Question:
How does the speed controller (C0074) affect different operating mode?
Answer:
1. U/f characteristic control
- without incremental encoder feedback (C0025 = 1) => no influence
- with feedback (C0025 <> 1) => limits the n-controller output to C0074 of nmax (C0011), i.e. the max. motor speed is (C0050 + C0074) * C0011 if the feedback fails
For control reasons (control reserve of PI controller), the I component in the speed controller is limited to 100 % max (≤ SW-V6.2) or 8 * C0074 (≥ SW-V7.0). If C0074 is set lower than the max. motor slip to be expected the speed controller adds a speed deviation up to its internal limit value. In case of a reversal this
saved speed error must first be removed, i.e. the speed controller can
prevent an immediate speed reversal at the motor shaft.
2. Vector control
- without and with incremental encoder feedback => no influence
If the incremental encoder feedback fails the maximum speed is limited by maximum slip in vector control.
Calculation example of the maximum slip (nN = 1470 rpm, fN = 50 Hz (polepair number p = 2); IN = 10 A; Imax = 1.5 * IN = 15 A):
Synchronous speed
nsyn = fN / p * 60 s/min = 50 Hz / 2 * 60 s/min
= 1500 rpm
Slip
s = (nsyn - nN) / nsyn = (1500 rpm - 1470 rpm) / 1500 rpm
= 0.02
max. slip smax ≈ 1.2 * s * Imax/IN * 100 % = 1.2 * 0.02 * 15 A / 10 A * 100 % =
3.6 %
Note: Factor 1.2 considers the overproportional rise of the maximum torque (C0057) in relation to the maximum current (C0022).
