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Ventricular Septal Defect (VSD)

Ventricular Septal Defect (VSD)

(See also Overview of Congenital Cardiovascular Anomalies Overview of Congenital Cardiovascular Anomalies Congenital heart disease is the most common congenital anomaly, occurring in almost 1% of live births (1). Among birth defects, congenital heart disease is the leading cause of infant mortality. read more Overview of Congenital Cardiovascular Anomalies .)

Ventricular septal defect (see figure Ventricular septal defect Ventricular septal defect A ventricular septal defect (VSD) is an opening in the interventricular septum, causing a shunt between ventricles. Large defects result in a significant left-to-right shunt and cause dyspnea. read more Ventricular septal defect ) is the 2nd most common congenital heart anomaly after bicuspid aortic valve Bicuspid Aortic Valve Bicuspid aortic valve is the presence of only two (rather than the normal three) valve cusps. Bicuspid aortic valve is the most common congenital cardiovascular abnormality. It is present in. read more , accounting for 20% of all defects. It can occur alone or with other congenital anomalies (eg, tetralogy of Fallot Tetralogy of Fallot Tetralogy of Fallot consists of 4 features: a large ventricular septal defect, right ventricular outflow tract obstruction and pulmonic valve stenosis, right ventricular hypertrophy, and over-riding. read more Tetralogy of Fallot , complete atrioventricular septal defects Atrioventricular Septal Defect Atrioventricular (AV) septal defect consists of an ostium primum type atrial septal defect and a common AV valve, with or without an associated inlet (AV septal type) ventricular septal defect. read more , transposition of the great arteries Transposition of the Great Arteries (TGA) Transposition of the great arteries (in this case, dextro-transposition) occurs when the aorta arises directly from the right ventricle and the pulmonary artery arises from the left ventricle. read more ).

Ventricular septal defect

Pulmonary blood flow and LA and LV volumes are increased. Atrial pressures are mean pressures. RV pressure and O2 saturation are variably elevated, positively related to defect size.

AO = aorta; IVC = inferior vena cava; LA = left atrium; LV = left ventricle; PA = pulmonary artery; PV = pulmonary veins; RA = right atrium; RV = right ventricle; SVC = superior vena cava.

A hole in the heart

A congenital VSD – a hole in the heart that’s present at birth – will typically close on its own, requiring surgery only if it causes blood-flow problems or related conditions.

A hole that forms during a heart attack (acquired VSD) is often fatal. These holes can form when the heart starves for blood and it begins to weaken and die. A rupture in the septum, the tissue between the heart’s pumping chambers, will almost always leak blood, further weakening the heart. Within several weeks, the affected heart muscle turns to scar tissue, which can cause heart failure or lead to death.

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Though acquired VSDs can happen to any patient after heart attack, higher risk groups include women, seniors, and patients with high blood pressure or chronic kidney disease. Symptoms may include bluish skin, lips, and fingernails, labored breathing, sweating, and extreme fatigue.

VSDs can often be avoided if patients go to the ER as soon as they detect heart attack symptoms. Without monitoring, a VSD can form and the patient's health will rapidly decline, resulting in advanced heart failure within a few months.

Open heart surgery for VSD can be risky – VSDs are not perfect holes, and suturing the affected tissue is like sewing through butter. If stitches can't hold, we can't do open heart surgery. With percutaneous VSD closure, we guide a catheter from the groin to the heart and insert a small, permanent device to close the VSD.

Both procedures are complex, with a 50% mortality rate for open heart surgery and a 30% mortality rate for percutaneous VSD closure. By comparison, most open-heart procedures register a 1% mortality risk.

Parents of children with congenital heart disease

Babies with a congenital heart defect can be very difficult to feed. Here, parents describe their experiences of feeding their newborn baby, overcoming problems and what feeding was like when their child went on to solids.

Not all parents had problems feeding their baby. Some babies fed well and put on weight, and experienced none of the problems that many cardiac babies have. One mother of a child with SVT recalled how her baby guzzled its milk and feeding had never been a problem.

Explains that her baby who had SVT never had any problems feeding.

Explains that her baby who had SVT never had any problems feeding.

