Announcement

Collapse
No announcement yet.

Contactors on my mill question

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • MaxHeadRoom
    replied
    Out of interest, Google "24vdc control circuits in industrial equipment" 😉

    Leave a comment:


  • Paul Alciatore
    replied
    Good decision from the engineering point of view. But if your children are anything like mine were/are, you better rethink the no feeding thing.

    A caution would be to look up the amount of current that/those 24 Volt control outputs can supply and compare it to the current the 24 V contactor coils will need. If there is not enough you would need to put a relay between them which uses a higher current rated source.

    As for all the worry about E-Stop, I personally do not like any kind of software or logic circuit based generation of such signals. In my mind an E-Stop is an EMERGENCY stop and should be used only in cases where life or limb is at stake. And in those cases, any damage to the machine or the electronic controls is of far less consequence. Thus an E-Stop should consist of the very simplest and MOST RELIABLE circuitry possible. I favor a NORMAL stop system for all other uses. If I design a control, like the one on my lathe, then the two can often be combined into one stop system. On my lathe any one of many switches in the open state if enough to stop it. If the relay/contactor 24V power supply goes bad, it stops. If the wiring breaks, it stops. Etc. Unfortunately my simple control circuit will not work with most VFDs.

    With a CNC system designed by others you may be forced to work with their limitations. Unfortunately, all to often the designs where price is often a big issue, rely on a number of components and/or software features. Failures in any of that can leave the machine running and unstoppable without pulling the power cord or switching the breaker in the building's breaker panel off.

    For better or worse, the designers of that system, are the best source for information on how to do wire it.



    Originally posted by Black Forest View Post
    OK I will ditch the 220v coil contactors and order 24v coil contactors. I was just trying to save money so I could feed my children. They are going to have to tuff it up now!

    Leave a comment:


  • J Tiers
    replied
    As for the relays and their ratings, do not go by the relay markings. Check the manual for the control. If the manual says you can put 230V through the relays, at the current you want to use, then the PC board they are on has been tested for that usage, and it is OK.

    That assumes the control in question is UL/CE recognized. If not, that's a different matter.

    Originally posted by MaxHeadRoom View Post
    .................................
    Note: If a AC device, solenoid armature etc, fails to shift over, it results in failure of the coil.
    Yep, it depends on the change of inductance and thus impedance when the coil pulls the moving magnetic material into the gap. If it does not pull-in, then you have many times the "sealed" current in the coil, and smoke is going to happen.

    DC contactor coils can sit forever on their rated voltage, and no bad results.

    But, if you have 220V coils, I agree that there is nothing wrong with using them. So long as the contactors can handle the higher 220V current, and the switches are rated for it, no issues.

    Leave a comment:


  • rdfeil
    replied
    On another note... I do not understand the obsession with low voltage control and/or DC control voltages. Yes, there are times when these are required by the equipment, but mixing AC and DC in a control cabinet requires separation of wiring to meet certification requirements.
    In Black Forest 's application the interface board manufacturer CLEARLY states that the relay outputs are good for 240 volts at 10 amps. This would be fine for a master contactor. Even more so in this application as BF has a mechanical spindle brake that will apply when the power is cut. That is a pretty definite controlled stop in an emergency situation.
    The bottom line here is that he has the 220 volt stuff so why not save the money and buy more tooling. The kids can fend for themselves 🤣.

    Leave a comment:


  • rdfeil
    replied
    Originally posted by wmgeorge View Post
    I do not see the need for DC coils, we used step down transformers to get the 24 vac. Making them DC introduces more issues. Every VFD I have used says Never to break the motor feed from the VFD under power.
    Take a look at ABB drives. They are not cheap, but they are exceptionally well designed and they actually have application notes for using one drive to power many motors with external switching. The only catch is you have to provide overload protection for each individual motor and wire appropriately.

    Leave a comment:


  • MaxHeadRoom
    replied
    Originally posted by wmgeorge View Post
    I must say in the many years I did industrial electrical I saw very few burnt out contactor coils or solenoids, not enough to warrant a complete change over to DC. If the AC ones did burn out it was usually do to some sort of mechanical failure that needed to be addressed anyway.
    Many of the ones I came across was due to usually the mechanical maintenance pushing a solenoid armature over manually to test something , causing instant failure of the coil.
    The result of converting from AC to DC was in many of the systems I retro-fitted from relay logic to PLC. the Large inventory of AC coils in the maintenance parts were never used after the retro.

    Leave a comment:


  • wmgeorge
    replied
    I must say in the many years I did industrial electrical I saw very few burnt out contactor coils or solenoids, not enough to warrant a complete change over to DC. If the AC ones did burn out it was usually do to some sort of mechanical failure that needed to be addressed anyway.

    Leave a comment:


  • MaxHeadRoom
    replied
    Originally posted by RB211 View Post
    I highly disagree with the industry standard of having the E-stop kill power to the VFD. If you want to kill power to the VFD, it should be its own switch. Most VFD's seem to want to stay powered off for a tiny bit before reestablishing power..
    One reason i use the 'Controlled Stop' as per NFPA79/CEC/NEC and my previous post.

