VFD Braking Resistor

[Peter.]

This is really an update on the Chekko Lathe ressurection.

Now the drive is operational... the motor and countershaft, and the flat belts are functionally tested, it's become apparent that I need to assist the vfd with respect to decellerarion. My little Randa slows to a stop in about 1.5 seconds so I asked the Chekko to do the same, but the countershaft and spindle pulleys, with a combined weight of around 50kg are having none of it. I can stop from around 1000 rpm (motor rpm) but any higher and I get errors. The 10" chuck (not yet fitted) will easily add another 30kg to this.

Quite obvious a braking resistor is required. But... how can they justify the rediculous prices they are asking for what is simply a big resistor?

The inverter spec stated a minimum 75ohm. That's no problem... but the wattage needs to be high. Very high. But how high?

In my scrap bins I found a heating element from an old towel rail. It's rated 600W @ 240V. But how does the 600W equate during braking when the voltages can be considerably higher? Anyway, it measures 93ohm so I figured what the hell?

As a test I connected it, with no heatsink, and got perfect braking up to the max setting 3000 rpm. It got warm, but not hot. With a chuck it'll have to work harder but early results are promising. Add to this the pulley ratio is now wrong as I had to remake the motor pulley at twice the original diameter so the max HZ will need to be reduced to approx half it's current value. So hopefully I'll be decellerating from closer to 1500rpm instead of 3000.

I've made a nice ally heatsink for the element and hope to test tomorrow with the chuck mounted.

Fingers crossed.

I've got a couple spare came out of a lift motor control cabinet. They are not super heavy suty quite the opposite. What OHM rating are you looking for?

[kf2qd]

YOu could use 2 of those stove elements by wiring them in series. Here's the rule - 2 equal resistors in parallel equal 1/2 the resistance. 2 resistors in series equal double the resistance. If you were to use 2 equal resistances in series you could use resistors that have half the wattage as each would then be doing half the work.

As it is, you braking resistor seems to be working just fine. If you start/stop often you might want to put a fan near by to get rid of the excess heat.

[Jonesy]

.

As it is, you braking resistor seems to be working just fine. If you start/stop often you might want to put a fan near by to get rid of the excess heat.

Yes indeed. Working better than fine. Get's luke warm at most with many stops. Didn't get the chuck on yet though. That's next.

[Tobias-B]

A caution about heating elements-
they are designed to get hot (duh)
and their resistance increases a LOT with heat.
(I hesitate to say exponentially, but it's an order of magnitude)

So when cold, they are darned near a dead short, and if the braking
circuitry is not designed to load into a dead short, it can overload the
switching devices that apply the load to the bus.

A resistor, however, is designed to increase resistance as little as possible
as it heats, so it's a more 'ideal load'

Added to all the above, it makes good sense that the 600 watt load works
well- since the load never heats up all that much due to a very low duty cycle,
it's presenting an appropriate resistance to the braking circuitry at its 'cold'
resistance, and working well. If it heats up, you may have to reduce your
braking percentage to keep bus voltage down, but nothing will be damaged.

t
resistance may or may not matter. Even if it is futile.

[Jonesy]

Thanks TB. I did hit this element with a heat gun while connected with a multimeter yesterday when trying to decide if it was a goer. I got it hot. Very hot, but not glowing! Didn't see any change in resistance. Stayed within an ohm of 93.


Either way... it's working a treat.

I was stopping the spindle on a sixpence ( a Dime.... for you lot), but as the chuck is a screw-on I've dropped it to 5 seconds.

Need to do a real-world test with the big 4 jaw in situ.

[Jonesy]

FYI... it's the 600W version of this...http://ebay.co.uk/itm/333980565521?_....c100290.m3507

[mikey553]

Nichrome resistance varies with temperature increase, but it is not that much.

https://www.google.com/search?q=nichrome+resistance+vs+temperature&sxsrf=AOaemvLAl2r_J_LAFpHMUZRUywp6fJeebA:1641760256880&tbm=isch&source=iu&ictx=1&fir=PT1bCruK-xsnpM%252CWKZuxVdHB0dnYM%252C_&vet=1&usg=AI4_-kTCjK6tYyB8T5z-k_7Z1zjtLx99ig&sa=X&ved=2ahUKEwiwmqvHwaX1AhWim-AKHco0BwQQ9QF6BAgPEAE&biw=1536&bih=714&dpr=1.25#imgrc=uFI72cD9VHkWDM

I still like the rectangular power resistors better since they are designed to be air cooled, have aluminum body with ribs and have provisions for bolting them down. Your heating element is designed to be submerged into liquid for heat transfer. For intermittent use in a machine tool it probably would not matter since your element is oversized.

[Jonesy]

Nichrome resistance varies with temperature increase, but it is not that much.

https://www.google.com/search?q=nichrome+resistance+vs+temperature&sxsrf=AOaemvLAl2r_J_LAFpHMUZRUywp6fJeebA:1641760256880&tbm=isch&source=iu&ictx=1&fir=PT1bCruK-xsnpM%252CWKZuxVdHB0dnYM%252C_&vet=1&usg=AI4_-kTCjK6tYyB8T5z-k_7Z1zjtLx99ig&sa=X&ved=2ahUKEwiwmqvHwaX1AhWim-AKHco0BwQQ9QF6BAgPEAE&biw=1536&bih=714&dpr=1.25#imgrc=uFI72cD9VHkWDM

I still like the rectangular power resistors better since they are designed to be air cooled, have aluminum body with ribs and have provisions for bolting them down. Your heating element is designed to be submerged into liquid for heat transfer. For intermittent use in a machine tool it probably would not matter since your element is oversized.

My element is in direct contact, through it's entire length, with a 40mm square ally block. I thought it should be in contact with something, and what better than ally?

[boslab]

Was told an old tumble dryer heater element?, I have no idea if the resistance is right but it can certainly dissipate the heat
mark

[macona]

Haas just uses range top heater elements for braking resistors on their mills. Cheap and effective.

[Jonesy]

Haas just uses range top heater elements for braking resistors on their mills. Cheap and effective.

Cheap is good. Effective is a bonus!

Unbranded generic high power resistors are cheap enough. But manufacturer specific units are a real con. My manual suggested I use 3 of their units in series to get the right setup. This would cost more than the inverter would (if I'd paid for it).

It does seem there are good cheap effective alternative options for most things in life. I tried to explain that to the wife and she just threw things at me.