He was a brilliant feeder. To be honest they say that children that have been, well I say they say, this was what the hospital told me that children that are in SVT which is, I don’t know what actually is classed, at what rate the heart goes up to, but all I know is it’s very fast at that, at that point, but that children in SVT are brilliant feeders. So he used to take down about 4 ounces of milk in about 3, 4 minutes. So you’d be like this and the bottled be gone which was amazing.

I did try and breast feed him but he had such a strong suck that it just, I had to put him on the bottle. And I mean he was fed with the tube for the first week they’d put a tube, a tube up his nose and just fed the milk into his stomach through that. But they’d given him a dummy to try and make sure he, he developed his sucking reflex. Which was fine, you know, against all advice they give this little new born a dummy. So I mean he never had a problem feeding. He used to, sometimes he used to get a bit sweaty he’d be quite clammy to the touch but, I mean to be honest, you know, he used to fly through the milk. It was never a problem.

And, they said it is really common that they just, it’s almost like they’re guzzling their milk but I think they’re burning it off so quickly that, I mean I can’t talk for other babies but that’s what I’ve been told.

Other parents described a constant battle to get their baby to take its milk, only taking a few ounces at a time, slow feeding, not gaining weight. Some mothers put supplements in their baby’s bottle to increase their weight.

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She overcame her baby’s feeding problems by using a combination of breast-feeding, expressed milk.

She overcame her baby’s feeding problems by using a combination of breast-feeding, expressed milk.

But in the early days as well, when he was diagnosed and he’d had his first lot of surgery the other huge concern is that in general babies with cardiac problems are slow to gain weight, they don’t feed well and Felix was fed with a naso-gastric tube as they all tend to be when they’re first born. He was very reluctant to take a bottle and he was slow to gain weight and when we came home initially he still had a naso-gastric tube and we were trying him — weren’t we? — with a bottle as well. And I attempted to breast feed him. And initially we were just switching from one to the other and didn’t seem to be getting anywhere. And eventually he did take to breast feeding and, and we ironed out his sort of feeding problems but it did take a long time and it took, and it took a lot of time as well because he’d only have very small amounts. You know, little and often and he was regularly sick. And you’d find that you’d no sooner finish one feed and wind him and you’d be ready to start the next one.

And we had extra calories — didn’t we? — to add into, we were giving him a bottle, to add into the bottle or if you were feeding him with a tube to put into the tube. But you used to find that he wasn’t able to digest it very well and he’d, he’d sort of bring that up. And I think you find that most parents say the same thing, that there’s that constant pressure all the time to get food down them so that they gain weight and it is a very slow process.

How did you overcome that?

Just persevering really but it was very, very time consuming and, I mean I did breast feed successfully but I felt that the support that you got with regard to feeding was actually very, very poor and the, I think it’s the, I think it’s the British Heart Foundation that are actually rewriting the feeding leaflet at the moment and they’ve consulted our local hospital about it and I have had some input into that. But at no time did anybody ever say that it was really even possible to breast feed a cardiac baby. Any literature that you were given was quite negative.

But anyway when I came home I took the tube out and I actually fed him with an oral syringe. That was what we did at home. Through, through his mouth but, but in a controlled way and that isn’t suitable for all children because sometimes they have reflux problems as well. I know that but that was what we did. And then when he went into hospital for subsequent surgery he did have the naso-gastric tube down immediately after surgery. But once he came off the ventilator I then insisted that he was then fed by an oral syringe. I expressed milk and then used my milk because it was easier for him to digest. And, and that was how he was fed. So I’d breast feed him when he had the energy and when he didn’t I’d supplement it with the oral syringe. And that worked for us.

Describes the battle she had feeding her newborn baby but said it got better when he went on to.

Describes the battle she had feeding her newborn baby but said it got better when he went on to.

