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11:49 pm
May 28, 2012
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Taming Those Misbehaving Motors

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It takes a lot to stump most maintenance pros. The points in this article, though, may help you become an even better troubleshooter.

Given the number of motors in most plants, it’s not surprising that they sometimes misbehave. While maintenance professionals are typically well equipped to tame the unruly motors that come their way, they’re occasionally puzzled by the following three behaviors:

  1. The motor is drawing high no-load current.
  2. The current of the three line leads isn’t balanced.
  3. The motor is running hot.

Let’s troubleshoot
When any of the above situations arise, it’s helpful to know how to assess them, as well as what their likely causes are and how to prevent future occurrences.

1. The motor is drawing high no-load current…
A good way to determine if a motor is running properly at no load is to check the current draw with an ammeter. Low-speed motors, typically with 8 or more poles (900 rpm and slower), draw relatively high no-load current. If possible, compare the suspected high no-load current reading with the motor manufacturer’s data and previous service-center repair records. The applied line voltage should also be compared with the motor’s rated voltage. Table I shows typical ranges for motor no-load current.

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

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If the motor has been rewound recently, the situation may be different. Magnetic-flux values that are too high or core loss that is excessive will often result in higher than normal no-load current. Good service-center repair practices can prevent the high-current issue by checking the magnetic-flux densities and correcting the winding data before the motor is rewound, rather than after it is fully assembled. A software application like EASA’s “AC Motor Verification and Redesign Program” can be used by a service center to quickly check the magnetic densities and “flag” values outside typical acceptance ranges.

The condition of the laminated stator core should be checked, and repaired or replaced if necessary, before a motor is rewound. Testing can be done with a commercial core tester or with a “loop test.” Although “lower-tech” than a core tester, the loop test can readily pinpoint poorly insulated or damaged areas of the stator core by causing to heat up when a load is applied.

Good repair practices like these minimize downtime while conserving copper and other valuable materials that would be needed to repair or replace a defective winding or core that is discovered after the motor has been reassembled.

2. The current of the three line leads isn’t 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. Next, 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 II.

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If the supply is the source of the unbalance, the supply voltages need to be better balanced. NEMA Std. MG 1 prescribes a 1% limit for voltage unbalance, noting 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 must be 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 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 5% of the average.

If the air gap—the space between the stator and the rotor—is eccentric, unbalanced currents can occur. In that case, the “high leg” will stay with the motor. Another possibility is an open connection that leaves out a circuit in a multiple circuit winding. An example is a 4-parallel 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 would detect this condition.

3. The motor is running hot…
There’s no way to determine if a motor is running hot just by touching the frame—and doing so is dangerous. Maximum temperature ratings are based on insulation class and apply to the winding temperature at the hottest spot inside the motor. As a general rule, the frame can be 20 to 40 C degrees cooler (less or more), depending on the design and the enclosure. Still, with some modern insulation systems, the surface temperature of the motor could be hot enough to burn your fingers or hand. (CAUTION: Never use a body part to check the temperature of a motor. Use a temperature-detection device.)

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Fig. 1. Using a thermocouple on the stator core in a terminal box to estimate winding temperature.

To illustrate how hot a frame could be, assume a high-efficiency design motor with Class F insulation has a 40 C ambient temperature and operates with a Class B temperature rise—i.e., a winding temperature rise of 90 C degrees. At full load, that means the total winding temperature would be about 130 C (90 C + 40 C), well below the 155 C design limit for Class F insulation. Assuming the frame temperature is 40 C lower than the winding temperature (which is rather liberal), the surface temperature of the frame would be about 90 C, or 194 F.  No one should touch a frame that hot.

A safer and more effective way to determine if a motor is running hot is to estimate the temperature of the winding. The estimated winding temperature will be about 5 to 10 C degrees (9 to 18 F degrees) hotter than the temperature at the outside of the axial center of the stator core. To estimate the winding temperature in critical applications, a thermocouple can be installed on the stator core in the terminal box (refer to Fig. 1). A temperature-detection device like the one shown in Fig. 2 can also be used to safely measure the temperature at the same location or to check other parts of the motor. If the winding temperature is higher than expected compared with the frame’s surface temperature, it’s possible that the stator core is loose in the frame—which would inhibit heat transfer.

Other causes of excessive heat in the winding can be 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 degrees (18 F degrees) hotter than normal, the winding under the same conditions will be 10 C degrees (18 F degrees) hotter and have approximately half of its normal thermal life. Contaminants that build up on the motor—or that block the 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’s power rating.

A pump or fan with a discharge valve or damper open too wide can increase load, as can 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 both the rotor and the stator.

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Fig. 2. Using a digital infrared thermometer to check motor-bearing temperature

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 the  I2R (current squared times resistance) copper losses of the winding. 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 of excessive winding temperature internal to the motor include contaminants that build up in the motor or block ventilation passages, missing or damaged air deflectors or a winding with incorrect data. Examples of incorrect winding data include a misconnected winding (e.g., a winding connected delta instead of wye); a winding with “dropped” turns (reducing turns increases magnetic-flux density and core losses); or incorrect voltage (e.g., a 208-volt winding being operated on a 240-volt supply system). A damaged stator core can greatly increase core losses and cause excessive heating and high current even at no-load (refer to common problem #1). MT

Thomas H. Bishop, P.E. is a Senior Technical Support Specialist at the Electrical Apparatus Service Association (EASA), based in St. Louis, MO. EASA is an international trade association of more than 1900 firms in 58 countries that sell and service electrical, electronic and mechanical apparatus. Telephone: (314) 993-2220; email: easainfo@easa.com.

For more info, enter 02 at www.MT-freeinfo.com

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