Rockwell Automation 1902 Syncpro II - Manual (Page 8)

8 Rockwell Automation Publication 1902-IN001B-EN-E - April 2013
Chapter 1 Product Description
Synchronous Motor Theory The synchronous motor is a commonly used industrial motor favored for its
higher efficiency, superior power factor, and low inrush currents. Typical
applications that benefit from the constant operating speed include refiners, head
box fan pumps, chippers, etc. Synchronous motors are particularly well suited to
low RPM applications. The synchronous brush-type motor is composed of a
three-phase stator winding, a DC rotor winding, and a squirrel-cage winding.
The stator winding is identical to that of an induction motor and, as such, the
direction of motor rotation depends on the rotation of the stator flux. The
direction can be changed by reversing two of the stator leads, just as it does with
induction motors.
The rotor contains laminated poles which carry the DC field coils that are
terminated at the slip rings. It also has a squirrel-cage winding composed of bars
embedded in the pole faces and shorted by end rings. The squirrel-cage winding
is also known as “damper” or “amortisseur” winding. This winding enables the
motor to accelerate to near synchronous speed so that the DC supply can be
applied to the field windings for synchronizing the motor to the line (typically
95%).
These field windings are connected through slip rings to a discharge resistor
during start up. The resistor is required to dissipate the high voltages that are
induced into the field windings from the stator, and it is removed from the circuit
when the DC field voltage is applied. The synchronous motor can be compared
to a transformer, with the three-phase stator resembling the primary and the field
winding acting like a secondary. Through this transformer action, an induced
voltage is generated in the motor field during starting. The induced signal can be
used to protect the squirrel-cage winding by monitoring the motor speed during
acceleration and to determine when the DC field can be excited for
synchronization. At zero speed, the frequency induced into the field is 60 Hz, at
95% speed the frequency induced is 3 Hz (for a 60 Hz system).
Once at 95% speed, the DC field is supplied with either 125 V DC or 250 V DC
and the discharge resistor is removed from the circuit. The excitation in the field
windings creates north and south poles in the rotor which lock into the rotating
magnetic field of the stator. The slip rings are used to connect the field windings
to the discharge resistor and static exciter. It is at these slip rings that the field
resistance of the motor can be measured to confirm the required field voltage and
current at rated power factor. If, for example, the field voltage is 125 V DC and
the current is 20 amps DC, then the resistance measured should be about 6
Ω,
based on Ohms Law.