Week-7 Challenge: DC Motor Control
H- BRIDGE:
H-bridge is a rather simple circuit, containing four switching element, with the load at the center, in an H-like configuration.
The switching elements (Q1..Q4) are usually bi-polar or FET transistors, in some high-voltage applications IGBTs. Integrated solutions also exist but whether the switching elements are integrated with their control circuits or not is not relevant for the most part for this discussion. The diodes (D1..D4) are called catch diodes and are usually of a Schottky type.
The top-end of the bridge is connected to a power supply (battery for example) and the bottom-end is grounded.
In general all four switching elements can be turned on and off independently, though there are some obvious restrictions.
Static Operation
The basic operating mode of an H-bridge is fairly simple: if Q1 and Q4 are turned on, the left lead of the motor will be connected to the power supply, while the right lead is connected to ground. Current starts flowing through the motor which energizes the motor in (let’s say) the forward direction and the motor shaft starts spinning.
If Q2 and Q3 are turned on, the reverse will happen, the motor gets energized in the reverse direction, and the shaft will start spinning backwards.
In a bridge, you should never ever close both Q1 and Q2 (or Q3 and Q4) at the same time. If you did that, you just have created a really low-resistance path between power and GND, effectively short-circuiting your power supply. This condition is called ‘shoot-through’ and is an almost guaranteed way to quickly destroy your bridge, or something else in your circuit.
SIMULINK MODEL:
The BJT when used for power switching application operates as IGBT. When it is conducting a forward voltage Vf is developed between collector and emitter (in the range of 1V). 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 simulink signal.
MODEL DESCRIPTION:
- DC motor used in the model is 5HP 24V and 1750rpm. It simulates a fan type load. The armature mean voltage can be varied from 0 to 240V when the duty cycle is varied from 0% to 100%.
- The H-bridge consists of 4 BJT/diode pairs.
- Two transistors are switched simultaneously. Q1 and Q4 or Q2 and Q3. When the Q1 and Q4 are fired, a positive voltage is applied to the motor and diodes D2-D3 operate as free-wheeling diodes when Q1-Q4 are switched off.
- When Q 2 and Q3 are fired, a negative voltage is applied to the motor and the diodes D1-D4 operate as free wheeling diodes when Q2 and Q3 are switched off.
OUTPUT:
Thus from the graph, it is seen that the from 0 to 0.5S, the current flows through D3 and from 0.5S to 1S, the current flows through Q3.
The motor starts in the positive direction with a duty cycle of 75%. At t =0.5S, the armature voltage is suddenly reversed and the motor runs in negative direction.
From the armature current graph, we can able to visualize that at the starting there is a spike increase in the current at 0S and also at 0.5S.
The voltage gets changed in the circuit, the speed of the motor also changes.
In the transition of positive to negative direction of rotation we observe a high surge in current, this is due to the transient switching occurring in the current.
1B. ARMATURE CURRENT:
From the armature current graph, we can see that at the starting there is a sudden spike increasing in current at 0S and also at 0.5S. This means that there is a transition from positive to negative direction of rotation we observe a high surge in current.
As there is already current flowing in the circuit & on changing of switches after 0.5S, the current in the circuit adds to the changed direction of current and we observe the surge in current.
The surge in the current can be reduced by reducing the duty cycle.
If the duty cycle is changed from 75% to 50%, the current surge can be reduced.
1C
COMPARISON OF 4 QUADRANT CHOPPER AND BJT H BRIDGE:
- The 4 quadrant choppers works in all 4 modes like
- Forward motoring
- Forward breaking
- Reverse motoring
- Reverse breaking
But BJT H bridge model works on two modes.
- Forward motoring
- Reverse motoring
- The current signals get transited from one to other switch if the direction of rotation of motor is changed from forward to reverse. While in H bridge, the direction of rotation of motor is changed by changing the polarity of current.
- The 4 quadrant chopper has switching frequency while the H bridge model has low switching frequency.
- The armature current in the 4 quadrant chopper have a little spike at transition from one to other switch because of high switching frequency while H bridge model have high spike of current.
- TWO QUADRANT CHOPPER:
Type C chopper is obtained by connecting type –A and type –B choppers in parallel. We will always get a positive output voltage V0 as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage v0 will be equal to Vs . The direction of the load current i0 will be reversed. The current i0 will be flowing towards the source and it will be positive regardless the chopper is on or the FD conducts. The load current will be negative if the chopper is or the diode D2 conducts. We can say the chopper and FD operate together as type-A chopper in first quadrant. In the second quadrant, the chopper and D2 will operate together as type –B chopper.
The average voltage will be always positive but the average load current might be positive or negative. The power flow may be life the first quadrant operation ie from source to load or from load to source like the second quadrant operation. The two choppers should not be turned on simultaneously as the combined action my cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.
