4-8
continues to exist. A shaft running through the pivot point of the bar magnet would rotate at the same
speed as the rotating field. This speed is known as synchronous speed. The shaft represents the shaft of an
operating motor to which the load is attached.
Remember, this explanation is an oversimplification. It is meant to show how a rotating field can
cause mechanical rotation of a shaft. Such an arrangement would work, but it is not used. There are
limitations to a permanent magnet rotor. Practical motors use other methods, as we shall see in the next
paragraphs.
SYNCHRONOUS MOTORS
The construction of the synchronous motors is essentially the same as the construction of the salient-
pole alternator. In fact, such an alternator may be run as an ac motor. It is similar to the drawing in figure
4-6. Synchronous motors have the characteristic of constant speed between no load and full load. They
are capable of correcting the low power factor of an inductive load when they are operated under certain
conditions. They are often used to drive dc generators. Synchronous motors are designed in sizes up to
thousands of horsepower. They may be designed as either single-phase or multiphase machines. The
discussion that follows is based on a three-phase design.
Figure 4-6.—Revolving-field synchronous motor.
To understand how the synchronous motor works, assume that the application of three-phase ac
power to the stator causes a rotating magnetic field to be set up around the rotor. The rotor is energized
with dc (it acts like a bar magnet). The strong rotating magnetic field attracts the strong rotor field
activated by the dc. This results in a strong turning force on the rotor shaft. The rotor is therefore able to
turn a load as it rotates in step with the rotating magnetic field.
It works this way once it’s started. However, one of the disadvantages of a synchronous motor is that
it cannot be started from a standstill by applying three-phase ac power to the stator. When ac is applied to
the stator, a high-speed rotating magnetic field appears immediately. This rotating field rushes past the
rotor poles so quickly that the rotor does not have a chance to get started. In effect, the rotor is repelled
first in one direction and then the other. A synchronous motor in its purest form has no starting torque. It
has torque only when it is running at synchronous speed.
A squirrel-cage type of winding is added to the rotor of a synchronous motor to cause it to start. The
squirrel cage is shown as the outer part of the rotor in figure 4-7. It is so named because it is shaped and
looks something like a turnable squirrel cage. Simply, the windings are heavy copper bars shorted
