I came into work midnights and was asked to check a Benshaw drive for a 600v 40amp motor on a large air handeling unit. After a few minutes it was obvious that L1 and L2 SCR'S were shorted.Since there is no bypass the only option was to wire straight off the fuse holders which I was not willing to do without written direction and was unable to get a hold of the on call supervisor. The motor has just been change a week ago by others and the sealtight fitting is tie wrapped to the box as they obviously didn't have the requireed reducers.The box is also not tight to the motor so this all needed to be sorted for fear of a another short. I ended up just locking and tagging it out as moving a 400lb motor in a confined space to get at the box by myself was out of the question.In the 15yrs i have worked in plants i have never seen a 40hp motor wired right off the fuse holders and wonder how often it done and for how long.Would you do it under these conditions.They didn't fix any thing
The Canadian code has provisions for using fuses as overload protection but I don't remember every seeing it used. Whether I would do it would depend on whether control equipment would be bypassed. I wouldn't bypass a pressure switch or any safety controls. Then, a large motor might not start off fuses rated for overload protection.
I believe that, no matter how bad things are, they can always get worse. What if you omit proper overload protection and the motor dies. Now, you have two problems and the second one is entirely yours.
#213455 - 05/05/1412:14 PMRe: Benshaw drive fried,wired off the fuse holders
THW,that is my fear and the odds aren't good. Just to be clear,this is in an MCC and the motor leads t1,t2and t3 have been removed from the VFD output and brought up to the fuse holders in the combination starter effectively making the disconnect the devise that starts the motor.All o/l protection,and the VFD are out of the circuit.The disco is fused at 50a and the motor FLA is 40a The issues I have with it are
-the lack of O/L protection
-across the line starting of the very large blower wheel that's intended to be soft started.
-the fact that we are a starting a 600v 40a motor by disconnect on a questionable installation.
- the fact that this building has both an automated switch gear that switches the service entrance between two separate grids on voltage loss with secondary emergency generator backup
The new motor is a week old and the VFD fried when we transferred back to normal power from generator after our annual high voltage maintenance shut down on Friday.I did find L2 very lose and burnt on the line side of the equipment disconnect.If they had returned my call I would have informed them that a drive of the same rating for a far less critical motor is sitting within easy reach of the incoming motor leads in the same MCC and it would have taken me all of 5 minutes to pull the leads into the other tub and do it properly. If we lose power and it happens often the transfer back to normal power from emergency can be very messy as the generator can be anywhere from 0 to 180 degrees out of phase with normal power.Furthermore we do have an in phase monitor to delay the transfer so i can guarantee that motor will take a hit sooner rather then later. I most likely be the poor SOB that has to replace both the VFD and new motor on the next go round and if big boss figures out what is going on he will want to know why I said nothing.Sadly going up the chain of command is a very risky proposition around here.I guess my real question is,am I making more out of this than I should?
Last edited by frank; 05/05/1412:20 PM.
#213457 - 05/05/1404:39 PMRe: Benshaw drive fried,wired off the fuse holders
1) If the unit is powered by a VFD then the AC waveform is rectified and a new AC (3 phase) waveform is synthesized.
This makes it entirely irrelevant what happens -- phase wise -- when the unit transitions from one primary loop to another.
2) This is obviously a fully engineered high powered electrical consumer -- being fed by a double-headed primary Service.
3) I've never seen (disco/ safety switch) fusing placed AFTER a VFD. Such a placement is a sure fire way to destroy the VFD!!!
EVERY time a fuse blows the electronics should be fried. VFDs DO NOT LIKE TO BE shut down without a taper. If such a situation occurs, some reactor/ inductive tank is required to stop "wave form recoil." (for lack of a better term)
I do not intend to get into a long, deep, involved physics lecture on electromagnetic phenomena. Suffice to say that VFDs emit extremely high frequency wave edges on their way to the creation of synthetic alternating current. Because of this, this current has to be smoothed out via inductors/ chokes/ coils/ reactors so that the exterior circuitry is not blown out by insanely high voltage peaks.
At the frequencies involved, these pulses can BOUNCE BACK from the dead end of ordinary conductors -- right back into polarized electronic elements that CAN'T take the reversed polarities.
You've got extensive damage in your VFD as a DIRECT CONSEQUENCE of the layout.
It will REPEAT until this goofball scheme is corrected.
There can never be a situation where the VFD is forced to juice an instant open circuit.
In sum, you've got a ticking bomb on your hands.
You need to study up on VFD practices. It sure sounds like your whole facility was installed incorrectly -- by those totally unaware that VFDs can't (normally) survive load-side blown fuses. (VFD fusing has to be up the line.)
The zany notion that it's your SCRs that are dead is cute.
I'd bet your last dollar that the logical control electronics are toast. They are tiny by comparison. Without them, your SCRs don't fire off. This makes the SCRs look like they're what's fried. Actually, SCRs are inherently robust and can take far more punishment than the control chips.
One last note: VFDs produce what the fuses would regard as circulating current. That is, they suffer the ill effects of the voltage peaks just like old time electrical motors.
Now that the high peak problem is understood, the NEMA crowd has engineered an entire suite of motor designs that is able to tolerate VFDs.
The synthetic waveform has many of the characteristics of a strongly lagging or leading motor. That is it needs more reactive power than real power. This reactive current juices the fuses ABOVE the level of real load. It's enough to cause fuses to blow at loads below the motor protection design threshhold.
The disco is fused at 50a and the motor FLA is 40a The issues I have with it are -the lack of O/L protection
In the US overloads are sized to 1.15 the motor's load.
40 amps translate to 46 amps if you multiply it. The next standard fuse size is 50 amps.
In my opinion this does not equate to o/l protection.What will happen when if one those fuses opens?Without a starter or motor protection circiut a single phase condition is very likely.In fact I would consider raising the fuse rating to the maxamum allowable short circuit protection if the motor wasn't being started by the disconnect which is not what it's designed to do.
#213460 - 05/06/1403:51 AMRe: Benshaw drive fried,wired off the fuse holders
Tesl,These VFD'S have been in service for 15yrs and are factory built units in Modle 6 Square d MCC'S.I know how to test SCR's and am factory trained on Banshaw drives.There is also no fusing after the output the fused disconnect is of couse part of the combonation starter(drive)and provides short circuit protection.Thanks the girls think im cute to but regardless of that fact I know how to test across scr's and the event recorder has also recorded the fault.On a side note Banshaws are very well known to fail in this way in fact shorted scr's are the prominant fault when they fail which is a rare.