3 phase signal input

[jxsmith]

Hi guys,

I'm hoping there's a simple answer to my issue. I've designed a plc driven device but I want it to only run when a three phase motor in another part of the machine is also running. I've added the appropriate interlocks in my code but my question is what is the easiest way to wire the input since the motor is three phase and the plc accepts 24vdc inputs? Any help would be appreciated.

 

[sthompson]

Hi guys,

I'm hoping there's a simple answer to my issue. I've designed a plc driven device but I want it to only run when a three phase motor in another part of the machine is also running. I've added the appropriate interlocks in my code but my question is what is the easiest way to wire the input since the motor is three phase and the plc accepts 24vdc inputs? Any help would be appreciated.

Welcome to the Forum!

If you are using a Starter, you could use an Auxiliary Contact to switch the 24VDC Input on. I am not a big fan of using 24VDC with 3 Phase Starters, but I have seen it done.

If you are using a VFD, they usually have Programmable Relay Outputs that you could use to switch the 24VDC Input on. Same Caveat as above, but again it is done fairly regularly.

HTH

Stu....

 

[Adam303]

Use a dry auxiliary contact on the contactor that turns on the motor.

 

[controlled]

Welcome to the Forum!

If you are using a Starter, you could use an Auxiliary Contact to switch the 24VDC Input on. I am not a big fan of using 24VDC with 3 Phase Starters, but I have seen it done.

If you are using a VFD, they usually have Programmable Relay Outputs that you could use to switch the 24VDC Input on. Same Caveat as above, but again it is done fairly regularly.

HTH

Stu....

Im curious as to why your not a fan of using 24Vdc with 3 phase sarters

 

[jxsmith]

Thanks guys. Just a little background, my project is basically being tacked on to an existing machine, otherwise I would just use the plc to drive the motor contactor. I'm also a bit limited for space in the cabinet.

 

[sthompson]

Im curious as to why your not a fan of using 24Vdc with 3 phase sarters

I have seen induced voltage cause problems in lower voltage DC Circuits. Admittedly, it is rare, but why take the chance of future problems.

As a general rule, I try and stick with 120VAC Inputs for Starter and Local Disconnect Aux Contacts.

JMHO FWIW

Stu....

 

[Ken Roach]

The first question to answer is if the existing three-phase motor uses a VFD, a soft-start, or an across-the-line starter.

A status contact on a VFD or soft-start will take up zero additional space, and an added auxiliary contact on a motor starter contactor will take up a fraction of an inch.

If you can't "piggyback" on the existing control circuits for this AC motor, you'll need to install something to monitor the current flowing through the AC motor's source leads, and that means a current transformer and a transducer or sensing relay. Both of those are far more expensive and have a larger footprint than adding an auxiliary to the existing control circuit.

 

[jdbrandt]

This question is a classic "define the problem" or "if you want better answers, ask better questions" type of question.
It all depends, I think we agree, what the question poser means by the phrase "...is (also) running..".
Running, versus, "the contactor is energized (aux contact solution)", or "the motor has voltage applied and is drawing current".
Perhaps the best way to know that the actual motor being monitored is "running" (also called "moving") would be a zero speed switch, or, in the real extreme, an encoder.
It depends on how important the answer is, and moreover, how disasterous it would be if the wrong answer to this question was generated. Would somebody get hurt? Would a machine or product get damaged?

 

[Ned_Flanders]

This question is a classic "define the problem" or "if you want better answers, ask better questions" type of question.
It all depends, I think we agree, what the question poser means by the phrase "...is (also) running..".
Running, versus, "the contactor is energized (aux contact solution)", or "the motor has voltage applied and is drawing current".
Perhaps the best way to know that the actual motor being monitored is "running" (also called "moving") would be a zero speed switch, or, in the real extreme, an encoder.
It depends on how important the answer is, and moreover, how disasterous it would be if the wrong answer to this question was generated. Would somebody get hurt? Would a machine or product get damaged?

Are you in the pub?

 

[jdbrandt]

Are you in the pub?

HaHa

 

[I is a noob]

I have seen induced voltage cause problems in lower voltage DC Circuits. Admittedly, it is rare, but why take the chance of future problems.

As a general rule, I try and stick with 120VAC Inputs for Starter and Local Disconnect Aux Contacts.

JMHO FWIW

Stu....

Couldn't you just run a shielded cable if using a 24VDC input was your only option?

 

[kittydog42]

You can use a standard current sensing relay, the kind that goes around one of the phase wires. In theory, you need a special VFD current sensing relay but in practice, you will probably find that the standard (lower cost) variety works fine. If you get one that can be adjusted then that is even better because you can set the on point. Functional Devices is a company to check for this.