Tom Bishop, P.E.
EASA Senior Technical Support Specialist

The most frequent concern about a motor with high current is high no-load current with a 3-phase motor. That topic has been addressed in three prior Currents articles beginning with “Test Run Tips: Common Causes For High No-Load Current On Rewound Motors” (June 2002); “Avoiding High No-Load Amps On Rewound Motors” (February 2004); and “Taming Those Misbehaving Motors” (December 2009). This article will cover the broad topic of high no-load current with 3-phase motors and the issue of high current with load with 3-phase motors. Also considered will be cases of lower than expected no-load current.

High no-load current – motor not rewound
A typical scenario when this issue arises is the case of a motor that has been reconditioned by the service center and the no-load current is higher than expected. Although it is not about high no-load current, the Currents article “No-load Current Basics: Practical Guidelines For Assessment” (February 2005) provides useful guidance, particularly in Table 1 which is duplicated above.
Note that the values in Table 1 are guidelines. Some motors may have no-load currents above or below the range and still be satisfactory. However, when the no-load current is outside these ranges it warrants further analysis. Another consideration when no-load current is high, or low, is the actual test operating voltage versus the motor rated voltage. If the applied voltage is not within 10% of the motor rated voltage, the no-load current can be much higher or lower than expected. For example, test operating a motor rated 200 volts on a 240-volt supply system is almost certain to result in relatively high no-load current.

A misconnection can also result in unusually high or low no-load current. For example, consider a 12-lead single voltage motor intended to be a connection parallel delta for 460 volts. However, since most 12-lead motors are dual voltage, the motor is mistakenly connected series delta for operation at 460 volts. The no-load current in that case is exceptionally low. The solution is to connect the motor for parallel delta.

Another possibility with a 6-lead or 12-lead motor is misconnecting with a delta instead of a wye. Although most of these motors operate/run with the delta connection, some use the wye connection. If that is the case and the motor is operated with the delta connection, the no-load current will probably far exceed the rated current. A good practice with 6- or 12-lead motors is to test operate them in the wye mode and check no-load current. If it is exceptionally low, reconnect for delta and repeat the no-load test. If, however, the no-load current appears to be normal in the wye connection, check with the customer to determine the actual connection. If the customer connection is delta, there is an error in the winding.

A less common scenario is a motor without a nameplate that has a relatively high no-load current for an assumed power rating. In this case, determine the frame size by measuring the width from bolt-hole center to bolt-hole center of the motor feet front to back and side to side, and measure the height from the base to the shaft center. Also check the no-load speed to determine the number of poles. The frame size and number of poles/speed can be used to closely estimate the motor power rating. If the new estimated power rating differs from the initial value, use the new rating to evaluate the no-load current.

High no-load current – rewound motor
The most common causes of high no-load current after rewind are incorrect winding data, core damage, and axial misalignment of stator and rotor. We will address these topics here; for additional possible causes, see the articles mentioned in the first paragraph of this article.

Incorrect winding data that increases the magnetic flux levels compared to design levels results in increased no-load current. Although the effect on full-load current is not nearly as significant, the higher flux levels also result in higher starting current that can cause protective devices to trip. In extreme cases this can cause contactors to blow open or weld closed.  

Delta connected windings with equalizers can be mistaken as being wye connected. The equalizing jumper connections can appear to be wye connections. Figure 1 is an example of an equalized 2 delta, 4-pole winding connection. The connection could be mistaken as a 4 wye during data taking. If connected 4 wye, the winding magnetic flux densities would increase about 15% and greatly increase the no-load current. In most cases, verifying the winding data would reveal the misconnection. Also, although the most common equalized connections are 2-circuit delta or 2-circuit wye, other numbers of circuits may be equalized, such as a 3 delta (6-pole).

Although errors when taking winding data are rare, two of the most common are reduced turns or increased number of circuits. A good practice to help avoid using incorrect rewind data is to verify the data.  This can be done by comparison to the EASA winding database or by calculating the magnetic flux and current densities, such as by using the EASA AC Verification and Redesign program.  An advantage of using this program is that if there is an error in the winding data, the program can be used to design a suitable replacement winding.

