Even in the winter months a motor can overheat. Especially if snow or ice covers the air vents and prevents air from circulating on open drip proof designs. Motors running at design temperature should have an average insulation life expectancy of 20,000 running hours. Running motors hotter than design will reduce insulation life and running motors cooler than design will increase insulation life.

Not all motors have the luxury of being installed into a nice cool winter environment, so a review of the design temperature is important. Most motors have an ambient temperature rating of 40°C and an insulation class (A,B,F or H) listed on the motor nameplate. If your motor nameplate says insulation class B you can open your Electrical Engineering Pocket Handbook by EASA and see the temperature rating for Class B insulation is 130°C. So how does 130°C insulation rating and 40°C ambient temperature rating relate? Well, this means that if you start your motor with an ambient (outside air) temperature of 40°C, the internal insulation temperature should not exceed the insulation rating of 130°C. In a T-Frame Totally Enclosed Fan Cooled motor with Class B insulation an 80°C temperature rise is expected. Temperature Rise is the differential temperature from the ambient temperature and the internal winding temperature. So at full load and 40°C ambient with an 80°C rise, the surface temperature could reach 120°C (40°C+80°C). The 10°C difference (120°C vs. 130°C) is for hot spot allowance that may occur deep in the stator winding slots.

Even in winter months don’t be too quick to test the surface temperature of your motor with a bare hand. Surface temperature depends on many cooling design factors, but for a standard T-Frame motor, surface temperatures may reach as high as 95°C (203°F) and still be considered normal. Ouch! So don’t let snow and ice cover air vents and do be careful what you touch.

To see what happens to overheated insulation visit the PdMA YouTube Channel at: https://www.youtube.com/watch?v=Vt0cO1jHnKA 

Source: PdMA

For many people the word “swirl” creates visions of whirlpools, storm winds, or even ice cream cones. But if you have been in the reliability industry, and specifically the electrical reliability industry, the word swirl means something totally different. The late G.B. Kliman is credited for much of the research documentation surrounding the swirl effect sometimes referred to as the 5th harmonic analysis of AC induction motor rotors. This research showed that the phase shift in the air-gap flux density surrounding a broken bar will result in pole-pass frequency sidebands at the 5th harmonic of line frequency. And of equal importance, it showed that these sidebands are less affected by other non-rotor bar related anomalies unlike the case with sidebands around line frequency. As an example, asymmetries like bearing eccentricity or an out of round rotor can mask or give a false indication of a rotor bar issue when looking at the sidebands around line frequency, but again, have little effect on the sideband clarity at the 5th harmonic of line frequency.

To see a case study discussing the swirl effect visit the PdMA YouTube Channel at:

Depending on the environment and criticality of your electric motor, space heaters should be strongly considered if you want to ensure maximum reliability and expected motor life. When a motor is secured (de-energized) there is an initial increase in temperature due to the loss of cooling air. However, very quickly after shutdown the temperature of the motor windings will start to drop. Once the temperature falls below the dew point, air moisture will condensate on the windings and machined surfaces creating a conductive path for current to ground. This drop in resistance to ground due to the surface moisture contamination can cause a low state of reliability for the next startup and increase the chance of a failure. When selecting a space heater, a simple calculation can give you a ball park estimate for power requirements to remain above the dew point.

H = DL/35

Where:

H = Heater size in kW

D = Diameter in Feet

L = Length in Feet (between end bell centers)

To see the results of excessive moisture on a 2000HP electric motor visit the PdMA YouTube Channel at:

https://www.youtube.com/watch?v=9SG3lNm8u8w

The most frequent concern about high current with a three-phase motor is high no-load current. But the broad issue of high no-load current isn’t the only three-phase motor issue to which plants should pay heed: High current with load and lower-than-expected no-load current are potential areas of concern, too. Let’s explore the sources of all of these.

High no-load current: Motor not rewound

One situation in which higher-than-expected no-load current can occur is with reconditioned motors. Although some motors with no-load currents above or below the guidelines may still be satisfactory, motors with no-load current outside of these ranges warrant further analysis.

When no-load current is high or low, consider the actual test operating voltage versus the motor’s rated voltage. If the applied voltage is not within 10% of the motor’s rated voltage, then 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 also can cause unusually high or low no-load current. For example, consider a 12-lead single voltage motor intended to be connected parallel-delta (Figure 1) for 460 volts. Because most 12-lead motors are dual-voltage, the motor could be mistakenly connected series-delta (Figure 2) for operation at 460 volts, which would result in exceptionally low no-load current. The solution is to connect the motor for parallel delta.

Click to read the full article and see the tables: https://www.plantservices.com/articles/2018/md-whats-causing-your-high-motor-current/

Continuing with the January fresh start for a new year of commitments we will focus on motor efficiency best practices. The following are some considerations for best practice in optimizing your motor efficiency and overall power consumption:

• Size your motor correctly.  Your motor operates much more efficiently when it is running above 60% of rated horsepower. The efficiency drops significantly when running below 50%.
• Reliability before Efficiency.  No matter how efficient your motor is, if it is running in a reduced reliability state you risk a short life expectancy, which will eliminate any savings. Establish a predictive maintenance approach to test your electric motors on a routine basis.
• Establish a Replacement Strategy.

○ Immediate Replacement – If a motor offers a quick return through energy savings it should be considered for immediate replacement. Examples include older inefficient motors that run continuously. Especially oversized motors.

○ After Failure Replacement – Motors offering only a moderate payback period can be replaced with a high efficiency motor at the time of failure.

○ No Replacement Necessary – Motors already exhibiting acceptable efficiency with an extended payback period should not be included in the replacement plan. Especially motors that run only intermittently.

System Efficiency vs. Motor Efficiency.  When evaluating electric motors for improved efficiency always look at the total system efficiency including the driven load for the best overall cost effective solution.

From our friends at pdma