So it was this battle; do you think he’ll have little bit more, let’s try him again. And he might have, you know, a couple of ml and you’d write that down and you’re sort of adding it up, and we’re nearly there, we’re nearly there, you know. Try him a little, little bit more, see if he’ll have a little bit more. And you would just sort of, willing him that please just have a little bit more, a little bit more, and it was just sort of a 24 hour thing all the time on your mind. Can I get another ml of milk down, will he take an, and then off course he’d be sick. And he did tend to, to vomit quite a lot because I think, you know, again he’d had this operation for the malrotation of the gut so his tummy had been played about with so he was a bit delicate. He was also on a lot of medicine. He was on diuretics, which do tend to upset the tummy anyway. So you know it was just this constant, constant battle of always on your mind of feed, feed, feed, feed, feed, you know and I mean it’s like that when you’ve got a baby anyway, you know, feeds every 3 hours, or whatever. But it was just so important for Sam, so important that he had this milk and just so difficult to get it, him to take it. And as I say, you just, sort of live your life, just thinking you know, that’s it, this is my life; my one aim in life to get so many millilitres of milk, you know into Sam and you do start to think, ‘You know is this it? Is this what’s going to be like, forever?’ But it’s not, you know, it does end and it does get better.

And it certainly did improved once he started to go onto solids. He could cope with that so much better. Purely, I think, because it’s not this constant sucking, you know, he could have mouthful of food and have a break and breathe. And then have another mouthful of food, you know, and he certainly coped with that a lot better than the milk.

Describes the problems she had with feeding her baby and recalls that the health visitors didn’t.

Describes the problems she had with feeding her baby and recalls that the health visitors didn’t.

And I spent a small fortune on different sorts of teats and sterilising needles and making holes bigger and doing different systems and I’m sure that the chemist thought I was barking. And I think by this time I probably was a bit post-natal, absolutely obsessed with her gaining weight and her feeding and what was happening. And finally the Health Visitor said ‘put her on a bottle, this is just you’, and I felt such a failure, I felt such a failure.

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So we did. We got formula milk and we started giving her bottles. Which was a nightmare because she wasn’t really interested. And feeds would take, feeds with a bottle would take sort of 3/4 of an hour or an hour, an hour and but she did seem to thrive a little bit more because at this time she was practically off the percentile chart. She wasn’t quite off it but she was. And various Health Visitors would, they would weigh her and they’d measure her head and because there was some growth then that was OK. And, this is with hindsight, everybody looked at their little part of what they should be doing and if there was a tick, if they’d checked it and it was OK, nothing wrong there, then that was it. Nobody ever sort of referred on. And also everybody looked at me, they didn’t actually look at my baby. They didn’t really consider her. And they didn’t refer on because they looked at her and yes there was some increase in weight, yes there was some, you know her head, head was obviously growing so therefore she was growing a little bit and because she wasn’t off the percentile, because she was just staggering along it, then I would get sort of repeated stories of ‘Oh babies that they knew that all of a sudden just raced away. You think ‘I don’t want to know that’. This is me.

Babies with a heart condition who are already breathless seem to find it difficult to control breathing and sucking at the same time. Several mothers explained that often their baby would be sick after feeding and then they did not know whether to try again. Exhaustion from frequent and stressful feeds was a serious problem for many mothers during their baby’s early months.

Many described feeling frustrated, worried, and overwhelmed by the need to get their baby to take its milk (see Interview 14). These feelings had, in some cases, been exacerbated by visits from the health visitor, who may not have had to deal with CHD babies and so had not appreciated the nature of the problem (see Interview 21).

In some cases, children with congenital heart disease also have other conditions which cause their problems with feeding, for example, mouth deformities (Down’s Syndrome, VCFS) or reflux and gut problems (Di George Syndrome, atrial isomerism). One mother whose son had surgery for his malrotation of the gut, said he used to vomit frequently which contributed to his difficulty feeding (see Interview 14).

Several mothers emphasised how important it was to realise that they weren’t the only one with a problem feeding their baby. Advice on feeding for children with heart problems is available on the Children’s Heart Federation website.

Many mothers of babies with a heart condition find it difficult to breastfeed. One mother we interviewed who had successfully managed to breastfeed said it had taken perseverance but that it seemed particularly important as being the one real contribution she could make at that time to caring for her baby (see ‘Messages to other parents’ and Interview 05). A few mothers felt that the written information on breastfeeding didn’t encourage mothers to persevere.

Double Outlet Right Ventricle (DORV)

Double outlet right ventricle (DORV) is a congenital heart defect, meaning it is present at birth. It occurs when the heart doesn’t form properly during fetal development. In most cases, the cause is unknown.

