Variable frequency drive main circuit failure analysis

Variable frequency drive includes main circuit, power circuit, IPM drive and protection circuits, cooling fan and other several parts. The structure is mostly unitized or modular. Incorrect or unreasonable setting will cause the VFD malfunction and failure easily, or can't meet anticipated operation effect. As a precaution, careful analysis before the failure is particularly important.

Variable frequency drives main circuit mainly consists of three-phase or single-phase bridge rectifier, smoothing capacitor, filter capacitor, IPM inverter bridge, current limitation resistors, contactors and other components. Many common failures are caused by the electrolytic capacitors. The electrolytic capacitor life is determined by the DC voltage and the internal temperature on the capacitor both sides, the capacitor type is confirmed during the circuit design, so, internal temperature inside the electrolytic capacitor is critical important. Electrolytic capacitor will affect the variable frequency drive life directly, generally, temperature increase 10 ℃, VFD life reduce a half. Therefore, on one hand, considering proper ambient temperature in installing, on the other hand, reduce ripple current by taking some measures. Adopt power factor improved AC/DC reactors can reduce ripple current, thereby extend the electrolytic capacitor life.

During variable frequency drive maintenance, usually it's relative easy to measure the electrostatic capacity of to determine the capacitor deterioration, when the electrostatic capacity is less than rated 80%, insulation impedance is below 5 MΩ, it needs to replace the electrolytic capacitors.

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Soft starter VS variable frequency drive

Soft Starter reduces electric motor starting current to 2-4 times during motor start up, reduces the impact to power grid during motor start up, avoid the motor being burned out, and provide protection in motors running process.

Variable Frequency Drive allows the electric motor smooth start up, control startup current growing from zero to motor rated current, reduce impact to the power grid and avoid the motor being burned out, also provide protect in  motor running process. Besides these functions, the main function of variable frequency drive is adjusting the motor running speed according to actual operation conditions, to achieve energy saving effect.

So, from the function side, variable frequency drives are much better than soft starters.

One essential difference between a soft starter and a VFD in this regard is, that the VFD delivers "nearly" sinusoidal voltages (and currents) to the motor, which makes it possible to develop high starting torques during the acceleration, even higher than nominal full load torque, depending on the application, while a soft starter only supplies fractions of the basic waveform, which serves to reduce the current to the motor significantly, but still at the nominal frequency. This will reduce the available starting torque dramatically until the motor is up to around two-thirds of nominal speed, or maybe even higher.

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Variable frequency drive installation requirements

Variable frequency drives are electronic devices, they have stringent requirements in installation environment which is specified in its user manual normally. In exceptional circumstances, if it does not meet these requirements, we must adopt appropriate suppression measures: vibration is the main reason to cause electronic devices mechanical damaged, for big shock and vibration occasions, we should use rubber anti-vibration measures; moisture, corrosion gas and dust will cause electronic devices such as corrosion, poor connection, insulation reduced and then cause short circuit, as a precautionary measure, we should do dust treatment and corrosion control for the control panel, and adopt closed structure; temperature is the key factor to affect electronic devices life and reliability, especially semiconductor devices, we should install the variable frequency drive according to its required installation environment or install additional air conditioning and avoid direct sunlight.

In addition to the above points, inspect the variable frequency drives air filter and cooling fan periodic is also very necessary. For special alpine occasions, to avoid the microprocessor can't work properly due to temperature too low, we should take necessary measures such as setting the air heater.

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Reduce cost of single and three induction motor

First you must optimize the design for the application. This is true for the electromagnetic and mechanical design. If you are making a general purpose motor then this will be more challenging because you will have to compromise to meet a variety of requirements. But the process is the same. You can design by hand using knowledge and experience or, better you can use the numerous design tools, many of which have perimetric design, variable ranges or optimization methods.
To evaluate your designs you need a cost equation. You simply multiply the weight of material and the cost or relative cost of the materials. Can you reduce the amount of the most expensive materials by making better use of the less expensive ones. Often you can.

