Rockwell Automation 1901 SyncPro - Manual (Page 6)

1-2 Product Description
1901-UM020C-EN-P – June 2013
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 ohms, based on Ohms Law.