Page 571 - The Mechatronics Handbook

0066_Frame_C20 Page 41 Wednesday, January 9, 2002 5:49 PM

I a
Supply
V
I f R a

V a M

FIGURE 20.68 Speed control using thyristors.

Mark
V a
Space

Average value of V a

Time

FIGURE 20.69 Voltage across armature terminals.

Efficiency of dc Machines
The losses in dc machines can be generally classified as
2
1. Armature losses: This is the I R loss in the armature winding, often referred to as the “copper loss.”
2. Iron loss: This loss is attributable to magnetic hysteresis and eddy currents in the armature and
field cores.
3. Commutator losses: This loss is related to the contact resistance between the commutator brushes
and segments. The total commutator loss is due to both mechanical friction and a voltage loss
across the brushes.
4. Excitation loss: In shunt-wound machines, this power loss is due to the product of the shunt
current and the terminal voltage.
5. Bearing friction and windage: Bearing friction is approximately proportional to the speed, but
windage loss varies with the cube of the speed. Both of these losses are fairly minor unless the
machine is fitted with a cooling fan, in which case the windage loss can be quite significant.

Despite the variety and nature of the losses associated with dc machines, they have nonetheless a very
good performance with overall efficiencies, often in excess of 90%.

AC Machines

Synchronous Motors
Synchronous motors are so called because they operate at only one speed, i.e., the speed of a rotating
magnetic field. The production of the rotating magnetic field may be actioned using three, 120° displaced,
stator coils supplied with a three-phase current. The rotational speed of the field is related to the frequency

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