Why is Impedance Imbalance in Electric Motors so Important.
Impedance is the opposition to current flow in a circuit.
When an AC alternating current is drawn by Impedance as a load, a voltage drop is produced that is out of phase with the current from 0 to 90 degrees.
This phase shift is dependent on the reactance being the Inductance and Capacitance contributors, at a certain frequency.
A straight piece of copper wire is almost a perfect Resistor, but does not possess reactance as there is no Coil (Inductance), or Capacitance other chargeable bodies nearby. It therefore can be measured with a multi meter in Ohms as pure resistance.
Why is Impedance Imbalance in Electric Motors so Important.
In an electric motor this all changes as the Copper wire is coiled adding Inductance, and the Insulation provides Capacitance, very little Impedance comes from a the pure resistance of the copper wire.
The total Impedance is therefore the Root Mean Square of Resistance and Reactance measured in Ohms, but is dependent on Frequency.
Very High Frequencies will pass through a capacitor as a dead short, while completely blocked by a coil. On the other hand very low frequencies pass through coils but are blocked by capacitors.
The All TestPro instruments have adjustable test frequencies to ensure the Impedance measurement is in the sweet spot of the motor, and typically a 200 hz test tone is used as standard.
The procedure for Motor Circuit Analysis is to measure each phase of a three phase motor using this Impedance measurement with the acceptable limit approximately 5% imbalance with the Rotor installed.
A measurement greater than this recommended limit means the motor will experience an unacceptable voltage drop that is not balanced creating inefficiencies and stressing the Winding.
This magnetic out of balance creates heat leading to a shorten motor Insulation life, and wastes energy.
There are many published papers from IEEE that state the Voltage Imbalance greatly affects electric motor efficiency determined by the Ohms law
V = I Z in simple terms Cos phi 0.
eg IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 16, NO. 4, OCTOBER 2001
Assessment of Voltage Unbalance
Annette von Jouanne, Senior Member, IEEE and Basudeb (Ben) Banerjee, Member, IEEE
So there is two business drivers should be considered when purchasing Electric Motors that being Energy Efficiency which is by far the biggest cost of Ownership, and Motor Life.
3Phi Reliability regularly test electric motors in the field and record results.
The following chart has plotted Impedance Imbalance versus two All TestPro Winding tests being Phase Angle & Current Frequency Response (Methods Listed in IEEE 1415:2006)
Both Methods are described in https://www.3phi-reliability.com/blog/phase-angle-test-an-effective-means-of-determining-electric-motor-winding-health
What is very interesting is the correlation between Impedance
Imbalance and Motor Reliability.
The occurrence of Winding defects almost disappears below 3% Impedance Imbalance, while above 8% imbalance shows a significant increase in winding stress and Losses.
The MotorGenie test report has a warning (Yellow) at 3% which correlates to the data collected by 3Phi Reliability, but no Alarm Limit is set in the Test Report (Red). Based on the data to date 3Phi Reliability recommend an Alarm limit of 8% Impedance Imbalance for Motors.
What this means is approximately 50% of Motors in industry are above this Limit, and concentrated in Newly purchased motors.
Note: Efficiency Class is not correlated to these results, if fact many new IE3 & IE4 motors have high Impedance Imbalance.
Impedance Imbalance creates higher losses and those losses end up as heat either in the Stator or Rotor.
It shows that the Impedance Imbalance is a direct contributor to Energy Efficiency & Motor Reliability.
Wait!!!! It’s not that Simple.
Impedance Imbalance is slightly more complex as the Stator (Motor Windings) can be the cause of the defect (Self Impedance), but also the Rotor has a major influence (Mutual Impedance).
If you rotate the Motor Shaft during an Impedance measurement the reading will often dramatically change based on rotor position. A well made rotor will impart Mutual Impedance in a smooth sine wave of Impedance known as a RIC test (Rotor Influence Check),
Many gross rotor defects can be identified with a RIC test.
When a rotor has a defect the magnetic flux passing from the stator drawing current from the electrical supply, those flux lines aren’t symmetrical around the rotor center line. Therefore the torque transmitted isn’t efficient and losses increase. The heat shortens the Motor life.
One of the most common defects is casting voids or porosity in the Aluminum used to form the rotor bars, and this is difficult to control in manufacture as many factors can lead to a poor outcome.
One indicator of a magnetic defect in the rotor is the use of excessive balance weights or drill holes in the rotor ends. These balance weights are used to Mechanically bring the rotor into specification but the Magnetic Imbalance remains hidden.
3Phi Reliability conduct a number of different Motor tests that can determine if the this Impedance Imbalance is Stator (Self Impedance) or Rotor (Mutual Impedance) related. To date tests show that Stators are generally well made, but Rotor quality is a significant problem.
3Phi Reliability testing shows motors below 5% imbalance suffer very few winding defects, and are randomly distributed by Motor Size.
In this example the Current Frequency Response shows a Winding defect, but the Phase Angle is a Pass (Green).
What you will notice is T1-T3 winding has a lower Impedance but the Phase Angle is slightly higher and the absolute I/F is also higher. The is a divergence between the winding tests and Impedance.
What is recommended is to rotate the shaft approximately 120 degrees placing the original rotor position under another Phase.
If the same pattern of divergence occurs then the Winding defect can be confirmed in the stator and the Rotor influence can be discounted.
In this example the winding defect can be confirmed, but the Phase Angle is a Pass. This indicates that the Insulation hasn't decayed but a turn to turn defect exists.
A conclusion is the T1-T3 coil has a manufacturing defect where a coil loss is present. This can be coil placement, damage in wire insulation, or air entrapment in the resin causing strands of the coil to short. As a maintenance manager your probably not going to go into that much detail but Acceptance Testing of New Purchases is a high priority. It is critical to weed out purchased defects.
A number of the High impedance imbalance motors failed after a few weeks, so the predictive testing using Phase Angle & Current Frequency Response is really showing it effectiveness.
Facilities that implement Motor Circuit Analysis, controlled lubrication and weed out the defects can easily eliminate 95% of motor failures.
3Phi Reliability highly recommend implementation of Acceptance Testing for purchased and Overhauled motors.
Before condemning a motor rotate the supply phases maintaining correct rotation, if the Impedance imbalance or Current remains in the same phase then you can confirm the Load has the defect.
Note: Voltage
Imbalances in supply, across terminations, Circuit Breakers, and
Contactors can also amplify the motor losses which 3Phi Reliability regularly test.
Once you pay for these defects it’s YOURS FOR LIFE.
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