Mike Howell, PE
EASA Technical Support Specialist

The phase balance test is briefly described in section 4.2.8 of ANSI/EASA AR100-2020. Other names for this test include open stator impedance test, ball test, small rotor test and dummy rotor test. The phase balance test is used in some form by many service centers both as a troubleshooting test and a quality control check before winding treatment. The typical approach is to apply a reduced and balanced three-phase voltage to the stator winding terminals with the rotor removed and then to evaluate the resulting current balance and magnitude. Acceptance criteria differ, but it is a reasonable expectation that the current should be balanced within 10% of the average current.

Additionally, nameplate current should be achieved at roughly 12 to 20% of rated volts per hertz. For example, if a motor is rated 460 V 60 Hz (7.7 V/Hz), you would typically obtain nameplate current somewhere between 55 V and 90 V if testing at 60 Hz. If the motor is rated 460 V 200 Hz (2.3 V/Hz), nameplate current would be expected at 60/200 = 30% of that range or 17 V and 27 V if testing at 60 Hz. 

Even though the test current will be very close to full-load current when performing this test, the magnetic field developed will be very low. This is because removal of the rotor significantly increases the reluctance of the magnetic circuit. In Figure 1, the top image is a model of one phase of a 40 hp (30 kW) 4-pole stator at rated current (52 A) and close to nominal back iron magnetic flux density (100,000 lines/in² or 1.6 T). With the rotor removed as shown in the bottom image of Figure 1, the same stator current results in a much smaller back iron magnetic flux density (12,000 lines/in² or 0.2 T). Consequently, this test is not suitable for evaluating the stator core. 

However, this test is very useful for identifying poor connection joints, connection errors and winding errors when testing at rated volts per Hertz, at around 75 to 100% of rated current, and with supplemental thermographic images. 

Basic Test Data Examples
In Figure 2, we have test results and calculations for a stator rated 20 hp (15 kW), 4 pole, 460 V, 60 Hz, 24 A. A reasonably balanced test current near rated current was achieved at 13.5% of rated voltage at 60 Hz. These test results are acceptable.

In Figure 3, we have test results and calculations for a stator rated 50 hp (37 kW), 4 pole, 400 V, 80 Hz, 68 A. In this case, the test was performed at 60 Hz, but the machine is rated 80 Hz. This had to be considered in the calculations. The equivalent rated voltage at 60 Hz is 400 V / 80 Hz * 60 Hz = 300 V. A reasonably balanced test current near rated current was achieved at 15.9% of the adjusted rated voltage for 60 Hz. These test results are acceptable.

Some tips for interpreting the test data are provided in Table 1. Because of the wide variance in designs, it should be noted that this test is good for finding significant problems, but it certainly is not a catch-all or substitute for the other tests normally performed on windings.

Table 1: Phase Balance Test Diagnostic Guide
Condition of Winding	Expect at 12-20% V/Hz
Correct	≈ 100% FLA
Connected WYE, should be DELTA	≈ 30% FLA
Connected DELTA, should be WYE	≈ 300% FLA*
Connected with half the required circuits	≈ 50% FLA
Connected with twice the required circuits	≈ 200% FLA
Winding turns or pitch slightly off	Probably won't catch it
Other winding issues might be identified by evaluating current balance and use of thermography.
* If you have a variable voltage test panel, stop increasing voltage at FLA and evaluate the voltage.

 

Common Test Variations
A small rotor will rotate smoothly and consistently if no coils or groups are reversed. Commercial devices like the Spindicator shown in Figure 4, small induction rotors and garage door rollers are used. This method is preferred over spinning a ball bearing.

A ball bearing will spin around the stator bore due to the stator’s rotating magnetic field as shown in Figure 5. However, reversed coils and stator surface issues can affect rotation, making this method less preferred. The momentum of the ball bearing can assist it past a dead spot or reversed coil, causing the technician to miss the issue. 

Enhanced Testing With Thermography
Sometimes, issues can be detected very quickly using thermography while performing the phase balance test. 

• Figure 6: Turn-to-turn shorts can sometimes be found using thermography. If the winding has not been resin treated and the area is accessible, repair may be an option. 
• Figure 7: High resistance or poor connections cause elevated heating, which can be detected through thermography. 
• Figure 8: Misconnected stator windings can easily be identified. This large 2-pole winding has equalizers incorrectly placed. 
• Figure 9: Another misconnected winding – the C phase has two circuits and was misconnected with more coils in one circuit than the other leading to parasitic circulating currents and electrical noise.

Summing It All Up
The phase balance test is a simple yet effective method for diagnosing some problems with existing windings and for inspecting new windings before resin treatment. Including thermography enhances diagnostic capabilities, making it a valuable addition to the testing process. This approach is not as expensive to perform as it once was and offers opportunities to provide valuable services to customers and to detect costly errors in the service center while repair is feasible.