This section contains the following topics:
1) What is the Problem
2) If it is a Problem, why do DCC system manufactures support it?
3) What does the DC motor do with this form of AC?
4) How does a DC locomotive move on track powered by DCC AC?
1) What is the Problem?
Running a DC locomtoive on DCC runs the risk of damaging the motor inside the the DC locomotive. The risk to a DC motor can best be understood by two facts:
1) in the fact a DC motor is designed for and expecting to see as a DC voltage to run from. DCC's "AC like" track voltage does NOT resembled DC in any way. To understand the technical differences between DC and DCC voltages, go here: DC versus DCC
2) The motor will generate a lot of heat without moving. Current is flowing in the motor but there is nothing being converted to mechanical energy to move the engine. It is all being converted to pure heat. Essentially the motor temperature will rise very quickly and not have the benefit of any potential air circulation that is created when the motor spins. Some motor cannot deal with that level of heat and can become thermally damaged with the worst being a burned out motor. This is especially true of core-less motor which are special high performance DC motors that can only run on pure DC.
See section 4 below for more detailed information.
2) If it is a Problem, why do DCC system manufactures support it?
It is true that many, but not all, DCC System manufactures support this feature. However, this does not invalidate the argument about the risk to the motor. So why do they do it?
A) If you read the warranty, DCC manufacture say they are NOT responsible for any damage cause to the rolling stock when using this system. In other words, if the locomotive motor burns up, do not expect the DCC manufacture to take responsibility.
B) To understand why DCC manufactures do this is to understand DCC history. The original purpose of supporting an Analog (DC) locomotive on a given DCC system was motivated by the market conditions when DCC systems were first introduced. Early DCC adopters did NOT want to lose the capability of running their huge roster of DC locomotives when converting to DCC.
4) What does the DC motor do with this form of AC?
1) Mechanically nothing really is happening for the motor does not move forward or backwards. It is easy to see the confusion on the motor's behalf by noticing the POLARITY of the DCC AC voltage is constantly changing. The raw DCC signal on the motor terminals is telling the DC locomotive to go FORWARD and then fraction of a second later telling the same motor to go into REVERSE and then another fraction of a second later to go FORWARD again and so on forever! That repetition of voltage polarity flipping is exactly like what AC does. The rate at which the polarity is flipping is called a frequency in units of Hertz or "Hz" for short. The term should not be to unfamiliar with you. For example, the typical 120V AC wall outlet frequency goes by the term "60Hertz or 60Hz" or 50Hz in other countries. DCC frequencies are much higher.
2) Audibly the motor is attempting to vibrate at DCC's AC frequency which averages about 7100Hz or 7.1KHz. This is a pretty high frequency but still an audible frequency such that you can hear the motor vibrate just like a speaker via all the mechanical linkages it is connected to. Hence this is the buzzing and/or whine you hear when the engine just sits there.
3) Electrically a lot is happening and this is where the danger shows up. Remember that DCC track voltage is at "FULL" at all time. Since the motor is not moving, the current flowing in the motor can build up to be the same as the STALL CURRENT level of the motor and stay at that point. Motors are NOT designed to run in this condition continuously. At worse, the motor will overheat and burn up.
For more information on motor/decoder current ratings, go here: DCC Decoder Ratings