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  1. #1
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    Default Need advice on VFD installation.

    What I have-

    A Cincinnati cylindrical grinder that uses 16" wheels, up to 2" thick. They are driven by a 5 hp motor through flat belts with no provision for changing speed. It has two contactors for forward and reverse controlled by a simple switch, forward, off, reverse. No start and stop buttons. The problem is that the wheels cost several hundred dollars and the customer naturally wants to get maximum life. As they wear down, the surface speed and stock removal rate drop, making the job take longer.

    We have a Mitsubishi 10 hp drive, overkill, but we already have it. It has an internal braking resistor and an analog output for frequency. The setup would have the on-off switch, a speed control pot, and a red LED digital panel meter showing speed. We have one of the Mitsubishi remote controls with an LCD display on another grinder and the operator doesn't like it, too hard to read. The owner feels strongly that the control and display should be easily read and the speed set so the operator doesn't need to bend over or step away from the main controls. I agree.

    With the amount of inertia in a 16" wheel, braking is highly desirable. On another smaller grinder that many people use, they have had trouble with them pushing the stop button and waking away, leaving the drives lit up indefinitley. On it, I put a timer on the contactor so that it keeps the power on during the braking cycle and then shuts down with no further action on the part of the operator. I'm not sure that would be the best idea here. The grinder has a limited number of operators and a noisy fan on the electrical compartment that reminds them to turn off the disconnect.


    I'm open to suggestions.

    Bill

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    What is the main question? The remote display? That sounds like the issue.

    You would need an A/D and a micro (preferably all in one) set up to read out in the desired units, so that the red LEDs can sub for the LCD system. With some cleverness, it might be possible to convert a standard panel mount meter to doing that, depending on the units needed.

    The braking resistor may be OK at that level, maybe even with the big wheels. Most of that size drive will have a large resistor on the heatsink if internal.

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    I never shut off power to my several VFD's Why?

    Set the ramp down time when the stop button is pushed and walk away. Leave the display lit. That way you KNOW it's ready to go when you hit the start button.

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    Quote Originally Posted by CalG View Post
    I never shut off power to my several VFD's Why?

    Set the ramp down time when the stop button is pushed and walk away. Leave the display lit. That way you KNOW it's ready to go when you hit the start button.
    That is the basic question, perhaps not expressed clearly. Do I leave the VFDs on, turn them on with the disconnect, or make a delay in turning them off to give time for braking. I see conflicting advice. The machine has a VFD on the workhead installed by persons unknown. The VFD goes on with the main disconnect and the motor is switched on and off by a contactor between the VFD and motor, a no no by everything I have seen from manufacturers but has been working for years.

    Re the display, there is an analog output proportional to frequency on the VFD and that will go to a red LED panel meter appropriately calibrated to read RPM on the wheel. No more gray on gray displays. The speed control will get a scale giving the settings for different wheel diameters. The grinder is mostly used on large shafts and guides that are worn. They grind them a couple of thousandths undersize, have them hard chromed, and grind to original dimension. Grinding an 8 inch diameter shaft 5 feet long takes a while and running at less than optimum surface speed is expensive.

    Bill

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    All right, no response. I put the question in general terms to see what someone might come up with. Now here it is specifically. I want the motors mechanically disconnected from power when someone is setting up. I don't want to trust a VFD that can have a component failure or be triggered by a line spike. The grinder shares a line with things like 100 hp motors that can generate some huge ones. The main disconnect is on the back of the machine and I doubt that people can be trusted to walk around it each time they want to change something. Contactors are available so I could install some sort of system shut off with a switch on the control panel. I could use the E-stop, but that would leave the grinding wheel free wheeling in a shutdown. I could install a system kill switch that accomplished the same thing as the main disconnect. Whatever, it needs to be something convenient so it is as easy as not doing it.

    Bill

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    Maybe a 4 pole contactor, it's coil rigged up to an E stop normally connected switch and small 24vac transformer.

    3 poles are post vfd connect the motor to vfd.

    The 4th pole is in series with the vfd E stop circuit, or in series with the potentiometer or whatever else is used to control the vfd.

    So you stop the vfd then press E stop to change the wheel.

    In the event that operators get lazy and use the E stop to stop the vfd (many people are afraid of opening a contactor on the output of the vfd for fear of supposed higher voltage spikes present) you can install a DV/DT snubber on the vfd side of the contactor.

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    That would do it, use the existing contactor for the disconnection and connect the existing forward-off-reverse switch to control the VFD. The operator tells me that they always shut off the main disconnect at night so it wouldn't be sitting on for long periods. I couldn't get in the electrical cabinet today because they have a crankshaft for some sort of press brake in work. It has multiple throws with main bearings between, all chromed and ground. The operator told me they had 32 hours in it. Wow.

