PLC usage limitation

Greetings!

I would like to inquire if PLC usage is applicable if I am going to run a motor of 4 units which has the following capacity: 45kw, 22kw, 18kw and 5.5 kw. It has solenoid valves as additional outputs. And, sensors as inputs.

If I am going to use relay type PLC (could be any model but preferably Mitsubishi), do I need to consider the maximum rated current of my outputs? How about with the 45kw motor?

What is the maximum PLC current usually available?

This is for cooling tower application.

I am hoping more ideas from this circulation.
Thank you very much.

Sheryl

None of these motors can be run directly off PLC outputs. You will need relay between them - PLC output goes to motor relay coil - motor and line across the relay. Do not forget to put MOV (or?) other device to protect PLC relays from collapsing motor relay fields.

You may want to use relays on the valves also - depends on how large their coils are.

YES you want to look at the individual current and volt requirement for each coil
and
total them to ensure they do not overload the plc power supply or current rating for the contacts on that output.

IF you are intending to use VFD for motor control then you do NOT need a relay between PLC and VFD - depending on VFD - read the drawings and specs.

PUMP CONTROL variable speed - you can save bunches of electricity with a proper setup carefully done. Downside it can be good way to wreck equipment.
- two cautions
1. Do not run pump too slow. Not enough water flow thru pump to cool seals and or not enough fan speed in motor to keep motor cool.
2. Do not run too fast - pumps obey affinity laws power is proportional to cube of speed change. For BOTH read the pump curves.

One other thought it is warm in Philipines BUT the humidity is high also. Evap cooling works best in high temp LOW humidity. Check your psychrometric graphs to ensure you are going to be OK.

Dan Bentler

I like to use interface relays between my plc and contactors - they are so much simpler and cheaper to replace! They tend to be rated at 6A 240V.

markie said:

I like to use interface relays between my plc and contactors - they are so much simpler and cheaper to replace! They tend to be rated at 6A 240V.

Click to expand...

Another very good route to go - especially if you have long runs of control wire.

Dan Bentler

I would suggest you use transistor outputs in the PLC and external relays. I always use Omron G2RV SL700 24VAC/DC relays as they have flywheel diodes already built in.
Pain in th 'A' when a relay in the PLC 'dies' in the middle of the night - you then need a spare PLC. Much easier, and cheaper, to replace an external relay.

Whenever possible I always go for transistor outputs driving 24V DC relays protecting them from the outside world. The flyback diodes are built in so no worries there.

I have always gone for PLC relay O/Ps which in turn drive interface relays usually 24v DC coils (which are easy to replace).
The interface relays then drive the contactors or whatever.

And as far as I am aware, have no problems/worries with that arrangement over the years.

Sir Dan:

Greetings!

Thank you for the responds. May I ask..With regards to the external relay, could you help on current rating consideration on relay if my motor to be run is 45kw? Also, on the PLC, yes, if I will use external relay first and use its contact connected to the PLC output address, is it already enough that the PLC will not be damage? I have not use this kind of motor rating before. I am only programming around 2.2KW..

I am wondering the PLC unit itself limitations (circuit inside maxx current rating).

Hope to hear more and thank you in advance.

Sincerely,

Sheryl

Sir MArc U:

Thank you for your inputs.

MAy I know if you had encountered programming motors of these ratings: 45kw, 22kw, 18 kw or 5.5 kw?

With your relay 24V DC, what is the current rating? What example of load rating you used on these? A sample diagram of your project is highly appreciated.

Thank you very much.

PS: To the rest of those who responded my inquiry, thank you very much.

Sincerely,

Sheryl
Philippines

OK
Sheryl here is the answer for the motor currents. Motor sizes are
45kw, 22kw, 18 kw and 5.5. See attd Excel spreadsheet. In there assume 3 phase at 450 volt. I did not include power factor.

You will need to recalculate using your actual line voltage which I do not know. I do not know the loads the motors drive so must assume doubling the amps as calculated should give you the current capacity for the relay you need to drive the motor.

I do not know what voltage you will use to supply control power to the motor relay coils. You will have to get factory literature and do some studying.

Dan Bentler

leitmotif said:

OK
Sheryl here is the answer for the motor currents. Motor sizes are
45kw, 22kw, 18 kw and 5.5. See attd Excel spreadsheet. In there assume 3 phase at 450 volt. I did not include power factor.

You will need to recalculate using your actual line voltage which I do not know. I do not know the loads the motors drive so must assume doubling the amps as calculated should give you the current capacity for the relay you need to drive the motor.

I do not know what voltage you will use to supply control power to the motor relay coils. You will have to get factory literature and do some studying.

Dan Bentler

Click to expand...

Sir Dan:

I appreciate the excel file you attach. I also attach mine with re-calculation of actual voltage and a 0.8 power factor. Now, let me continue. From that excel file values, a 45kw motor is equivalent to 60.32hp. Thus, I need to use a contactor with 150A full load current. Say my contactor at 200-240V has an auxiliary contact of example 4NC + 4 NO or 2NO + 2 NC whichever is needed, is it now safe that these auxilliary contacts will now be directly connected to my PLC outputs? From some post above, they suggested to used transistor type PLC.

Next thing that runs in my mind, at around what current rating do these contacts (from contactor auxilliary to PLC outputs) has? Yes, in actual it can be tested. But, what about in manual computation? The bottom of my inquiry focuses on the maximum amperage capacity of the PLC.

