Switching high power motors with PLC

Hi,

Is it possible to switch on a 30 Kw motor using a plc output with only 2A contact capacity? How can I make this possible?

Use the PLC output to turn on a larger relay and then use that relay to switch on a contactor big enough to switch the load.

Thanks for helping, i have thought of that as well. Looks like my brain's not working well..Thanks a bunch

theripley said:

Thanks for helping, i have thought of that as well. Looks like my brain's not working well..Thanks a bunch

Click to expand...

When you are working with a load of any description you need to understand what you are doing.

I was considering giving you a good ribbing for your question, but, in fareness other people will read this.

Firstly - PLC contacts are normally rated for a resistive load (2A)
AC Loads are not purely resistive so can not normally be directly driven by PLC contacts.

Secondly - A Load listed in wattage can easily be calculated to Current using I = W / V

And Last But Not Least - Frequency of operation
Number of times that a contact operates in a given time.
With a high current load - this will damage contacts very quickly.

As a result - most people are in favor of using additional relays - cheaper to replace and can be correctly sized.

I believe (sorry to you OP) that if you have to ask you are doing the wrong Job and need a lot more training.

Erika,

For a 30 kw motor, you may be able to energize a motor starter with your PLC output (without an additional relay). Check your motor starter contactor current inrush rating to see if it is more than 2 Amps.

If you use a Variable Frequency Drive (VFD) to run the 30 kw motor, then almost certainly you can control it directly with your PLC.

If you don't need to change the speed of the motor, you can use a Soft Starter (better than starters) which you can turn on/off directly from the PLC output.

We were swithcing a 30kw motor starter through a 2A contact on a VFD (it was a permissive).
The 2A vfd contacts broke open after about 6 months of operation (required replacing a $10k VFD board)
If I am ever up against a 2A contact to start anything, it's getting a 10A relay to do the actual switching.

-John

theripley said:

Hi,

Is it possible to switch on a 30 Kw motor using a plc output with only 2A contact capacity? How can I make this possible?

Click to expand...

Maybe if you used enough outputs in parallel???
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Or:
Maybe you need to hire a qualified electrician to help with this project.

theripley said:

Hi,

Is it possible to switch on a 30 Kw motor using a plc output with only 2A contact capacity? How can I make this possible?

Click to expand...

Ripley
The short answer is NO you cannot start a 30 kw motor off a single contact on a PLC. First it is 3 phase (ASSMUME) so you would need 3 contacts. Second on 480 line it would draw 30 amp and on 240 about 60. A 2A contact may last about one half second.

SOLUTION
You will need a motor controller whose input matches the output of PLC.

OPTIONS
1. Relay or a packaged motor starter (the starter would be best - you have all the needed components)
2. Soft starter also a good solution - will keep the starting current low
3. VFD - possibly the best option. You have stop start reversing braking overload protection all in one box. All are controllable with control signals (ie PLC outputs) from PLC.

Dan Bentler

If I use a vfd to control these motors, don't I need contactors on the line & load side of the inverter?

YOu generally do not need a relay for line side of VFD for the "average" installation. The VFD substitutes for the relay and overloads. You do need circuit disconnect means and over current protection - however - if replacing relay control, these can be downsized per VFD maker instructions.

ALMOST NEVER put a relay between a VFD and the driven motor. The one exception can be where driving multiple motors from one VFD - this takes a bit of care when. designing and setup.

If you did put in a relay you ahve gained nothing - motor will still coast to stop - just like it would if you turn VFD OFF. You will lose all ability to dynamic brake if you open the relay. In general there is no advantage to putting in relay on VFD output and several disadvantages.

Uhh are you really intending to drive multiple 30 kW motors with one VFD?? If so give this some very careful thought. I do not recommend this - you lose over VERY expensive VFD and you have lost all motors. VFD must be sized for number of motors X Kw and you should also watch for total current - money will add fast. In addition now you must put in overcurrent protection for each motor (more money) and maybe relay for that motor (more money)

Tell us waht this machine does and how you want motors to run and we can help more.

Dan Bentler

If I use a vfd to control these motors, don't I need contactors on the line & load side of the inverter?

Click to expand...

No.

Line side of VFD: fuses or circuit breaker sized for rated current of motor nameplate (which should match VFD rated full current).

Load side of VFd: just some wire to connect motor to VFD. It may need to be special VFD-rated cable, if long run or not in metal conduit.

depends on your countries rules.
Here - we have to have them fitted to the line side.
The Idea of fitting to the load side, can be destructive to the vsd.
BUT
I have used three contactors on the load side and a plc to drive three different motors via the one VSD. (checked for Zero Speed Prior to change over)
That worked well.