DORV is a complex abnormality that has many variations and typically involves other heart defects as well.

What happens in DORV?

There are two main arteries that carry blood out of the heart: the aorta and the pulmonary artery, also known as the “great vessels.”

In a normal heart:

  • The aorta is connected to the left ventricle, the chamber of the heart that pumps red, oxygen-rich blood out into the body
  • The pulmonary artery is connected to the right ventricle, the chamber of the heart that pumps blue, oxygen-poor blood to the lungs

In babies with DORV, both the aorta and pulmonary artery are connected to the right ventricle.

A.D.A.M Interactive Anatomy®

In addition, almost all babies with DORV also have a ventricular septal defect (VSD), an abnormal opening in the wall between the left and right ventricles that allows blood from each side of the heart – oxygen-rich and oxygen-poor blood – to mix together.

For babies with DORV, this hole is helpful, providing the only way for oxygenated blood to reach the right ventricle, where the aorta is connected, and be pumped out into the body.

However, because the blood is mixed, it is not as high in oxygen as it should be.

Risks to the Baby

In babies with DORV, the heart has to work harder than normal to pump enough oxygenated blood out into the body, which can cause the heart to become enlarged. Too much blood may also be pumped to the baby’s lungs.

Without treatment, risks include high blood pressure and heart failure.

Types of Ventricular Septal Defects (VSDs)

There are four types of ventricular septal defects that occur in DORV, based on where the opening is located:

  • A subaortic VSD is located below the aorta, allowing oxygen-rich blood to flow from the left ventricle, through the VSD, and into the aorta. Blood from the right ventricle flows mainly into the pulmonary artery.
  • A subpulmonary VSD is located below the pulmonary artery, a condition known as a Taussig-Bing anomaly. Oxygen-rich blood flows from the left ventricle, through the VSD, and into the pulmonary artery. Oxygen-poor blood from the right ventricle flows mainly into the aorta.
  • A doubly committed VSD means there are two VSDs, one below the aorta and one below the pulmonary artery.
  • A remote (non-committed) VSD means the hole is not near the aorta or the pulmonary artery.
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Types of DORV

DORV differs in each baby based on the location of the VSD, the positioning of the aorta and pulmonary artery (which are normally side-by-side), and other cardiac defects involved. The impact these variables have on the normal blood flow through the heart will determine the baby’s prognosis and the intervention required.

There are three major types of DORV:

    – a condition involving four abnormalities occurring together, including narrowing of the pulmonary valve or artery (pulmonary stenosis), decreasing pulmonary blood flow
  • Transposition of the Great Arteries (TGA) – when the positioning of the aorta and pulmonary article are switched or “transposed”
  • VSD DORV – the most common type of DORV, with pulmonary over-circulation (an increase of blood flow to the lungs) and no outflow tract obstruction

Risk Factors

DORV occurs in an estimated 6 to 15 infants per 100,000 live births. It is more common in males.

Babies with certain genetic abnormalities are at increased risk for double outlet right ventricle, including trisomy 13 and trisomy 18.

Genetic counseling and testing is recommended for all women pregnant with a baby with DORV.


Newborns with DORV typically show signs of the condition within a few days of birth.

While the signs and symptoms can vary for each baby, in general they may include:

  • Rapid breathing or difficulty breathing
  • Heart murmur
  • Rapid heartbeat
  • Sweating
  • Extreme tiredness
  • A blue tint to the skin, lips or nails (cyanosis), due to lack of oxygen in the blood
  • No interest in feeding
  • Weight loss or poor weight gain


During Pregnancy

Double outlet right ventricle may be detected before birth during a routine prenatal ultrasound. Your OB/GYN will refer you to a pediatric cardiologist, who will perform a fetal echocardiogram (cardiac ultrasound) to better view and evaluate your baby’s heart structure and blood flow.

You and your baby will be monitored closely throughout your pregnancy. Your care team will include maternal-fetal medicine specialists, pediatric cardiac surgeons, pediatric cardiologists, and neonatologists, for a well-planned, well-coordinated approach to prenatal care, delivery and treatment after birth.

After Birth

In other cases, DORV may not be detected until the baby is born and shows signs and symptoms such as breathing problems, a blue tint to their skin, or a heart murmur, the sound of blood moving abnormally through the heart.

