Comparing Generators and Alternators                  01/03 R. Kwas


Generator / Alternator Similarities: Both generate alternating current (AC), as any rotating generating device does, but both have internal provisions for rectifying this into the direct current (DC) required.  Both outputs are RPM dependent (an alt's. less so, see below). They weigh about the same so either one will hurt like a sonnova&*$@& if dropped on your foot!

Differences: A generator uses brushes and a internal commutator to rectify output  into DC, an alternator uses internal solid state diodes. Input (field) on gen. is housing (stator), input on alt. is rotating armature. Output on gen. is rotating armature, output on alt. is stator.

Generator advantages: Can be checked in-situ by doing a "Reciprocity Test"...that is...running it as a motor while still installed.  They worked adequately for their era. Used ones in unknown condition are inexpensive (I've got a couple in which I threw in to the corner of the garage when upgrading, which I'll send to the first person who will buy me a case of Warsteiner Bier and pay for shipping!). They make great anchors for small water craft or buoys.

Generator disadvantages: Output is squat at any RPM under 2000 (because pulleys must be selected so that at high engine RPMs, mechanical rectification will still work), and therefore HIGHLY engine RPM dependent. Output (high) current must go though brushes riding on an (abrasive) interrupted commutator, so brush life is shorter, and output current must also run through a relay contact, to disconnect it from battery when engine is not running to prevent it from loading and discharging battery. Replacement gens. or rebuilding are relatively expensive and are most certainly a special order item. There is little or no chance of finding one, even on Interstate80, if you have a failure on the road on a Sunday night.

Alternator advantages: Alt. has output even at idle (pulleys can be sized to allow this because solid state rectification through diodes is not speed limited at typical pulley speeds), and is therefore much less engine RPM dependent. Only (field) input (low) current runs through brushes (which run on a less abrasive continuous slip-ring). Higher efficiency allows higher output for a given case size (up to 100A can be wrung out of a package the size of a Bosch generator!), and diodes which rectify stator output into DC also serve to isolate windings from battery when engine is off.  Early alts. also used relay regulators, but modern all-electronic regulators have no mechanical contacts, are therefore highly reliable, require no adjustments, and are small enough to fit internally, and so can help minimize system complexity even down to "one wire". Alts. are relatively inexpensive, are stocked in various quality levels (with corresponding cost) even at the smallest automotive supplier. You might even be able to scrounge one for $20 from just about any GM parts car behind a gas station in East Squeedunk on a Holiday.

Alt. disadvantages: Cannot be checked in-situ by doing the "Reciprocity Test"...that is, running it as a motor...must be removed for checking.  Not original  for a vintage Volvo, but other than that, I can't honestly and seriously think of any.


Output data for reference:




I was not able to locate a graphical representation*, but here is data from the Factory Volvo Manual:

Bosch Dynamo Type G14V30A 25-027-36

Engine : Dynamo Ratio:  1:1.8

20A Output Warm:  2400RPM

20A Output Cold:  2550RPM


* if the reader has one, or knows location of one please, advise us so that we can include it here.


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