Edit: I submitted this before the above reply was written; it answers my follow up question....apparently the IDG has a permanent magnet which is used to power the electromagnet. Frankly I'm also surprised at how little generator operation is emphasized during training, granted I'm not an engineer and in an automated airplane such as the -400 there's really not much application for the knowledge. Thanks.
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Thanks for the explanations so far, very helpful.
So for a generator to have a "field" which can be turned off, it must use an electromagnet instead of a permanent magnet? Obviously, if a permanent magnet is used, the field is always there.
Does anyone know where the power for the electromagnets comes from on a 744/GE CF6 IDG? Does it come from the same power source as that of the EECs (although our manuals don't discuss it I've been told there are separate generators in each engine which power the EEC's independently of the regular aircraft electrical system)? Does it come from the DC electrical system (I recall the "IDG disconnect switches" being located on the "Main Battery Bus")?