drive control problem - Electrician Talk - Professional Electrical Contractors Forum

martindag · 10-30-2009, 06:56 PM

hi guys,

im trying to find a solution at my problem. i want to control a 1 hp with a drive now, that motor which was already in place with a contactor 600v ac. the control for that motor come from a PLC with output card 120vac ONLY. the way it programmed when PH reading get 6 to 9 it start more or less than that it stop. i dont really have acces to modify that PLC or plug on it.

so my only option is find some king of relay ... that the coil would be for 120vAC and output for 15vdc+ to activate the digital control of my drive that work on 15vdc.

could a simple Relay switch 120vAC do the work for that ? im scare that the magnitic field of that coil ***** my output 15vdc that go throught it.

in brief...

i work in reverse ! lol
i need something like a relay with a input 120vac ON/OFF that activate a 15vdc+ output.

any suggestion ? low price stuff welcome !

Jlarson · 10-30-2009, 08:11 PM

Why do you need a VFD? If you hook it up like you are trying to all you will do is stop and start the motor. Now, you can probably program the drive to ramp up and down on the call to stop or start but you will not have variable speed based on Ph level. To do that you would need an anolog signal from the PLC or the Ph instrument or whatever to do that. If you need to ramp the motor up to speed go with a soft start with 120VAC control, less work and cheaper.

martindag · 10-30-2009, 09:37 PM

i totaly agree with you a soft starter would be much more better than that VFD to control that motor, the only problem is my boss already ordered that VFD and i have no choice to do something with it now lol ( thats great when they dont ask you for advice) so... i tried it anyway it seem to work prety good actualy a standard relay 120vac input and i use it to control my digital signal 15vdc from my drive. prety weird i agree usualy small power control big power lol but in this case my boss gonna be happy i guess and it wont be expensive

Jlarson · 10-31-2009, 01:53 AM

I hate it when people order stuff without thinking then they come to you and you have to make it work. I just found out I have to do the same thing to a VFD for energy control. I have to take a PLC and shut off the drive by controling a 24vdc signal then drop out an input contactor to kill the drive and its transformer.

What's this motor do anyway, it sounds like a water/wastewater app.

martindag · 11-01-2009, 08:00 PM

yeah exactly for the neutralizer tank for a waste water treatment, on the original plan i saw it was suposed to have a motor starter for that motor lol and they didnt want to buy it to safe on the cost ... but now they realized a 1725rpm to whip that was way to fast ...

mcclary's electrical · 11-01-2009, 08:41 PM

Quote:

Originally Posted by martindag

yeah exactly for the neutralizer tank for a waste water treatment, on the original plan i saw it was suposed to have a motor starter for that motor lol and they didnt want to buy it to safe on the cost ... but now they realized a 1725rpm to whip that was way to fast ...

Develope your 15 volts via a control xfmr, then run it through a standard din rail mount ice cube relay with 120 volt coil, make the dc voltage on a set of NO contacts and it should work by the PLC

Jlarson · 11-02-2009, 12:22 AM

Yeah, I would say 1725rpm is a little too much for a mixer. When I do a tank mixer I like to use a big gear motor, no VFD needed.

If you need automation stuff in the future use Automation Direct, Allied Electronics, and Omega. They all have online ordering and catalogs, I get just about everything automation related I need from them.

John Valdes · 11-06-2009, 01:28 PM

Quote:

Originally Posted by Jlarson

Yeah, I would say 1725rpm is a little too much for a mixer. When I do a tank mixer I like to use a big gear motor, no VFD needed.

If you need automation stuff in the future use Automation Direct, Allied Electronics, and Omega. They all have online ordering and catalogs, I get just about everything automation related I need from them.

I agree on the reducer if the mixture is real thick and hard to get started.
OP. You can let the PLC control the drive. Or you can just let the PLC enable the drive and use the drive controls to operate. What type of drive do you have? Does it have a built in PID loop mode?

martindag · 11-06-2009, 04:51 PM

its for a neutralizer tank more exactly it mix Waste water with caustik and coagulant to make a reaction to make little ball of S*** lol so too fast we get like cream and we dont want that, and too slow its not good also.

the coagulant control is analog but is controled by the flow at the affluent of the system only. so faster the flow go more coagulant you get. and the mixer is a 4-20 ma analog input from a ph sensor with set point you can put like 6 ph to 9ph and will ON / OFF output when needed or something like that ( dont have much time to watch all the function)

but anyway what i did i added a relay switch 120v with my contactor coil
to make my drive control ON/OFF because it would be too much problem to remake everything i bypassed my power 600v at the entrance of my contactor, so i use it only now for the contact on it, its fuse protected anyway and the rest my VFD will get in fault,

best would be to isolate everything but that take time and time is money and my boss dont like to spend money so ... lol

Jlarson · 11-06-2009, 06:22 PM

So you took the old contactor with a 120 VAC coil and used that to swithch the 15 VDC. I would have done it almost the same way if I had to. As long as it works and it's safe.

martindag · 11-06-2009, 06:41 PM

yeah that coil is still activated but the contactor got no power on it, and my relay coil activation is in parralel with it 120v and control my 15vdc with a N.O. contact

look weird lol but work and not expensive.

cdnelectrician · 11-06-2009, 08:31 PM

Solid state relay maybe? You won't have any interference from a coil with a solid state relay.