Electrical and Mechanical Defects in an electrically driven machine have a direct impact on energy consumption. This can be from sub-standard maintenance practices, such as misalignment or belt tension, or degrading components such as bearings, belts, impellors, gears, and other parts. In this article we will discuss a simple motor/pump combination on a Variable Frequency Drive (VFD) application which has bearing defects as well as impellor defects. There are additional issues within the machine as well as specific losses associated with the VFD that are captured, which we will cover in another post.
The ability to calculate energy losses associated with defects, with energy costs having a direct impact on the bottom line and energy costs normally being part of the operating costs observed at the C-level, is important. Greenhouse gas emissions, particularly CO2, is also viewed at the highest levels within most organizations, and can be directly calculated in relation to defect losses.
In this case, here is the machine information (click on images to expand):
The above information includes the motor nameplate and number of blades associated with the split case pump. The bearing multipliers are built into the EMPATH system as well as the rotor bars and stator slots from the database of over 8400 motors. Data was collected from the output of the VFD with the following basic information:
Additionally, as both voltage and current data was collected, we were able to obtain torsional information. Using the amplitude and frequency, the kW for each defect can be calculated as the (RPM*Torque)/5252, where Torque is in ftlbs. The value would be horsepower which is multiplied by 0.746kW/hp to obtain the kilo-Watts at that specific frequency for that associated defect.
The EMPATH, however, carries the ability to look at Power (kW) FFT analysis. This provides the ability to look at the relationship in dB between full power to related defects in kW (dB). One of the differences between Voltage and Current FFTs and the Power FFT is that it is not amplitude modulated and, therefore, does not have sidebands. In the linear spectrum, the actual kW for each frequency can be evaluated and the peak relates to the kW losses associated with the defect.
In the above low frequency spectrum, bearing ball, cage and outer race peaks are visible as well as a blade frequency. This can be viewed in the power spectrum as shown here:
The losses from the above Linear Power Spectra are:
- Bearing Cage = 0.25kW
- Bearing Ball = 0.58kW
- Bearing Outer Race = 0.44kW
- Impellor blade = 1.32kW
The total for bearing losses is 1.27kW
In the high frequency spectra, the impellor will be reviewed. This would represent an impulse associated with blade defects.
And the associated power spectra peak would be:
With the first impellor blade defect showing as 1.32kW and the impulse peak being 2.9kW, they are added together to show 4.22kW.
The total Power used by the bearing and impellor defects are 5.49kW at this load. The total motor power output is 114.47kW, which mean the defects consume about 4.8% of the total power consumption of the motor.
The energy impacts are easily calculated from here. Assuming the motor operates 8,760 hours per year with a demand rate of $10/kW and a usage of $0.10/kWh, then the energy costs associated with the defects are:
Demand = 5.49kW*$10/kW*12months = $659 per year
Usage = 5.49kW*8760hrs*$0.10 = $4810
Total additional energy costs are ~$5,469
We know, in this application, that this is the minimum load and speed, but also that the equipment will be off for periods. In the end, it will balance out.
At the same time, we also know that the defects are feeding on themselves. The loss energy is going into the defects and not useful energy. The result is that this energy is accelerating the defects and will continue to grow worse over time.
In addition to the energy costs associated with the defects, the greenhouse gas emissions can be easily calculated. The USA average is 0.909 Tons CO2/MWh. This can vary by region. For instance, in the Midwestern USA, the average is 0.8 Tons CO2/MWh. For the location of the specific pump, per the US EPA website (www.epa.gov), the loss is 0.909 (average).
Tons CO2 for defects = (5.49kW*8760hours/(1000kWh/MWh))*0.909 = 43.7 Tons of CO2.
The total CO2 for the operation of the equipment is = (114.47kW*8760hours/(1000kWh/MWh))*0.909 = 911.5 Tons of CO2
The correction of the bearing and impellor defects result in an improvement of 5.8%. Other defects contained within the application, including possible changes in the type of output filters will have additional impacts and improvements.
The Power analysis capability and this solution are unique to the EMPATH and EmpathCMS systems. For more information contact MotorDoc LLC directly at info@motordoc.com.