Synchronous Motor V-Curves


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Introduction

Synchronous motors are used in applications such as textile mills where constant speed operation
is critical. Most small synchronous motors contain squirrel cage bars for starting. In this
experiment, synchronous motor starting is demonstrated. After starting, the motor is locked into
synchronism by applying a rotor field current. The field current is then varied to illustrate
control of the reactive power from positive to negative. When the reactive power is zero, the
machine is operating with unity power factor and the armature current drawn from the source is a
minimum. The armature current is plotted versus field current to obtain the classical "V-curves".


Laboratory Software

a screen-shot of the software for this experiment. The synchronous machine
armature voltage, armature current, real power, and reactive power are displayed as well as
graphs of the armature voltage and armature current. The field quantities are also displayed and
a control is built-in for adjustment of the field current. After going through the start-up
procedure, the field current will be adjusted and the data is saved at each step for plotting the
machine V-curves. The standard "Add", "Clear", "Print", and "Save" buttons are included. Two
V-curves will be obtained in this experiment and can be marked by selecting "no-load" (shown in
Figure 6) or "load". Technically, the no-load curve has a bit of load due to a dc machine which
the synchronous machine is connected to.


Laboratory machines


the a diagram of the motor test stand used for this experiment. The synchronous
machine is actually a wound-rotor induction machine. It will operate as a synchronous machine
when a DC current is supplied to the rotor. Besides the rotor windings accessible form the
connector box, the machine also has short-circuited damper windings on the rotor. Both sides of
each stator winding (as, bs, cs, an, bn, and cn terminals) are brought out on the connector box for
connection in wye or delta. However, in this experiment, the machine will be connected in wye.
The rotor is wye-connected internally, and the three terminals are brought out (ar, br, and cr).
Synchronization lamps are connected in-between the stator windings and the line connection
(terminals a, b, and c). A three-phase switch bypasses these lamps when switched on. This setup
can be used for synchronizing the generator to the line or for synchronous motor starting. The
synchronous machine is rated at 208-V (line-to-line rms), 60-Hz, 250-W. It is a 4-pole machine
and thus has a synchronous speed of 1800-RPM.