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14-3. Various Feedback Functions
The PAC27 has various feedback functions (constant voltage (standard), constant current, constant power, power linearity), which are so easy to
operate since an external CT or PT need not be attached.
The feedback control functions detect thyristor current or voltage internally in the regulator and control it to a value set by a control signal from
the controller or some other means.
If line voltage or the load varies, a change in output can be held to a minimum. The functions are most suitable for load characteristic compensation
and precision control.
14-3-1. Constant voltage (VFB: voltage feedback) control (standard)
This function is equipped on a standard basis. Output voltage corresponding to a control input signal is obtained as a true effective value.
The function is usable for almost all types of loads but requires the current limiting function (option) in the case of a load with a large rush current.
14-3-2. Constant current (CFB: current feedback) control
Output current corresponding to a control input signal is obtained. When the load changes, this function works to keep output current at a constant
level by changing output voltage. It is used for heaters in which rush current is large and/or current fluctuates violently. It is also effective in
stabilizing load current. Even for pure metal heaters with large rush current, the use of the current control function (option) is not required. Because
current corresponding to a control input signal is tried to output, a power regulator is used for regulation if the capacity of the PAC27 does not
match with that of the heater.
14-3-3. Constant power (PFB: power feedback) control
Output power, i.e., a calorific value from the heater, which corresponds to a control input signal is obtained. Since it is assumed that the
function is used against a load of variable resistance, rated voltage x rated current x 1/2 is set as a 100% value of power. The function is
capable of being used for almost all types of heaters and allows for precision control of temperature.
As the constant power control is meant for a heater load, a correct power level is unable to be attained for other loads (such as a motor)
than heaters.
Selection of this control system for an SiC heater makes compensation for heater deterioration possible without using a tap change
transformer for the heater. (This may not be possible due to certain relationships between line voltage and heater voltage.)
Example: When the line voltage is 200 V AC and the rated current of an SiC heater is 140 V 20A (2.8 kW):
The rated voltage of the thyristors x rated current x 1/2 is set as a 100% power value, that is, 2.8 (kW) x 2/200 (V) = 28(A).
Therefore, a PAC27 of 200-220 V 30A should be selected.
In this case the 100% power value is 220 (V) x 30 (A) x 1/2 = 3.3 (kW). In order to match with the heater power, the power
regulator should be tightened to 2.8 (kW)/3.3 (kW)=85%. Owing to this, at 100% of input control signal, the initial value of the
heater attains 140V 20A (2.8 kW). When the heater deteriorates to raise the resistance value, it can be used at 200 V 14A (2.8
kW) maximum. During this period, output power corresponding to control input is obtained without requiring any other operation
such as tap changing by a transformer.
14-3-4. Power linearity (VVFB: voltage square feedback) control
It is arranged so that the square of output voltage corresponds to constant resistance.
Since power against constant resistance is in proportion to the square of voltage, power corresponding to control signals is obtained when used
for constant resistance heaters (nickel chrome, iron chrome, etc.).
Although the controllability is lower when compared to constant power control since resistance inevitably changes between a few percent and ten
percent or so in actual heaters, power which is substantially in proportion to control input signals may be obtained. It cannot be obtained by
ordinary constant voltage control.
It is a type of constant voltage control. Unlike constant power control, it does not require calculation of the capacities of the PAC27 and the heater.
Installation of a power regulator is also unnecessary.
