1.4 Calculated basic parameters of designing
Basic parameters describing operating properties of rectifiers are:
- Average values of the rectified voltage and current Ud and Id;
- An EFFICIENCY;
- Power-factor Kp;
- An external characteristic – correlation between average rectified voltage and average rectified current of a load Ud=f (Id);
- Control characteristic – dependence between average rectified voltage and control angle Ud=f(α);
- Pulse factor- ratio of an RMS value of harmonic components of rectified voltage (current) to the average value of the rectified voltage (current):
Distortion factor is equal to the ratio of an RMS value of a basic current harmonic to an RMS value of a complete primary winding current of transformer.
Power factor is equal to ratio of active power witch is carried by first harmonics of primary current and voltage to total power of those harmonics.
The parameters of individual elements of the rectifier are defined by the given operating conditions of a consumer of a direct current and possible anomalous duty of it (emergencies).
The main task of design calculations of the converters is that to definite the correlations enabling to find electrical parameters of the rectifiers, the transformer and also parameters of the filtering, stabilizing devices and regulating system account to the duty of the load, and then to choose these devices under the catalogue or, if it is necessary, to construct them.
Parameters describing operation of rectifiers:
- Average ITAV, root mean square ITRMS and maximum ITM values of a current flowing through a valve under rated duty of load and under possible overloads;
- Peak-inverse voltage of a valve if it is off-state URM;
- Peak-positive voltage of valve UDM (in the case of application of controlled rectifiers).
Parameters of power transformer:
- RMS values of voltage and current of a secondary (rectifier’s) winding of transformer U2 and I2;
- RMS value of a current of a primary winding of the transformer I1;
- Total powers of windings S1 and S2 and electrical rating (type power) of the transformer ST.
1.5 Some definitions
1). Let’s consider electromagnetic processes in the converter in angular coordinates
,
where ω- cyclic frequency, t- current time.
2). The angle, at which an uncontrolled rectifier is turned on, names as a point of natural turning on.
The angle when controlled rectifier is turned on just as a control pulse is applied names as a control angle (turning on, regulating, ignition angle). This angle is fixed concerning a point of natural turning on.
3). The average value, or direct component of a current (voltage) is a square between a current (voltage) waveform and abscissa axis averaged during of waveform period.
Figure 1.2
4). The root mean square value of periodical current is same value of a direct current, which flowing during of waveform period through the given part of a circuit, produces as amount of heat as considered alternating current.
Amount of heats is produced by the direct current is
Q==I2RMSּּRּT;
Amount of heat is produced by the alternating current is
But the condition Q= = Q ~ must be executed.
From here
As U=IR, so
5). Let's suppose that the uncontrolled rectifier is a special case of controlled rectifier at
ά = 0.
6). Let’s consider thyristor as logical switch.