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Table of Contents
 

Table of Figures
 

Abstract

A change in our client's chemical process has shortened the life expectancies of two motors used in identical applications to 10% of their expected life span. The motors in question are located in very harsh environments, (inside chemical reactors with high temperatures and pressures). We know that the failures are caused by over heating of the motors. After evaluating the system in question and the natural of failures, we surmise that poor power quality is not the main factor in motor failures, but may contribute to it. It is suspected that because of the large dV/dt the life span of the insulation is sacrificed. Different methods of analysis were utilized to make educated assumptions and conclusions of the system as built.
 

Problem Statement

An induction motor is used inside of a pressure vessel to agitate various concentrations of ethylene, vinyl acetate, n-butyl acrylate, and methyl acrylate gas. Various types of catalysts are utilized to bond these gases together to form polyethylene (a common type of plastic). The motor experiences pressures of 30,000 psi, temperatures in excess of 250 degrees Fahrenheit for periods of 24 hours, and 185 degrees Fahrenheit during continuous operation. The stator is known to be magnetically saturated beyond the original design.

Originally, the motor was a 75 HP, 208 volt, 8-pole motor. When our client decided to implement a new type of stirring technology inside this chemical reactor, it was determined that more HP and speed would be needed. The motor was "redesigned" and rewound as a 6-pole machine, fed at 480 volts and re-rated at 100 HP, based upon a dynamometer test. A Hall effect horsepower (HP) calculator is used to continually monitor loading levels. The typical operating range is 120-155 HP. Data logs have shown HP loading levels to be at 150+ HP for periods over 1 week at a time. The loading levels are known to be the root cause for the premature motor failures, but the potential increase in profit is driving our client's and our efforts.

The only source of cooling for the motor is the feed gas to the reactor. This enters the top of the reactor, as shown below in figure 1.
 

 

Figure 1: Chemical Reactor Layout
 The reactor temperature profile is approximately 490 degrees Fahrenheit in the bottom of the reactor, to 325 degrees Fahrenheit in the top of the reactor (just below the motor). The feed gas comes into the reactor at 105 degrees Fahrenheit, and proceeds through the motor as a cooling agent. The gas flow rate is approximately 4000 pounds per hour. Temperature in the motor is know to vary from 155 degrees Fahrenheit at the top of the stator, to about 270 degrees Fahrenheit at the bottom of the stator, where the failures are occurring.

There is very limited space to change the physical dimensions of the motor. This demonstrates the need to remove all unwanted electrical heat from the motor.

This motor is fed (at 480 volts) from a pulse-width modulated (PWM) drive. The drive is mounted 300 feet from the motor, and physically can not be located next to the motor. The motors are in a restricted area (in a concrete blast bunker) because of the hazards associated with the chemical process. As a result, taking measurements and gathering data by conventional means is not possible.

The physical systems are shown in figure 2.

Figure 2: 100 HP Power Supply
  

Technical Solutions

A common method for dealing with high levels of harmonic distortion is to simply de-rate a transformer, or a motor. Within this application, de-rating of the motor is not an option since the desired load is fixed. The following methods have been utilized to analyzed gathered data.

Symmetrical components were used to study harmonic frequencies that exist in the system. Calculating the magnitudes of the positive, negative and zero sequence components while applying the principles of superposition allowed us to determine the effect on the motor performance of each harmonic frequency. The following procedure was used to calculate the symmetrical component spectrum, and symmetrical component diagram:

Step seven yields 50 symmetrical component voltage diagrams that look similar to the 41st harmonic frequency (2460 Hz) as shown Figure 3. This particular frequency was of interest to us because of its magnitude (~29%) compared to the fundamental frequency (60 Hz), and because it is predominately made up of a "negative" rotation sequence (in reference to the desired direction of rotation). Please note, that although the figure shows a larger positive sequence voltage than negative sequence voltage, the desired direction of rotation for this motor is ACB, which is evident if the symmetrical component voltage diagram for the fundamental frequency were examined. An additional output of step seven (the symmetrical component program) was a symmetrical component voltage spectrum. This spectrum allowed us to quickly analyze the most severe harmonic frequencies, and compare them with the other harmonic quantities found in the system.


