Page 64 - Electric Machinery Fundamentals

4U ELECTR IC MACHINERY FUNDAMENTALS

~}:------

(a)
':.}:-----

: k_(-b)-----
(t:

Iii
F",,(tok (e) FIGURE 1- 23 (
)
The linear de machine operating at no-load
conditions and then loaded as a motor.
(a) Velocity V(I) as a fu nction of time;
F 10ad -"11"---'-----'-----'-------1 (b) induced voltage emit); (c) current i(t);
(d) (d) induced force Find(r).

(I-51)
An amount of electric power equal to eindi is consumed in the bar and is replaced
by mechanical power equal to FindV. Since power is converted from electrical to
mechanical form, this bar is operating as a motor.
To summarize this behavior:

1. A force Fjo(,d is applied opposite to the direction of motion, which causes a net
force Fnel opposite to the direction of motion.
2. The resulting acceleration a = Fnellm is negative, so the bar slows down (v!).
3. The voltage ei"' = d BI falls, and so i = (VB - ei"'j, )/R increases.
4. The induced force F ind = iilB increases until I ind I = I [oad I at a lower
F
F
speed v.
5. An amount of eleeu·ic power equal to eindi is now being converted to me-
chanical power equal to Findv, and the machine is acting as a motor.

A real dc motor behaves in a precisely analogous fashion when it is loaded:
As a load is added to its shaft, the motor begins to slow down, which reduces its in-
ternal voltage, increasing its current flow. The increased current flow increases its
induced torque, and the induced torque will equal the load torque of the motor at a
new, slower speed.