CURRENT DRAW OF OPEN DELTA BANK

[sblocker]

I HAVE A 120/240 OPEN DELTA TRANSFORMER BANK FEEDING A VFD THAT DRIVES A 10 HP PUMP. THE CURRENTS ARE UNBALANCED (47.9; 24.8; 56.3) WHILE THE VOLTAGES ARE WITHIN 1.2% (243; 246; 240). THE CURRENTS WERE TAKEN ON THE INPUT OF THE DRIVE. iS THIS CURRENT UNBALANCE DUE TO OPEN DELTA CONNECTION? THANKS IN ADVANCE FOR ANY HELP.

 

[rbulsara]

I may be wrong (as I have not thought this thru, but that is not a reason not to express my opinion

Are you reading the currents and V on the secondary side or the primary? I think you are measuring them on the seconday side. Please check the primary too. I suspect one of the primary line is open (the phase that reads low value).

I belive, if not for the VFD, the motor would have tripped on single phasing or burnt by now.

 

[buzzp]

It does not matter what the drive is taking in only whats coming out as far as your motor is concerned. By your statements, all of these measurements are on the input side of your drive. No big deal. I am not a drive designer but some where in there they need single phase power for firing IGBTs, SCRs, and just plane control power. This is probably why your seeing the unequal current draw on the input. The open delta transformers does not help here either.
I would not concern yourself with the input power to drive. Check the output voltage/current.

 

[busbar]

Offhand, the listed voltage imbalance is quite small. You probably won’t get much sympathy from the utility [assuming it’s their transformer] unless there is at least double the posted voltage imbalance.

 

[aolalde]

sblocker:

10 HP at 240 Volts should be driving around 27 amps from a regular three phase power supply. Since the PF of the drive is close to unity that current at the driver input should be a little bit more reduced. This does not match the reported currents of 47.9/24.8/56.3 amps at 240 Volts.

Is the transformer reducing or increasing the voltage? From what input voltage to what output voltage? What is the set up output of the driver? Voltages and Currents into the motor?

 

[busbar]

 
Rule of thumb — the listed voltage imbalance should only cause about 8% current imbalance.

 

[buzzp]

Busbar,
The rules are out the window because these measurements (at least the currents) were taken on the input of the drive not the output.

I guess to me, who cares what the input current into the drive is. If the voltage is within specs of the drive operating range then the drive should be fine, on the converter side. If he is worried about his motor then he needs to make these measurements with a TRUE RMS meter on the OUTPUT of the drive.

 

[GOTWW]

According to ECM-PQ corner, the Ammeter readings taken by an average reading ammeter can be up to 40% off at the line side of a VSD due to the harmonics.

 

[rbulsara]

I still suspect some measuring error in voltage..

Thinking backwards....It appears that if you visualize the open delta or V connection(secondary), say 240V exist between the open ends and no voltage at the common. This will provide 120V across two phase windings of a delta connceted motor and 240V across the third winding.

This would result in 44-54Amps for 10 HP at 120V (per phase) and 24 amps on the phase that has 240V. This appears weird but something to think thru... or rule out.

I am interested in what is measured at the output of th drive (motor terminals) and on the primary side of the transformer bank..

Also describe the 'open delta' set up.

 

[rbulsara]

I should concede that above is the most bizzare 'statement' I have ever made..so far..

 

[busbar]

rbulsara, you may "red flag" your own post to be deleted and repost as desired.

 

[rbulsara]

LOL..I may..but I think the orignial post has some inconsistencies...

50A at 240V does not make sense for a 10HP 3 phase motor....unless drive is running on 240V single phase...

10 HP at 240V single phase approximately equals 42 or 44 A.

 

[mc5w]

Your input rectifier may have a bad diode. Deenergize the drive, open it up, and check for this condition.

Both 240 volts corner ground and 120/240 volts 4-wire delta have a tendency to push a line frequency voltage imbalance through the motor drive and into the motor's capacitance to ground. A 3-phase line reactor will help compensate for this and a 3-phase reactor between the drive and the motor will help prevent high frequency current from flowing in the equipment ground and in the supply wires. 3-phase inductances will also help to compensate for the unbalanced inductance of an open delta transformer bank.

Your drive should also have a DC smoothing ( current sourcing ) reactor between the rectifier diodes and the power supply capacitor. This capacitor should have enough extra inductance to tolerate a reasonable amount of voltage imbalance.

Also remember that an open delta transformer bank is for the purpose of providing a modest amount of 3-phase power at a modest price.

Mike Cole, mc5w@earthlink.net

 

[busbar]

buzzp—

I wanted to infer that the very minimal voltage imbalance at the open-delta-bank secondary is likely not much of a contributor to the high drive-input-current imbalance.

—busbar

 

[buzzp]

Okay busbar, I understand.
He does not say where this voltage was measured but does day the current was on the input of the drive. Sounds like he measured the voltage on the output of the drive (judging by the numbers being so balanced). I guess time will tell what he has and what he is trying to do.

 

[sblocker]

ALL MEASUREMENTS WERE MADE ON INPUT OF DRIVE. OUTPUT OF DRIVE IS CONSTANTLY CHANGING, WHILE INPUT STAYS CONSTANT. LOAD ON MOTOR IS ASSUMED CONSTANT(WELL IS FULL OF WATER).DRIVE MANUFACTOR CLAIMS THAT ALL RECTIFIERS ARE GOOD.

 

[rbulsara]

if the load on the motor is constant why does output of teh drive is constantly changing? and what is changing?

at least post the followig:

readings at output of the drive , current and voltages. even if it is changing post readings of some instance..

 

[GOTWW]

Digging the hole deeper rbulska? I do that too. Sblocker but what did yo measure the currents with? did you take peak, & rms & average?, peak over average? maybe you should get a PQ meter or take snap-shot oscope readings. Maybe your current transformers are saturated. You put the burden of proof on us it seems. As my acquaintance Rbulska says "quit BS's and provide a complete data set, using reliable calibrated equipment".

 

[buzzp]

Your motor could care less what the input power to the drive is. If your worried about the motor then check the Vs and Is on the output of the drive. If your have drive problems, then worry about the input to the drive.