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

Three of the most common motor problem calls received from members by EASA’s Technical Support Department are:

1. “The motor is drawing high no-load current.”
2. “The current of the three line leads is not balanced.”
3. “The motor is running hot.”

If you have ever faced one or more of these issues, and it’s almost certain you have, read on.

1. “The motor is drawing high no-load current.”
This call is often associated with a motor that has just been rewound. The frequent causes of the high current are a winding with high magnetic flux densities or damaged core lamination insulation. Checking the magnetic flux densities and testing the core prior to rewind are good practices that can prevent the high-current issue.

The EASA AC Motor Verification and Redesign Program quickly calculates magnetic flux and current densities, and “flags” values outside of typical acceptance ranges. Use a commercial core tester or loop test to check the laminated core condition. If using the loop test method, refer to EASA’s Tech Note 17 for step-by-step guidance.

Magnetic flux values that are too high or core loss that is excessive will often result in higher than normal no-load current. By checking magnetic densities, the winding data can be corrected before the motor is rewound, rather than after it is fully assembled. Likewise, a defective core can be repaired or replaced as the result of a core test, rather than learning after assembly that the core is defective. The cost of rewinding or replacing a defective winding or core is all the more costly with the present high price of copper. If EASA members are uncertain of a test procedure or a test result, contact EASA’s Technical Support Department.

The high no-load current could have causes other than the defects mentioned above. Low-speed motors, typically with 8 or more poles, draw relatively high no-load current. Before taking the motor apart (or the even more costly step of stripping out the windings), check with the motor manufacturer, your own previous repair records, or members can contact EASA Technical Support to evaluate the high current. Also, compare the applied line voltage to the motor rated voltage. See Table 1 which is reprinted from the February 2005 CURRENTS article “No-load Current Basics: Practical Guidelines for Assessment.” It provides typical ranges for motor no-load current.

Higher-than-rated line voltage will increase no-load current and lower-than-rated voltage will reduce no-load current. As obvious as that sounds, it is something we often overlook when test running a motor, such as one rated 208 volts and tested at an actual line voltage of 240 volts or above.

Note: In addition to the February 2005 CURRENTS article mentioned above, more information on no-load current can be found in these articles, available in the “Technical Articles From CURRENTS” section of “Members Only” at easa.com:   “A Closer Look At The No-Load Current” in the May 2001 edition and “Avoiding High No-Load Amps On Rewound Motors” in the February 2004 edition.

2. “The current of the three line leads is not balanced.”
The current unbalance could be due to the motor or the supply line.  To determine which one is the source, arbitrarily label the supply lines A, B and C, and the motor leads 1, 2 and 3. Connect A to 1, B to 2 and C to 3, then operate the motor and measure the current in the three lines. De-energize the motor and connect A to 3, B to 1 and C to 2, then operate the motor and again measure the current in the three lines. If the high current and low current readings follow the same line leads, the supply is the cause; if the high and low readings follow the motor leads, the motor is the source. This is illustrated in Table 2.

If the supply is the source, the supply voltages need to be better balanced. NEMA standards prescribe a 1% limit for voltage unbalance; they note that current unbalance can be expected to be 6-10 times the voltage unbalance on a percent basis. If the supply voltage unbalance exceeds 1% or the current unbalance exceeds 10%, the supply voltages must be corrected to less than 1% unbalance, or the motor de-rated.

If the motor is the source of the unbalance, the turns per phase or per parallel circuit are probably not balanced or the winding is misconnected. An error when making coils could lead to some coils having more or less turns than others, consequently resulting in unequal turns in a circuit (versus other circuits) or a phase. The unbalanced turns will result in unbalanced currents much the same as with unbalanced supply voltages.

An unbalanced or misconnected winding can usually be detected using a surge tester. Measuring the lead-to-lead resistance with a digital low-resistance ohmmeter (DLRO) may also detect unequal turns. The lead-to-lead resistance should be within 5% of the average.

Unbalanced current can occur if the winding has unequal grouping with more circuits than are permissible for the slot-circuit combination. Also, although uncommon, if the air gap is eccentric, unbalanced currents can occur. In that case, the “high leg” will stay with the motor. This is especially common when a winding has half as many circuits as poles (e.g., a 3-circuit connection on a 6-pole motor). Another possibility is an open connection that leaves out a circuit in a multiple circuit winding. An example is a 4-pole 4-delta connection with one circuit of one phase not connected. The result is a winding with 3 circuits in one phase and 4 circuits in each of the two correctly connected phases. Testing lead-to-lead with a DLRO would detect this condition.

Note: Additional information on unbalanced current (and voltage) can be found in the article titled “Unbalanced Voltages and Electric Motors,” in the December 2007 edition of CURRENTS. It, too, is available in the “Technical Articles” section of “Members Only” at easa.com.

3. “The motor is running hot.”
How often have you heard this statement from a customer? With some modern insulation systems, the surface temperature of the motor could be hot enough to cause a burn if a finger or hand is placed on it. Thus, a caution:  Never use a part of your body to check the temperature of a motor. Use a temperature-detecting device.

Standards define the temperature limits for windings, but not for the surface of a motor. If the outside of the axial center of the stator core can safely be reached with a temperature-detecting device (Figure 1) the winding temperature can be estimated. That estimate is that the winding temperature will be approximately 5° to 10° C (9° to 18° F) hotter than the temperature measured at the outside of the axial center of the stator core.

Winding temperature limits vary by size and type of motor. To determine if the winding is too hot, see the CURRENTS article titled “Understanding Motor Temperature Rise Limits,” in the November 2003 edition, also available in the “Members Only” section at easa.com. If the winding temperature is higher than expected compared to the surface temperature of the frame, it is possible that the core is too loose in the frame. That inhibits heat transfer.

Causes of excessive winding heating can be either external or internal to the motor. External causes include high ambient temperature, contaminants, mechanical overload, high inertia loads, high- or low-supply voltage, unbalanced voltages, or a damaged stator core.

Total winding temperature is the combination of winding temperature rise plus ambient temperature. If the ambient is 10° C (18° F) hotter than normal, the winding under the same conditions will be 10° C (18° F) hotter, and will have approximately half of its normal thermal life. Contaminants that build up on the motor or that block ventilation passages increase the temperature of the winding and other components, such as bearings, resulting in premature failure. Mechanical overload simply means the driven load is greater than the motor power rating.

A pump or fan with a discharge valve or damper open too wide can increase load, as would putting too much load weight on a conveyor. High-inertia loads such as fans or blowers that result in extended starting time increase heating of the rotor as well as the stator.

High- or low-supply voltages will result in either excessive core losses or reduced torque capability, respectively. Unbalanced voltages increase current in at least one phase, increasing I2R winding copper losses. They also create “negative sequence” currents (a topic beyond the scope of this article) that heat the stator and rotor surface at twice line frequency.

Causes internal to the motor also include contaminants that build up in the motor or that block ventilation passages, missing or damaged air deflectors, a damaged stator core, or a winding with incorrect data. A damaged stator core can greatly increase core losses and cause excessive heating and high current even at no-load (see common problem #1 above). Examples of incorrect winding data include a misconnected winding such as a winding connected delta instead of wye, a winding with “dropped” turns (reducing turns increases magnetic flux density and core losses), or incorrect voltage such as a 208 volt winding being operated on a 240 volt supply system.

Categories: Troubleshooting & failure analysis

Tags: Unbalanced voltage Motor temperature No-load current

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