Two mosfet blocks and diodes are connected as shown in the circuit diagram.
The DC voltage is set as 24V.
The load is considered as series RLC branch and inductor L. Thus the block works as a load. 12V is set as the voltage for the load.
The pulse generator block is used to create gate pulse. The NOT gate is used so that the two MOSFET will not get closed at the same time otherwise shortcircuit will occur due to the use of NOT gate. When one mosfet is ON, the other mosfet will be in off state.
The amplitude of pulse generator is kept as 10s and the period as 0.02s and pulse width is 50%.
The voltage waveform is not going less than zero. The minimum value of voltage is zero and the maximum is always in the positive direction.
In the current waveform, the current value increases and decreases. The current values are in positive and negative.
The pulse amplitude is 10 and thus from pulse waveform we can see that the pulse amplitude is 10.
- BLDC MOTOR
A brushless DC motor (known as BLDC) is a permanent magnet synchronous electric motor which is driven by direct current (DC) electricity and it accomplishes electronically controlled commutation system (commutation is the process of producing rotational torque in the motor by changing phase currents through it at appropriate times) instead of a mechanically commutation system. BLDC motors are also referred as trapezoidal permanent magnet motors.
Unlike conventional brushed type DC motor, wherein the brushes make the mechanical contact with commutator on the rotor so as to form an electric path between a DC electric source and rotor armature windings, BLDC motor employs electrical commutation with permanent magnet rotor and a stator with a sequence of coils. In this motor, permanent magnet (or field poles) rotates and current carrying conductors are fixed.
The armature coils are switched electronically by transistors or silicon controlled rectifier the correct rotor position in such a way that armature field is in space quadrature with the rotor field poles. Hence the force acting on the rotor causes it to rotate. Hall sensors or rotary encoders are most commonly used to sense the position of the rotor and are positioned around the stator. The rotor position feedback from the sensor helps to determine when to switch the armature current.
This electronic commutation arrangement eliminates the commutator arrangement and brushes in a DC motor and hence more reliable and less noisy operation is achieved. Due to the absence of brushes BLDC motors are capable to run at high speeds. The efficiency of BLDC motors is typically 85 to 90 percent, whereas as brushed type DC motors are 75 to 80 percent efficient. There are wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large power ranges.
Construction of BLDC Motor
BLDC motors can be constructed in different physical configurations. Depending on the stator windings, these can be configured as single-phase, two-phase, or three-phase motors. However, three-phase BLDC motors with permanent magnet rotor are most commonly used.
The construction of this motor has many similarities of three phase induction motor as well as conventional DC motor. This motor has stator and rotor parts as like all other motors.
Stator of a BLDC motor made up of stacked steel laminations to carry the windings. These windings are placed in slots which are axially cut along the inner periphery of the stator. These windings can be arranged in either star or delta However, most BLDC motors have three phase star connected stator.
Each winding is constructed with numerous interconnected coils, where one or more coils are placed in each slot. In order to form an even number of poles, each of these windings is distributed over the stator periphery.
The stator must be chosen with the correct rating of the voltage depending on the power supply capability. For robotics, automotive and small actuating applications, 48 V or less voltage BLDC motors are preferred. For industrial applications, 100 V or higher rating motors are used.
Rotor
BLDC motor incorporates a permanent magnet in the rotor. The number of poles in the rotor can vary from 2 to 8 pole pairs with alternate south and north poles depending on the application requirement. In order to achieve maximum torque in the motor, the flux density of the material should be high. A proper magnetic material for the rotor is needed to produce required magnetic field density
Hall Sensors
Hall sensor provides the information to synchronize stator armature excitation with rotor position. Since the commutation of BLDC motor is controlled electronically, the stator windings should be energized in sequence in order to rotate the motor. Before energizing a particular stator winding, acknowledgment of rotor position is necessary. So the Hall Effect sensor embedded in stator senses the rotor position.
Most BLDC motors incorporate three Hall sensors which are embedded into the stator. Each sensor generates Low and High signals whenever the rotor poles pass near to it. The exact commutation sequence to the stator winding can be determined based on the combination of these three sensor’s response.
Working Principle and Operation of BLDC Motor
BLDC motor works on the principle similar to that of a conventional DC motor, i.e., 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 case BLDC motor, the current carrying conductor is stationary while the permanent magnet moves.
When the stator coils are electrically switched by a supply 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.
Consider the figure below in which motor stator is excited based on different switching states. 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 causing motor to rotate.
Observe that motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). By this way motor moves in a clockwise direction.
Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low and High level signals whenever rotor poles pass near to it. These signals determine the position of the shaft.