Alternating operation of 2 motors daily

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I would not feed it into a contactor that supplied power to the VFD. Just use the alternator signal to enable the specific drive. Incidentally, we do not use overload relays with VFDs as the VFD has a built in overload and overload relays are only listed for use with motor loads and not VFD loads.
 
Tainted said:
It depends on the pressure of the pipe. The pressure transducer will give the pump the signal to run.
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If you are just planning to turn the pump on when the pressure drops to a certain level, it seems likely that turning the pump on will cause the pressure to go up thus likely turning the pump back off. This seems like another case where we have to drag enough information out of a poster to be able to give good suggestions.
 
Tainted said:
is something like this acceptable? I just drew it.
View attachment 2566432

M1 and M2 on the ladder logic is the coils for motor M1 and M2
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1. If you have two VFDs, you do not need the contactors ahead of them. Powering up VFDs from contactors every day will shorten their life significantly.

2. You will not need the OL relays (so long as you are using UL listed VFDs), they will protect the motor.

3. That is not how a single 4-20nA circuit to two separate drives would work, you would need to do it in a loop and adjust the gain to account for the burden of the drives. That is also complicated if you have to remove one drive from the circuit for repairs. That’s one reason I would add a micro PLC.

4. Other than the time clock, what is telling the VFDs to start or stop? Or is the plan to run them non-stop 24/7/365? Again, a Transducer does not do the CONTROL, it just provides a variable signal. There needs to be at the very least a PI control loop involved, probably a PID controller.
 
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.
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Two separate VFDs allow for continued operation of a VFD dies - gives some redundancy. May or may not be important.


SceneryDriver
 
Jraef said:
1. If you have two VFDs, you do not need the contactors ahead of them. Powering up VFDs from contactors every day will shorten their life significantly.

2. You will not need the OL relays (so long as you are using UL listed VFDs), they will protect the motor.

3. That is not how a single 4-20nA circuit to two separate drives would work, you would need to do it in a loop and adjust the gain to account for the burden of the drives. That is also complicated if you have to remove one drive from the circuit for repairs. That’s one reason I would add a micro PLC.

4. Other than the time clock, what is telling the VFDs to start or stop? Or is the plan to run them non-stop 24/7/365? Again, a Transducer does not do the CONTROL, it just provides a variable signal. There needs to be at the very least a PI control loop involved, probably a PID controller.
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Forgive me I was in a rush when creating the diagrams... I thought a 4-20mA signal can also start and stop the motors as well as vary them in speed? I don't see why I can't just hook up the pressure transducer to the VFD?
 
Tainted said:
Forgive me I was in a rush when creating the diagrams... I thought a 4-20mA signal can also start and stop the motors as well as vary them in speed? I don't see why I can't just hook up the pressure transducer to the VFD?
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The speed command and the run command are separate things. You can have a run command and a speed command for zero hertz and the motor will not spin.
 
Tainted said:
Forgive me I was in a rush when creating the diagrams... I thought a 4-20mA signal can also start and stop the motors as well as vary them in speed? I don't see why I can't just hook up the pressure transducer to the VFD?
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Again, SOME VFDs can be set up to be used this way BECAUSE they have a build-in PID controller and logic capabilities. That is not universally true of all drives however. In addition, when you have two drives and want them to alternate, that logic must be duplicated in each drive and something else will have to tell the drives which one is in control.

If you don't understand the concept of a PID controller, think of your "cruise control" in your car. You set the speed you want to maintain, the cruise control takes over control of your throttle to give the engine more or less fuel to maintain the speed at that setpoint. Without that, you (the driver) are controlling the throttle to maintain the speed, but in your head, YOU are doing a lot of internal calculations whether you are aware of it or not. In either case, the Cruise Control or your brain are the PID controller. But in your pressure situation, without some form of "intelligence", with your pressure transducer just going to the VFD to give it a speed commend, as the pressure drops, the drive speeds up, so the pressure rises, the transducer signal goes up so the VFD slows and the pressure drops, then this process just continues going up and down, up and down, never settling. This is called "hunting". The PID control loop is the intelligence that keeps the system stable.

If all of this is above your head, you may need to step back and get someone else involved in designing your system.
 
Jraef said:
1. If you have two VFDs, you do not need the contactors ahead of them. Powering up VFDs from contactors every day will shorten their life significantly.

2. You will not need the OL relays (so long as you are using UL listed VFDs), they will protect the motor.

3. That is not how a single 4-20nA circuit to two separate drives would work, you would need to do it in a loop and adjust the gain to account for the burden of the drives. That is also complicated if you have to remove one drive from the circuit for repairs. That’s one reason I would add a micro PLC.

4. Other than the time clock, what is telling the VFDs to start or stop? Or is the plan to run them non-stop 24/7/365? Again, a Transducer does not do the CONTROL, it just provides a variable signal. There needs to be at the very least a PI control loop involved, probably a PID controller.
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Don’t they make a 4-20 ma device that takes one 4-20 ma signal, and converts it to two with the same output? That may be simpler than adjusting the gain? Whichever drive is active would still “see” the transducer. Might be a lag between the two at the time of switchover, one would ramp down, while the other is ramping up? Wether the pressure would drop too much affecting the process whatever the process is?
 
hillbilly1 said:
Don’t they make a 4-20 ma device that takes one 4-20 ma signal, and converts it to two with the same output? That may be simpler than adjusting the gain? Whichever drive is active would still “see” the transducer. Might be a lag between the two at the time of switchover, one would ramp down, while the other is ramping up? Wether the pressure would drop too much affecting the process whatever the process is?
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you could wire the 4-20 mA signal to a resistor, say a 250 Ohm resistor and then wire the resistor in parallel to a voltage analog input on both drives. This would give a 1-5 VDC signal to both drives.

or maybe just wire the 4-20 mA signal in series with both drive current analog inputs if this is supported.
 
hillbilly1 said:
Might be a lag between the two at the time of switchover, one would ramp down, while the other is ramping up? Wether the pressure would drop too much affecting the process whatever the process is?
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Yeah not knowing what this thing is makes it just about impossible. Lots of questions need to be answered. How much pressure fluctuation is acceptable? Is there a reservoir or accumulator in the system. Are the 2 pumps for redundancy or to handle peak flow requirements, or for both? ect ect ect
 
hillbilly1 said:
Don’t they make a 4-20 ma device that takes one 4-20 ma signal, and converts it to two with the same output? That may be simpler than adjusting the gain? Whichever drive is active would still “see” the transducer. Might be a lag between the two at the time of switchover, one would ramp down, while the other is ramping up? Wether the pressure would drop too much affecting the process whatever the process is?
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Yes, they do make 4-20mA signal splitters...
But it would be less expensive to get a micro PLC and solve the other problems.
 
Jraef said:
Yes, they do make 4-20mA signal splitters...
But it would be less expensive to get a micro PLC and solve the other problems.
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Would you not have to buy and program the software to run the plc? If you already have it, it would not be a big deal, but I would say the OP doesn’t have it. Just trying to keep it simple.
 
Tainted said:
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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Assemble a simple flip-flop latch to alternate running the motors. Better still, buy one. We have bought a 2,500 MVA diesel genset with two starter motors; the starter motors alternated starting the genset every time.
 
You could just use 2 pressure transducers. One for each drive.
Then the clock to switch the run command to each drive.
 
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