AMZ-FX Guitar effects. (2007) Mosfet Body Diodes. Accessed 22/12/09.

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1 Pulse width modulation Pulse width modulation is a pulsed DC square wave, commonly used to control the on-off switching of a silicon controlled rectifier via the gate. There are many types of SCR s, most use current as a control medium. There are SCR devices which work using voltage instead of current to determine on and off switching. AMZ-FX Guitar effects. (2007) Mosfet Body Diodes. Accessed 22/12/09. The above diagram shows a MOSFET, the source is the negative cathode, the drain represnets the positive anode, the gate is the switching voltage input. The gate voltage determins the maximum current allowed throught the drain, when the gate voltage falls below a threshold voltage the device will switch off. These are commonly used in DC motor control bridges. The pulse will switch on the device, which will the operate until the current or in the case of MOSFET s the voltage, running through it falls below that devices holding current, i.e. the current required to keep the device in operation. If the current falls below this level the device will stop conducting and act a diode, resisting the flow of electricity. Below is a diagram depicting an SCR in operation, the holding current level can be seen clearly PUPR. (2008) Special diode types. Accessed 21/12/09. Pulse width modulation, for gate firing is usually controlled using a small simple electronic chip. A pulse width generator construction follows.

2 Unknown. (n.d.) Control Technologies Manual-PWM AC Drives. 1 st ed. Control technologies. The pulses are commoly created by supplying an sine wave and a triangle wave, combining these two waves gives the following resault. It can be seen that these two waves combined gives a resaultant square pulsing waveform. The square wave is on when ever the sine wave exceeds the triangle wave and off when the triangular wave exceeds the sine wave. Unknown. (n.d.) Control Technologies Manual-PWM AC Drives. 1 st ed. Control technologies. Pulse width modulation in inverters and rectifiers Pulse width modulation is commonly used to create a sine wave from a series of small pulses of DC voltage. The figure below shows how this is achieved, it can be seen that all of the DC voltages are at higher amplitude than the maximum amplitude of the AC wave. The period of time the DC voltage is on in the duty cycle determines the magnitude of the DC wave, the longer the pulses are on the greater the AC output will be, but a consequence of the longer pulses are an increase in harmonic wave forms. It can also be seen in the figure that the shape of the sine wave is created by the length of each pulse, the pulses nearing either edge of the wave being on for a shorter amount of time and the middle pulse being on for the greatest amount of time.

3 Siemens. (unknown) DC Drives. Surry, Siemens. When the AC wave form reaches equal amplitude with the amplitude of the DC wave form then the AC output will increase only in frequency and not voltage. The frequency is increased by shortening the duty cycle of the pulses; this is shown in the example below Siemens. (unknown) DC Drives. Surry, Siemens. In the above diagram the duty cycle of the PWM has been halved and as a result the frequency of the AC output has doubled, without a loss in voltage output The above PWM technique is used in inverter circuits to fire the SCR s to create an AC output of the desired voltage, frequency and shape of wave output, from a standard DC input. Below shows a gate controlled SCR, and the effect a pulsing gate voltage has on the output. The SCR is fed a standard AC waveform but the device will not conduct without a gate voltage. In this case the gate voltage is pulsed at the same frequency as the AC waveform, but with about a 45 0 lag, this has the effect of reducing the wave by a quarter, this in turn reduces the RMS and average output voltages proportionally. This can be said to be a form of voltage control.

4 Bhatt, A. (2008-9) Single phase AC Motor speed controller. Accessed 28/12/09. The above technique is used to control the level of DC voltage output from a rectifier. A half wave rectifier output is shown below Free Electronics Projects-Electronics circuits and hobby designs diagrams. (2008) Half wave rectifiers. Accessed 5/1/09. If the two techniques are combined it is possible to chop the wave form, by firing the SCR s at a given angle of the AC waveform using PWM, this will lower the output DC voltage depending on the amount of waveform removed. This is the way a rectifier can be controlled using PWM to both convert AC voltage to DC voltage and adjusts the magnitude of the DC voltage output. Pulse width modulation in DC motor control SCR s and pulse width modulation can be used to control the speed of motors. The speed controller takes a signal which is represents the speed required and then runs the motor at that speed. The controller can directly or in-directly control the motor speed, if the motor speed is directly controlled then the controller is known as a feedback controller or a closed loop controller, these types tend to be more accurate but they are also more complicated and expensive, these are generally used in large motor circuits. If the control is in-direct then the controller is known as open loop speed control, this is a simpler and less expensive, and is used in small inexpensive motor arrangements. The motor drive output will vary dependent on the type of motor used. Below is a two loop speed control system, the box depicting the SCR is the motor control bridge commonly an h-bridge which will be discussed in greater depth later.