    , Europe has mainly used DC for solenoids and contactors for Many decades, One manufacturer that Black Forest may be familiar with is the German Co.. Hegensheidt Manuf. they produce many machines used in the railway industry and a big supplier to N.A. RR's All of their machines have used DC control voltage for solenoids and contactors for Many decades.
    When I first tried to introduce DC devices into N.A. when maintaining machines, I got "We have always done it this way"!
    Once large manuf. Co's saw the drop in maintenance due to machines no longer down due to frequent burnt AC solenoid valves etc , they changed their minds!
    Note: If a AC device, solenoid armature etc, fails to shift over, it results in failure of the coil.

    Leave a comment:


  • wmgeorge
    replied
    I agree with Doozer, low voltage CNC circuit boards should not have Line Voltage.

    I will continue to not open a disconnect on the output side of a VFD while the motor is running regardless of what is posted here. For the folks running DC on the switches feeding control circuits be sure to check and make sure they are rated for DC and at the voltage needed.

    I thought Emerson sold rebranded/ imported Chinese VFDs. Turns out they do : China | Emerson SG
    They went Chinese because of the cost and warranty issues with the ones they made.
    Last edited by wmgeorge; 05-04-2022, 03:14 PM.

    Leave a comment:


  • J Tiers
    replied
    UL 508 has a separation requirement, and a requirement on wiring etc being rated for the highest voltage in the enclosure.

    CE probably has the same sort of requirement.

    Leave a comment:


  • Doozer
    replied
    There was a comment about the relays on the control board being rated
    for 230volts and 10 amps or whatever, so why not run the contactors
    using 230volts through the control board relays.
    Well, it is not good practice (safety) to run high voltage through control
    boards. The machines I build as of late, I use a 24vdc input PLC controller.
    Input power is 480v. This goes through a 480vac to 24vdc 10amp switch
    mode power supply. Then I power the PLC and all the other controls with
    24vdc. In fact, I put the 480 vac power supply and 480v VFD in a separate
    interlocked NEMA cabinet. I use a separate cabinet for the 24v control
    components. That way, the high voltage danger when troubleshooting
    issues with the control part of the system is eliminated. Each voltage
    has its own cabinet. Not saying to go to that extreme, but just dividing
    the cabinet and keeping high voltage from going through the control
    boards make good sense.

    --Doozer

    Leave a comment:


  • RB211
    replied
    I highly disagree with the industry standard of having the E-stop kill power to the VFD. If you want to kill power to the VFD, it should be its own switch. Most VFD's seem to want to stay powered off for a tiny bit before reestablishing power.
    Besides, if you have a braking resistor, I'd rather have that function in place when the E-stop is hit.
    With my Teco 7300, something inside blew up, sending shrapnel around inside when I did the industry standard of cutting power to it, AND having set it to resume after power back on. The combination of the two didn't work well on it, I learned my lesson, NONE of my VFD's are killed with the E-stop, they are only commanded to stop the spindle with the Brake resistor.

    Leave a comment:


  • MaxHeadRoom
    replied
    Originally posted by Black Forest View Post
    OK I will ditch the 220v coil contactors and order 24v coil contactors. I was just trying to save money so I could feed my children. They are going to have to tuff it up now!
    I would use 24v DC coils just needs a bridge rectifier on the AC 24v
    The only advantage an AC coil has is at switch on, after that DC are more efficient, also if you happen to need 24vdc for any other electromechanical devices, they are less prone to burn out on DC.
    Also if you need indicators, they are mainly 24v LED, which require DC.
    Look at the outputs of the VFD for something equal to 'At zero speed' etc.
    Not sure if the same regulation exists concerning E-Stop where you are?
    Last edited by MaxHeadRoom; 05-04-2022, 11:35 AM.

    Leave a comment:


  • J Tiers
    replied
    Originally posted by wmgeorge View Post
    ................... Every VFD I have used says Never to break the motor feed from the VFD under power.
    You probably need to use modern ones.

    I know of several folks who use large VFDs to power their entire shops, used as just a 3 phase 60 Hz AC source. They turn machines on and off at will, all connected to the one VFD that is always on.

    Every solar power inverter is fine with things being turned on and off. They are made just like a VFD, and have no special design.

    I've designed several VFDs that were intended to have the load turned off and on. Nothing special was needed other than handling the motor inrush current. No issues in service.

    My boss at the consulting company worked for Emerson for a long time designing VFDs, and he never had to make any special precautions against damage either.

    That has not been an actual issue since back when VFDs used bipolar transistors. Those were not as tolerant of such things, although even there, it should not have been a problem.

    Remember, the VFD "cuts power" thousands of times per second, with voltage spikes etc. If it was going to be an issue, VFDs would not even work.

    But, if a manual still says never to do it, follow the manual on the off-chance that the maker knows that their cost-cutting has created a possible problem.

    Leave a comment:


  • wmgeorge
    replied
    Originally posted by Black Forest View Post
    OK I will ditch the 220v coil contactors and order 24v coil contactors. I was just trying to save money so I could feed my children. They are going to have to tuff it up now!
    I do not see the need for DC coils, we used step down transformers to get the 24 vac. Making them DC introduces more issues. Every VFD I have used says Never to break the motor feed from the VFD under power.
    Last edited by wmgeorge; 05-04-2022, 11:13 AM.

    Leave a comment:

Working...
X