Implement two-quadrant chopper (buck-boost converter topology) DC drive

Simscape / Electrical / Specialized Power Systems / Electric Drives / DC Drives

The Two-Quadrant Chopper DC Drive (DC6) block represents a two-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors. This drive features closed-loop speed control with two-quadrant operation. The speed control loop outputs the reference armature current of the machine. Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived. This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.

The main advantages of this drive, compared with other DC drives, are its implementation simplicity and that it can operate in two quadrants (forward motoring and reverse regeneration). In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained. However, for all two-quadrant DC drives, reversible and regenerative operations (reverse motoring and forward regeneration), which are required in most DC drives, cannot be obtained.

In Simscape™
Electrical™ Specialized Power Systems software, the Two-Quadrant Chopper DC
Drive block is commonly called the `DC6`

motor drive.

The Two-Quadrant Chopper DC Drive block uses these blocks from the Electric Drives/Fundamental Drive Blocks library:

Speed Controller (DC)

Regulation Switch

Chopper

The machine is separately excited with a constant DC field voltage source. There is thus no field voltage control. By default, the field current is set to its steady-state value when a simulation is started.

The armature voltage is provided by an IGBT buck-boost converter controlled by two PI regulators. The converter is fed by a constant DC voltage source. Armature current oscillations are reduced by a smoothing inductance connected in series with the armature circuit.

The model is discrete. Good simulation results have been obtained with a 1-µs time step. In order to simulate a digital controller device, the control system has two different sampling times:

The speed controller sampling time

The current controller sampling time

The speed controller sampling time has to be a multiple of the current sampling time. The latter sampling time has to be a multiple of the simulation time step.

**Output bus mode**Select how the output variables are organized. If you select

`Multiple output buses`

, the block has three separate output buses for motor, converter, and controller variables. If you select`Single output bus`

, all variables output on a single bus.**Model detail level**Select between the detailed and the average-value inverter. Default is

`Detailed`

.**Mechanical input**Select between the load torque, the motor speed and the mechanical rotational port as mechanical input. Default is

`Torque Tm`

.If you select and apply a load torque, the output is the motor speed according to the following differential equation that describes the mechanical system dynamics:

$${T}_{e}=J\frac{d}{dt}{\omega}_{r}+F{\omega}_{r}+{T}_{m}$$

This mechanical system is included in the motor model.

If you select the motor speed as mechanical input, then you get the electromagnetic torque as output, allowing you to represent externally the mechanical system dynamics. The internal mechanical system is not used with this mechanical input selection and the inertia and viscous friction parameters are not displayed.

For the mechanical rotational port, the connection port S counts for the mechanical input and output. It allows a direct connection to the Simscape environment. The mechanical system of the motor is also included in the drive and is based on the same differential equation.

**Use signal names as labels**When you select this check box, the

`Motor`

,`Conv`

, and`Ctrl`

measurement outputs use the signal names to identify the bus labels. Select this option for applications that require bus signal labels to have only alphanumeric characters.When this check box is cleared (default), the measurement output uses the signal definition to identify the bus labels. The labels contain nonalphanumeric characters that are incompatible with some Simulink

^{®}applications.

The **DC Machine** tab displays the parameters of the DC
Machine block of the Fundamental Blocks (powerlib) library.

**Smoothing inductance**The smoothing inductance value (H). Default is

`10e-3`

.**Field DC source**The DC motor field voltage value (V). Default is

`150`

.

The **IGBT/Diode device** section of the **Converter**
tab displays the parameters of the Universal Bridge block of the Fundamental
Blocks (powerlib) library. For more information on the Universal Bridge block
parameters, refer to the Universal Bridge reference
page.

**Regulation type**This pop-up menu allows you to choose between speed and torque regulation. Default is

`Speed regulation`

.**Schematic**When you click this button, a diagram illustrating the speed and current controllers schematics appears.