Figure 4. - Magnitude of "Negative" Symmetrical Component for Each Harmonic Frequency

  After analyzing the symmetrical components of the current, it became apparent that pulse width modulation (PWM) control is not a major problem. In order to have heating from harmonics, harmonic currents must flow. In our system, there are very low levels (< 7% THD) of harmonic currents. However, a concern with PWM control is that the high dV/dt will create standing waves in the cable between the motor and the drive. Some users of PWM drives have seen voltages as high as 2600 volts (in a 480-volt system) at the load terminals. This can be attributed to the high dV/dt and mutual capacitance of the cable configuration.

These observations drove us to conclude that the effects of voltage distortion provided a minimal amount of heating. As a result, the power quality team shifted its focus onto our secondary goal, improving the performance of the motor. This required the derivation of an equivalent motor circuit to establish some baseline performance characteristics. The client provided the team members with load test data that enabled an equivalent circuit to be calculated.


 

Motor Model

 Calculated values are being reanalyzed due to some speculations with the results.  If you would like to know the values please email us at: pwrquality@iastate.edu.
 

Figure 5. – Motor Impedance Model
 In order to determine impedance characteristics for the motor shown in the schematic in Figure 2, results of dynamometer testing on this motor were used, and the values determined are as shown in Figure 5. Unfortunately, to accurately determine some of these values, no-load testing results are required and that information was not acquired. The values shown were determined with test values when the motor was running as close as possible to no-load conditions (with a rated speed of 1200 rpm the motor was running at 1199 rpm). As a result of this, some of the values are not as accurate as possible, for instance, it is seen that the per phase stator winding leakage reactance (X1) has a lower value that would be seen with true no-load test results. In addition, the values for the rotor and load impedance could be calculated through the use of a locked-rotor test. These impedances could not be estimate from given test data.

 It is apparent that in order to accurately determine the impedance values of this motor, more testing must be done. Approximate values could be used to provide a reasonable estimate of the needed quantities. If the client does not consider it feasible to obtain test results needed for this type of analysis, then estimates will have to be used.

Cable Model
 
 

 
Figure 6. – Cable Impedance Model

  In order to properly simulate the performance characteristics of this circuit, it was determined that the impedance characteristics of the conductor cable directly before the motor and right after the variable speed drive must be obtained. This is a round, one conductor cable of the #2/0 Aluminum type. Their configuration is as follows: the cables lay in three inch conduit trays, two conductors per phase, and take on characteristics of underground cable. The impedance values and model schematic are as are as shown in Figure 6. This model represents the impedance values of the two parallel cables per phase consolidated into one model through parallel combinations of reactance. Strictly according to length this cable could be modeled as a short transmission line, thereby eliminating the shunt capacitance. However, due to the fact that these are underground cable and therefore lie within close proximity to each other, the admittance for the shunt capacitance is included.

After reviewing the various circuit elements, the following statement can be made. The rotor resistance only affects when peak torque is available to the load. Since the client can not provide the team with an accurate load torque curve, we can not make any recommendations as far as changing the rotor characteristics.

One of the main elements limiting a motor’s ability to perform at higher HP levels, is core saturation. Flux is proportional to HP, while flux depends on magnetomotive force (mmf). Shown below in Figure 7, is two BH (or hysteresis loops) curves for a different core materials.

Figure 7 – BH Curves

  The load determines the amount of flux needed, and the motor will attempt to provide this flux by increasing the mmf (H), which is directly proportional to current. Different core materials require different levels of H to achieve the desired flux (B) levels. Figure 7 shows an exaggerated difference between two materials. The yellow curve represents economical non-grain orientated silicon steel (such as M19 presently used by our client). It is important to note that the core losses are calculated from the area enclosed the BH loop, and expressed as Joules per cycle. The green curve is more expensive grain-orientated silicon steel (such as M5 or M6). While most companies push for the grain-orientated steels only to make motors and transformers more efficient, there is another benefit. Using materials with a BH curve like the green curve shown in Figure 7 allows a motor to develop and more easily define the magnetic fields in the core. An improved material would then allow for a smaller driving force (less mmf, which relates to less current) to obtain similar HP loads. Lowering the mmf would not only reduce core losses, but also the I2R (Cu) losses as well.

