unbalance
unbalance
(OP)
Hi,
I`m testing a new 1.5 k W 400 V star connected motor and the three phases are reading 2.9 A, 2.9 A and 3.8 A respectively.The three phase voltage is balance . Is it normal ?
Another similar motor running since a couple of months is taking 2 A, 2 A and 3.5 A.
A common rule is that a 10 % unbalance is permissible.
Thks.
Bob
I`m testing a new 1.5 k W 400 V star connected motor and the three phases are reading 2.9 A, 2.9 A and 3.8 A respectively.The three phase voltage is balance . Is it normal ?
Another similar motor running since a couple of months is taking 2 A, 2 A and 3.5 A.
A common rule is that a 10 % unbalance is permissible.
Thks.
Bob





RE: unbalance
http://www.franklinelect.com/Manual/AIM_33.htm
RE: unbalance
A small voltage unbalance can produce a large current unbalance. Current unbalance may be in the neighborhood 5 - 10 times as large as voltage unbalance. That may affect your conclusion on whether the voltage is unbalanced. For another check on the voltage, try rotating the stator leads (keeping same rotation). If the high current follows the power lead than it is the power system.
In my experience you are much more likely to have unbalanced current at low load and often it will go away as load increases. I haven't seen much written about this and I don't know exactly why it is. How are you testing your motor? If there is no load then I'm not surprised at your unbalance.
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RE: unbalance
The derating factor for 2% voltage unbalance is 0.95 and for 5% Voltage Unbalance the derating factor is 0.76
For example 1HP motor with 2% Voltage Unbalance becomes 0.95HP and 0.76 HP if the voltage unbalance is 5%.
The current is expected to be unbalanced 6 to 10 times the voltage unbalance at operating speeds.
RE: unbalance
The 1.5 k W motor is being used as the forced-cooling fan on a 400 k W motor.
The three phase voltage is balanced.
Bob
RE: unbalance
RE: unbalance
RE: unbalance
An egg-shaped rotor will rotate affecting gradually and evenly the trhee phases of the winding.
RE: unbalance
3² x 2 = 18
The greater the unbalance, the higher the motor winding temperature and the sooner the insulation will fail. NEMA standards recommend a maximum voltage unbalance of 1 percent without derating the motor. The motor can be derated down to 75 percent for a maximum of a 5 percent voltage unbalance. If the voltage unbalance exceeds 5 percent it is not recommended that the motor be operated. A rule of thumb states that for every 10°C a motor is operated over the rated temperature rise, insulation life (and therefore motor life) is reduced by half.
David Baird
mrbaird@hotmail.com
Sr Controls Engineer
EET degree.
Journeyman Electrician.
RE: unbalance
"The cause of current unbalance can be quickly identified in three phase systems. The process is referred to as ‘rotating phases.’ The trick is to identify the current readings of each phase, such as: A = 10 Amps; B = 12 Amps; and, C = 15 Amps. You then switch the conductors such that phase A is moved to phase B, phase B is moved to phase C and phase C is moved to phase A. This preserves the rotation of the motor. Restart the motor and re-measure current.
If the current balance remains the same, with the A, B and C phase currents remaining low, medium and high, then the unbalance is due to the motor. If the unbalance rotates such that A = 15, B = 10 and C = 12 (or close), then the current unbalance is due to the power supply.
Tip provided by Howard W Penrose, Ph.D.
T-Solutions, Inc.
http://www.tsoln-inc.com"
raghunath_n00@rediffmail.com