Tom Bishop, PE
EASA Senior Technical Support Specialist

Three of the most common three-phase motor problems we receive inquiries about 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.”

Even if you have never faced one of these issues, read on because it is almost inevitable that you will, and you will want to know what to do about it.

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 Verification and Redesign – Version 4 program quickly calculates magnetic flux and current densities and “flags” values that are outside of typical acceptance ranges. A quick reality check for the airgap density is to use the winding database and search on "HP (kW) poles, core length, bore diameter", and compare the densities to your data. If your data results in more than 20% higher AGD, the fault lies with your data. A commercial core tester or loop test can be used to check laminated core condition. If using the loop test method, refer to Core Testing in Section 7 of the EASA Technical Manual for step-by-step guidance.

Winding magnetic flux values that are too high, or core loss that is excessive, 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 has been 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. If you are uncertain of a test procedure or a test result, contact our technical support group at technicalsupport@easa.com or +1 314 993 2220.

The high no-load current could have causes other than the defects mentioned above. Low speed motors, typically 8 or more poles, draw relatively high no-load current. Check with the motor manufacturer, your own previous repair records, or contact us in technical support to evaluate the high current before taking the motor apart, or the even more costly step, stripping out the windings. Also, compare the applied line voltage to the motor rated voltage. Other possible causes include: axial misalignment of stator and rotor core (should be aligned within 3% of the stator core length); a shorted stator core; incorrect connection or turns.

Table 1 is adapted and expanded from the February 2005 Currents article “No-load Current Basics: Practical Guidelines for Assessment” and provides typical ranges for motor no-load current. The greater the number of poles, the higher the ratio of no-load to full-load current. Winding magnetic flux densities, particularly air gap and back iron, also affect the ratio of no-load to full-load current. The higher the flux density, the higher the no-load current compared to full-load current. Motors of higher power ratings tend to have lower ratios of no-load to full-load current.

Higher than rated line voltage will increase no-load current and voltage below rated will reduce no-load current. As obvious as that sounds, it is something we often overlook when test running a motor, such as a motor rated 200 or 208 volts and being tested at an actual line voltage of 240 volts or above, or a motor rated 380 volts and being tested at an actual line voltage of over 415 volts.

Note: Additional information on no-load current can be found in the following technical articles at easa.com/currents: “A Closer Look At The No-Load Current” in the May 2001 edition of Currents and “Avoiding High No-Load Amps On Rewound Motors” in the February 2004 edition of Currents.

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 current 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, and they note that current unbalance at normal operating speed 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 should be corrected to less than 1% unbalance, or the motor should be derated.

If the motor is the source of the current 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 greater or fewer turns than others, 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 2% of the average for random windings and within 1% of the average for form coil windings. Note: Some concentric windings may exceed the 2% limit.

Unbalanced current can occur if the winding has unequal grouping with more circuits than are permissible for the slot-circuit combination. Another source of unbalance is an incorrect grouping sequence for the jumpers selected (i.e., adjacent-pole versus skip-pole). 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 three circuits in one phase and four circuits in each of the two correctly connected phases. Testing lead to lead with a DLRO or thermal imaging of a reduced voltage open stator test would detect this condition.

Note: Additional information on unbalanced current (and voltage) can be found at easa.com/currents in the technical article titled “Unbalanced Voltages and Electric Motors” in the December 2007 edition of Currents.

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 (see Figure 1), the winding temperature can be estimated. That estimate is that the winding temperature will be approximately 5°C to 10°C (9°F 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 technical article “Understanding Motor Temperature Rise Limits” in the November 2003 edition of Currents at easa.com/currents. If the winding temperature is higher than expected compared to the surface temperature of the frame, the core may be 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 or unbalanced voltages. 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 probably be 10°C (18°F) hotter and will have approximately half of its normal thermal life. Contaminants that build up on the motor or 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 result in extended starting time, increasing 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 the line frequency.

Causes internal to the motor also include contaminants that build up in the motor or block ventilation passages, 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 200 or 208 volt winding being operated on a 240 volt supply system.

AVAILABLE IN SPANISH

Categories: Technical topics, AC motors, Repair procedures & tips

Tags: Motor temperature No-load current

Rate this article:

2.0

Print