The Electrical laboratory at my college required a Class-E chopper in order to demonstrate the Armature Voltage control method of a DC Shunt Motor using PWM to the students during practical hours. This post gives a brief description of the construction of the Chopper.
It consists of 3 units:
- Control and Gate Drive PCB
- Power Stage PCB
- Auxiliary Supply
1. Control and Gate Drive PCB :
The control structure is built around the ATMEGA16A microcontroller and it is responsible of taking user inputs (Stop, Forward, Reverse, Duty Cycle). It is also responsible for latching and controlling the gate drive signals to select the direction of motor rotation.
The gate drive type is a isolated gate drive for each IGBT. Hence, there are 4 individual gate drivers (TLP250) for each IGBT on the Power Stage of the chopper. The isolated supplies for each gate driver arrive at the control board, and the gate drive signals are sent to the respective IGBTs on the Power Stage. The Chopper is switched at 3kHz.
Since this setup is for demonstration purposes of the Chopper, the armature current (Ia) is tapped via a WCS2702 hall effect CT. Even though the full load current of the motor was 8 Amps, it was supposed to be seldom loaded, hence I chose to use a CT of +/- 2 Amps which would have a higher sensitivity and wouldn't require any amplification in order to observe the waveform. Also, the no load current would be within the linear range of the CT and the current waveform could be observed without any deformation/saturation.
2. Power Stage PCB :
The Power Stage is a Full Bridge constructed out of 4 IKW75N60T (75 A/600 V) IGBTs. Although the body diode of the IGBT is sufficient to freewheel the inductive spike generated by the armature during every switching cycle of the converter, I have placed 2 MUR460 ultrafast diodes parallel to the drain-source junction of each of the IGBT. Each IGBT has a RC snubber made out of 0.082uF/400V polyester capacitor and a 4.7E/5W resistor. The IGBTs are placed under the PCB on a heatsink (Mounting screws visible via holes on the PCB).
4. Operation:
The connections were made according to the above block diagram. The field winding of the motor was excited directly via a 200V DC Supply. The Chopper was given the same supply as it's input.
The armature of the motor was connected differentialy between the Full Bridge.
User inputs:
1. Stop
2. Forward
3. Reverse
4. Duty Cycle (Potentiometer based)
Upper Waveform: PWM Output at 70% Duty Cycle (5V Logic)
Lower Waveform: Armature Current (Ia) tapped via WCS2702
3. Auxiliary Supply :
These are 4 linear supplies isolated from each other (separate ground point) in order to fulfill the power requirements of the control logic and the isolated gate drive circuitry. The auxiliary supply was initially supposed to be designed on a flyback converter having isolated secondaries, but going towards the simpler solution of 4 separate linear supplies seemed a feasible option due to lack of time.
05V/750mA x 1 - Microcontroller, Switching Logic, Buffers, LCD
15V/750mA x 1 - Power Stage Low-Side Gate Drive
15V/500mA x 2 - Power Stage High-Side Gate Drive
4. Operation:
The connections were made according to the above block diagram. The field winding of the motor was excited directly via a 200V DC Supply. The Chopper was given the same supply as it's input.
The armature of the motor was connected differentialy between the Full Bridge.
User inputs:
1. Stop
2. Forward
3. Reverse
4. Duty Cycle (Potentiometer based)
Final Setup
Upper Waveform: PWM Output at 70% Duty Cycle (5V Logic)
Lower Waveform: Armature Current (Ia) tapped via WCS2702
Forward Motoring (Ist Quadrant)
Reverse Motoring (IIIrd Quadrant)
*Sorry for the poor quality images. All the photos were taken in haste.
