Figures 4 and 4a complete a one-quarter turn
or 90° of a two-pole motor. Continuation of this
completes one revolution.
The rotor shown in the above figures is that of
a two-pole synchronous motor having definite
North and South poles developed by direct
current flowing in the field winding. The current
flows from the observer in the left hand portion of
the winding, towards the observer in the right
hand portion. Using the “right-hand rule” this
develops a North pole at the bottom of the rotor
and a South pole at the top.
The magnetic poles developed in the stator are
exactly opposite. However, unlike poles attract,
so the North pole of the rotor lines up under the
South pole of the stator.
As the North pole of the stator shifts 30°
clockwise, the rotor is pulled along with it, and
(assuming no load on the rotor shaft and no
losses) the rotor and stator poles line up exactly as
shown. Each rotor pole lags behind its
corresponding stator pole by 20 to 30 electrical
degrees at full load. This is called the “load
angle,” and it is a function of motor load, where
increased torque requirements result in greater
magnetic pull and a greater displacement from
magnetic center.
A normal two pole stator differs in many
respects from that shown in Figures 1 through 4.
Instead of one slot per phase there are six or seven.
The coil pitch is about 120° instead of 60° and will
overlap the coils of the various phases. The
configuration shown results in rotation by a series
of jerks. The use of more closely spaced coils will
result in a smoother, more uniform turning action.