Braking Resistor for Mill
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  1. #1
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    Default Braking Resistor for Mill

    I am upgrading a mill currently that has a maximum spindle RPM of 8000. I am using a Hitachi VFD with a 25 ohm 500W braking resistor. From maximum speed, deceleration under 15 seconds is causing a fault. I would like to know how to decrease deceleration time to closer to 5 seconds. I am playing with the idea of connecting two brake resistors in parallel to do this, but am currently unsure if it will help.

    Any insight would be appreciated.

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    What Hitachi drive are you using? What fault occurs? Putting 2 resistors in parallel will double the current draw through the braking transistor. If it is not rated for that additional current, you'll smoke it.

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    Collect information about your equipment such as model and serial number of VFD. Note the operating voltage to the VFD and phase. Get the mill's HP rating, voltage you are running at via VFD, phase, any data you have on amperage whether it be measured and/or nameplate. With everything you can put down on paper, call Hitachi and ask for assistance in selecting a braking resistor. You don't say what the tooling load is but 8000rpm is considerable. Look at other methods of reducing the mechanical energy if possible.

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    the folks that fixed my Bridgeport style cnc came up under warranty and replaced a (vfd driven) spindle braking resistor. but it seemed as I got better at making the machine go fast as I could, it happened again. back they came, but brought a "bigger" resistor, id guess physically 2x the size. worked well, but went again running a program with a lot of max rpms and tool changes. they came back again, and the new and last resistor appeared to be ceramic in construction, like 3/4" in dia, and about 5" long. none of these changes seemed to change the braking speed, but that big azz resistor was the final solution (so far).

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    What making the resistor physically bigger does (amusing it is the same resistance and material) is increase the heat dissipation capacity. If it is too small things start melting or burning.

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    I'm an idiot when it comes to electronics... other than to know that when you let the magic grey smoke out of them, they quit working. But, I have noticed that quite a few machine tool builders use ordinary kitchen stove heating elements as breaking resistors. They are cheap, readily available and (obviously) designed to dissipate heat. I have no idea on the ohms or anything else...

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    Quote Originally Posted by G00 Proto View Post
    .....I have noticed that quite a few machine tool builders use ordinary kitchen stove heating elements as breaking resistors. They are cheap, readily available and (obviously) designed to dissipate heat. I have no idea on the ohms or anything else...
    It certainly is a budget option. Fadal is the only builder I've seen use them. I've mostly stayed away from budget machines for the past 20 or so years so not sure who else uses them. More efficient designs send power back into the line as much as possible rather than kick the energy off as heat.

    Unfortunately the OP has not gotten back with answers to some questions. Without those answers it's pretty tough to help him.

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    Hmmm, not sure I agree that two resistors in parallel would smoke it. You’d have to make sure the combined resistance was the same. It certainly allows more current to flow but the way I see is that the resistance (lets say 10 ohms) is the gateway in which that flow is timed. Building out a larger room behind that gate doesn’t damage the inverter if the rate at which that door opens is equivalent to the manufacturers window of recommendation. The fact is the inverter is bleeding off all that extra capacitance and if it has too much before it bleeds off it will toss an overload or over current error. That bleed off is turned into heat and then cooled off in ambient air. A second resistor makes room if the first is being over filled. Best in terms I can explain and I’m no engineer but keeping the same wattage, and ohm, or identical resistor and properly installing in parallel shouldn’t have ill effects. It is entirely plausible that it won’t benefit either.

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    Quote Originally Posted by Uguessedit View Post
    .......Best in terms I can explain and I’m no engineer ......
    That is apparent.

    Here's just a part of what you are missing. You can not add a resistor in parallel with an existing resistor and keep the same resistance. Google "parallel resistors" to confirm.
    Last edited by Vancbiker; 05-20-2020 at 01:27 PM. Reason: added content

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    Yes, you can switch more resistors in parallel to dissipate more heat, thats no problem..
    BUT:

    This will not solve your problem..

    1. Every parallel resistor will lower the total resistance, which also equals more current, when you change nothing else..

    2. The resistor does not see to be your limiting factor...your control seems to throw an error as you exceed its max. Break current setting...

    What exactly do you need to break? Huge mass?

    Anyway:

    You need to:

    1. Find out which braking current capacity your drive has.
    2. Check if this capacity is enough for your goal of braking 8000 rpm with your mass within 5s
    3. Check if the limiting setting is set to its max. allowed value
    4. Check if your braking resistor is sized for this max value
    5. If not, replace braking resitor witch suitable type for the max. possible braking load
    6. If drive braking capacity is not enough, replace drive with a beefier modell or use some kind of mechanical break?

    Good luck


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