Application and Operation
6.2 The Control Modes
Product User Manual
Operating Instructions, Version AE 12/2009, A5E01454341C
113
6.2.5 Closed Loop Control (CLVC or CSMC)
In some applications, when stable, low speed (below 1 Hz) operation under high torque
conditions is required, an encoder may be used to provide speed feedback.
The control diagram of Figure "Block Diagram of Vector Control Algorithms for Induction and
Synchronous Motor Control" remains the same, except for the slip calculation block, which is
disabled so that encoder speed feedback is directly used as an input to the speed regulator.
When an encoder is used with the drive, the control loop type is required to be set to CLVC
(for closed loop vector control with an induction motor) or to CSMC (for closed loop vector
control with a synchronous motor). Spinning load should be enabled when this control mode
is enabled.
6.2.6 Slip Cpmpensation
NEMA B induction motors require slip of the rotor speed (RPM) relative to the stator speed
(frequency) to develop torque. The amount of slip is directly affected by the loading of the
machine. For induction motors, the control provides Slip Compensation to the speed
reference to allow the motor to run at a constant speed, regardless of the torque output
required.
Slip Compensation operates as an open loop speed regulator that increases the electrical
output frequency of the drive as the load increases, or decreases the frequency as the load
drops.
Effect of Slip Compensation on Motor Speed with NXG Control
With Slip Compensation, the electrical frequency is always greater than the desired shaft
speed (mechanical frequency) for all non-zero loads. Therefore, at 100% speed demand, the
NXG OLVC will maintain the shaft speed at the rated synchronous speed of the motor, not
full load speed.
Example:
A 6-pole motor rated for 60 Hz has a synchronous speed of 1200 rpm. The Full Load speed
from the motor nameplate is 1192 rpm. Sending a speed demand of 100% will produce a
mechanical (shaft) speed of 1200 rpm with slip compensation. This will result in a higher
output (electrical) frequency to the motor, to provide the necessary torque to achieve the
desired speed. The slip frequency is directly proportional to the required torque, up to the
rated torque current.
Theory:
Sending the drive a speed demand of 100% means that Synchronous or Rated Speed is
desired. This is calculated by equation 1 below.
Synchronous Speed, Ns, is defined by the formula:
1. N
S
= 120 * f
RATED
/ # of poles
Slip is defined as a percentage (at rated torque) of the difference between synchronous and
full-load speed (NFL) divided by the synchronous speed:
1. Slip (%) = 100 * (N
S
- N
FL
) / N
S
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