Home made brake for 3 phase motors.

[Jraef]

Most of the drive systems we do are for industrial applications and don't have dynamic braking as standard.
Some of the smaller ones do have the brake chopper fitted as part of the power circuit but they still need an external resistor to dissipate the energy.
The last one we did was a 200kW unit and we had to buy a separate brake chopper module and a fairly big dynamic brake resistor.

I know brake choppers are separate on larger drives, but it is still a possibility because the drive is able to be programmed to control the external chopper. This Danfoss drive has no braking functionality in programming, nor does it appear to have terminals available for the DC bus, that's what I found odd. It's not like that is difficult or expensive to provide.

 

[Besoeker]

I know brake choppers are separate on larger drives, but it is still a possibility because the drive is able to be programmed to control the external chopper.

For the system I mentioned above, the control was all in the chopper module. The control algorithm was pretty simple. If the DC link went above a set value, the chopper kicked in to regulate that voltage. There was no signal from the inverter, and none required, to stir it into action.

 

[lt2000]

Well ..... there is also the fact that too many companies display no loyalty nor pass profits on to employees so it is often a case of companies getting what they pay for. It is hard for a person to care about the company when the company does not care about them.

Absolutly true

 

[BJ Conner]

Been there done that.

Been there done that.

"Building Engineers tried adjusting dampers to slow return fan but that only helped a little."

It's not an electrical problem-it's a mechanical problem. They may need to replace some dampers.
I started to work on a set of paralled fans that had a similar problem. IF one fan was running and the other started it tripped. The reason was it was running backwards. The plant wanted dynamic breaking, a mechanical break or something.
Both fans had inlet and outlet dampers that were supposed to seal, they didn't. One fan was supposed to supply 1/2 the system air- it didn't. It was recirculating a lot of air and running the other fan backwards in the process.
Replacing dampers may be a lot cheaper than an electrical solution.

 

[Fulthrotl]

Could you explain this to my employer? Somebody needs to.

my observation is that once an employer has reached
the point of no perception, little or nothing can be done
to save him.

i'm sorry, i'm afraid it's hopeless.

 

[renosteinke]

Key word here is "VFD."

When you add a brake (resistor) to a VFD, every time you energize the resistor the VFD will see a voltage spike. If you're already near the limits of the VFD, you will then see an over-voltage fault, and everything will shut down.

 

[Besoeker]

Key word here is "VFD."

When you add a brake (resistor) to a VFD, every time you energize the resistor the VFD will see a voltage spike.

Don't know how you conclude that. The brake chopper switches in the brake resistor when it senses the DC link voltage going above a set voltage limit to prevent it from rising further in regen mode.

 

[renosteinke]

I "conclude" that based upon what I found, and corrected, in a machine shop last week.

It was actually pretty simple .... nominal voltage (source) 480v, rating of the VFD 400v, and the over-voltage protection set at 500v. Hit the'stop' button and -voila- an immediate 'over voltage' alarm.

The problem was made worse by their using a large resistor, in an attempt to make the machines stop 'instantly.'

It also probably didn't help that the supply was an ungrounded delta.

Moral of the story: Forget DIY and buy the right gear the first time. (For this customer, that applies to the distribution system as well, but that's another story).

 

[Besoeker]

I "conclude" that based upon what I found, and corrected, in a machine shop last week.

It was actually pretty simple .... nominal voltage (source) 480v, rating of the VFD 400v, and the over-voltage protection set at 500v. Hit the'stop' button and -voila- an immediate 'over voltage' alarm.

OK.
Maybe a bit of bad engineering design? Wrong sequence of control.
The normal stop ought to shut down the drive as the first step in the sequence. If there is a contactor that should drop out after the the drive has been shut down so it doesn't break current and if it doesn't break current no transients involved.

The problem was made worse by their using a large resistor, in an attempt to make the machines stop 'instantly.'

Where exactly was the resistor connected?

 

[mike_kilroy]

I "conclude" that based upon what I found, and corrected, in a machine shop last week.

It was actually pretty simple .... nominal voltage (source) 480v, rating of the VFD 400v, and the over-voltage protection set at 500v. Hit the'stop' button and -voila- an immediate 'over voltage' alarm.

The problem was made worse by their using a large resistor, in an attempt to make the machines stop 'instantly.'

It also probably didn't help that the supply was an ungrounded delta.

Moral of the story: Forget DIY and buy the right gear the first time. (For this customer, that applies to the distribution system as well, but that's another story).

you didn't say how you 'fixed' it. there is no voltage spike on a properly functioning DB braking circuit. Period. End of story. When the IGBT turns on at a given voltage level, it shorts the resistor across the dc bus. this makes the voltage go down, not up. NO way to get a voltage spike out of this.

The proper 'fix' for your situation would be buy them the proper vfd; you do not run a 400v vfd on 480v with 500v OV trip point and expect it to work. 480v makes a 650vdc bus that should have trip point set to 800-850vdc and OV fault at 900v.

a 'big' resistor has nothing to do with it either. big can mean in wattage or resistance. big resistance means it sucks less joules out when shorting the buss - still not causing a voltage spike. big wattage just means it doesn't glow red at lower wattages, again nothing to do with voltage spikes.

grounded or not delta or wye has nothing to do with it either.

I'm afraid your customer has the wrong equipment for the job.

 

[Besoeker]

a 'big' resistor has nothing to do with it either. big can mean in wattage or resistance. big resistance means it sucks less joules out when shorting the buss - still not causing a voltage spike. big wattage just means it doesn't glow red at lower wattages, again nothing to do with voltage spikes.

Maybe the "big" resistor was too low a value for the brake chopper and that fried the chopper IGBT and that was misdiagnosed as a spike causing the problem?
Just suggesting that as possibility....