**Nominal speed**The nominal speed value of the DC motor (rpm). This value is used to convert motor speed from rpm to pu (per unit). Default is

`1750`

.**Initial speed reference**The initial speed reference value (rpm). This value allows the user to start a simulation with a speed reference other than 0 rpm. Default is

`0`

.**Low-pass filter cutoff frequency**Cutoff frequency of the low-pass filter used to filter the motor speed measurement (Hz). Default is

`40`

.**Sampling time**The speed controller sampling time (s). This sampling time has to be a multiple of the current controller sampling time and of the simulation time step. Default is

`100e-6`

.**Proportional gain**The proportional gain of the PI speed controller. Default is

`10`

.**Integral gain**The integral gain of the PI speed controller. Default is

`50`

.**Acceleration**The maximum change of speed allowed during motor acceleration (rpm/s). Too great a value can cause armature over-current. Default is

`1000`

.**Deceleration**The maximum change of speed allowed during motor deceleration (rpm/s). Too great a value can cause armature over-current. Default is

`-1000`

.

**Low-pass filter cutoff frequency**Cutoff frequency of the low-pass filter used to filter the armature current measurement (Hz). Default is

`500`

.**Reference limit**Symmetrical current reference (pu) limit around 0 pu. 1.5 pu is a common value. Keep in mind that the lower limit is automatically reduced for low speeds (see speed controller description). Default is

`1.5`

.**PWM switching frequency**The switching frequency of the two IGBT devices (Hz). Default is

`5e3`

.**Sampling time**The current controller sampling time (s). This sampling time has to be a submultiple of the speed controller sampling time and a multiple of the simulation time step. Default is

`20e-6`

.**Power and Voltage nominal values**The DC motor nominal power (W) and voltage (V) values. These values are used to convert armature current from amperes to pu (per unit). Default for

**Power**is`200*746`

. Default for**Voltage**is`440`

.**Proportional gain**The proportional gain of the PI current controller. Default is

`2`

.**Integral gain**The integral gain of the PI current controller. Default is

`200`

.

`SP`

The speed or torque set point. The speed set point can be a step function, but the speed change rate will follow the acceleration / deceleration ramps. If the load torque and the speed have opposite signs, the accelerating torque will be the sum of the electromagnetic and load torques.

`Tm`

or`Wm`

The mechanical input: load torque (Tm) or motor speed (Wm). For the mechanical rotational port (S), this input is deleted.

`Vcc, Gnd`

The DC voltage source electric connections. The voltage must be adequate for the motor size.

`Wm`

,`Te`

or`S`

The mechanical output: motor speed (Wm), electromagnetic torque (Te) or mechanical rotational port (S).

When the **Output bus mode** parameter is set to **Multiple
output buses**, the block has the following three output buses:

`Motor`

The motor measurement vector. This vector is composed of two elements:

The armature voltage

The DC motor measurement vector (containing the speed, armature current, field current, and electromagnetic torque values). Note that the speed signal is converted from rad/s to rpm before output.

`Conv`

The IGBT/Diode device measurement vector. This vector includes the converter output voltage. The output current is not included since it is equal to the DC motor armature current. Note that all current and voltage values of the converter can be visualized with the multimeter block.

`Ctrl`

The controller measurement vector. This vector contains:

The armature current reference

The duty cycle of the PWM pulses

The speed or torque error (difference between the speed reference ramp and actual speed or between the torque reference and actual torque)

The speed reference ramp or torque reference

When the **Output bus mode** parameter is set to **Single
output bus**, the block groups the Motor, Conv, and Ctrl outputs into a single bus
output.

The library contains a 5 hp and a 200-hp drive parameter set. The specifications of these two drives are shown in the following table.

**5 HP and 200 HP Drive Specifications**

5 HP Drive | 200 HP Drive | ||
---|---|---|---|

Drive Input Voltage | |||

Amplitude | 280 V | 550 V | |

| |||

Power | 5 hp | 200 hp | |

Speed | 1750 rpm | 1750 rpm | |

Voltage | 240 V | 500 V |

The `dc6_example`

illustrates the two-quadrant chopper drive used with the
200-hp drive parameter set during speed regulation.

[1] Boldea, Ion, and S.A. Nasar, *Electric
Drives*, CRC Press LLC, 1999.

[2] Séguier, Guy, *Electronique de
puissance*, Dunod, 1999.