Damage to the stator core can be detected by using a loop or core loss test.  Hot spots that exceed about 15°C (27°F) or losses that exceed about 4-5 watts per pound (9-11 watts per kilogram) probably will result in increased no-load current.  If the core losses or temperature rise are high, the core needs to be repaired or, in a worst-case scenario, replaced. The axial alignment of stator and rotor should be checked and adjusted if the cores are offset by more than about 5% of the core length.  Also, if an apparent offset is found, check and compare the stator and the rotor core lengths.  If they vary by more than 5%, that can result in increased no-load current.  Preferably, the stator and rotor core offsets or differences in lengths should not vary by more than 2%.  Form coil stators designed to use magnetic wedge topsticks will have increased no load current, reduced efficiency, and increased operating temperature if the magnetic wedges are not replaced.  A simple check to verify the presence of magnetic wedges is to apply a magnet to the topsticks.  If the magnet is attracted to the wedges, the wedges are magnetic.

A less common cause of high no-load current is excessive air gap.  Inspect for evidence of machining of the outside diameter of the rotor, which would increase the air gap.  

Another possibility is mismatched parts.  For example, making one motor from two “identical” motors that have defective parts, such as one with an open rotor and one with a failed stator winding.  The result can be an excessive air gap or difference in stator versus rotor core length, with the result being excessive no-load current.

High current with load
The most probable causes of high current with load are mechanical overload, excessively high magnetic flux densities or, less frequently, an open rotor. An error in winding data that results in lower-than-design-level magnetic flux can also cause high current with load.

If inspection of the driven equipment does not indicate a mechanical cause for motor overcurrent, it may be necessary to load test the motor to confirm that it has acceptable current at full load. Use caution when evaluating actual motor current versus rated current. The NEMA MG1 motor and generator standard allows a +/- 10% tolerance on rated full-load current. For example, if the nameplate rated current is 100 amps, the actual current could be as low as 90 amps (-10%) or as high as 110 amps (+10%) and still be acceptable.

A winding with magnetic flux levels at least 10% greater than the original design densities will usually draw higher than rated current at full load and will have higher than typical no-load current. A method of checking the performance of a “magnetically strong” motor such as this is to measure the exact rpm at full load. If it is significantly above rated speed, the motor may be magnetically strong and therefore have relatively high flux densities, possibly causing magnetic saturation and higher current. Determining if the speed is significantly high requires some further investigation and some basic calculations.  

The NEMA MG1 standard allows 20% variation in motor slip rating at full load.  For example, if a 4-pole, 1800 rpm synchronous speed motor has a full load rating of 1750 rpm, the slip is the difference between synchronous and rated speed. In this case the slip would be 50 rpm (1800 – 1750); 20% of the slip would be 10 rpm (0.2 x 50). Therefore, the actual speed could be between 1740 rpm (1750 – 10) and 1760 rpm (1750 +10). A speed greater than 1760 rpm would be considered significantly high and could indicate a magnetically strong winding design. Before drawing any conclusion, check the line-to-line voltage. If the line-to-line voltage is more than 10% above the motor rated voltage, that could be the cause of the high full-load speed and possibly the high current.

An open rotor can cause a pulsation in the output torque of a motor as the open bar passes under each phase of the motor winding. The net effect is a reduction in the steady-state output torque; on average, the motor draws higher current that also often pulsates. If an open rotor is suspected with an assembled motor, a single-phase open rotor test should be performed. This test consists of applying about 1/6 - 1/4 of rated AC voltage or variable voltage up to a current value of 75 - 125% of rated current, to two line leads of an induction motor while slowly turning the rotor manually. A clamp-on analog ammeter is used to measure any fluctuations in current. A fluctuation in current of more than 3 percent for a used rotor (1% maximum for a new rotor) usually indicates a broken bar; it will occur each time the open bar passes under an energized pole. 

Just as excessive magnetic flux can result in high current with load, so also can the lack of sufficient flux result in high current with load. The torque capability of a motor is proportionate to the square of the magnetic flux level. For example, if the winding flux is 10% lower than the design flux, the torque capability of the motor will be reduced to about 81% (0.9 x 0.9 x 100 = 81) of rated. If the motor power rating was 100 hp, it would now behave as if it were designed for 81 hp.  At full rated load the motor would be overloaded about 23% (100/81 x 100) due to its magnetically weak winding. The results would include overcurrent and operation at much lower than rated speed.

Conclusion
Electric motors that draw higher than expected current either at no load or with load are a common issue. Determine if the high current issue is limited to the no-load condition and, if so, whether or not the motor was rewound. Then follow the steps in the applicable paragraphs of this article to guide you to a solution. Similarly, if the high current condition is with load, or both with load and no-load, follow the applicable paragraphs in this article. If none of these approaches is completely successful, contact us in EASA’s Technical Support Department.

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Categories: AC motors, Stators/windings, Testing, Motor testing

Tags: High no-load current

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