Application and Operation
6.2 The Control Modes
Product User Manual
114 Operating Instructions, Version AE 12/2009, A5E01454341C
With slip compensation, the slip frequency is subtracted from the output frequency (f
OUT
) to
ensure that the mechanical speed matches the desired speed. In simple terms, this is done
by taking the per unit (P
U
) Torque (T
PU
) times the slip and subtracting it from the speed
feedback (in frequency), effectively adding it to the speed reference:
1. S
MOT
= f
OUT
- (Slip * T
PU
)
2. S
ERR
= S
DMD
- S
MOT
In equation 4, S
ERR
represents the error signal processed by the speed regulator. The
implication for this is that for a speed command of 100%, based on the synchronous speed,
the applied electrical frequency will be higher than rated frequency due to the increase
created by the slip compensation (equation 3 and 4). This will result in the motor running at
true requested mechanical speed with the electrical frequency adjusted to provide the torque
necessary to produce that speed.
Limiting Frequency by Disabling Slip Compensation
If the motor is to be limited to a specific frequency, then the slip compensation can be
disabled. In the same example, the Full Load speed must be set to 1200 rpm. This
effectively disables the slip compensation by reducing equation 2 to produce a slip of zero.
Then equation 3 and 4 reduce to:
1. Slip = (1200 - 1200) / 1200 = 0
2. S
MOT
= f
OUT
- 0 = f
OUT
The end result will be that the drive will regulate to the output frequency rather than the
motor shaft speed (mechanical speed). No compensation for slip is done.
Conclusion
With Slip compensation:
● Output shaft speed will equal the percentage of synchronous speed requested
● The frequency will vary depending on load, but the speed will be fixed
● Motor Speed in rpm should be monitored
Without Slip Compensation:
● The Output Frequency will equal the speed demand percentage of rated frequency
● The mechanical (shaft) speed will vary with load, but the frequency will be fixed
● Motor Frequency in Hz should be monitored
6.2.7 Frequency Regulator
A Frequency Regulator generates the motor’s torque-producing current reference. The
Stator Frequency Reference (Ws_ref) is generated from the output of the Slip Compensator.
The Stator Frequency (Wout) comes from Phase Lock Loop (an estimate of the actual stator
frequency). The Frequency Regulator is evaluated at 1/5
th
of the inner current loop update
rate.
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