When a motor is operated in an over saturated state, the relationship between the mmf and the flux is no longer linear. Large changes in current result in relatively small increases in flux. These large changes in currents also contribute to an increased rate of change in core losses. Our client can monitor HP input levels, and this phenomenon is known to exist. By simply changing the core material type, the region of operation will drop back to a more linear region of the BH curve, and thus lower losses considerably. This will reduce the electrically generated heat in the core, and make modest improvements in the HP output.

More investigations will continue in this area, as outlined in the future work section of this report.

Deliverables

Our team will provide our client with the following:
 

Proposed Budget

A budget breakdown follows:

Table 1: Proposed Budget
 
Predicted
Actual
Travel: One trip to Clinton, Iowa …………………………
$120.00
$120.00
Poster Supplies: ……………………………………..……
$ 50.00
$65.00
Equipment: All metering equipment supplied by Equistar 
$ 0.00
$0.00
Telephone and postage: …………………………………..
$ 20.00
$31.25
Materials for laboratory experiments: ……………..…….. 
  • Core laminations 
  • Insulators 
  • Clamps 
  • Wires
$ 0.00 
 
$0.00
Total Expenses: ………………………………………….
$190.00
$216.25
$151.25 of total expenses was available from outside sources, the project team paid the remaining.
 

Human Effort Expended

Referring to the Gantt chart, it is seen that in the beginning the average number of hours each team member was to contribute to the project exceeded nine (9) hours per week. This expected number of hours that were to be committed to the project was an optimistic estimate given that all members are taking other senior level courses. In comparing the predicted to the actual it will be seen that our team nearly met our goal of contributing the estimated number of hours that were to be spend working on this project, 7 hours per week per member compared to 9.

There were some exceptions in which, due to other circumstances, our team was unable to meet the week’s estimated obligated hours. For example the week of March 10 was the week directly before the beginning of spring break and other classes were requiring more time and effort with exams and heavier homework loads. This diminished our involvement on the project during this week. From the Gantt chart it will also be seen that there are no hours listed for trips. Initially our team planned on visiting the manufacturer that has been rebuilding our client's motors. Unfortunately this was not possible due to time constants. However, our team will be meeting with our client on Wednesday May 6, 1998 to discuss our findings and recommendations.

Schedule Spring 1998 Gantt Chart Based on a Four person Team

Table 2: proposed schedule Spring 1998
Predicted 
Actual
 
27-Jan
3-Feb
10-Feb
17-Feb
24-Feb
3-Mar
10-Mar
17-Mar
24-Mar
31-Mar
7-Apr
14-Apr
21-Apr
28-Apr
8-May
Research
6
6
4
6
6
4
6
6
4
6
6
4
6
6
  
 
4
5
3
5
5
4
5
6
4
5
5
4
4
N/A
  
Project Plan
8
12
Due
                        
 
8
12
                          
Progress Report    
2
2
2
2
2
2
2
2
2
2
2
2
2
     
1
0.5
2
2
2
1
1
0
2
2
2
N/A
N/A
Design Review                              
                               
Formal Report          
2
6
      
12
12
12
Due 
  
           
2
6
      
12
10
14
N/A
  
Final Presentation                      
4
8
Due
  
                       
3
7.5
6
  
Class Presentation            
4
8
Due
            
             
4
8
2
            
Poster              
4
6
6
5
Due
      
               
3
5.5
10
Due
        
Web Page Maintenance
13
10
      
10
      
10
    
10
    
 
14
10
      
6
      
6
    
6
    
Reviews & Meetings
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
 
8
8
8
8
8
8
8
8
8
 8
8 
8
8
N/A
N/A
Task 1 Problem understanding
12
12
12
                        
 
10
11
10
                        
Task 2 Develop Models   
2
7
7
7
                    
   
2
5 
7
6.5
                    
Task 3 Test and verify each model      
6
8
6
                  
       
6
7
5
                  
Task 4 Lab experiments using models        
4
4
12
                
         
3
3
0
                
Task 5 Analyze Results              
10
 10
            
               
8
8
            
Task 6 Trips                
6
8
40
        
                 
0
0
0
      
12
Task 7 Develop future objectives                          
8
14
                       
5
5
6
4
                               
Total team hours per week:
47
50
33
29
35
36
38
38
36
40
73
30
46
24
24
 
44
48
27
26.5
31.5
30
25
34
28.5
29
27
32
46.5
12
16
Average hours per member per week
11.75
12.5
8.25
7.25
8.75
9
9.5
9.5
9
10
18.25
7.5
11.5
6
6
 
11
12
6.75
6.625
7.875
7.5
6.25
8.5
7.125
7.25
6.75
8
11.625
3
4

Future Work

Our future work will be split up into two main activities. First more modeling and analysis work on the pulse width modulated drive system will be done and then new work on a similar system that includes a six-step inverted drive to operate a larger motor.
 