    Bill

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    Another question. The drive I have is a Mitsubishi FR-A520, unused but probably 10 years old. Is it more likely to be damaged by switching its output than a later one?

    Bill

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    Maybe. There have been advances in IGBTs recently, but in general that new is likely to be no worse than the later ones. The reputation comes from very old types, I believe.

    No guarantees, but I do not foresee a problem. What does the manual say? Some are silent, some allow that, and some expressly forbid it. I always figure the last group know something specific.

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    Quote Originally Posted by 9100 View Post
    All right, no response. I put the question in general terms to see what someone might come up with. Now here it is specifically. I want the motors mechanically disconnected from power when someone is setting up. I don't want to trust a VFD that can have a component failure or be triggered by a line spike. The grinder shares a line with things like 100 hp motors that can generate some huge ones. The main disconnect is on the back of the machine and I doubt that people can be trusted to walk around it each time they want to change something. Contactors are available so I could install some sort of system shut off with a switch on the control panel. I could use the E-stop, but that would leave the grinding wheel free wheeling in a shutdown. I could install a system kill switch that accomplished the same thing as the main disconnect. Whatever, it needs to be something convenient so it is as easy as not doing it.

    Bill
    A contactor is NEVER an appropriate Lock-Out / Tag-Out point for servicing a machine, you MUST use a lockable disconnect switch. OSHA would have you and the business owner castrated for doing this (if someone gets hurt). The right way to do it is a lockable disconnect on the output of the VFD, with an aux. contact on the disconnect that is fed back to the "Enable" circuit on the VFD, which disables the output transistors just prior to the main contacts opening. It prevents the potential damage to the transistors.

    The reason for not disconnecting power from the line side of the VFD is because ALL VFDs have what is called a "pre-charge" circuit that prevents damage to the capacitors from the initial application of power. That generally consists of a series resistor used as a current limiter, then that resistor is bypassed with a relay contact after about 1 second. Every time you cycle power, that pre-charge resistor heats up and if you do it every time you want to run the VFD, eventually that resistor burns out, or maybe the bypass contact welds, and the pre-charge current limiter is no longer there. The capacitors explode shortly thereafter and the VFD is trash.

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    If you used series connected contactors with safety contact monitoring and a safety relay this could possibly meet muster if inspected by OSHA.

    Much easier would be to purchase a drive with built in safety rated STO input. (Safe Torque Off).

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    Quote Originally Posted by Jraef View Post
    A contactor is NEVER an appropriate Lock-Out / Tag-Out point for servicing a machine, you MUST use a lockable disconnect switch. OSHA would have you and the business owner castrated for doing this (if someone gets hurt). The right way to do it is a lockable disconnect on the output of the VFD, with an aux. contact on the disconnect that is fed back to the "Enable" circuit on the VFD, which disables the output transistors just prior to the main contacts opening. It prevents the potential damage to the transistors.

    The reason for not disconnecting power from the line side of the VFD is because ALL VFDs have what is called a "pre-charge" circuit that prevents damage to the capacitors from the initial application of power. That generally consists of a series resistor used as a current limiter, then that resistor is bypassed with a relay contact after about 1 second. Every time you cycle power, that pre-charge resistor heats up and if you do it every time you want to run the VFD, eventually that resistor burns out, or maybe the bypass contact welds, and the pre-charge current limiter is no longer there. The capacitors explode shortly thereafter and the VFD is trash.
    This isn't about servicing the machine. It has the normal lockable disconnect. I am referring to the normal start-stop like making a pass, stopping and measuring the diameter, then making another pass. The scenario Johansen suggests would have the drive lit all the time and the contactor between the drive and the motor, which I had not considered because in the past I have seen warnings to never do that.

    Motion, STO is a completely new concept for me. Another example of the difficulty of starting with a galena crystal set and spending the first two decades of my electronic experience using tubes. Ohm's law and Maxwell's equations are still valid, but from there on it is almost a trip through the looking glass.

    Bill

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    Bill . . . we design safety systems for a broad range of machines - we recently did one for a traditional engine lathe and now doing one for a VTL.

    This explanation should get you started on understanding the Safe Torque Off functionality: What is Safe Torque Off in Variable frequency drive?

    There are sophisticated controllers that can stop multiple axis machines on a specific trajectory (like turning the feedrate to zero on a CNC) in a safety rated fashion. We have done carbon fiber layup CNC machines with up to 55 axes of control this way. This allows a safe stop to occur while machining / making a part without risk of damage to the part.

  16. #14
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    Last question, I hope. Where this has ended up is with a contactor controlling the power to the VFD and a couple of timers, one putting a delay on startup and the other holding power on for the braking cycle. The idea is to light up the drive and get the capacitors charged before starting motor ramp up. The question is, what would be an appropriate delay?

    Bill


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