Shorted Dynamic Braking Resistor

[bigwood1]

I currently work in a foundry setting where we operate with two manned overhead cranes. On occasion metal dust from out very dirty foundry will build up on the external resistors that we have for the VFD's installed on the cranes and basically short circuit. This has caused a drive failure on multiple occasions. Is there a way to protect the drive in a situation such as this? Could it be as simple as installing short circuit protection in line between the resistor bank and the VFD?

 

[4-20mA]

Can you mitigate the dust accumulation instead? Maybe a shroud? Maybe canned air duster? What will short circuit protection give you if the actual issue will continue to occur?

 

[augie47]

any chance of enclosing the resistor................

Braking Resistor Box

Braking Resistor Box for testing equipment, elevator and crane.

jdcresistors.com

 

[synchro]

The company at the link below makes dynamic braking resistors with an IP66 dust and liquid ingress rating. Perhaps you could contact them whether they, or some firm that they work with, could provide a solution for your application.

Resistor REOHM BW 155 Braking resistor for drives with frequency converters of medium to high power. Mounting inside and outside the control cabinet is possible… – REO-USA

reo-usa.com

 

[synchro]

Could some metal dust also be getting into the VFD itself even if it's in an enclosure? The electronics inside a VFD would be much more vulnerable to conductive dust than would a braking resistor, due to the higher impedance levels involved.

 

[zbang]

Can you mitigate the dust accumulation instead?

That's the best path- stop the problem from happening.

 

[Jraef]

What happens is that when you short the DB resistor circuit, it is a direct short on the braking transistor and it will destroy it. You can TRY to use a super fast acting semiconductor fuse on the DB circuit, but my personal experience is that they just add to the expensive stuff that needs to be replaced, because they never act fast enough to actually save the transistor.

So yeah, mitigate the dust issue.

 

[drcampbell]

Two possibilities come to mind:

Build a sealed aluminum box with finned heat sinks on the outside and aluminum-housed resistors on the inside.

Build a semi-sealed, positive-pressure resistor box cooled by filtered shop air.

... You can TRY to use a super fast acting semiconductor fuse on the DB circuit, but ... they never act fast enough to actually save the transistor. ...

One of the nice things about power semiconductors is that they're super fast.
One of the problems with power semiconductors is that they're super fast.

Even if you can find semiconductor fuses that clear fast enough, you would only be preventing equipment damage, not preventing downtime.

 

[gadfly56]

Two possibilities come to mind:

Build a sealed aluminum box with finned heat sinks on the outside and aluminum-housed resistors on the inside.

Build a semi-sealed, positive-pressure resistor box cooled by filtered shop air.



One of the nice things about power semiconductors is that they're super fast.
One of the problems with power semiconductors is that they're super fast.

Even if you can find semiconductor fuses that clear fast enough, you would only be preventing equipment damage, not preventing downtime.

^^^^This right here.

 

[coop3339]

I think that eliminating the cause is best but there may be other options.

I wonder if there is a way to monitor the current flow through the brake resistor and alarm or disable the VFD when the limits are exceeded. Do VFDs have any alarm points that can be set to monitor this? I haven't installed or programmed one in 20 years or more.

I would imagine the resistance lowers overtime with the build up of dust? Maybe make it a maintenance routine to blow out the resistor box and check the resistance? could do weekly or monthly? This way you could catch it before it goes too far out of tolerance and causes a failure.