Week-7 Challenge: DC Motor Control

Ans 1.A

Speed control of a DC motor using BJT H-Bridge

In this model 4 switches are used and for that 4 pulse signals are generated .

Here H-bridge is designed to control speed of the motor

IGBTs acts as BJT as switching devices

4 IGBT switches are used for inverting the DC voltage 0 to 240 v supply. The current flows from collector(C) to emitter(E).

The gate is controlled by the pulses (0 and 1) according to the armature voltage of motor given by pulse generator.

In the Image we can see the armature voltage control with a step time of 0.5 and how the initial to final values are given and accordingly pulses are generated.

In the above image we can see that four switches are used Q1, Q2, Q3, Q4. Pulses P1, P2, P3, P4 are connected with corresponding switches.

In first state, Q1 and Q4 are closed by giving 1V to IGBT'S, current from battery passes through Q1 and Q4. Diode D2, D3 are used as freewheeling diode in this state. In this case Forward motoring and forward breaking are held.

In Second state, Q2 and Q3 are closed by giving 1V to IGBT's, current from battery passes through Q2 and Q3. Diode D1, D4 are used as freewheeling diode in this state. In this case reverse motoring and reverse breaking are held. The current direction change its direction in this state.

Both the states are occurring in 0.5 seconds in their sample time. Thus DC supply converted and feeded to the armature of the motor.

Here freewheeling diodes are used as a safety purpose of this inverter circuit as when the flow of current on low values protects the circuit and machine by feed backing the current or in high voltage fluctuations voltage gets split as protection to the motor and circuit components.

In motor operation, armature terminals are connected with a 240V along the inverter circuit and field windings are connected with 300 V DC supply to act as a electro magnet for the motor.

The load torque is given as mechanical input to the motor for its reference.

Thus through this magnetic fields generated, variable current carrying conductors in rotor armature produces forces according to Fleming left hand rule principle. Thus, the motor starts to rotate.

Here run this demo model and study the results from scopes.

Result :

Speed of motor:

Initially increases to 0 to nearly 900 rpm for 0.5 secs (Forward motoring action takes place).

Then speed reduces to 0 rpm (Forward dynamic breaking action takes place).

Then speed increases from 0 to -900 rpm up to 1 sec (Reverse motoring action takes place).

Then reverse dynamic breaking occurs (Not shown in scope).

Load torque of motor:

Initially torque increase from 0 to 0.5 secs at a value of 6 Nm in forward motoring action.

Then torque reduces to 0 Nm in forward dynamic breaking mode.

Then torque increases in reverse direction to a value of -6 Nm in reverse motoring action till 1 sec.

Again, torque reduces to zero. (Not shown in scope).

Ans 1.B.

Here the initial current shoot up is due to sudden increase in voltage which in turn is regulated by duty cycle. In this case motor driven in 75% of duty cycle. (i.e.) 0.75*240= 180 V.

Whenever switching action occurs the armature current has a huge spike i.e. at 0sec forward motoring and 0.5sec reverse motoring.

The Speed of DC Motor can be controlled by varying the Supply Voltage and for this PWM method is used.

By Reducing the pulse width the armature current shoot up can be controlled.

In Pulse generator Block the pulse width is reduced to 50% (i.e. 120 V) so the gate pulse value and the diode and IGBT current value will also be varied.

The above graph shows the DC motor output when pulse width is reduced to 50%.

When Duty Cycle is 50% the armature starting current has no spike but during the switching action at 0.5sec there is small overshoot but the Spike has reduced much as compared to the spike when duty cycle was 75%.

Again changing the duty cycle to 35%

The shoot up of current get diminished on both the ends.

Thus to reduce the shoot up armature currents we have to change the duty cycle gradually from 0 to 100%,that in turn increases voltage gradually and speed also changes accordingly.

Ans 1.C.