Tests to confirm the diagnosis may include:

  • Pulse oximetry, to measure the oxygen level in the baby’s flow
  • Chest X-ray
  • Electrocardiogram (ECG or EKG), to measure the heart’s electrical activity
  • Echocardiogram, to view the heart’s structure and blood flood
  • Cardiac MRI or CT scan, for more detailed imaging of the heart
  • Cardiac catheterization, to further evaluate the heart and defect


Surgery is required to correct double outlet right ventricle, enabling blood to flow normally out of the heart to the body and lungs.

The timing and type of surgery depends on each baby’s defect, other heart abnormalities, and overall health. If symptoms are severe, the surgery may be performed the first few days after birth. In complex cases, a series of surgeries may be required.

Follow-Up Care

Babies born with DORV will need to continue to see a heart specialist throughout their lives, transitioning at adulthood to a cardiologist who specializes in the treatment of adults with congenital heart defects.

Texas Children’s – #1 in the Nation for Pediatric Cardiology and Heart Surgery

Texas Children’s Pavilion for Women and Texas Children’s Fetal Center are nationally recognized centers for the care of fetuses and infants with congenital heart defects, providing mother and baby expert, collaborative care all in one location.

By Michael Brandt, PE

The use of Variable Frequency Drives (VFDs) to control electric motors has increased over the last few decades. Not only can they provide control, they can increase efficiency and reduce costs through operational energy savings. We have used them on industrial projects to reduce the water hammer in a pumping application or to start a conveyor belt or production process line.

A simple Google search will show you lists of reasons why you should use VFDs, including their ability to:

• Vary a motor’s speed to match the application, saving energy that can’t be done when using other motor starting methods

• Match the speed of the motor to the process with a single unit versus using a system that requires more energy to run and needs more equipment to reduce its speed

• Eliminate speed reduction equipment, such as gear boxes, belts, valving, etc., that require maintenance but also add inefficiencies that the motor must drive

• Adjustable torque limiting

• Reduce power line disturbances over across-the-line motor starting

• Operate in reverse operation

• And several others

VFDs can also limit a motor’s starting in-rush current, reduce electrical peak demand charges, and can ramp up the motor speed to a control setpoint.

Yet, with all the benefits of using a VFD to run an electric motor, there are some inherent disadvantages to consider that will help prevent premature motor failure. One of the problems we’ve experienced deals with motor bearing issues on industrial projects. However, after experiencing these issues firsthand, we’ve found there are two main mitigation techniques that can be put into place to minimize or eliminate these problems.

VFD problems morrison maierle electrical engineering

This photo shows a 600 HP, vertically mounted, 480VAC, 3-phase process motor that is connected to a pump to move processed water from a capture facility to a heap leach area. This motor uses a variable frequency drive (VFD) to power the motor and allows it to operate at different speeds. Motor bearing issues can crop up if steps aren’t taken to mitigate the problem.

Understanding Motor Bearing Currents

Circulating currents that flow through motor bearings—between the stator and the rotor—are not a new phenomenon; they have been present since motors were invented. Before we address the solutions, it’s helpful to review how motor bearing currents work.

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Circulating bearing currents—related to the use of VFDs—are primarily caused by the speed of the inverter switching in the output stage of a VFD. Drives use Pulse Width Modulation (PWM) with semiconductor circuitry to generate the variable frequency and voltage in order to power the motor at variable speeds. This high switching rate induces a voltage on the motor shaft by parasitic capacitive coupling between the rotor shaft and the stator winding (Read the abstract here).

When the voltage rises above the oil film’s dielectric constant (resistance), usually 20-30V, the film breaks down and an electrical discharge occurs, producing a current to flow through the bearings from the rotor to stator. The motor stator, or stationary portion that is part of the frame, is bonded to ground through the equipment grounding conductor in the power feeder. The discharge (that occurs when the current passes through the bearings) instantaneously heats and melts the surface of the bearing race at that point and starts to create pitting in the race. This is known as Electrical Discharge Machining (EDM).

Over time, this pitting creates a fluting on the face of the bearing races. When the fluting becomes severe enough, bearings can fail, which causes increased motor vibration. If left unresolved, the motor will fail.

