[Episode 9] A Shortcut to Becoming a Super Engineer!? Learning from the User’s Perspective

Summary of the Previous Episodes

Last time, Ichinose attended a meeting with users together with Teacher Sugiken for the first time. He was full of motivation, but he couldn’t keep up with the conversation at all during the meeting and became depressed. Ichinose realized what he was lacking and asked Professor Sugiken to explain the contents of the meeting again.

Character Introduction

• Ichinose (the protagonist) is a new engineer. He has been aiming to become an engineer since he was in middle school, and finally joined ROHM. He is so passionate that he forgets to eat and sleep while studying on motor drivers. Currently, only Ichinose can see Dora and Tako.
• Ninomiya is in the same year as Ichinose. Her grades are always at the top. She has a strong personality, but she is also a hard worker and has a high opinion of Ichinose. She is in secret a big fan of Sugiken.
• Teacher Sugiken is a super engineer at ROHM. He is usually kind, but he is passionate and takes pride in his work as an engineer. Sugiken used to be able to see Dora and Tako, but now he can’t.
• Dora is a motor driver fairy who loves people who are passionate about motors. He has a crush on Tako, but is always at the mercy of the insensitive Tako.
• Tako is a motor fairy and childhood friend of Dora. She is knowledgeable about motors, and her knowledge surpasses that of Dora. Although she is a reliable older sister, she is insensitive when it comes to love, and is unaware of Dora’s feelings.

Required Performance of Brushless Motors: Quietness, Reliability

In my eighth talk of the other day, I explained that there are three types of performance that are sought from brushless motors: efficiency, quietness, and reliability. Among these, I talked about efficiency. Now I’ll explain the other two types of performance, quietness and reliability.

Contents of Episode 9

• ・Motor Quietness: Noise And Cause
• ・Noise Measurement
• ・Reliability of Motor Circuits
• ・Electromagnetic Compatibility (EMC)

Motor Quietness: Noise And Cause

In general, when an object is moved using a motor, quietness is desired. So, what kind of noise hinders quiet operation? Standards for judging a noise are different depending on whether the noise is itself irritating and whether or not the noise is masked by other noises.

The noise of wind being blown by a fan and the suction noise of a vacuum cleaner are not so very irritating. And, mechanical equipment in a factory is not required to be extremely quiet. But the low-pitched buzzing sounds from air conditioners at night can be unpleasant.

In other words, sounds may be recognized as noise, or on the other hand may not pose a problem, depending on the environment and on how a machine is used. The developer of a motor driver must be aware of how the sounds that the motor may emit will be perceived in various settings and applications.

Here are some examples of problematic noises that occur when motors are running.

Broadly speaking, there are two kinds of sounds emitted by motors.

One is the sounds that occur because a motor is a vibration source. These are sounds that emerge from the machine in which the motor is installed, and correspond to vibrational sounds, impact sounds, and resonant sounds.

The other is the sounds produced by the motor itself. These are sounds that can be heard even when the motor is run independently; they may include electromagnetic sounds and bearing sounds.

Such sounds are classified as noise in the diagram below. Causes of these various types of noise include torque pulsation, torque fluctuation, coil vibration, and friction sounds. We can say that among these, only cogging is unrelated to motor drivers. Of course, it’s not just motor drivers that cause noises. If a certain motor driver’s specs have been changed to those of another motor driver, the sounds may change, and so the motor driver ranks as an important component related to the origins of noise.

<Vibration sounds and impact sounds>
The vibration sounds that occur because a motor is a source of vibrations become noise when torque pulsations of the motor propagate as vibrations to the machine body, producing a clattering or a buzzing sound. Impact noises are noises caused by sudden changes in torque, and sound as though the equipment body is being tapped or struck. Vibration noises and impact noises both occur when the torque pulsations or changes in torque of a motor are comparatively large.

<Resonant sounds>
Resonant sounds occur at resonance frequencies of the motor and the equipment or the load (such as a fan). For example, when a motor is turning a fan, if the torque pulsation frequency coincides with a resonance frequency, a whirring or humming noise may occur. Such resonant noises tend to occur even for relatively small torque pulsations.

In addition, changes in the motor rotation rate, lightening of the load, or a change in the specifications of the shaft that supports the load may cause the frequency of torque pulsation to coincide with a resonance frequency, so that resonant sounds occur.

<Torque pulsation>
Torque pulsation that causes noise occurs when there is pulsing of the strength of the winding magnetic fields, or when there is pulsing of the relative angle between winding magnetic fields and the permanent magnet magnetic field (see Episode 7 What Is Torque Pulsation?, Sinusoidal Conduction).

Apart from this, there is also torque pulsation that is called cogging torque. Cogging torque occurs when there are changes in the amount of magnetic flux accommodated by the stator core and the permanent magnets of the motor. Here “amount of accommodation” uses as an index the ease of passage of magnetic flux (called the permeance) leaving the permanent magnet N poles, passing through an air layer to enter the stator core, and passing through the stator core to the S poles. A rotor position at which magnetic flux easily passes through is a position of easy accommodation. When the rotor moves from a position of easy accommodation to where the magnetic flux does not pass through easily, a torque appears that tries to move the rotor back to the position of easy accommodation. This is called the cogging torque. Ordinarily, in a two-pole three-slot brushless motor such as that shown below, there are six positions with good accommodation within one rotation. Hence cogging results in six pulsations per full rotation. If a brushless motor has four poles and six slots, there are 12 pulsations per rotation. In this way, the number of cogging torque pulsations differs depending on the number of poles and the number of slots.

