Understanding PWM modulation in Electric drives

Pulse width modulated converter:

The voltage source converters are used in electrical drives.These converters utilize capacitors in the DC-link to store temporarily electricalenergy. Switching the power electronic devices allows the DC voltage tobe modulated which can result in a variable voltage and frequency waveform.The purpose of the modulator is to generate the required switching signalsfor these switching devices on the basis of user defined inputs.

For this purpose,the voltage–time integral was introduced which in turn is tied tothe average voltage per sample U(tk) that may be written as U(t)=(1/T_s) ∫ u(t) dt ranging between t_k to (t_k + T_s) where Ts is a given sample interval and u(t) represents the instantaneousvoltage across a single-phase of a load.The introduction of the variableTs assumes the use of a fixed sampling frequency which is normally judiciallychosen higher than the fundamental frequency range required to controlelectrical machines.The upper sampling frequency limit is constrainedby the need to limit the switching losses of the converter semiconductor devices.

The ability to control the converter devices in such a manner that the loadis provided with a user defined mean reference voltage per sample U∗(tk)is instrumental to control current accurately. The switchingstates of the converter must be controlled by the modulator to ensure thatthe average voltage (per sample) equals the user defined average referencevalue to ensure that the actual and reference incremental current change(per sample interval) are equal. The average voltage per sample U(tk) varies asfunction of the converter switch on/off time within a sample interval.

The space vector modulation is considered, together with the need to impose a modulatorstrategy that can handle the finite switch on/off times of practical converterswitches

Understanding Pulse Width Modulation (PWM)

Pulse Width Modulation (PWM) is a commonly used technique for generally controlling DC power to an electrical device, made practical by modern electronic power switches. However it also finds its place in AC choppers. The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast pace. The longer the switch is on compared to the off periods, the higher the power supplied to the load is.

The PWM switching frequency has to be much faster than what would affect the load, which is to say the device that uses the power. Typically switching has to be done several times a minute in an electric stove, 120 Hz in a lamp dimmer, from few kilohertz (kHz) to tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies.

The term duty cycle describes the proportion of on time to the regular interval or period of time; a low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on.

The main advantage of PWM is that power loss in the switching devices is very low. When a switch is off there is practically no current, and when it is on, there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero. PWM works also well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle.

PWM has also been used in certain communication systems where its duty cycle has been used to convey information over a communications channel.

Power delivery

PWM can be used to adjust the total amount of power delivered to a load without losses normally incurred when a power transfer is limited by resistive means. The drawbacks are the pulsations defined by the duty cycle, switching frequency and properties of the load. With a sufficiently high switching frequency and, when necessary, using additional passive electronic filters the pulse train can be smoothed and average analogue waveform recovered.

High frequency PWM power control systems are easily realisable with semiconductor switches. As has been already stated above almost no power is dissipated by the switch in either on or off state. However, during the transitions between on and off states both voltage and current are non-zero and thus considerable power is dissipated in the switches. Luckily, the change of state between fully on and fully off is quite rapid (typically less than 100 nanoseconds) relative to typical on or off times, and so the average power dissipation is quite low compared to the power being delivered even when high switching frequencies are used.

Modern semiconductor switches such as MOSFETs or Insulated-gate bipolar transistors (IGBTs) are quite ideal components. Thus high efficiency controllers can be built. Typically frequency converters used to control AC motors have efficiency that is better than 98 %. Switching power supplies have lower efficiency due to low output voltage levels (often even less than 2 V for microprocessors are needed) but still more than 70-80 % efficiency can be achieved.

This kind of control for AC is power known delayed firing angle method. It is cheaper and generates lot of electrical noise and harmonics as compared to the real PWM control that develops negligible noise