5 Barnes, M. (2003) Variable speed drives and power electronics. Oxford, Elsevier. In a DC motor the speed is directly proportional to supply voltage, by, From this it can be said that a fall in the vaule of the voltage will be proportional to that of speed, i.e. if the voltage is halfed then the speed will half. A speed controller works by varying the average voltage supplied to the motor. This can be achived by inserting a varieable resistor inline with the supply, this is inefficent and inaccurate. A prefered method is to use a pulse width modulation device to switch the, via a SCR bridge, supply at a high frequancy, the motor is only able to reconise the average supply, as the frequancy is to rapid to allow the motor to speed up or slow down at times when the voltage rises or falls, and so when the on-off time is 50%-50% and the voltage switches between 5V and 0V the motor will work at 2.5V. If the on time is incresed then the motor speed increases. This is how pulse width modulation controls the speed of an DC motor. This switching is controlled by use of a on-off SCR device i.e. one pulse will switch the device on the other will switch it off, this allows a small current or voltage to switch a large current. below can be seen an example of how a pulse width modulator works, it can be seen that if the pulses are held for less time within a cycle then the average voltage reconised by the motor will lessen and thus so will the speed, as the pulses are held for a longer period of the cycle the average voltage increases and the speed follows. Society of robots. ( ) INTRODUCTION TO MICROCONTROLLERS. Accessed 3/1/10.

6 The breaking time and the acceleration of the motor will be dependent on the inertia of the rotor, by the friction and the load torque. Eduypedia. ( ) Power control. Accessed 15/12/09. The above graph shows the switching of a motor at a low frequancy. It shows that the average of the motor is 150rpm. If the switching frequancy is increased then the motor will not be given time to slow or speed up an so would register a constant speed of 150rpm. H bridge configuration By Ian Loram-Martin, using CAD The above diagram shows the H bridge set up, the components are generic and can be changed for similar devices based on the needs of the motor. It is constructed using four on-off SCR devices (can be any type depending on requirments of current, voltage and/or switching time requirments), these can be switched on and off in certain arrangments to gain differing resaults, it should be noted that the motor positive is on the left and the negitive on the right, also it should be noted that the field winding (L 1 ) diode is used to block the supply when all of the SCR s are switched off. The switching of the SCR s are controlled, most commonly, by use of PWM control circuits (as previously discussed). This also allows the speed of the motor to be controlled using the PWM to fire the SCR s in such a way that the magnitiude of voltage to be fed to the motor is changed in turn changing the speed. Below is the switching required to make the motor rotate in its forward direction.

7 By Ian Loram-Martin, using CAD The above diagram shows the motor in a forward motion state. The PWM is applied to Q 1 and Q 4 this switches the SCR s into a conducting state, the amount of time the SCR, in this case Q 1 is pulsed on and off for controls the average voltage (the voltage reconised by the motor), if the SCR is in the on position for 50% of the time and off for 50% of the time then the supply voltage is halfed by the time it reaches the motor. Switching of these two devices current to flow through the path shown (flowing from top to bottom). By Ian Loram-Martin, using CAD The above diagram shows the current flow when the supply from Q 1 is switched off by the PWM controler, Q 4 will still be in the on state the remaining current is dissapated through this path using the intrinsic diode in Q 3.

8 By Ian Loram-Martin, using CAD When the PWM is applied to the SCR Q 2 and Q 3 is in the on state, it causes the current to flow through the motor but the polarity is reversed, this causes the motor to work in the reverse direction. The control remains the same as in the previous circuit for forward motion of the motor. By Ian Loram-Martin, using CAD The above diagram again shows the dissapation of the current, this time the SCR Q 3 is kept on and the current flows through the intrinsic diode of Q 4. This occurs when the PWM is in the off state to the Q 2 SCR. The SCR can also be switched to cause sudden braking in the motor, this is done by switching on either Q 1 and Q 2 or Q 3 and Q 4. This is due to the fact that the motor is a generator and when it turns without a supply it generates a voltage. If the terminals of the motor are connected (shorted), then the voltage generated counteracts the motors freedom to turn. It is as if a voltage is applied which is similar but in opposition to the generated voltage, this act causes the motor to slow i.e. breaking it. Also switching on of either Q 1 and Q 3 or Q 2 and Q 4 will cause a short circuit of the motor arrangment, this can cause the motor not to rotate, heating of wiring, batteries, and switches possibly causing damage to each, and can in extreme cases can cause the an explosive reaction. This is very dangrous and should always be avoided.

9 By Ian Loram-Martin, using CAD The above diagram shows the flow of current when the motor is under braking conditions. When braking the motor becomes a generator. The produced current is redirected back into the batterys, charging them, this is know as regenerative braking, which will be explored further in part C of question 2. It should be noted that regenerative braking can occur through SCR s Q 4 and Q 1 as well. The H bridge is the only arrangment where the motor can be reversed, regenerative braking can be implmented and speed controlled. It also allows a good torque through out the speed range. This gives it a major advantage over all other types of DC motor control.