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

· Speed Controller (DC)

· Regulation Switch

· Current Controller (DC)

· Chopper

Inputs: Armature voltage, speed reference and motor torque

Output: Motor measurement vector, Converter measurement vector (IGBT/diode measurement vector), Controller measurement vector

Working:

· 4 IGBT switches are used

· Pulses are given from PI current controller which generates pulses by comparing armature current and current reference.

· Current reference can be generated by using regulation switch, which includes torque reference and PI speed controller pulses which in turn internally compares speed of motor and reference speed (given as input)

· Speed of motor is controller mainly by current controller.

· In this model 4 quadrant operation is performed .

· Forward motoring and forward braking mode (regen braking) Q1 and Q4 are ON and Q2 and Q3 are OFF

· Reverse motoring and reverse braking mode Q2 and Q3 are ON and Q1 and Q4 are OFF

Comparison with H-Bridge circuit:

· In DC7 four quadrant model speed is controlled by varying current pulses from PI current controller and in H-bridge model speed is controlled by varying duty cycle voltages.

· Here due to 4 quadrant operation regenerative braking occurs whereas in H-bridge circuit for regenerative braking to occur we need to change the polarity of current flow.

· In DC7 four quadrant model smoothing inductance is used for smooth increase and decrease in current flow which controls the motor speed smoothly which is missing in H-bridge model

· In H-bridge model higher armature current ripples are observed whereas In DC7 four quadrant model lower armature current ripple are there due to high switching frequency of DC-DC converters

Ans 2 : 2 quadrant chopper model

Graph :

Hence, we could see clearly that the voltage is always positive but the current becomes both positive and negative.

• Here either one should be ON at a time, either CH1 or CH2.
• If CH1 is ON the current flows from source positive to DC motor, passes through L and E to negative terminal of source.(1st quadrant)
• If CH1 is OFF the, FD diode conducts and complete the circuit (L-E-FD-L) as stored energy in inductor L. The voltage across is also positive voltage, current direction also positive.
• Now If CH2 is ON, Current flow from E (Load current) completes the circuit(L-CH2-E).
• If CH2 gets OFF, the current passes through D2 towards source completes the circuit
• (E-L-D2-positive source -negative source-E).
• In this stage regenerative breaking occurs. (2nd quadrant).
• This cycle continuous again up to stop time.

Working: Pulses for the two IGBT switches are given by using pulse generator and NOT logical operation for alternate switch ON/OFF.

Ques 3 : BLDC Motor :

BLDC motor named as brushless dc motor having high efficiency than other types of motors. This motor is also known to be synchronous motor.

The stator coil is wounded with coils, they are stationary but producing variable DC voltages for rotating the permanent magnet as changing the magnetic field of stator poles continuously.

3 winding terminals are coming out as U, V, W, and another end are connected in single point The angle shift between two coils makes 120 degrees.

The current moves from U to W makes magnetizing the coils attracts the rotor consisting of permanent magnet makes moment to align with the resultant arrow(larger arrow).

Rotation is maintained by continually switching the flux so that the permanent magnet is constantly chasing the rotating magnetic field induced by the coils. In other words, energizing of U, V, and W must be continually switched so that the resultant flux keeps moving, producing a rotating field that continually pulls on the rotor magnet.

To attain smooth continuous rotation of rotor we have to control the current flow through all the three poles in same time.

we can acheive continuous change in current flow in all stator poles at same time in 120 degree phase shift.

Control of BLDC motor:

By using the inverter circuitry to also adjust the voltage into each coil, we can control the magnitude of the current.

A typical way to adjust the voltage is with pulse width modulation (PWM). We alter the voltage by lengthening or reducing the pulse ON time ( “Duty cycle”: the ON time

expressed as a ratio of the ON+OFF switching interval). Increasing the duty cycle has the same effect as raising the voltage, reducing the duty cycle has the same effect as lowering the current.

PWM can be implemented using MPUs equipped with dedicated PWM hardware