Table of Contents
Advertisement
3.1 OVERVIEW
3 SYNCHRONOUS MOTOR APPLICATIONS
3
Figure 3–6: FIELD DISCHARGE RESISTANCE – MEDIUM STARTING TORQUE
Selection of the value of the field discharge resistance is a decision that may require judicious application of
several factors present on a particular drive, such as torque, excitation systems, and control components.
The importance of speed for applying field cannot be over-emphasized. Rotor and load masses cannot be
accelerated fast enough to allow synchronization, if slip is in excess of ten percent.
Synchronous-motor controllers which can accurately apply field at an optimum speed and a favorable angle
permit matching the motor to the load with a greater degree of precision than might otherwise be possible. The
increase in load which can be pulled in due to precision application of field will vary from one motor design to
another along with system inertia.
Applying excitation at the point of zero induced current (favorable angle) takes advantage of motor capability in
two ways:
1. It catches ("traps") salient-pole flux at significant magnitude (provided there is a field discharge resistor of
adequate value) and uses it for torque during a 180° acceleration period.
2. It catches the rotor in correct angular position to be pulled forward into step.
In addition to permitting closer matching of motor to load, optimum application of excitation also reduces power
system disturbance which occurs when the motor goes through a complete slip cycle with the field energized.
If the motor is large relative to the power system, surges transmitted to the system will be at a minimum if field
is applied to prevent slip at pull-in.
3-
SPM Synchronous Motor Protection and Control
GE Multilin
4
Courtesy of store.ips.us
Advertisement
Table of Contents
