Random pulse-width modulation (RPWM) is a

modulation In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the '' carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informat ...

technique introduced for mitigating

electromagnetic interference Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrost ...

(EMI) of

power converter Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may ...

s by spreading the

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...

of the noise signal over a wider

bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...

, so that there are no significant peaks of the noise. This is achieved by randomly varying the main parameters of the

pulse-width modulation Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts. The average value of voltage (and current) fed ...

signal.

Description

Electromagnetic interference Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrost ...

(EMI) filters have been widely used for filtering out the

conducted emissions Conducted emissions are the effects in power quality that occur via electrical and magnetic coupling, electronic switch of semiconductor devices, which form a part of electromagnetic compatibility issues in electrical engineering. These affect the ...

generated by

s since their advent. However, when size is of great concern like in

aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines. ...

and

automobile A car or automobile is a motor vehicle with wheels. Most definitions of ''cars'' say that they run primarily on roads, seat one to eight people, have four wheels, and mainly transport people instead of goods. The year 1886 is regarded ...

applications, one of the practical solutions to suppress

is to use random pulse-width modulation (RPWM). In conventional

(PWM) schemes, the

harmonics A harmonic is a wave with a frequency that is a positive integer multiple of the '' fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', ...

power is concentrated on the deterministic or known frequencies with a significant magnitude, which leads to

mechanical vibration Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic, such as the motion of a pendulum—or random, suc ...

noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...

, and EMI. However, by applying

randomness In common usage, randomness is the apparent or actual lack of pattern or predictability in events. A random sequence of events, symbols or steps often has no order and does not follow an intelligible pattern or combination. Individual rand ...

to the conventional PWM scheme, the

harmonic A harmonic is a wave with a frequency that is a positive integer multiple of the '' fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', ...

power will spread out so that no

of significant magnitude exists, and peak

at discrete

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from '' angular frequency''. Frequency is measured in hertz (Hz) which is ...

are significantly reduced. In RPWM, one of the switching parameters of the PWM signal, such as switching frequency, pulse position and

duty cycle A duty cycle or power cycle is the fraction of one period in which a signal or system is active. Duty cycle is commonly expressed as a percentage or a ratio. A period is the time it takes for a signal to complete an on-and-off cycle. As a formu ...

are varied randomly in order to spread the energy of the PWM signal. Hence, depending on the parameter which is made random, RPWM can be classified as ''random frequency modulation'' (RFM), ''random pulse-position modulation'' (RPPM) and ''random duty-cycle modulation'' (RDCM). The properties of RPWM can be investigated further by looking at the

power spectral density The power spectrum S_(f) of a time series x(t) describes the distribution of power into frequency components composing that signal. According to Fourier analysis, any physical signal can be decomposed into a number of discrete frequencies, ...

(PSD). For conventional PWM, the PSD can be directly determined from the

Fourier Series A Fourier series () is a summation of harmonically related sinusoidal functions, also known as components or harmonics. The result of the summation is a periodic function whose functional form is determined by the choices of cycle length (or '' ...

expansion of the PWM signal. However, the PSD of the RPWM signals can be described only by a

probabilistic Probability is the branch of mathematics concerning numerical descriptions of how likely an event is to occur, or how likely it is that a proposition is true. The probability of an event is a number between 0 and 1, where, roughly speaking, ...

level using the theory of stochastic processes such as

wide-sense stationary In mathematics and statistics, a stationary process (or a strict/strictly stationary process or strong/strongly stationary process) is a stochastic process whose unconditional joint probability distribution does not change when shifted in time. ...

(WSS)

random processes In probability theory and related fields, a stochastic () or random process is a mathematical object usually defined as a family of random variables. Stochastic processes are widely used as mathematical models of systems and phenomena that appea ...

