Phase balance testing.

[Ohm aye God]

Hi all,
Can you test for phase balance on a 3ph motor while its connected to the supply cable (dead) and still has the links in?
I'm getting conflicting answers.
If so what method?

Cheers

 

[che12345]

Dear Mr. Ohm aye God

" Q. Can you test for phase balance on a 3ph motor while its connected to the supply cable (dead) and still has the links in? "
A1. a) "...the supply cable (dead) ..." = power off?
b) "...still has the links in ... " = motor terminal links is in place either linked in star or delta connection?
A2. What do you wish to measure?
A2.1 The winding resistance, inductance or impedance ? Or the insulation resistance?
A2.2 The power source [voltage between line-to-line and lines-to-neutral] or the [three-phase running currents?] Note: 1. in this case, the motor must be on load.
2. you can measure the (lines-to-neutral) voltage at the power source only. Neutral wire is NOT available at the motor terminal box.
Che Kuan Yau (Singapore)

 

[waross]

If you mean phase current unbalances then the answer is mostly NO.
Voltage and/or phase angle errors on the supply cause the majority of current unbalances in a healthy motor.
The exception would be a defective motor. Shorted turns in a motor will cause current unbalances but shorted turns will normally progress to complete failure quite rapidly.
This is probably the reason for your conflicting answers.
The problem may be a failing motor or, more likely, a supply unbalance issue.

Unbalanced supply phase voltages are the major cause of phase current unbalances in motors.
Long distribution circuits often use field located voltage regulator banks spaced at intervals along the length of the distribution circuit.
The action of the voltage regulators is to correct the phase to neutral voltages.
In the event of unbalanced loading creating unbalanced voltages the regulators will correct the line to neutral voltage but in so doing will cause unequal phase to phase voltages and unequal phase angles.
The measurement of interest is the line to line voltages.
When the line to line voltages are equal they form an equilateral triangle and so the phase angles must also be equal.
Equal line to neutral voltages do not imply equal line to line voltages nor do they imply equal phase angles.

Mitigation:
Oversized motors.
The motor may be oversized so that the load current plus the unbalanced current on the worst phase is less than the rated current of the motor.
This will still result in greater than normal heat generation in the rotor.

Dedicated wye/delta transformers.
A wye/delta transformer with a floating primary neutral will correct the unbalances.
This solution brings other issues.
A wye/delta transformer with a floating primary neutral is known to generate transient switching over voltages.
It is recommended that the wye/delta transformer have no loads connected when it is energized.
It is recommended that dedicated wye/delta transformers be used only for loads that are sensitive to supply unbalances.
This would be mostly motors.
A good scheme is to use a wye/delta transformer to supply each MCC, motor or group of motors.
An under-over voltage protection relay may be used to energize the control voltage transformers to avoid subjecting them to switching transients.
Three phase line to line connected rectifier banks may benefit from a wye/delta transformer.
Three phase line to neutral connected rectifier banks may benefit from a wye/delta transformer if there is a line to neutral unbalance.
The wye/delta transformer should be oversized in relation to the connected loads.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Ohm aye God]

Che12345,
Thank you,
I'm only looking to gain a simple resistance value (impedance) on each of the windings.
The motor is full connected in a delta configuration with the supply cable connected and links in place.
There is no voltage.

 

[che12345]

Dear Mr.Ohm aye God

O. " I'm only looking to gain a simple resistance value (impedance) on each of the windings.
The motor is full connected in a delta configuration with the supply cable connected and links in place...."

C1. For (each winding)* [resistance], you may use any digital multi-meter on the market, as the [resistance] is likely to be >0.1 to <50 Ohm. Note: the meter output is dc, therefore the measured value is the winding [resistance]. You can also measure (each winding)* [impedance] by connecting (each winding)* to a very low step-down transformer output <5Vac 50/60Hz and record the V/I= Z ohm (impedance).
C2. Attention: You [must] temperately remove the links and the cables on the motor terminal box before carrying out the measurement. This is to ensure that you are measuring (each winding)* i.e. (U1-U2), (V1-V2), (W1-W2). Re-connect the cables and three links back to the original (vertical) position, in [delta formation] i.e. (U1-W2), (V1-U2), (W1-V2) after measurement.
C3. Caution: refrain from measuring in [delta formation], as you may arrived at confusing or difficulty to establish the exact value of (each winding)*/faulty winding.
Che Kuan Yau (Singapore)

 

[NCTHAI]

@Bill - Great explanation and information.

@Mr.Ohm aye God

I believe that you want to measure the motor parameters (resistance / impedance / reactance) and IR when motor is dead but motor cable remains connected. You are measuring at motor terminal with cables connected. In this condition, the measurements can done as cable is connected but open at other end. Theoretically, you should have no impact on readings obtained due to cable remain connected and under ideal conditions you should get the symmetrical results (Similar for all three phases); However, in practice,readings will be affected by cable length, routing, temperature, adjacent cables operating voltages, laying method and many typical parameters and operating conditions.

My take is use such measurements to know general health of motors while doing the routine maintenance but do not rely on them if you have to make any decisions. If you want to make important decisions related to motor condition, repair or replacement, it is recommended to eliminate all possible unknowns and variables - cable connection in this case.

NC

 

[waross]

INSULATION TEST
For an insulation test, the links may be left in place for the first test.
If the first insulation test is satisfactory there is no need to go further.
The ratio between winding resistance and insulation resistance is so great that there is no difference in the insulation reading if the winding resistance is included.
If the initial test is unsatisfactory then remove the links and test each phase individually.
NOTE:
The initial test may be done with the supply conductors connected.
Again, if the test is unsatisfactory then the supply conductors must be removed to determine whether the low reading is a motor problem or a cable problem.
WINDING RESISTANCE TEST.
I have often found winding resistance tests with a multi-meter to inconclusive.
A much better result may be gained by a voltage drop test.
One link is removed so that the windings are in a broken delta configuration. (Open delta in IEC land?)
A low voltage (6 or 12 Volts) is applied so that the current passes through all three windings in series.
The voltage drop across each winding is then measured and compared.
The test may be conducted with DC applied and then with AC applied.
A shorted turn will often show a greater difference on the AC test.
A second test may be performed by applying low voltage AC to each winding in turn and comparing the current draw of each winding.
A caveat: In some instances a bad turn will short out with line voltage applied but will not short out with low voltage applied.
This is a fairly rare event.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Ohm aye God]

Thank you all,
Between you all you have answered all I need to know.
I appreciate your input and not poking fun at me for not being a motor testing genius