VFD problems morrison maierle electrical engineering

The vertical lines above are the key indicators of fluting on a bearing race. When fluting becomes severe, bearings can fail and leads to increased motor vibration. Photo credit: AEGIS Bearing Protection.

A Real-World Case

We’ve experienced instances of motor bearing issues that eventually lead to motor failure. For example, one of our clients was experiencing premature motor failure in some vertically mounted 600Hp process motors. These are 480VAC, 3-phase, VFD driven motors. The problem was discovered when the motors’ vibration sensors were showing levels above the setpoints in the control and monitoring system. When the motor was removed from service and sent in for repair, the upper motor bearings revealed substantial fluting on both the inner and outer bearing races that indicated a problem.

In this configuration, the upper bearing assembly was under a higher load than the lower bearing assembly. The higher load was causing a thinner oil film, resulting in a lower dielectric value of the oil film. The lower resistance path was providing a connection for the circulating bearing current to pass through the bearings from the rotor to the stator, which resulted in EDM. This caused fluting in both the inner and outer bearing races.

VFD problems morrison maierle electrical engineering

The red arrows show fluting on a 600 HP motor’s inner and outer bearing race.

Two Mitigation Strategies

There are numerous strategies for mitigating bearing currents in an electric motor. All strategies involve breaking the current path through the bearings from the rotor to stator to ground or providing a low-resistance path to ground from the rotor to the motor case to ground.

  1. The first method involves breaking the conductive path by insulating the rotor from the stator. This is accomplished by providing an insulating material, typically ceramic or another non-conductive resin material between the rotor’s bearing race and the motor case. Another method is to provide ceramic bearings between the rotor and stator. With either of these methods, the circulating shaft current will find another path to ground. In many applications, the motor shaft is directly coupled to the equipment it is driving. Direct coupling, through metallic shafts and couplings, provides a low resistance path to ground in the other connected equipment’s bearings which can damage that equipment.

For example, grounding brushes use spring-loaded metal bristles or a metallic block that are in constant contact to the rotor and can be internally or externally mounted to the motor housing. The performance of these are affected by contaminates, oxidation, and heat at higher motor speeds.

A good solution involves using a shaft grounding ring that incorporates conductive microfibers to provide a low-resistance current path to ground. In this situation, the microfibers are encapsulated in a housing that contact the entire rotor’s circumference and provide a larger contact area than grounding brushes. These grounding rings are mounted on the exterior of the motor housing around the shaft and bonded to ground through the motor housing. The microfibers build up less heat which allows longer service life at higher motor speeds and appear to have less problem with oxidation.

Additionally, with any of these strategies, filters on the VFD output are recommended to reduce the PWM voltage spikes before they reach the motor. One option is to add these filters, also called reactors, at the VFD output or at the motor. Typically, these are incorporated into a VFD enclosure or Motor Control Center (MCC) bucket that contains the VFD.

Fixing a Real-World Problem

So back to our client’s motor bearing fluting issues. After an investigation, we found that when the motor was rebuilt, the insulated bearing material was not properly installed which resulted in a low resistance path that allowed the current to pass through the upper bearings that was the cause of the fluting. As a remedy, after the motor was properly rebuilt, they added a grounding brush to keep any current from passing to the coupled piece of equipment. To date, the motor is still operational and has no further issues.

With any motor driven by a VFD, we’ve found that it’s critical to mitigate the motor bearing currents. And before any remedy is put into place, you should evaluate each solution in order to maximize service life, reliability, and reduce maintenance costs.

If you have VFD-driven motors that seem to have a decreased lifespan, or need a new application for a VFD driven motor, we have some ideas to help you arrive at the best possible solution.

Mike Brandt, PE is an electrical engineer whose experience covers the design of municipal and industrial water-wastewater treatment facilities, airfield lighting, building power, lighting, and control systems. As a member of the design team on a wide variety of Morrison-Maierle projects, on large-scale industrial projects, he specializes in low and medium voltage distribution, transformation and standby power systems. Mike and his wife live in Helena and are parents to two college-age daughters and two beagles.