RFM

Among the different RPWM techniques, RFM (random frequency modulation) is the most common method of the three major types, used in many

topologies to pass the

electromagnetic compatibility Electromagnetic compatibility (EMC) is the ability of electrical equipment and systems to function acceptably in their electromagnetic environment, by limiting the unintentional generation, propagation and reception of electromagnetic energy whic ...

(EMC) test. In this type of modulation, the switching frequency of the PWM signal is varied

randomly In common usage, randomness is the apparent or actual lack of pattern or predictability in events. A random sequence of events, symbols or steps often has no order and does not follow an intelligible pattern or combination. Individual rando ...

in order to spread the emitted noise of the

s in which it is applied. RFM is very easy to implement and it offers significant reduction of the

peaks compared to conventional PWM. However, application is limited to

s which does not require fixed switching frequency for their normal operation. A greater degree of switching frequency variation can affect the proper functioning of the devices and components inside the

circuit Circuit may refer to: Science and technology Electrical engineering * Electrical circuit, a complete electrical network with a closed-loop giving a return path for current ** Analog circuit, uses continuous signal levels ** Balanced circu ...

RPPM

RPPM is also commonly deployed in

s to pass the EMC compliance tests. This

technique also offers significant reduction of the conducted emission and, consequentially, the radiated emission of

s. However, compared to RFM, RPPM is less effective in EMI reduction. This is because the PSD of RPPM contains both the density and harmonic components, and the spectrum cannot be fully spread unlike that of RFM where the spectrum has only the density component. However, in this

scheme, both the switching frequency and the

pulse width The pulse width is a measure of the elapsed time between the leading and trailing edges of a single pulse of energy. The measure is typically used with electrical signals and is widely used in the fields of radar and power supplies. There are two c ...

are fixed so that the converter components like

inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a ...

s and

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of a ...

s can function properly.

RDCM

In RDCM, the

or the

of the PWM signal is varied randomly in order to spread the noise spectrum. This kind of

is less common compared to the previous ones. This is because RDCM is less effective at spreading the noise. Moreover, randomly varying the

may cause output

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge t ...

fluctuations and ripples. Besides, in some

topologies, the

variation is the primary means of controlling the input-output

s and

currents Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

using closed loop control systems. An example of this could be the drive for a brushed DC motor. Since the power to the motor is already being "chopped" at a specific frequency to vary the voltage and current, introducing randomization into the process could cause detriments to the system's performance.

In variable-frequency inverter systems

This type of modulation is becoming more common in

variable-frequency drive A variable-frequency drive (VFD) is a type of motor drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and, depending on topology, to control associated voltage or curre ...

s of all sizes and applications. in consumer-sized VFDs that include it as a feature, it presents as a user-selectable parameter, often with several different operating levels. Some drives may also use more than one method at once. However, while the term "RPWM" is generally used to label this type of modulation in a technical sense, the technology does not yet have a label in the world of VFD parameter names. For example,

Fuji Electric , operating under the brand name FE, is a Japanese electrical equipment company, manufacturing pressure transmitters, flowmeters, gas analyzers, controllers, inverters, pumps, generators, ICs, motors, and power equipment. History Fuji Electric ...

labels its noise-reduction parameter as "motor tone", while both

Mitsubishi Electric , established on 15 January 1921, is a Japanese multinational electronics and electrical equipment manufacturing company headquartered in Tokyo, Japan. It is one of the core companies of Mitsubishi. The products from MELCO include elevators a ...

and Teco Westinghouse label it as "soft PWM". The use of the term "soft PWM" could potentially cause confusion to those not familiar with this technique, as

zero crossing control Zero crossing (or burst-firing) control is an approach for electrical control circuits that starts operation with the AC load voltage at close to 0 volts in the AC cycle. This is in relation to solid state relays, such as triacs and silicon contr ...

is sometimes labeled as "soft switching". A term that could be potentially used as a standard label could be "scrambled PWM" or "carrier scrambling", as the word "scramble" in this sense is nonspecific to the method of RPWM used, yet informs the user that there is indeed a specialized process occurring which affects the shape and properties of the output waveform. In addition, the word has a home in the

telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than tha ...

