Precautions During Maintenance of a VFD

"If your not sure what a Capacitors is, I would not be working on these units. Your into unsafe area you need not be in."

Absolutely. If you don't know how a VFD is constructed or how it functions, you should not expose yourself to this potentially fatal environment. Get trained FIRST, this is not a good candidate for "on-the-job learning".

The intermediate circuit capacitors are the big electrolytic capacitors within the VFD that hold enough charge to potentially kill.

Take a look at this Wiki article and scroll down to the section on regenerative VFD's.

I recently observed a maintenance technician place a screw driver between the two legs of the 600 volt DC bus. By the time I had the chance to scream STOP his fingers were burned to the bone,

Does that give you a clue! PLEASE STAY AWAY FROM IT!

"I recently observed a maintenance technician place a screw driver between the two legs of the 600 volt DC bus. By the time I had the chance to scream STOP his fingers were burned to the bone,

Does that give you a clue! PLEASE STAY AWAY FROM IT!"

15+ years ago when I worked for ABB Drives I was witness to a seasoned veteran drives technician who did a similar thing on a 600HP DC drive at a steel rolling mill. He realized what he was doing at the last second and attempted to get it off, but got caught in the "DC Death Grip". We used a 2x4 to pry him off, but it was way too late, he died on the spot. Horribly by the way. It's etched in my mind to this day.

About 25 years ago I took 600VDC arm to arm on a large DC drive. Luckily it was the back of my hand that contacted so I pulled off. It was one of the most painful shocks I have ever had. It strained all the muscles in my chest. Witrhin the first couple seconds I thought I was going to die. I swear my heart stopped for a couple seconds, the room darkened, I was frozen in space, with excruciating pain. The chest took several weeks before the pulled muscles started to recover.

I will take a jab from a TV set or car ignition any day before another 600VDC. Atleast you just jump with the shock.

(I have made a concerted effort since that incident to avoid shocks. One hand in pocket, rubber mats, rubber gloves and leather over top, even better, get someone else to attach the scope probes.)

I think this is a good answer even if it does not directly talk about the capacitors. Safety always ranks as number 1.

The capacitors you ask about are the big round things at the bottom of the drive.

If someone has given you the key to the door of the drive, I suggest you close the door, lock it and give that person the key back, sign off the permit to work and go look for another job in a cake factory.

The only positive thing in your favour is you actually read the instruction manual, but it is horrifying obvious you know nothing about a VSD, capacitors or the danger they represent, so please go make fairy cakes, at least you will live longer!

The two terminals (BRK+, BRK-) happen to be across the capacitor bank. The auxiliary braking resistors and transistor connect to these points also. It is a convenient point to measure the residual voltage on the capacitors.

This is not to be done casually.

You do not apparently have the skill set to be working on the drive.

Please take the appropriate training before proceding. These circuits are lethal.

HI, The capactors are the DC Bus system that is at the heart of the VSD Drive, if not for these capactors the VSD would not operate. The reason for the 5 minute delay is to allow these capactors to discharge, if there is no braking resistor these capactors require a longer discharge time, that is why they say 5 minutes.

The problem is that when a motor is stopped there is a, regeneration process that happens and this charges the capactors, if there is no brake resistor these capactors are discharged through the bleed resistors which are of a very high value and require the 5 minute bleed time to rest at at a 0 voltage

Cheers Joe

Not quite true, the brake circuit only energizes if the capacitor voltage gets too high, about 900V. It does not reduce this voltage to zero.

"Not quite true, the brake circuit only energizes if the capacitor voltage gets too high, about 900V. It does not reduce this voltage to zero."

True. And besides, a motor only regenerates if the VFD keeps the windings excited, i.e. controlled decel or braking. If the VFD turns the motor off completely (i.e. coast to stop), it only regenerates for a few cycles.

Thanks, point taken.

No problem,

When one tries to simplify sometimes the result is close enough if not 100% accurate. The full explanation gets too complicated. Our OP wasn't too sure of what a capacitor is. (That electrical device that can store as much energy as a small handgrenade.) Just try to explain the algorithms for sensorless vector control. The idea is simple enough, but then it gets so tangled up in math and sampling and processing issues and dozens of settable parameters it is truly amazing it works as well as it does.