This cogging torque is not the same as the torque pulsations that are affected by the motor driver specifications, but you should remember this cogging torque as one cause of noise.

<Electromagnetic sounds>
Noises that occur due to vibrations of motor coils caused by magnetic forces are called electromagnetic sounds. Electromagnetic sounds occur when a current flowing in windings changes suddenly. Such a sudden change in current can also occur when the voltage applied to winding terminals changes under PWM control. Because of this, when a sound occurs at a frequency matching a PWM frequency, it is also called a PWM sound or a switching sound. Coil vibrations can also cause tapping on motor structures to produce sounds. In these cases, the sound frequency may not match a PWM frequency.

<Bearing sounds>
Bearing sounds occur due to rubbing or other contact of the parts of bearings. If there are deformations or scratches caused by mechanical pressure on bearing parts, or if lubricant is lacking, the sounds may be louder. You may think that such sounds are unrelated to motor control, but in fact there are cases in which a voltage applied to windings causes electrostatic charging of bearing components, which induces sparks that cause damage to bearing surfaces, so that as a result noise is increased. This phenomenon, called electro-erosion or electric erosion, is thought to be related to motor control.

So keep in mind that motor driving control is closely related to noises and their causes.

Noise Measurements

Now I’ll explain the methods used to measure noise related to motors.

Noise measurements are performed indoors, where outside sounds can be blocked. There are two types of rooms used. One is called an anechoic chamber, and in addition to being designed so as to block sounds, also functions to suppress the echoing within the measurement room of sounds emitted by the object for measurement (sound absorption). The other type of room is a soundproof chamber, which can block sounds, but does not have the sound absorption performance for muffling echoes of an anechoic chamber.
When performing noise measurements, first you’ll have to decide in which state the object for measurement will be. As examples, a motor might be used by itself, or the motor might be incorporated in some product, or the equipment with the motor might actually be mounted on a wall in a house, as in the case of an air conditioner indoor unit; measurements should then be performed with the selected state reproduced.

The positions of the microphones that are used to gather (collect) sounds are also important. Because sounds have directionality, microphones should be installed where they can easily detect noise. Also, microphones may be positioned taking into consideration the locations of persons in the area when the motor is being used. To perform more accurate measurements, rather than positioning a microphone at a single position, measurements may be made with microphones directed toward the object from various directions.

Power supplies and measurement devices are generally located outside the measurement room so that the sounds they make do not interfere with measurements. They are connected to microphones within the measurement room and to measurement devices outside the room, and noise is digitized and analyzed.

The original signals of measurement results are voltage signals like that shown below. The vertical axis plots the magnitude of the sound, and the horizontal axis represents time. From this signal, we can compare the magnitudes of sounds and can check the timing of the emission of sounds.

Analyzing measured sounds
In order to determine the frequency components contained in measured sounds, we use octave band analysis and FFT analysis. These frequency analysis techniques are extremely useful for identifying the origins of noise.

Octave band analysis involves creating bar graphs that represent the amplitude of sounds in different frequency bands; from these, we can read off the frequency bands in which the noise occurs. And in FFT analysis, line graphs are used to represent narrower sound values at different frequencies. This is useful for identifying noise at specific frequencies, for example. Using these analysis methods, we can measure overall (OA) values as comprehensive noise values.

In measurements, auditory correction (frequency correction, frequency weighting) may be performed. This is because there is some discrepancy between measurement values of actual sound pressure levels and the auditory sensation of sounds that we hear, and so the magnitudes are corrected according to the frequency sensitivity of the human ear. The correction characteristics include A-weighting and C-weighting, according to the extent of the correction. At the time of noise measurements, you’ll have to choose from among A-weighting, C-weighting, and Z-weighting (that is, no correction; a “flat” characteristic).

In noise measurement and verification tasks, octave band analysis and FFT analysis noise values are determined, AO values are found, and in some cases measurements are also performed using different correction characteristics.

Noise verification
If we prepare motor drivers with different performance parameters and swap them out to compare motor noise levels, we can discover the characteristics of the different motor drivers. More specifically, if we compare motor drivers that use 120 degree square-wave driving and sinusoidal driving, we will find differences in the frequencies of noise caused by torque pulsation, and if PWM frequencies are different, the frequency differences can also be confirmed in the sound data. If we need to know whether or not these sounds affect the general noise level, we can compare OA values.

Up till now I have been talking about measurement methods, but when investigating actual noise, our sound perception is also important. Sound perception is just our sense of sound when we hear sounds with our own ears. Before performing measurements using instruments, we first check a sound by just listening. We can gradually raise or lower the motor rate of rotation to check sounds over the entire range of rotation rates, and can check for directionality by moving around the object and listening. And after we have eliminated a source of noise, we can also use our own ears rather than data to judge whether the noise level really has been improved.

