Page 66 - Electric Machinery Fundamentals

42 ELECTRlC MACHINERY FUNDAMENTALS

To summarize this behavior:
1. A force Fopp is applied in the direction of motion; Fnc, is in the direction of
morioo.
2. Acceleration a = F ne , / In is positive, so the bar speeds up (vi).
3. The voltage ei,d = v BI increases, and so i = (ei,d i - VB)IR increases.
i
4. The induced force F ind = illB increases until I ind I = I Flood I at a higher
F
speed v.
5. An amount of mechanical power equal to FindV is now being converted to
electric power eindi, and the machine is acting as a generator.
Again, a rea] de generator behaves in precisely this manner: A torque is ap-
plied to the shaft in the direction a/motion, the speed of the shaft increases, the in-
ternal voltage increases, and current flows out of the generator to the loads. The
amollnt of mechanical power converted to electrical form in the real rotating gen-
erator is again given by Equation (1-52):
(I-52)

It is interesting that the same machine acts as both motor and generator. The
only difference between the two is whether the externally applied forces are in the
direction of motion (generator) or opposite to the direction of motion (motor).
Electrically, when eil1d > VB' the machine acts as a generalOr, and when eind < VB' the
machine acts as a motor. Whether the machine is a motor or a generator, both in-
duced force (motor action) and induced voltage (generator action) are present at all
times. This is generally true of all machines- both actions are present, and it is only
the relative directions of the external forces with respect to the direction of motion
that determine whether the overall machine behaves as a motor or as a generator.
Another very interesting fact should be noted: This machine was a genera-
tor when it moved rapidly and a motor when it moved more slowly, but whether it
was a motor or a generator, it always moved in the same direction. Many begin-
ning machinery students expect a machine to turn one way as a generator and the
other way as a motor. This does not occur. Instead, there is merely a small change
in operating speed and a reversal of current flow.

Starting Problems with the Linear Machine
A linear machine is shown in Figure 1-25. This machine is supplied by a 250-V
dc source, and its internal resistance R is given as about 0.10 .n. (The resistor R
models the internal resistance of a real dc machine, and this is a fairly reasonable
internal resistance for a medium-size de motor.)
Providing actual numbers in this figure highlights a major problem with ma-
chines (and their simple linear model). At starting conditions, the speed of the bar
is zero, so e ind = O. The current flow at starting is

= VB = 250 V = 2500 A
i star, R 0.111