My brother is an aeronautical engineer with about 25 years experience...he admits he still marvels at airplanes, or birds, or bees flying.

Thanks GW

Cheers Joe

and here we go.........trully professional.........

if u look at my question, i said "I was looking into a ABB VFD fault tracing document" not actual VFD. i know i cant be near the VFD. so there s no harm in gaining knowledge................so if u can rethink and just enlighten me a lil, it would be of great help.............

ok, but you could have mentioned that earlier! WE are only concerned for your safety!

Anonymous Poster #1 said

"and here we go.........trully professional.........

if u look at my question, i said "I was looking into a ABB VFD fault tracing document" not actual VFD. i know i cant be near the VFD. so there s no harm in gaining knowledge................so if u can rethink and just enlighten me a lil, it would be of great help............."

Wait, so you are implying that we should have known your INTENT in posting this and now you are SCOLDING us for not knowing that and thinking of your safety?

I happen to be able to explain this in absolutely boring detail, I have been a VFD designer and have been in the industry since the early 1980's. But when I get scolded for trying to foster safety upon someone who APPEARS to be about to attempt something dangerous, I am not inclined to be "trully professional", whatever "trully" means.

Learn some manners sir.

And learn to use spell check...

The VFD uses a three phase (six diode) bridge that rectifies the AC to DC. The capacitors are used to filter this to a more pure DC. The capacitors charge 1.4 x AC line volts, so for a 575V input the capacitor charges to about 800 volts.

The output is a six transistor/FET/IGBT bridge arrangement with 2 devices per output phase in series across the DC bus with the midpoint being the AC variable frequency output.

The terminals you mention are connection points for an external braking circuit that just happen to be the same as the intermediate capacitor bank that has all the energy stored.

So before you work on the drive you want to be sure it is discharged.

By code we have to provide some sort of discharge network so those caps don't store the energy indefinitely. We also provide warning labels to caution you not to be foolish.

Shorter discharge times are possible, but then efficiency of the drive drops.

During a controlled stop the capacitors charge to a higher voltage. If a braking circuit and resistor network is connected once the voltage exceeds a predetermine high voltage it dumps energy to the resistor to bring the voltage back to nominal rating.

The variable frequency output to an induction motor creates a voltage and resultant current waveform. If the frequency output is slightly faster than the "electrical" rpm of the motor (ie: slip) we generate motoring torque. If the frequency exactly matches rotation speed no torque is created, however current still flows to magnetize the motor, but since it is mostly inductance there is no real power and the current lags by ~90 deg. If the load on the motor increases (decreases) then a positive (negative) slip occurs and torque is developed and real power is supplied (absorbed).

The energy is either supplied by or absorbed onto the intermediate DC bus and capacitors. Since the diode bridge cannot return the energy to the AC line the capacitor voltage rises. If a braking circuit is included then the circuit turns on and discharges this energy onto the resistors dissipating it as heat. The circuit turns off once the voltage is back to nominal.

The whole concept of the in phase real power component and the orthogonal magnetizing component is the basis for flux vector and Sensorless vector control. These vectors are simply a mathematical description of the rotating magnetic fields and torque created by the three phase variable frequency supplied to the motor.

A humble advice to OP (post # 17), which he MUST follow:

"Pass on this thread to your friends/co-workers....... who are dealing with actual VFD/similar equipments. Even if they know the dangers involved, at least, it will elevate their level of bewareness".

A company I have been working with has asked that all VFD spares be cap charged once a year. Would you agree this is pointless and possibly dangerous if only for a couple of minutes?

No, it’s a good plan. Electrolytic capacitors age if not powered. All you need to to is connect them and leave them powered for 1 hour per year.

Side note: appending a 10 year old thread is a good way to be ignored. You should have started a new thread and referenced this one.

A simple telephone call to the Technical Helpline at <…ABB…> can answer these questions and many more in double-quick time.

There has never been so much telephony in the world, and never so much resistance to use the facility.

While CR4 does not offer training either in the use of the telephone or the maintenance of <…ABB…VSD…> equipment, those at <…ABB…> certainly can do the latter. This post suggests the latter would be a Good Thing, possibly preventing equipment damage, business loss, injury or even death of the practitioner.

What is this “telephone” device you speak of?