Reliability of Motor Circuits

Another performance parameter required from motors is reliability. “Reliability” means that the device or product doesn’t break easily, is safe, and is guaranteed to function properly.

A motor is a device that performs a mechanical operation, and is also an electrical device, and so motors are required to be reliable both mechanically and electrically. I’ll be talking about both these types of reliability.

<Motor mechanisms>
The reliability of a motor mechanism is related to efficiency, which includes age-related degeneration; quietness; guarantees of mechanical strength; thermal resistance to combustion and melting of resins; and resistance to dielectric breakdown of parts that are not expected to pass an electric current. These quantities are ensured on the basis of data obtained by the motor manufacturer from verification testing.

<Motor circuits>
Reliability parameters for motor circuits include electrical tolerances, thermal tolerances, noise, and stress tolerances; they are all highly relevant to ensuring reliability of a motor driver.

*Electrical tolerances

Electrical tolerances are rated absolute maximum values for the voltage that can be applied (withstand voltage) and the current that can be passed (withstand current) for an IC. In some cases there is a need to ensure that circuit breakdown or dangerous operations do not occur even when connectors are inserted or removed, and in a larger sense, this amounts to requiring surge resistance.

Static electricity: This is the ability to withstand breakdown when a motor receives static electric charge from outside. Electrostatic testing to measure the ability to withstand charging may involve applying an electrostatic charge to an entire motor circuit, or applying such a charge to an isolated IC (to the pins of a motor driver IC).

Lightning surges: The ability to withstand a high voltage that is assumed to be applied to a motor in the event of a lightning strike.

Latch-up: Erroneous behavior in which a parasitic thyristor of a semiconductor device causes false turn-on. In addition to the voltage and current of the IC terminals, the tolerance up to the occurrence of erroneous operation is measured.

*Thermal tolerance

Thermal tolerance refers to the temperature that the windings and electronic components can withstand. In addition to the temperature of the surroundings, this temperature takes into consideration the heat generated by the motor itself. Large currents flow in the power transistors of the motor circuits and in the motor windings, and so large amounts of heat may be generated. When this causes the temperature to rise dramatically, an overcurrent protection function may be added to limit currents and ensure motor reliability.

*Electromagnetic noise

Motors must satisfy two demands relating to electromagnetic noise: they must be tolerant of noise coming from external sources, and they must not emit too much noise themselves. Both types of noise may propagate either through the air or through wiring.

*Stress

Stress refers to mechanical strength. IC packages must be durable, and no cracks must appear in the solder that connects printed circuit boards and electronic components.

Among these aspects of reliability, I’ll now talk about electromagnetic compatibility (EMC), which includes electrical tolerances and electromagnetic noise.

Electromagnetic Compatibility (EMC)

Electromagnetic compatibility (EMC) refers to both the characteristic of an electronic device of not emitting electromagnetic noise that may affect other equipment, and conversely to the characteristic of continuing normal operation even when an electronic device is subjected to electromagnetic noise from another product.

A motor driver is a device that, upon PWM control, frequently switches voltages applied to winding terminals, and so such a device will be a source of electrical noise. And a motor that is being controlled by a motor driver is a rotating device, and as such any abnormal operation could potentially become physically dangerous. We can say that for a motor driver, EMC is an important characteristic that must be considered thoroughly.

The characteristic of emitting electromagnetic noise is called electromagnetic interference (EMI). Measurements are performed in a radio wave darkroom that blocks radio waves from the outside. Tests are standardized by JIS and other organizations; the relevant standards have to be satisfied.

EMI includes radiative emissions that propagate through space, and conductive emissions that propagate in cables and the like. For the former generally noise in the frequency range 30 MHz to 1000 MHz is measured, whereas the latter noise is measured in the range 150 kHz to 30 MHz.

The characteristic of being affected by electromagnetic noise is called electromagnetic susceptibility (EMS). This also includes the static electricity and lightning surges that I talked about earlier.

Radiation and conduction immunity tests are performed to check erroneous behavior of equipment caused by radio waves emitted from antennas and noise impulses applied to cables.

There are three types of electrostatic tests of electronic components, as one part of EMS. HBM is used in tests which assume the application of static electricity from a human body; MM is for tests of static electricity applied from a device; and CDM is used in tests to study the release and capture of charge accumulated on an object itself.

In HBM a capacitance of 150 pF or so is charged, and the charge is then applied via a discharge resistance of 1.5 kΩ. Using MM, a capacitance of about 200 pF is charged and then applied with no resistance (or through an inductor). And in CDM, a virtual capacitor is formed by enclosing an insulating plate between an electrode plate and an electronic component, in this state a voltage is applied to the electronic component to charge the virtual capacitor, and thereafter a terminal of the electronic component is connected to a conductor at a reference potential to cause discharge.

So tests are performed with different situations reproduced by changing the capacitance and the discharge resistance. The capacitances and resistance values that I’ve given you may be different depending on the standards being used.