AirIndiaOne From India, joined Mar 2005, 146 posts, RR: 0 Posted (9 years 9 months 2 weeks 1 day 4 hours ago) and read 2528 times:
Today In my Electrical Engineering class, i was told about a mathematical relation between two very separate quantities, electrical supply frequency and weight. It is known that aircrafts use 400 Mhz supply frequency, but how exactly does it help in reducing the weight of the aircraft?
Any references on this are most welcome
"You don't have to be crazy to be in aviation, but it helps", JRD Tata
Delta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 1, posted (9 years 9 months 1 week 4 days 2 hours ago) and read 2492 times:
Hello, I haven't been on A-net for a good while, I see some of the old-timers are still active.
Most airborne motors are 400 Hz (not MHz) induction types, so they run at basically synchronous speeds less 5 - 10% slip. At 400 Hz, the synch. speeds are 24,000, 12,000, 8,000, 6,000 rpm, and so on, depending on the number of poles.
As you can see, motors supplied with 400 Hz run faster than their earthbound counreparts running at 60 Hz. Since the size (weight and volume) is a related to output torque, the higher speed results in higher power, given the same torque. There are caveats, however (there's no free lunch) - the faster 400 Hz motors reject more heat, so provision must be made for additional cooling, and consideration must also be given to the fact that if the motor is air-cooled, the air density is lower (assuming the motor is outside the pressurized area) and its cooling effectiveness is less. A saving factor is that large airborne electric motors usually have intermittent duty cycles, so their heat generation is lower.
I work on electric driven hydraulic pumps, and the largest AC units are in the range of 12 - 15 kVA. These units are cooled by the hydraulic fluid that they pump, resulting in even more weight savings as the liquid can remove the same heat from a smaller surface area than air. There are models using oil jackets around the motor stator, and others where the entire motor - rotor, shaft, and all, are flooded. These are used on Boeing aircraft.
Airbus uses air-cooled motors, but their rotor has an internal liquid cooling loop, so the air only cools the stator. In induction motors, the rotor generates the bulk of the heat, so this is an ideal solution.
Smaller motors, in the 7.5 kVA range as used on the CRJ-200 and DHC-8 motorpumps, are air-cooled.
As for references, you can peruse some of our brochures on our website - they provide mostly application data, but you will find them helpful.