field, where a

scrambler In telecommunications, a scrambler is a device that transposes or inverts signals or otherwise encodes a message at the sender's side to make the message unintelligible at a receiver not equipped with an appropriately set descrambling device. Wher ...

is any device (typically analog) that is used to encode a signal so that it will be unintelligable if intercepted before it can reach the intended recipient without an appropriately tuned de-scrambler. That is to say that "scramble" would not be out of place if used to label RPWM in a more general sense. Regardless of the label, upon observation of the manual in consumer-sized vfds that include it, the focus of the parameter description does appear to be on acoustic noise reduction, rather than EMI reduction and motor health. In modern rail traction converters, this method presents in a change in the sound that the motors emit when driven by inverters which utilize it. As opposed to the normally steady, carrier-based whine of a classic SPWM convertor, the sound is more of a hiss, akin to

white noise In signal processing, white noise is a random signal having equal intensity at different frequencies, giving it a constant power spectral density. The term is used, with this or similar meanings, in many scientific and technical disciplines, ...

. Because rail traction convertors operate at such high power levels, EMI is more readily created in such systems. In these applications RPWM is highly beneficial to the motor's health and the level of emitted EMI.

Coexistence issue

RPWM techniques are very effective in reducing the EMI of

s. However, when

s with this special type of

coexist with

communication systems A communications system or communication system is a collection of individual telecommunications networks, transmission systems, relay stations, tributary stations, and terminal equipment usually capable of interconnection and interopera ...

, there may be a severe

conflict between the power system and the

communication system A communications system or communication system is a collection of individual telecommunications networks, transmission systems, relay stations, tributary stations, and terminal equipment usually capable of interconnection and interopera ...

. This detrimental effect can be observed in

power line communication Power-line communication (also known as power-line carrier or PLC) carries data on a conductor that is also used simultaneously for AC electric power transmission or electric power distribution to consumers. A wide range of power-line communicati ...

(PLC) systems, where both

s and

coexist. Indeed, recent studies have confirmed that RPWM applied to

s to minimize

can detrimentally interfere with the PLC system. FER comparison between PWM and RPWM

The interference can be worsened when the switching frequency and the

of the PLC system overlaps with that of the power system. Most

s use a switching frequency that is below 150 kHz, which is in the

low frequency electromagnetic compatibility Low-frequency electromagnetic compatibility (LF EMC) is a specific field in the domain of Electromagnetic compatibility, electromagnetic compatibility (EMC) and Power quality, power quality (PQ), which deals with electromagnetic interference phenom ...

range. This could cause coexistence issues mainly in narrow band PLC systems, (specialized PLC protocols which are being used for

smart grid A smart grid is an electrical grid which includes a variety of operation and energy measures including: * Advanced metering infrastructure (of which smart meters are a generic name for any utility side device even if it is more capable e.g. a ...

application, such as Prime PLC and G3-PLC, in frequencies below 150 kHz.) In conventional PWM, the noise from

overlaps with the PLC

band at discrete multiples of the switching frequency only. This results less interference to the PLC system. However, In RPWM, the noise is more spread and both the PLC and the noise from

shares wider

. This creates more disturbance to the PLC system. Therefore, it is advisable to carefully observe the properties of any coexisting systems in order to choose a switching frequency for randomly modulated

s that does not overlap with that of a coexisting PLC system.

References

{{reflist

External links

* https://www.analog.com/en/technical-articles/spread-spectrum-frequency-modulation-reduces-emi.html * https://www.analog.com/en/technical-articles/easy-to-use-spread-spectrum-clock-generator-reduces-emi-and-more.html Signal processing

Description

RFM

RPPM

RDCM

In variable-frequency inverter systems

Coexistence issue

See also

References

External links