Page 345 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

• Tension control
• Torque control
• Monitoring
Lower Installation Costs
• Contactors/control relays

• Overload relays
• PID modules
• Control panel complexity

Single-phase rectifier

Single-phase rectifiers for producing d.c. supplies for inverters are usually uncontrolled, i.e. composed of diodes
only; the variation of voltage and frequency is carried out by the output stage, usually by pulse width modulation.
However, single-phase controlled rectifiers are still employed as variable voltage sources for d.c. servomotors
used in control systems. a shows the circuit of a single-phase fully controlled bridge rectifier. The average rectifier
output from Appendix B (with m = 2 for single phase) is 2E (cos α)/π, where E is the peak a.c. line voltage and
p
p
α is the firing-delay angle. The e.m.f. of the motor is considered constant and the net voltage available to drive
current through the impedances is the rectifier output voltage shown less the e.m.f. it is assumed that the motor
has sufficiently high inductance to maintain current over short intervals when the rectifier’s output voltage is
negative.
Power flow is therefore from the load to the supply (inversion); this condition can only be maintained if the motor
e.m.f. is aiding the flow of current, i.e. generating. If inversion is not required, a simpler ‘half-controlled’ bridge
formed of two SCRs and two diodes may be used.
The waveforms assume that the supply current can reverse instantaneously when there is a change in the SCR
conduction pattern. However, presence of inductance in the supply prevents sudden reversal and the result is
that the supply voltage is committed for a short interval in supporting the L di/dt in the supply inductance. During
this ‘overlap’ interval, all four devices conduct (the load current rising from zero in one pair and decaying towards
zero in the other pair) and the rectifier output voltage is zero. If the a.c. current is i and the d.c. current I ,
d
Diagram of single phase rectifier and wave form

330

CITS : E & H - Electronics Mechanic - Lesson 223 - 236