With a similar approach you can review the mechanical design and you must be aware the these two activities can become intertwined. It is understanding this tight interrelationship that makes a good machine designer. So you must ask are you using the materials effectively? If for example you have poor cooling, optimization of the electromagnetic design will not get you to the lowest cost machine. Don't forget about fan design, air flow, thermal transfer and similar items. Mechanical also involves the amount of material in the parts. Can the amount of material be reduced and still maintain strength? And so on...

Finally you look at presses. First are your processes themselves reducing the effectiveness of the materials. Poor processes show up in high stray losses, high iron and copper losses. Do you have a good die casting process? What is you vendor doing? Do the know and how can they help you. And sometimes you should ask them how you can help them. If you design is hard to make well, who's fault is that. Look at winding, excessive material? Insulation, to thick or thin? Do you have good contact between stator and housing?

That is no one thing that gets to a low cost design. It is like playing sports, you have to learn the fundamentals and execute them well. Once you do that, then you can look at automation, more exotic processes and materials. It is a great team project. Pull together someone with sales, electromagnetic, mechanical, and manufacturing process experience and have a go at it. It is great fun and exciting. You will be surprised at what you will find.

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Motor testing and repairing

Are you having noticeable performance problems with these motors? The size and type of motor are critical as mentioned, a cast rotor with the right testing can pick up voids in the bar and resistance rings, not necessarily a problem as most mass produced cast bar rotors will have some sort of voids in the bars, and the motors are fine, the red flag comes up when using these black box tests, which picks up what appears to be a problem but is actually just a normal condition from the manufacturing process.

I have very little faith that any one test on an assembled motor, can tell the user everything about the condition of the internals, or health of the motor.

When you consider all the testing the health field can use, such as a full body scan, many times it leads to false alarms and more expensive testing.

I could ask a few dozen questions on the age, type, past testing, past history of the motors in question, but if you are basing the health or life expectancy of any motor by only the use of testing without a visual of the internals of a motor, those questions need to be addressed to the supplier of the testing equipment.

I believe in predictive maintenance, by vibration charting, insulation value testing, surge testing, all charted and plotted over time.

When you have insulation values at 100 megohms in March, and then 500 in July, it is likely the ambient conditions have effect on the readings. Dependent on the ambient conditions and area the motors are located, humidity in March is gone in July. So plotting the readings over time will give a plot to see if the trend is downward regardless, or it could be the readings in March are fairly constant, the readings in July are constant, but there is no downward plot of the insulation value.

When you get insulation values in March of 100 megohms, and again in July but the megger readings are now 60, then a user would want to decrease the time between testings, starting with say quarterly, once you develop a plot, if that plot changes downward, then it is time to test maybe even weekly as it may show some kind of insulation breakdown, or contaminates that would call for a visual inspection and possible cleaning/repair of the motor.

Same with plotting surge tests.

Same with plotting vibration testing.

But the answers to these questions where the test results are confusing at best, need to be addressed to the testing equipment provider.

I have yet to see any demonstration of a total motor health testing device, that did not have some caveat dependent on the speed or other design factors of particular motors.

Maybe these tests were not confusing prior to now, if so, I doubt two identical motors would fail/start to fail with the same exact type of problem.

Again I could ask a dozen questions such as are the motors new, is this the first time you have results that make no sense, and as much of the total history of the motors and testing programs you have in place.

When it comes to rotors, testing is critical, and often when problems are found with the motor, and all testing points to the rotor, often simply repairing the rotor will not resolve the problem.

In speaking with many engineers over several decades, a large manufacture of large electric motors, have decided once a rotor is identified as the problem, rebarring, or any single repair is usually unsuccessful, and their procedure is to scrap the rotor completely.

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VFD external electromagnetic inductive interference

If there are interference sources around the variable frequency drive, they will invade into the filter on variable frequency drive input side to reduce high harmonics, thereby to reduce the noise impact from the power lines to the electronic equipment; and install radio noise filter on VFD output side to reduce its output line noise for the same.

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Design a PMSM to 10000 rpm high speed

Synchronous speeds are a function of the applied frequency and the number of poles, governed by the equation

120 * (frequency in hertz) / (poles) = (speed in rpm).

