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In case of TRC (Time Ratio Control) ______ is varied.
Duty cycle
Firing angle
Supply frequency
Supply voltage
Duty cycle
In Time Ratio Control (TRC) of DC choppers, the average output voltage is varied by changing the duty cycle TONтАЛ/T. This is achieved by either varying the ON-time TONтАЛ keeping the total chopping period T constant (Pulse Width Modulation), or by varying the total period T while keeping either TONтАЛ or TOFFтАЛ constant (Frequency Modulation). Thus, duty cycle is the primary parameter varied in TRC.
In Time Ratio Control (TRC) of DC choppers, the average output voltage is varied by changing the duty cycle TONтАЛ/T. This is achieved by either varying the ON-time TONтАЛ keeping the total chopping period T constant (Pulse Width Modulation), or by varying the total period T while keeping either TONтАЛ or TOFFтАЛ constant (Frequency Modulation). Thus, duty cycle is the primary parameter varied in TRC.
D=TTONтАЛтАЛ=TONтАЛтЛЕf тАФ Duty cycle definition
VoтАЛ=DтЛЕVsтАЛ=TONтАЛ+TOFFтАЛTONтАЛтАЛтЛЕVsтАЛ тАФ Average DC output voltage of a step-down chopper
A DC chopper acts as a high-speed ON/OFF semiconductor switch connecting a fixed DC source to a load. By controlling the ratio of the conduction time TONтАЛ to the total switching period T, the average DC output voltage VoтАЛ=DтЛЕVsтАЛ can be regulated smoothly between 0 and VsтАЛ.
Time Ratio Control (TRC) directly controls the duty cycle D to regulate output voltage.
Constant Frequency Control (PWM) keeps total period T fixed and varies TONтАЛ.
Variable Frequency Control (FM) keeps either TONтАЛ or TOFFтАЛ fixed and varies T (f).
PWM is generally preferred because filter design is easier due to fixed switching frequency.
Provides smooth and stepless control of DC output voltage.
High efficiency due to low power dissipation in electronic switching state.
Fast response time compared to traditional line-frequency phase-controlled converters.
Variable frequency control (FM) introduces harmonics over a wide frequency band, making filter design complex.
Commutation circuits or switching losses can be significant at very high switching frequencies.
DC traction motor control (electric trains, trolley buses).
Switched Mode Power Supplies (SMPS).
Regenerative braking systems in electric vehicles.
| Feature | Constant Frequency Method (PWM) | Variable Frequency Method (FM) |
|---|---|---|
Parameter Varied | Conduction time TONтАЛ | Total period T (Frequency f) |
Chopping Period (T) | Constant | Variable |
Harmonic Filter Design | Easier (fixed frequency) | Difficult (wide frequency spectrum) |
Option B (Firing angle) is varied in Phase Control of AC-to-DC Controlled Rectifiers (using Thyristors/SCRs), not in DC Choppers.
Option C (Supply frequency) is varied in Variable Frequency Drives (VFD) for AC motor speed control.
Option D (Supply voltage) is an input parameter and is typically fixed in chopper control systems.
A is correct тАФ Time Ratio Control operates by varying the duty cycle (D = Ton/T) to regulate the average output voltage of a DC chopper.
In competitive exams like GATE/ESE, remember that PWM (Constant Frequency) is widely used because a variable frequency (FM) generates low-frequency harmonics that interfere with communication lines and require bulky filters.