Technical review of this article provided by Brian Literski, PE

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What we know

There’s so much we don’t know about e-cigarettes, but there are some things we do know:

• We need more research. E-cigarettes are relatively new to the market and the devices are changing all the time.

• Expert consensus is that regulated, nicotine-containing e-cigarettes are considerably less risky than smoking traditional cigarettes.

• That said, e-cigarettes are not risk free. For people who don’t smoke, vaping will probably introduce health risks.

• Not all e-cigarettes are the same. The outbreaks of vaping-related illness in the US over the past year have been largely attributed to vaping cannabis. E-cigarettes that contain cannabis sometimes also contain an additive called vitamin E acetate, which is known to be harmful to lungs. Vitamin E acetate is banned from e-cigarettes in Europe.

• Nicotine is not the harmful ingredient in cigarettes, or in e-cigarettes. It is addictive, so it gets its bad name because it’s part of what makes people keep smoking. But it’s the other ingredients in cigarettes that cause the increased risk of death and disease.

Much of the vaping-related illness in the US is attributed to vaping cannabis. Shannon Laura/Shutterstock

VSDs (Variable Speed Drives) and Their Purpose

Variable Speed Drives (VSDs) are electronic power controllers that allow for precise control of the speed and torque of alternating current (AC) induction motors that typically run at fixed speeds. AC induction motors drive a variety of rotating machines including fans, pumps and compressors and find application in most heating, ventilation and air-conditioning (HVAC) systems.

How Do VSDs Work?

Therma Blog 18 11 12 Graphic1

In AC induction motors, an alternating electric current is passed through a series of electromagnets to create a rotating magnetic field that is used to drive a shaft. The rotational speed and torque of the motor depends on the frequency and voltage of the supply. Since most electricity supplies are at fixed frequency and voltage, the speed and torque of induction motors connected directly to a conventional electricity source will also be constant.
VSDs are installed in series with the motor and intercept the incoming electricity supply. The internal circuitry of the VSD modifies the incoming voltage and AC frequency based on the load requirements, and the modified AC current is supplied to the motor. In this way, the motor speed and torque can be adjusted to match the load / demand it is servicing with high precision.

Why VSDs?

Therma Blog 18 11 12 Graphic2

In the absence of a VSD, an AC induction motor driven machine operates at a fixed speed and the output is controlled by external mechanical means (vents, valves, brakes etc) which effectively dissipate energy. The motor continues to run at full speed regardless of the demand resulting in poor efficiency. With a VSD, the output of the system is controlled by directly changing the speed or torque of the motor resulting in reduced power consumption and significant energy savings.
Consider the fan in a building HVAC system as an example:
Without a VSD the fan operates at a constant speed and vents are used to control the flow rate of air delivered to the building. The vents work by changing the so-called minor losses in the distribution system such that the flow rate is reduced at the constant fan speed. The vents are effectively dissipating or deliberately “wasting” energy to achieve the desired flow rate and the efficiency of the system is poor when the air supply demands are low.
In contrast, a VSD can control the fan speed in response to the demand. When the demand decreases, the flow rate can be reduced by slowing the fan, effectively reducing the energy input to the flow, rather than mechanically increasing the losses in the system. This results in less energy being wasted and therefore higher efficiency.


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In addition to the potential energy saving benefits of VSDs, other advantages include more precise control of the machine output, soft starting capabilities resulting in prolonged equipment life, greater operational flexibility and easier interfacing with wider process control systems (such as Building Management Systems), and regenerative braking of the motor.

Best Application For Energy Savings

VSDs are best suited to variable torque loads, which include components such as fans and pumps, which often operate at part load. In these components power consumption is proportional to the operating speed cubed and significant energy savings can be realized by reducing the motor speed.
In constant torque load situations (e.g. conveyors, air compressors), power consumption is directly proportional to motor speed and measureable energy savings can be achieved with VSDs if partial load operation is common. VSDs are not beneficial in constant power load systems such as machine tools.
Consideration should be given to the amount of time the system is expected to operate at part load since the efficiency of the VSD itself, although typically high, can incur a slight penalty under full load operation.

By Michael Owen, PhD
Michael is a mechanical engineer working in academia. His research encompasses various aspects of fluid dynamics and heat transfer with a strong emphasis on industrial heat exchangers.

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