Week-7 Challenge: DC Motor Control

Question 1:

Speed control of a DC motor using BJT H-bridge:

The above circuit is used to drive the DC motor. Here 4 switches with 4 diodes are being used and the load that is a motor in the center. 

Two transistors are switched simultaneously: Q1 and Q4 or Q2 and Q3. When Q1 and Q4 are fired, a positive voltage is applied to the motor and diodes D2-D3 operate as free-wheeling diodes when Q1 and Q4 are switched off. When Q2 and Q3 are fired, a negative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.

Simulation model:

The Bipolar Junction Transistor (BJT) when used for power switching applications, operates as an IGBT. When it is conducting (BJT operating in the saturated region), a forward voltage Vf is developed between collector and emitter (in the range of 1 V). Therefore, the IGBT block can be used to model the BJT device.

The IGBT block does not simulate the gate current controlling the BJT or IGBT. The switch is controlled by a Simulink signal (1/0). The DC motor uses the preset model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to square of speed). The armature mean voltage can be varied from 0 to 240 V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%.

Simulation results:

The duty cycle of the system is 75% the output is as follows:

Here the output voltage 180V 

In here it is observed that the speed and armature current are in the positive side of the so that the motor run at the positive side. At the 0.5 sec the armature current is in the negative side which had the motor to run in the negative side.It has a higher shoot of the current.

The duty cycle is of 50% the results are as follows:

Here the output volatge is 120V.

In here the initial shoot of the current is low and the speed is around 450 rpm.

As the duty cycle decreases there is an achievement of the smooth current curve. This shoot up happens because of the switching in the circuit that cause heat losses during switching. To avoid these losses, heat sink is used to dissipate the heat. From this the motor can change its direction of rotation easily and very smoothly.

Question 1C:

Four Quadrant Chopper DC drive VS H-bridge model:

Four Quadrant Chopper DC drive model:

In this model the DC motor of 200 HP is separately excited with a voltage source of 150 V DC. The armature current is provided by a converter controlled by two PI regulators. Here the converter is fed by a DC bus of 515V which is obtained by rectification of AC voltage.In order to limit the DC bus voltage during dynamic braking mode, a braking chopper has been added between the diode rectifier and the DC7 block.

DC 7 block	H bridge block
In this the motor would achieve the change of rotation through the change in the polarity of the motor	In this the motor achieve the change in rotation through the switching of the IGBT and diodes.
Regenerative braking is possible	Regenerative braking is not possible
High voltage is being achieved	A bit low voltage is achieved
The efficiency of the system is high	The efficiency of the system is low

Question 2:

Two quadrant operational chopper:

The chopper that can work in two quadrant as above is called as the Class C chopper. This circuit has 2 diodes and 2 switches which can control the flow of current. Here the diodes would work as the freewheeling diodes. The switches are used to operate and according to which the load would get the supply.The two quadrant operation indicate the forward motoring and forward braking condition. The current will be both positive and negative but the voltage would always be positive. 

If CH1 and D2 are on the output voltage is Vdc i.e supply voltage.

If CH2 nd D1 are on then the output voltage is zero.

The simulation model is as follows:

 In the above model the supply dc voltage is 30V and the pulse generator has the amplitude of 10 along with the period of 0.02 sec and the duty cycle is about 50%. The mosfet switches along with the diodes are used. The inductor along with E is considered as the motor load.Thee model is simulated for a time period of 0.5 sec.

The output of the system is as follows:

 

In here it is observed that the voltage value is always in between 0 to 30 V which indicate it is always positive. The current has varied in between -55 to 55 amp. So the current has followed both positive and negative side.

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

Hence a two quadrant converter is modeled.

Question 3:

BLDC motor:

Brushless DC electric motors also known as electronically commutated motors (ECMs, EC motors). 

Primary efficiency is a most importent feature for BLDC motors. Because the rotor is the sole bearer of the magnets and it doesn't require any power. i.e. no connections, no commutator and no brushes. In place of these, the motor employs control circuitry. To detect where the rotor is at certain times, BLDC motors employ along with controllers, rotary encoders or a Hall sensor.

Construction:

In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature windings are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy. 

The main design difference between a brushed and brushless motors is the replacement of mechanical commutator with an electric switch circuit.  A BLDC Motor is a type of synchronous motor in the sense that the magnetic field generated by the stator and the rotor revolve at the same frequency.

Brushless motor does not have any current carrying commutators. The field inside a brushless motor is switched through an amplifier which is triggered by the commutating device like an optical encoder.

The layout of a DC brushless motor can vary depending on whether it is in “Out runner” style or “Inrunner” style.

• Outrunner – The field magnet is a drum rotor which rotates around the stator. This style is preferred for applications that require high torque and where high rpm isn’t a requirement.

• In runner – The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the out runner style, but is capable of spinning at very high rpm.

Working:

BLDC motor works on the principle similar to that of a Brushed DC motor. The Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force. In the BLDC motor, the current carrying conductor is stationary  and the permanent magnet is moving.

When the stator coils get a supply from source, it becomes electromagnet and starts producing the uniform field in the air gap. Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape. Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate.

With the switching of windings as High and Low signals, corresponding winding energized as North and South poles. The permanent magnet rotor with North and South poles align with stator poles which causes the motor to rotate.