Adjust the ratio of frequency to poles to achieve the desired speed.
(example: a 4-pole design would require a line frequency of 333.33 Hz ... which means operating on either an adjustable speed drive or on a dedicated high-frequency power system.)

Once you've got the electro-magnetics sorted out, it's a matter of manufacturing to the mechanical constraints associated with the rotational speed.

Well, depending on the power rating, and on the required reliability, I believe it's very simply. The biggest problem would be to get a variable frequency drive, or other power supply to provide a 3-phase output frequency of about 500Hz.
An automotive alternator should be able to operate relatively reliably at your required speed, and it can probably deliver around 1.5kW at that speed.
In order to make it permanently magnetised, we just have to disassemble the rotor, take the rotor windings out, and replace them with some ring-shaped permanent magnet. We may possibly also use a number of individual smaller permanent magnets embedded in some non-magnetic material such a copper or aluminium between the two half-shelves of the rotor. Depending on the construction of the alternator, we may need some machine shop to pull the rotor halves apart, perhaps machine some material away to make room for the permanent magnet(s), and to press the assembly back together again, and to balance it afterwards.

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What’s PG card in variable frequency drive?

PG is short for Pulse Generator, generally it is used for measuring rotational speed. The most common PG card is optical encoder.
PG card is a part of vector variable frequency drive, to convert the encoder different form signals to suitable for the controller, like: electrical level conversion, analog digital conversion, optical isolation, etc.
Vector control variable frequency drive is a high-performance drive which can be comparable with DC converter.
In the vector control, it requires a motor speed feedback to the variable frequency inverter drive, this speed feedback is achieve by adding a rotary encoder (PG) to the motor, which means PG card feedback vector control VFD. In order to simplify the system, the feedback can be formed by operation of the inverter output signal, this control is called none PG card feedback vector control VFD, the performance has a slight gap than PG card feedback, but configuration is simple.

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Variable frequency drive cooling fan maintenance

Variable frequency drive cooling system mainly includes heat sinks and cooling fans, wherein the cooling fan service life is short. The fan generates vibration, noise increases and finally stops when approaching end-life, then the VFD drive tipped in IPM overheat. The cooling fan service life is limited by the bearing, which is about 10000 ~ 35000 hours. When the variable frequency drive continuous operation, we need to replace the fan or bearing in two to three years. To extend the cooling fan life, some VFD's fan only operation when the VFD turn on, but not the power on.

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Electrical machine software

You can categorize the electrical machine software into 2 basic types:

1) FEA packages that may or may not have a front end for analyzing motors. These are available from companies like Vector Field (now Cobham), Infolytica and a few others.
2) Motor design specific software such as the SPEED software, RMxprt and MotorSolve from Infolytica.

In the first category, the FEA packages are expensive because they are general purpose modeling packages. The motor add-on is usually limited mostly to the building the model and perhaps some specialized post-processing for motors. Their main advantages are:

1) 2D and 3D versions.
2) The user is free to define what analysis he wants to perform since they have very advanced general post-processors.

Their main disadvantages are:
1) Cost, they can get very expensive depending on the options you require.In some cases, the motor design module is a cost option.
2) Although they have general post-processors, many users require a lot of training in order to be able to get useful information.
3) Geometry input can be a lot more complicated since the front-ends typically have a limited number of geometries available.

The second category, the motor design software, is specifically designed for motor analysis. It can be magnetic circuit based such as SPEED and RMXprt or full finite element based such as MotorSolve. The magnetic circuit type of software has been available for a long time but it has only been recently that full FEA based motor design packages have become available.

The general advantages of software of this type are:

1) Template based input so the user simply chooses the motor geometry, stator and rotor and sets the parameters for the geometry. The input is therefore very simple but limited to the templates that are implemented in the package.
2) Post-processing is specialized and presented in a form that a motor designer can use it.

The general disadvantages of this type of software is:

1) No specialized post-processing is available directly from these packages unless added by the software provider in a new release.
2) Geometries are limited to the templates and adding templates may be very difficult and has to be done by the software provider.

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