with can be tampered with. There is no way you can prevent it.

In any case what happens if one of the motors fails? You probably want to have some way to switch it to the other one. Otherwise someone will have to tamper with it in the dead of the night to get it running.
 
If you do t need to vary the speed, don’t put in a VFD.

What tells the motor(s) to turn on and off now? If it’s a manual process, just put in a double throw safety switch. If it is automated, use mechanically interlocked contactors.
 
Jraef said:
If you do t need to vary the speed, don’t put in a VFD.

What tells the motor(s) to turn on and off now? If it’s a manual process, just put in a double throw safety switch. If it is automated, use mechanically interlocked contactors.
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I want to make them run with a pressure transducer. so basically it would need to be controlled by a pressure transducer and alternate operation. can I interlock them with a VFD, pressure transducer and a time clock?
 
hillbilly1 said:
www.zoro.com

Results for "Alternating relay"

Zoro Tools Industrial Supplies
www.zoro.com www.zoro.com
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Joethemechanic said:
These are made to alternate motors every run cycle. We used them alot for pump stations

www.rammeter.com

Time Mark Corp. Model 441-240V Multi-Stage Motor/Pump Alternator - SPST, 240VAC

Time Mark Corp. Model 441-240V Multi-Stage Motor/Pump Alternator. Trust Ram Meter Inc. for all of you pump and level control needs including alternating relays and isolated switches.
www.rammeter.com www.rammeter.com
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But what if I need to run them on a pressure transducer, and at the same time alternate them daily?
 
Tainted said:
But what if I need to run them on a pressure transducer, and at the same time alternate them daily?
Click to expand...
The transducer is wired ahead of the relay, the relay just selects which pump to run. The input to the relay comes from the time clock. Or even since you are using a vfd, the relay can be wired to the “enable” input on the vfd.
 
Tainted said:
But what if I need to run them on a pressure transducer, and at the same time alternate them daily?
Click to expand...
You can make the pt signal an input to both vfds and just select one of the VFDs to run.

Personally, I would have a four way switch that would allow me to select which VFD was used, select auto for alternating, and off to turn off both. Maybe even some way to turn on both.
 
hillbilly1 said:
The transducer is wired ahead of the relay, the relay just selects which pump to run. The input to the relay comes from the time clock. Or even since you are using a vfd, the relay can be wired to the “enable” input on the vfd.
Click to expand...
So if input 1 is on it will close contact 1 and if input 2 is on it will close contact 2?
 
Tainted said:
But what if I need to run them on a pressure transducer, and at the same time alternate them daily?
Click to expand...
You'll have to read the specs on them. The ones I worked with were quite a bit older. I know the ones I put in ran two pumps. Every time a float switch closed it would start a pump. when the water level went down and the float switch opened it shut off the pump. When the level rose again it would start the other pump. There was also another float switch for when the level got too high it would run both pumps. That float switch also served as a backup system so if one pump failed the other pump would run.

I had air compressors on a similar control years ago that was pre electronic all mechanical.

What you have is not an unusual situation. There have been controls on the market to do this for probably 100 years.

Google "Alternating Relay" or Alternating Control" and you'll find lots of products
 
Don't forget you'll need to program a dead band into the controller somehow. You don't want to switch motors while one of them is running.
 
Tainted said:
I want to make them run with a pressure transducer. so basically it would need to be controlled by a pressure transducer and alternate operation. can I interlock them with a VFD, pressure transducer and a time clock?
Click to expand...
Ok, pressure transducer operations and a VFD adds complexity to this. The transducer would provide a variable signal, good for telling a VFD what speed to run, but the on-off decision then needs some intelligence involved. Some VFDs can likely do this, others cannot, so without info on what VFD you plan on using it’s difficult to tell.

Personally I would put a micro PLC into the mix and have it control a couple of mechanically interlocked contactors on the output of one VFD, with the appropriate safeguards to prevent the contactors from opening or closing under load. Alternatively you could use two VFDs, one for each motor, but depending on the HP we are talking about here, that might cost a lot more.
 
If you haven't figured it out by the responses you have gotten, how this kind of thing can be handled could vary a lot, and probably the end solution will be determined largely by factors you have not bothered to tell us, and might not have even considered yourself, cost and complexity being just two of the factors.
 
Jraef said:
Ok, pressure transducer operations and a VFD adds complexity to this. The transducer would provide a variable signal, good for telling a VFD what speed to run, but the on-off decision then needs some intelligence involved. Some VFDs can likely do this, others cannot, so without info on what VFD you plan on using it’s difficult to tell.

Personally I would put a micro PLC into the mix and have it control a couple of mechanically interlocked contactors on the output of one VFD, with the appropriate safeguards to prevent the contactors from opening or closing under load. Alternatively you could use two VFDs, one for each motor, but depending on the HP we are talking about here, that might cost a lot more.
Click to expand...
is something like this acceptable? I just drew it.
IMG_8882.jpg

M1 and M2 on the ladder logic is the coils for motor M1 and M2
 
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