CURRENT DRAW OF OPEN DELTA BANK
CURRENT DRAW OF OPEN DELTA BANK
(OP)
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.





RE: CURRENT DRAW OF OPEN DELTA BANK
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.
RE: CURRENT DRAW OF OPEN DELTA BANK
I would not concern yourself with the input power to drive. Check the output voltage/current.
RE: CURRENT DRAW OF OPEN DELTA BANK
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.
RE: CURRENT DRAW OF OPEN DELTA BANK
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?
RE: CURRENT DRAW OF OPEN DELTA BANK
Rule of thumb — the listed voltage imbalance should only cause about 8% current imbalance.
RE: CURRENT DRAW OF OPEN DELTA BANK
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.
RE: CURRENT DRAW OF OPEN DELTA BANK
RE: CURRENT DRAW OF OPEN DELTA BANK
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.
RE: CURRENT DRAW OF OPEN DELTA BANK
RE: CURRENT DRAW OF OPEN DELTA BANK
rbulsara, you may "red flag" your own post to be deleted and repost as desired.
RE: CURRENT DRAW OF OPEN DELTA BANK
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.
RE: CURRENT DRAW OF OPEN DELTA BANK
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
RE: CURRENT DRAW OF OPEN DELTA BANK
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
RE: CURRENT DRAW OF OPEN DELTA BANK
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.
RE: CURRENT DRAW OF OPEN DELTA BANK
RE: CURRENT DRAW OF OPEN DELTA BANK
at least post the followig:
readings at output of the drive , current and voltages. even if it is changing post readings of some instance..
RE: CURRENT DRAW OF OPEN DELTA BANK
RE: CURRENT DRAW OF OPEN DELTA BANK