Why Do Railcars Use Inverters with Alternators?

[john13]

Hi everyone

I am currently working with diesel railcars. To power the electrical systems, we have engines connected to alternators, which then connect to static inverters. The auxiliary power system runs off the static inverters, initially at 415V AC but distributed throughout the railcars at 415V AC, 240V AC, 21V AC, 65V AC and 24V DC.

I don't understand the purpose of the inverters here. I understand that the alternators convert the engines' DC potential to AC. So do the inverters simply stabilise the frequency of the AC power flowing from the alternators? Or do they do something else?

Many thanks for any help you can offer with this.

 

[anorlunda]

If you remove the diodes, the frequency of the AC will vary with engine RPM. The rated frequency is also unlikely to be the 50 or 60 hertz you expect.

 

[Tom.G]

I understand that the alternators convert the engines' DC potential rotational energy to AC.

As @anorlunda stated, the AC output of the alternators will change frequency with engine speed. The AC is fed to the inverters, and the inverters convert it AC at the desired stable frequency and voltage(s).

Diesel-Electric drive systems take power from the alternators, through an inverter, and convert it to AC at a variable frequency. The frequency is varied according to the desired train speed. The higher the frequency, the faster the traction motors will turn. These are called Variable Frequency Drives, or VFDs.

VFDs are also used in industry, often to drive pumps or conveyer belts at a desired , controllable, speed.

 

[john13]

Many thanks for these very informative replies.

I'm a trainee driver, not an engineer, so this sort of stuff is a bit beyond my area of professional competence. But I do like understanding the underlying principles.