PM vs IE Alternators and What You Need To Know

Caterpillar has recently published a white paper on the subject titled :  CAT® INTERNALLY EXCITED (IE) ALTERNATORS.  

Joining us today from his home in Northern Ireland via the Cat Electric Power Hotline is the paper's author - Roger Rosborough.  Roger has a Masters and PhD in Engineering from Queens University in Belfast N. Ireland and has16 years experience with Caterpillar working in Mechanical Design, the Technology department developing cooling systems and most recently as a Marketing  Industry Consultant.

Today we'll be discussing why, in most cases, Cat's IE alternator should be given equal consideration to most PM alternators.

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Transcript

Lou:  Good Day and welcome everyone to Power Bytes!  I am your host Lou Signorelli  and Power Bytes is your destination Podcast for power generation discussions.  As always please know how much we appreciate you, our listeners.  We hope you find our topics helpful and interesting.  There are several ways for you to get in touch with the show. You can send us an email at powerbytes@cat.com, visit us at Cat Electric Power on Facebook or LinkedIn.  Please leave us a 5-star rating wherever you listen to your favorite podcast; It really does help.

Lou: In a recent episode of Power Bytes we talked about the different kinds of excitation systems and tried to illustrate the basics of what they are and how to think through choices related to selecting the right excitation system for your application.  Today, we are taking things a bit further and going to look more analytically at the performance characteristics of the Permanent Magnet (PM) vs the Internally Excited (IE).  

Lou: Caterpillar has recently published a white paper on the subject titled :  CAT® INTERNALLY EXCITED (IE) ALTERNATORS.  For a copy of this paper simply go to Cat.com and put "excitation" in the search bar.

Joining us today from his home in Northern Ireland via the Cat Electric Power Hotline is the paper's author - Roger Rosborough.  Roger has a Masters and PhD in Engineering from Queens University in Belfast N. Ireland and has16 years experience with Caterpillar working in Mechanical Design, the Technology department developing cooling systems and most recently as a Marketing  Industry Consultant.

Lou: Roger, Thanks for joining us today!

Roger: Thanks for the invitation Lou

Lou:  Roger, for those that did not hear our first in this series,  let's start with the basics.  Can you give us a brief recap on what is an excitation system and the different types customers will come across?
 
Roger:  An excitation system is the group of components that are used to magnetize the rotor in an alternator.  Most modern alternators are brushless, consisting of a voltage regulator, an excitor, a rectifier, and some source of energy to power the excitor.  The systems are typically categorized by the energy source.  There are Self-excited or Shunt systems that receive power from the output of the alternator.  The other two systems are separately excited systems because they are isolated from the alternator outputs.  There are Permanent Magnet systems which receive power from a Permanent Magnet Generator.  The final group is called Internally Excited because they use an Auxiliary Winding which is internal to the alternator stator, but isolated from the output connections.   You can get more detail on all these systems in the White Papers, and in the other Podcast on excitation systems. (May want to use titles of papers and podcast here)
 
Lou:  Thank you for that recap.  In your paper, you focused on a comparison of the Permanent Magnet or PM to the Internally Excited or IE. Why is a comparison of the PM to the IE even important? 
 
Roger: PM excitation systems are often called out by name in specifications from consulting engineers when requesting quotes, this means that anything other than PM would require an exception to be in consideration, that is if it gets considered at all.  An IE system provides a similar performance to PM when dealing with non-linear loads and provides identical short circuit capability.  Both of these systems are isolated from the output of the alternator and therefore should have equal consideration.  The comparison is important as it should raise awareness of IE and its capability. 
 
Lou: Thanks Roger, so why do think consultants are tending to specify PM when IE seems to deliver the same performance?    
 
Roger: It seems that IE systems are not well known or understood.  PM has been the traditional option of choice for some applications based on their performance. It is a separately sourced system, and it is not impacted by disturbances on the output of the alternator.  For this reason it operates well with high non-linear loads like you might find in a Data Center.  They also have high fault capability and high transient response capability which makes them desirable for large systems or large motor applications. 
 
Another reason why PM is specified is due the presence of a permanent magnet generator which ensures that there are no concerns with the alternator losing its magnetism.  With a Self-excited system, the residual magnetism of the system is needed to begin the excitation process.  If a Self-excited system is poorly maintained or is not operated for an extended period of time this residual magnetism can degrade to the point that the alternator isn’t able to operate.  This scenario is uncommon for most customers but is a concern for critical applications.  
 
While the PM is good for all these reasons, the IE system is also a separately sourced system and provides nearly identical performance to the PM.  Furthermore, the Cat SR5 IE alternators have permanent magnets embedded within the exciter field to eliminate any concerns regarding loss of magnetism.
 
Lou: Roger, at a high level, looking at the two types PM or IE, what are the key differences? 
 
Roger: That is a great question.  For two systems that provide such similar performance there are some big differences in how this is achieved.  The PM system uses a permanent magnet generator attached at the end of the alternator.  The permanent magnet is attached to the same shaft as the alternator rotor and spins inside its own stator to generate an AC output provided to the regulator.  The IE system uses a series of auxiliary windings which are in the main stator of the alternator.  As I stated previously, these windings are isolated from the main stator but are generating current from the passing of the rotor just like the main stator windings.  Both systems provide AC power to the voltage regulator separately without impact from the output of the main stator or its connected loads.
 
Lou: When you did your research for the white paper, what did you find were the key advantages of Cat's IE alternator? 
 
Roger: The IE and PM are so similar in their capabilities it is unlikely most would be able to distinguish the two systems.  The biggest difference, and thereby by could be seen as an advantage of the IE, would be in the footprint savings.  In PM system there is a large permanent magnet generator attached on the end of the rotor shaft, this is not the case in IE.  It could also be seen that the IE system has fewer components than conventional PM, which could be argued that fewer components means it is statistically more reliable.
 
Lou: Would you recommend IE in all applications?  Or is it best in particular kinds of situations? 
 
Roger: In applications where a PM is considered, the IE should be also be considered as an acceptable alternative.   Both systems having separately sourced energy for the excitation system means they will provide exceptional performance for nearly all applications.  
 
It should be noted that there are some applications where a self-excited system may be desirable than a PM or IE.  These may be where a customer is trying to have a low fault current, for example peak shaving.  For this reason, customer’s should understand their excitation systems and the performance differences they will achieve. 

It also should be noted that IE systems are not offered with every alternator package.  IE systems are not restricted to but are most commonly available with Random Wound alternators and in low voltage applications.  
 
Lou: So Roger, I've been taking a few notes as we've been talking. Let me see if I understand what your research revealed:
- The PM is certainly the industry standard and most often requested alternator type
- The Internally excited alternator  should be considered as an alternative for two real reasons:
○ It has fewer parts and very similar performance to the PM and as such should intuitively have greater reliability.
○ Cat's internally excited alternator is smaller than PM alternators and  thus makes it a great choice when space is an issue 

- Did I get that right…

Roger:  Yes, I would recommend consulting with a local dealer to discuss excitation options and consider the wording in specifications to allow more options like the IE systems in the future. 

Lou: There you have it folks.  Thank you Roger for taking us through that today and thank you to you, our listeners.  If you would like a copy of the whitepaper we have been reviewing, go to cat.com and type "excitation" into the search bar. I did check this morning and a link to these two papers and many more is the first link in the results.    If you'd like to reach us here at Power Bytes, please write us at powerbytes@cat.com or visit Cat Electric Power on Facebook or Linkedin. Till next time, thanks for listening to Power Bytes and have a great day!