unbalance

[maypot]

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

 

[SteveKW]

You may find this helpfull...

http://www.franklinelect.com/Manual/AIM_33.htm

 

[electricpete]

Some comments on current 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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[aolalde]

Current unbalance is not directly refered. The NEMA MG-1 std allows 1% Voltage Unbalance before the motor is derated.
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.

 

[maypot]

I queried the manufacturer of the motor and surprisingly they answered that the deviation in the current is acceptable.
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

 

[aolalde]

For 2.9, 2.9 and 3.8 amps the average current is 3.2 amps and the phase unbalance 18.75%. If the voltages at the motor terminal leads are 100% balanced, the winding impedances in the three motor phases are not identical. They could be unbalanced around 1.875% to 3.12%. That questions the manufacturing quality of such a motor.

 

[mc5w]

My experience is that small motors can have large amounts of current imbalance partly because the motor is built cheaply. Not as attention is placed on centering the rotor and even on a 5 Horsepower motor the rotor is going to be egg shaped.

 

[aolalde]

May be an egg-shaped stator bore, becuse the reluctance to the three phases is static and different.

An egg-shaped rotor will rotate affecting gradually and evenly the trhee phases of the winding.

 

[dbaird]

Phase voltage unbalance causes three-phase motors to run at temperatures greater than their published ratings. This excessive heating is due mainly to negative-sequence currents attempting to cause the motor to turn in a direction opposite to its normal rotation. These higher temperatures soon result in degradation of the motor insulation and shortened motor life. The percent increase in temperature of the highest current winding is approximately two times the square of the voltage unbalance. For example, a 3 percent voltage unbalance will cause a temperature rise of about 18 percent.

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.

 

[RRaghunath]

There is a tip I came across in Maintenance forums.com newsletter on the subject. The tip discusses the unbalance in motor currents and how to rule out the possibility of defective motor winding.

"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