Current System

The team is creating a Matlab program to calculate the harmonic impedance. This program would calculate the ground impedance seen by each motor harmonic. This would tell us if certain harmonics could pass through the insulation with noticeably lower resistance. These harmonics would be adding significantly to motor heating.

We are also attempting to quantify heating in terms of Joules per harmonic or Joules per a sum of harmonics. This is a very aggressive goal, involving a lot more work to calculate realistic numbers. The results would be very powerful information.

We will also do more work with properties of magnetic materials. Such work could possibly allow us to increase the HP rating of the motor without changing the physical size of the motor. Different materials will also have different levels of core losses, and saturation points. Continued work in this area will conducted along side the Material Science department and the Ames Laboratory at Iowa State University. With luck, we might even find a material that would increase motor life dramatically.
 

Six-Step Inverter System

The six-step inverter drive supplies a motor, which operates in a similar physical and chemical environment (known as LD 3 by the client). The motor is larger and is not experiencing the number of failure as the current system. From a power quality stand point, this system has a great potential for improvement. With the six-step inverter high harmonic distortion is experienced on both the voltage and current waveforms. This means that adding filters to the system might be economically justifiable. Again a very important task would be to quantify heating in terms of Joules per harmonic. This information requires extensive background calculations.
 

Circuit Element Modeling

In order to fully utilize the impedance models that were developed as mentioned earlier, the impact of these circuit elements on the occurrence of harmonics must be examined. Therefore, the impedance values of those circuit element models will be converted to the frequency domain using Fourier transform methods. In this way the circuit can be used to determine possible sources of harmonics and the effects they have on the circuit as a whole, and the principle of superposition can be used to determine the effects that each individual harmonic has on the system.

Conclusion

Over the course of the semester, our team has determined that the pulse width modulated (PWM) voltage control does not affect the performance of the motor or add extra heating. It was also found that by altering the magnetic domain within the core material, the BH curves would increase slope thus decreasing core losses in the motor. This brought us to the conclusion that new core materials would allow the motor to operate in the linear portion of the BH curves. This results in a more efficient motor while improving the maximum HP output.
 
 

In summary, the team gained knowledge in many areas. These areas include:
 
 

In conclusion, this problem is anything but a text book problem. At many points during the semester we found ourselves asking questions that no one could answer. These questions resulted in meetings with researchers working in the material science department. The benefit of these meetings was the knowledge gained from their work and experiences. In the future the team plans to work with the material science department and conduct specific core material tests for this project.


 

Team Members
 
*Graduated*
Chris Angland EE462 
1104 Pinon Dr. Apt. #1 
Ames, IA 50014 
Email: klub@iastate.edu 
Phone: (515) 292-0938		 *Graduated*
Darin Massner EE 462 
209 Apple Ave 
Ames, IA 50010 
email: dmassner@iastate.edu 
phone: (515) 233-6909 
 Matt Eibes EE461 
422 Stonehaven Dr. Apt. #18 
Ames, IA 50010 
email: meibes@iastate.edu 
phone: (515) 233-3064		 Andy Kelly EE 461 
221 Stanton 
Ames, IA 50014 
email: awkelly@iastate.edu 
phone: (515) 268-0295

Faculty Advisors
 
Prof. John Lamont 
122 Coover Hall 
Iowa State University 
Ames, IA 50014 
email: lamont@ee.iastate.edu 
phone: (515) 294-3600		 Prof. Glenn Hillesland 
111A Coover Hall 
Iowa State University 
Ames, IA 50014 
email: hilles@ee.iastate.edu 
phone: (515) 294-7678

Client Information
 

Clinton Plant 
Highway 30 West and Anamosa Road 
P.O. Box 2919 
Clinton, IA 52733-2919 
Phone: (319) 244-2571 
http://www.equistarchem.com