Electric motor question

[da_kraut]

Hello everybody,

while working on wiring up some make up air units at a local hospital I noticed the one motor was 100 hp.  This motor is a three phase 600 volt unit.  Physically it is close to two feet in diameter.  Now here is my question.  The SD90MAC and AC6000 both theoretically have 1000 hp per axle.  How do you put such a huge motor in such a small space?  Even if the drive motor per axle is only five feet in diameter would the trucks be able to have enough space to mount such a monster?

Thank you for your replies

Frank 

[Paul Milenkovic]

You spin it fast, give it a lot of current, and you blow a lot of air through it.

A certain amount of copper windings and a certain amount of iron will produce a given amount of torque.  If you push more current through that same amount of copper, the torque will increase at the expense of heating the wires and the rest of the motor.  There may be another limit of magnetic saturation of the iron, depending on the kind of motor design.  If you heat the motor up too much, you will melt, burn, or otherwise damage insulation, "burning out" the motor, either through making a short or perhaps making an open circuit if a length of wire melts and interrupts the circuit.

If you generate a certain amount of torque, spinning the motor fast and then stepping it down through reduction gears as done on a traction motor will produce more power.  There are limits to that in terms of eddy current losses through frequent changes in the magnetic field through iron pole pieces of the motor.  There is also a limit in terms of the motor flying apart - something they call "bird's nesting" because a broken motor is a tangle of wires and pole pieces that looks like some types of bird's nests.

Beyond that, locomotive traction motors use heavy amounts of air cooling.  Based on the various Diesel Spotter's Guide editions and cutaway drawings, EMD F units had blower motors.  More modern locomotives have a centralized blower drawing air from a centralized filter, EMD's have a blower duct bulging out from the hood, the air travels through ductwork under the running boards, and then goes to the motors using flexible ducts to allow the truck to swivel.

The other difference with your 100 HP fan motor is that while traction motors are limited in diameter, they are perhaps longer than your fan motor.  Just like a little race car motor may generate hundreds of horsepower and a big tractor engine may generate only 40 horsepower, electric motors vary in how hard they are stressed.  A locomotive traction motor gets spun fast and gets a lot of cooling air blown through it to allow it to be pushed hard. 

[JonathanS]

Also your 100 HP motor may have many more poles than a traction motor.  In AC motors the number of poles determines the RPM of the motor.  With 60 Hz power a 2 pole motor will run about 3600 RPM, a 4 pole motor at nearly 1800, a 6 pole motor at almost 1200, etc.