Rockwell Automation 150 MNL/SMC DIALOG+/APP-PROD GUIDE - Manual (Page 161)

Chapter
9
Solid-state Starters Using SCRs
Solid-state Starters
Using SCRs In solid-state starters, silicon controlled rectifiers (SCRs) (se
Figure 9.1) are used to control the voltage output to the motor. An
SCR allows current to flow in one direction only. The amount of
conduction of an SCR is controlled by the pulses received at the gate
of the SCR. When two SCRs are connected back to back (se
Figure 9.2), the AC power to a load can be controlled by changing the
firing angle of the line voltage (see Figure 9.3) during each half cycle.
By changing the angle, it is possible to increase or decrease th
voltage and current to the motor. The SMC Dialog Plus controller
incorporates a microprocessor to control the firing of the SCRs. Six
SCRs are used in the power section to provide full cycle control of the
voltage and current. The voltage and current can be slowly and
steplessly increased to the motor.
Figure 9.1 Silicon Controlled Rectifier (SCR)
Figure 9.2 Typical Wiring Diagram of SCRs
SCR
Power Inpu
3-Phase
L1
L2
L3
SMC Dialog Plus Controller
Power Section
T1
T2
T3
Motor