Section 12. PLC controller
12.6. Example of PLC use
An example presented in this section describes how to
control such quantities as output speed and time of motor
acceleration by means of built-in PLC.
TASK:
To modify the process of motor start-up in such way, that
the speed characteristics of a motor would look just like it is
shown in fig. 12.7.
In fig. 12.7 it is possible to distinct three zones: the
First zone - slow acceleration - (Dynamics 1), II zone - the
established speed, and also III zone – fast acceleration
(Dynamics 2).
Frequency converter MFC710 allows to set up 2 different rates
of acceleration and braking: Dynamics 1 and Dynamics 2.
These rates are determined by parameters 1.30, 1.31, 1.32,
1.33. Parameter 1.36 decides which dynamic is currently active.
The parameter 1.36 is a pointer and consequently it can be set
up so that one of PLC units decides about dynamics.
In fig. 12.7 it is shown, that after the motor acceleration (with dynamics1) to speed N1 it is necessary to make time limit
T and then to accelerate the motor (with dynamics 2) to speed N2. It is necessary to modify parameter 2.2 (Referencing-
unit A) so that one of PLC units define an relative level of speed up to which the motor should be accelerated (this level
corresponds to the nominal frequency of the motor).
In fig. 12.8 the structure which implements a given task it presented. The unit 1 is the comparator which reacts
to the value of first speed. In the observed case acceleration with dynamics 1 is implemented to speed of 20.0% (ST1)
of nominal speed. The signal which informs about achievement of the first value of speed enables Unit 3. The Unit 3 is
the counter which counts downwards from value 5 (ST3) every 1 second (constant speed for 5s). The output signal of
the Unit 3 enables the Unit 4 and switches type of dynamics (Dynamics 1 or Dynamics 2). The unit 4 is a switch which,
depending on a signal on input, passes to the referencing-unit A the first or second value of speed (ST1/ST2). Value of
balance time (ST3), first (ST1) and second (ST2) speed can be modified by connecting in proper place, for example, an
analog input or one of user referencing-units. Condition: ST2> ST1 must be fulfilled.
To realize such structure it is necessary to:
1.
define the Unit 1 (par. 6.1 = 12, par. 6.2 = PCH.176, par. 6.3 = PCH.320, par. 6.4 = 0),
2.
define the Unit 2 (par. 6.5 = 23, par. 6.6 = PCH.256),
3.
define the Unit 3 (par. 6.9 = 27, par. 6.10 = PCH.91, par. 6.11 = PCH.257, par. 6.12 = PCH.322),
4.
define the Unit 4 (par. 6.13 = 9, par. 6.14 = PCH.321, par. 6.15 = PCH.320, par. 6.16 = PCH.256),
5.
define the Unit 5 (par. 6.17 = 23, par. 6.18 = PCH.258),
6.
set up parameter 2.2 (Referencing-unit, A) on PCH.259 as it described in section 3.2.1, 3.2.8,
7.
set up parameter 1.36 (Choice of dynamics) on PCH.260,
8.
switch on PLC setting up parameter 5.144 on YES.
In above mentioned example the parameter 5.120 will define a threshold of speed N1 [resolution 0.1 %, that is 1000 =
100.0 %], the parameter 5.121 will define a threshold of speed N2 [resolution 0.1 %, that is 1000 = 100.0 %], a
parameter 5.122 time T with resolution of one second.
Fig. 12.8 - Structure of control implementing a predefined task
64
Frequency converter MFC710 type. TWERD Power Electronics
N [%]
t
N1
N2
I
t2
t1
T
Acceleration end
0
II
III
Fig. 12.7 - Selected characteristic of acceleration
