Drive questions

CENTER

Lifetime Supporting Member
Join Date
Nov 2003
Location
Long Island NY
Posts
142
Good evening gentlemen. I just want to start off by saying that I have learned a lot by reading these posts just about every day. But I have two questions that are not PLC related. I hope one of you engineering gru's may be able to answer for me. Got a typical service call today on an old digital 150 hp DC drive. Circuit breaker trips immediately every time they jog. I get there and there are two other drives doing the same thing two more drives that are shutting down for no reason and two other drives that are running fine. To make a long story short after doing all the normal checks and of course when you run out of ideas start changing parts and then get on the phone. So after going over everything with the tech he told me to check phase to ground. Which I would have never thought of doing. Phase to phase was 480, phase to ground was 180, 720 and 720. Found out that the initial installation of 1000 KVA transformer 208 Delta the 480-volt Y they pulled the neutral into the switchboard but never grounded it. Located a 100 amp circuit breaker for an HVAC unit that was causing the trouble. Turn that off and everything runs fine. Now my two questions are.
1 I can see getting 480 to ground but 720?
2 Why would a drive with no reference to ground at all fire or stay latched 2 SCR of different phases at the same time?
This is been bugging me all day I hope somebody has an answer for me thank you.
 
That is weird. I don’t see how you could be reading 720V unless your meter is acting cheeky or it’s not a true rms meter.

I have never worked on big power DC drives, but I do have a bit of experience with high power AC to DC rectifiers. 18,000A 20V

The drive most likely works in the same manner. Each phase is fed into a controller that monitors each sine and fires an SCR on a phase once the sine has just peaked, the SCR(thyristor) will open up once the sine has reached 0.

What you end up with is a saw tooth signal if viewed on a scope. Here is a pic of a signal on one of our rectifiers.
2z86h3t.jpg


This firing must be synchronized with the sine. If your reference is WAY off like you described, you could inevitably end up with a short. Although, I would wager that a DC drive of a standard caliber would disable itself if the phase got out too bad.

The controller is also likely not seeing the full phase voltage. There are probably resistors in line to bring it down to 100V or so from it’s NOMINAL voltage.

Probably doesn’t answer your question, but it is interesting. I’d grab a buddies meter and check it again if the situation comes up again.

It honestly blows my mind that people find themselves in situations of such peculiar problems with electrical equipment, but I haven’t left my work island for ten years.
 
Last edited:
A transformer secondary with no point grounded basically can float to any voltage AC or DC that the leakage will supply. I've measured 480VAC ungrounded delta floating at 1580VAC and, in another case, at 800VDC to Gnd.

While AC drives will run on this crazy dangerous stuff, most DC drives and especially the DC motors will not tolerate it. I think that is where you are at, just now.
 
I figured it must be some sort of phantom voltage. And yes I did check it with a second true RMS meter. The synchronization comes from resistors mounted on the snubber board mounted on the bridge. What the bridge sees the sink circuit sees.
 
I figured it must be some sort of phantom voltage. And yes I did check it with a second true RMS meter. The synchronization comes from resistors mounted on the snubber board mounted on the bridge. What the bridge sees the sink circuit sees.

Bingo sir.
 
The high voltage to ground was likely caused by the AC drive's input filter network, which woud be RC networks and MOV's to ground. They will charge up until they either limit the voltage (high impedence), of blow up. We see this regularly on ungrounded delta power...kaboom!

It'll drive you nuts.

If they want to run ungrounded, then the snubbers in the AC drive should be disabled/disconnected.
 
There are no AC drives. The HVAC CB I shut down was a old standard 2 compressors and a blower on contactors. And on the DC drive. The MOVs are phase to phase with nothing to ground.
 
Fist I believe you strange things happen
Try this open the primary side of the 208 V transformer bank.
Close the 480 V transformer bank
Measure and record the 480V transformers output, phase to phase and phase to ground.
Then check to see if everything on the 480 transformer are running fine.

Then revers the process open the 480V transformer back
and repeat the above measurements

I will bet with only one back on line as a time each bank will run fine and the voltages are all good. But when you combine the 2 banks together is when you have problems.

I ran into a similar problem about 40 years ago the turned out the we lost one of the primary side phase. with both the 208 and the 480 banks connected at the same time it would regenerate the 3rd open leg on the primary side and back feed the system. it was a bear to fine and even harder to convince the power company that the problem was on their end. in the end they found an open primary fuse in a substation that feed the system.

Good Luck
 
The voltage level you are reading is not “phantom”, it’s real. It is a capacitive value to ground BECAUSE there is no hard reference point. It can be just as destructive to electronic components.
 
Thank you jraef for the explanation I guess phantom was the wrong word. I just didn't understand how I could have more then 480 to ground when I had the grounded leg on the HVAC equipment. Any idea on my second question?
 
2 Why would a drive with no reference to ground at all fire or stay latched 2 SCR of different phases at the same time?
This is been bugging me all day I hope somebody has an answer for me thank you.

Is there possibly a DC component on the AC voltage? The SCR's need reverse voltage to commutate off.
 
Thank you jraef for the explanation I guess phantom was the wrong word. I just didn't understand how I could have more then 480 to ground when I had the grounded leg on the HVAC equipment. Any idea on my second question?
There was probably something on that piece of equipment that was resonating with the caps on that DC drive bus and pumping up the capacitive voltage to ground (think "tank circuit").
 

Similar Topics

Hi all, I have a project that use 755 drive to control a induction motor(pump) to maintain certain flow rate. Flow meter is promass 100, unit is...
Replies
1
Views
2,193
I am replacing a US Drive VFD with an Emerson 201m VFD and this is the first time I have ever installed one. I have it working so far in keypad...
Replies
0
Views
1,199
We use sinamics S120 with CU310 for conveyor system of rollling mill(to tranport steel coil). The system use sensorless vector control ? During...
Replies
3
Views
1,750
Anyone please tell me the name of a book for rolling mill drives. It should include how to calculate drive ratings and accelerate and declerate...
Replies
0
Views
1,076
Two questions regarding sinamics s120 drive with sensorless vector control for ac induction motor 1) If i increase inertia of the motor, does...
Replies
2
Views
1,502
Back
Top Bottom