Danfoss VLT 6000 HVAC Servisní příručka stáhnout pdf (Strana 114)

VLT

6000 HVAC

114

★ = factory setting. ( ) = display text [ ] = value for use in communication via serial communication port

MG.60.B1.02 - VLT is a registered Danfoss trade mark

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■ PID for process regulation, cont.

Inverse regulation

Normal regulation means that the motor speed

increases when the reference/setpoint is higher than

the feedback signal. If there is a need for inverse

regulation, in which the speed is reduced when the

feedback signal is lower than the reference/setpoint,

Inverse must be programmed in parameter 420 PID

normal/inverse control.

Anti Windup

The process regulator is factory preset with an active

anti-windup function. This function ensures that

when either a frequency limit, current limit or voltage

limit is reached, the integrator will be initialised for a

frequency that corresponds to the present output

frequency. This avoids integration on a deviation

between the reference/setpoint and the actual state

of the process, the controller of which is not possible

by means of a speed change. This function can be

disabled in parameter 421 PID anti windup.

Start-up conditions

In some applications, optimum setting of the process

regulator will mean that it takes an excessive time for

the required process state to be reached. In such

applications it might be an advantage to fix an output

frequency to which the VLT frequency converter is to

bring the motor before the process regulator is

activated. This is done by programming a PID start-

up frequency in parameter 422.

Differentiator gain limit

If there are very quick variations in a given application

with respect to the reference/setpoint signal or the

feedback signal, the deviation between reference/

setpoint and the actual process state will quickly

change. The differentiator may thus become too

dominant. This is because it reacts to the deviation

between the reference/setpoint and the actual

process state. The quicker the deviation changes, the

stronger the resulting differentiator frequency

contribution. The differentiator frequency contribution

can thus be limited to allow the setting of a

reasonable differentiation time for slow changes and

a suitable frequency contribution for quick changes.

This is done in parameter 426, PID Differentiator gain

limit.

Lowpass filter

If there are ripple currents/voltages on the feedback

signal, these can be dampened by means of a built-in

lowpass filter. Set a suitable lowpass filter time

constant. This time constant represents the limit

frequency of the ripples occurring on the feedback

signal.

If the lowpass filter has been set to 0.1s, the limit

frequency will be 10 RAD/sec., corresponding to (10/

2 x π) = 1.6 Hz. This means that all currents/voltages

that vary by more than 1.6 oscillations per second will

be removed by the filter.

In other words, regulation will only be carried out on a

feedback signal that varies by a frequency of less than

1.6 Hz. Choose a suitable time constant in parameter

427, PID Lowpass filter time.

Optimisation of the process regulator

The basic settings have now been made; all that

remains to be done is to optimise the proportional

gain, the integration time and the differentiation time

(parameters 423, 424 and 425). In most processes,

this can be done by following the guidelines given

below.

1. Start the motor.

2. Set parameter 423 PID proportional gain to 0.3

and increase it until the process shows that the

feedback signal is unstable. Then reduce the value

until the feedback signal has stabilised. Now lower

the proportional gain by 40-60%.

3. Set parameter 424 PID integration time to 20 s

and reduce the value until the process shows that

the feedback signal is unstable. Increase the

integration time until the feedback signal

stabilises, followed by an increase of 15-50%.

4. Parameter 425 PID differentiation time is only used

in very fast-acting systems. The typical value is

four times the value set in parameter 424 PID

Integration time. The differentiator should only be

used when the setting of the proportional gain

and the integration time have been fully optimised.

NB!

If necessary, start/stop can be activated a

number of times in order to provoke an

unstable feedback signal.

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