Transistorize PLC are good for fast switching.How important is the built in diode and if in case I'll use a relay type PLC, what probably be the disadvantage?

Thank you very much also to those who responded in my thread.

Sincerely,

Sheryl Caño
Philippines

Sheryl

OK let us start with the output contact on the PLC. Let us make it solid state type instead of electromechanical and let us rate it at 24VDC and 2 amp. This contact may have protection but for the cost of a MOV I think the additional insurance is cheap.

This contact will energize coil on an interposing relay (let us call it 45 A). These can be what are commonly called ice cube relays. An output contact on 45 A will control the coil on the motor relay (45 B). The 3 main contacts on 45 B will go to the 45 kW motor and be rated at 150 amp. Also on 45 B will be auxiliary contacts. YOu can use the 45 B aux contacts as inputs to PLC to indicate if 45 B is open or closed. These aree completely separate from the main contacts (150 amp) and are generally rated at around 10 amp

CAUTION if it is vital for PLC to know that motor is running AND pump is pumping (or whatever the driven load is) this is NOT true indication of that. TO get that you need to either monitor motor current or RPM or pump flow. Need to get more info.

Dan Bentler

Sir Dan:

Greetings!

Allow me to clear my doubts.

Let me start on the 2nd paragraph. This is how I understand ( pardon for I did not get the term 45 A and 45B).Isn't it 45A mean NO while 45B means NC? This is how it goes: From the output of the PLC, say I will ON Y1 address. When Y1 is ON (output address of PLC), it will turn ON external relay 1 which is say 24V DC with 10A rating. The contact of relay 1 which is normally open is connected to the coil of my contactor (which I stated above: "Say my contactor at 200-240V has an auxiliary contact of example 4NC + 4 NO or 2NO + 2 NC whichever is needed" with 150A full load currect.In which when contactor coil is ON (from R1 contact) will turn ON the 45kw motor. Did I got it right?

Now, back to 1st paragraph:

What does it mean: "This contact may have protection but for the cost of a MOV I think the additional insurance is cheap."

Does the MOV you mean is the MOV command we use on PLC programming? What insurance it says on MOV?

Thank you for the VFD input stated on your first reply.

And, pardon also if I use the circulation on PLC hardware questions not on programming command topics.

Hope to hear more.

Thank you very much.

Sincerely,

Sheryl Caño
Philippines

sheryl86_ann said:

Sir Dan:

Greetings!

Allow me to clear my doubts.

Let me start on the 2nd paragraph. This is how I understand ( pardon for I did not get the term 45 A and 45B).Isn't it 45A mean NO while 45B means NC? This is how it goes: From the output of the PLC, say I will ON Y1 address. When Y1 is ON (output address of PLC), it will turn ON external relay 1 which is say 24V DC with 10A rating. The contact of relay 1 which is normally open is connected to the coil of my contactor (which I stated above: "Say my contactor at 200-240V has an auxiliary contact of example 4NC + 4 NO or 2NO + 2 NC whichever is needed" with 150A full load currect.In which when contactor coil is ON (from R1 contact) will turn ON the 45kw motor. Did I got it right?

Now, back to 1st paragraph:

What does it mean: "This contact may have protection but for the cost of a MOV I think the additional insurance is cheap."

Does the MOV you mean is the MOV command we use on PLC programming? What insurance it says on MOV?

Thank you for the VFD input stated on your first reply.

And, pardon also if I use the circulation on PLC hardware questions not on programming command topics.

Hope to hear more.

Thank you very much.

Sincerely,

Sheryl Caño
Philippines

Click to expand...

Hi Sheryl - MOV means "Metal oxide varistor" which will prevent damage to an output transistor when it switches off an inductive load (Contactor coil).

Dan is saying you can use safely the auxillary contacts to indicate if the contactor is on or off. i personally feedback the ststus of the overload to the plc for control and indication purposes, as well as hard wiring the "Start" output through the overload.

It sounds like you need sombody familiar with heavy current/control panel design to give you a bit of guidance?

Mark

well sheryl you may need some help from someone in your area more familiar with motor power.

The best option for your motor size would probably to bought an already made motor starter sized with your motor nameplate. Peoples selling that, will help you out finding the right one and this would ensure safety rules in your area being followed

Because you can't use a relay for these 3 phases motors...

The motor starter needs:
a box
a short protection first (3 fuses switch or 3p breaker)
A motor contactor
Overload protection
Lights and buttons selector
All sized for your motor and wired the right way

Then you can mount your plc with isolating ice cube relay on his output. 24vdc output with 24vdc relay coil is my prefered choice.
Each ice cube relay will make availaible an isolated NO contact to triger each separate motor starter.
The control voltage will comes from the motor starter control transformer so you won't really have to worry with voltage and power of your ice cube relay. (Control voltage is usually 110 to 230 vac and most ice cube relay are fine with that....

The 4no or 2no-2nc auxilliary contact you talked about are signaling status to connect a running light or a running feedback to your plc.
You can rely on these contact if you are using only a complete motor starter without any other power cutout. If something happend like a short, overload or switch being opened, the motor contactor should be wired to automatiquely open and tells you the motor has stopped. If you have more device, you should use a current relay or something else to confirm motor running.