The chopper is a device that converts a fixed DC voltage to a variable DC voltage. It does this by using self-commutated devices such as MOSFETs, IGBTs, power transistors, GTOs and IGCTs. These devices are controlled by a low power control signal and do not require a commutation circuit. The chopper operates at high frequencies, which improves the performance of motors by reducing ripple and eliminating discontinuous conduction. Another important feature of the chopper is that it enables regenerative braking at very low generating speeds when the drive is supplied with a fixed voltage to a low DC voltage.
Motoring Control The transistor Tr is used to control the separately excited DC motor. This transistor is operated periodically, with a period of Tr and remains open for a duration of Ton. The waveforms of motor terminal voltage and armature current are illustrated in the figure below. During the on-period, the motor terminal voltage is V and the motor operates accordingly.
In this interval, the armature current is allowed to freewheel through the diode Df, meaning that the motor is disconnected from the power source. During this period, the motor terminal voltage is zero, as the current is allowed to travel in a single direction. This interval is referred to as the “freewheeling interval”, as the motor is not being powered by the source. During this period, the armature current can drop from ia2 to ia1. After this period, the Tr switch is turned on, and the motor returns to the duty interval.
The ratio of the duration of the chopper on period (ton) to the chopper period (T) is referred to as the duty cycle, and this determines the decrease in motor current from ia2 to ia1 during this interval.
Regenerative Braking
The figure below depicts a chopper for regenerative braking operation. The transistor Tr is switched on and off periodically with a period T and an on-time of ton. The waveform of the motor terminal voltage va and armature current ia for continuous conduction are also shown in the figure. To increase the value of La, an external inductance is included in the circuit. When the transistor is on, the armature current increases from ia1 to ia2.
The motor, now functioning as a generator, converts mechanical energy into electrical energy. The stored magnetic energy of the armature circuit inductance is increased as a result, while the remainder of the energy is dissipated in the armature and transistors. This process allows for the conversion of mechanical energy into electrical energy.
When the transistor is turned off, the armature current flows through diode D and the source V. This causes the armature current to decrease from ia2 to ia1. During this time, the energy stored in the electromagnetic field is released and combined with the energy supplied by the machine to power the source. This process is divided into two intervals: 0 ≤ t ≤ ton, which is known as the energy storage interval, and ton ≤ t ≤ T, which is referred to as the duty interval.
Forward Motoring and Braking Control
The chopper operates using two transistors, Tr1 and Tr2, and two diodes, D1 and D2. When the chopper is in the motoring mode, the transistor Tr1 is activated and the diode D1 is engaged. This combination allows power to flow through the system and activates the motor. When the chopper is in the braking mode, the transistor Tr2 is activated and the diode D2 is engaged. This combination allows power to flow through the system in the opposite direction, providing regenerative braking. The control of the transistors and diodes allows the chopper to switch between motoring and braking modes.
Dynamic Control
The figure shows a dynamic braking circuit and its waveform. During the interval between 0 ≤ t ≤Ton, the current ia increases from ia1 to ia2, during which energy is stored in the inductance and the remainder is dissipated in resistors Ra and TR.
During the time period between Ton≤ t ≤ T, i, the current ia decreases from ia2 to ia1, resulting in the energy stored in the inductances being dissipated in the form of heat in the braking resistance RB, Ra, and the diode D. The transistor Tr is used to regulate the amount of energy being dissipated into RB, thus allowing it to be controlled and adjusted to an effective value.
