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Capacitor discharge

DPittman

DPittman

Ultra Member
I haven't done a YouTube video search yet as I figured I'd start here first.
I'm going to be moving my milling machine and drill press soon both of which have vfds on them. I'm going to have to dk some rewiring for them in the new location. I just remembered that the capacitors of the vfds apparently can store lethal energy in them after being disconnected.

Does unplugging the units and then trying to power on the machinery drain the stored capacitor energy?
 
DPittman said:
I haven't done a YouTube video search yet as I figured I'd start here first.
I'm going to be moving my milling machine and drill press soon both of which have vfds on them. I'm going to have to dk some rewiring for them in the new location. I just remembered that the capacitors of the vfds apparently can store lethal energy in them after being disconnected.

Does unplugging the units and then trying to power on the machinery drain the stored capacitor energy?
Click to expand...
If you can get to them a screwdriver with a rubber handle works to discharge them, stick the shaft across the contacts a few times.

I misread the VFD part and was talking more on the motors, you probably don't wanna have to open up the vfds?
 
DPittman said:
I haven't done a YouTube video search yet as I figured I'd start here first.
I'm going to be moving my milling machine and drill press soon both of which have vfds on them. I'm going to have to dk some rewiring for them in the new location. I just remembered that the capacitors of the vfds apparently can store lethal energy in them after being disconnected.

Does unplugging the units and then trying to power on the machinery drain the stored capacitor energy?
Click to expand...
I would suggest not shorting the capacitor to discharge.

Unplug and wait perhaps 10-30 minutes. You can easily verify by measuring with DMM set to DCV (minimum range 400) on the DC bus terminals these are almost always available typically near the ac input terminals. Should be about 340Vdc when powered (assuming your source is 240V). You can watch it discharge.

Typical example shown below, every VFD will obviously have different terminal arrangement.
vfd_bus.webp
 
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I don't recommend shorting the caps. You might damage something else in the circuit. I think @slow-poke has the right approach and @thestelster has the correct info for his Hitachi. Unless someone has solid reference material for your drives, I'd just unplug them for an hour or so. If And unless you know what you are doing, why not just unplug them from the wall one day and move them the next day. That's likely WAAAAY overkill but definitely safer than other methods too.
 
Heck, wait until the LED of the VFD display goes out, and push the start button of the machine. I would think that any power left in the capacitors would drain from that.
 
thestelster said:
Heck, wait until the LED of the VFD display goes out, and push the start button of the machine. I would think that any power left in the capacitors would drain from that.
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You’ll be shocked to find out that the caps can have significant charge long after the screen goes dark. Trust me.
 
BaitMaster said:
You’ll be shocked to find out that the caps can have significant charge long after the screen goes dark. Trust me.
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But that's why I added to throw the switch to the motor that the VFD is controlling. Or is there some other electonic gizmo, that doesn't release the power stored in the caps when the vfd power is off??

I remember working on my car and the service manual said to pump the brakes a couple times to discharge any stored power in the electrical system before doing any work.
 
thestelster said:
But that's why I added to throw the switch to the motor that the VFD is controlling. Or is there some other electonic gizmo, that doesn't release the power stored in the caps when the vfd power is off??

I remember working on my car and the service manual said to pump the brakes a couple times to discharge any stored power in the electrical system before doing any work.
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Electronic gizmo. All controlled via an onboard computer.

But @combustable herbage is right. Just be careful while you do it.

They have an in built discharge resistor that will drain it to a safe level after a certain amount of time. Adding another resistor speeds up that process by a lot.
 
- Pressing the start button likely won't do anything with no power in, including discharging the caps because the bus will be too low to enable the outputs.

- 100k resistor will take a while, there will be more current drain from the display.
 
slow-poke said:
I would suggest not shorting the capacitor to discharge.
Click to expand...
I'll pile on that also.

Every VFD I have handled, including the offshore ones, have a discharge resistor across each of the caps.
But to be absolutely sure, touch a 2.2K (2 watt, to 10 watt) resistor across the capacitor leads directly - using an insulated pair of needle nose pliers, please! You should not see a spark. If you do then there are no bleeder resistors in your circuit. You will have to touch the capacitor's leads several times to take out the bulk of the stored energy.

The Teco units I use expressly describe their bleeder resistors, and that 30 seconds is enough for the unit to be safe. Mine are 6 years old, so that time interval might be different....
 
Even if there is no explicit discharge resistor exists, the dielectric material has some leakage current within the capacitor itself. It will take more time, but it will eventually drain. No common capacitor will contain meaningful energy after a few hours
 
mbond said:
Even if there is no explicit discharge resistor exists, the dielectric material has some leakage current within the capacitor itself. It will take more time, but it will eventually drain. No common capacitor will contain meaningful energy after a few hours
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As an experiment and to scare the pants off of some young apprentices we hooked up a bridge rectifier to a large phase converter capacitor without a bleed resistor connected. We left it on the bench for 4 hours and then shorted it with a screwdriver. Definitely still charged.
 
BaitMaster said:
As an experiment and to scare the pants off of some young apprentices we hooked up a bridge rectifier to a large phase converter capacitor without a bleed resistor connected. We left it on the bench for 4 hours and then shorted it with a screwdriver. Definitely still charged.
Click to expand...
Back in high school I had work experience at a TV repair shop, in the days of cathode ray tubes & massive electrolytic capacitors. We'd charge 'em up & then discharge them various ways, trying to draw out the longest spark. Big fun...
 
JustaDB said:
Back in high school I had work experience at a TV repair shop, in the days of cathode ray tubes & massive electrolytic capacitors. We'd charge 'em up & then discharge them various ways, trying to draw out the longest spark. Big fun...
Click to expand...

I still remember dad doing that to the RCA Victor tv before sending us to check the tubes at the drugstore. I still have the repair book.
 
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