Reply to "AC vs DC"

@Deere Lines posted:

Isn't it ironic that old full size trains started out as DC, but when rebuilt they are converted to AC tractive motors. Why can't the old Pulmor motors be upgraded in a similar fashion. Seems to me that it would be possible, with the right electronics.

The D.C. traction motors found in first and second (and some third) generation diesel electric locomotives were D.C. universal motors very similar in construction to Lionel's universal "Pullmor" motor. Locomotives never used can motors as can motors are just not as efficient as universal motors. The D.C. universal traction motors can be operated using A.C. current, but they are much less efficient. In many ways, Lionel's Pullmor motor could be considered a D.C. universal motor as it operates more efficiently off of D.C. than A.C. 

The A.C. drive of a locomotive displaced D.C. units because the A.C. traction motors are much more efficient than any D.C. motor. The traction motors on the ET44AC or SD70ACe are asynchronous motors (induction motors, a Tesla invention) are 90+% efficient. Compare that to ~75% for comparable universal motors or ~65% for comparable can motor.  The high efficiency of asynchronous motors means more mechanical motion performed with less heat generation which is one reason why they displaced D.C. traction motors relatively quickly. 

@prrjim posted:

The key is the conversion from DC to AC traction motors.    They replace the traction motors.

The prime mover rotates a generator that produces A.C. current. The A.C. current is rectified to D.C. As you said that is a key step. The D.C. current is then rectified back to A.C. This is the next key step. The purpose of this back-and-forth from A.C. to D.C. and back again to A.C. is that it is easier to vary the frequency of the A.C. feed to the motors. One can use, say, 60 cycle A.C. and convert it to something else such as 12 cycle AC. However, changing one frequency of AC to another frequency is more difficult than taking D.C. and converting it some frequency of A.C. 

The rotational speed of the asynchronous motor is controlled by varying the frequency of A.C. feed to the motor.  The D.C. is converted to variable frequency A.C. (VFD) which is fed to the asynchronous traction motors that drive the wheels. The video is a simple description. 

A hypothetical advantage for a heavy haul locomotive is that with a VFD is that full power can be given to a motor and have a low frequency A.C. fed to the motor so it turns very slowly. This allows full power to start a train. As the large mass begins to move the frequency can be increased which will increase the rotational rate of the motors. 

VFD's are very common in the RC hobby. They commonly use what are commonly called brushless D.C. motors which are in fact AC motors. The reason that they are called brushless DC motors is that DC from the batteries is used to feed the VFD. Using the same extension of thought the induction motors on a locomotive could be considered brushless DC motors even though an induction motor will never operate off of DC being fed directly into the motor. VFD's are certainly applicable to the model railroad hobby. Märklin created a motor and VFD for their trains. 

Lionel attempted a VFD product in their Odyssey motor, but ultimately failed and the product never made it to market. 

Last edited by WBC
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