What is the difference between designing switchmodes and motor drives?
The main difference is that in motor drives the principal inductor in the "output filter" is outside the power converter enclosure whereas the switch mode designer gets to put all of the filtering inside the power supply. Typically meeting EMC requirements with motor speed drives requires careful design.
We have a power supply and we need to design a controller for the loop. We have used our spectrum analyzer but the controller we design from the result is not stable when we think it should be. Why is this?
Spectrum analyzers often measure inaccurately. Do the open loop forward transfer measurement once manually with an oscilloscope by injecting a small signal and measuring the output.
We are going to connect a controlled power converter with controlled turn on and turn off switches (IGBTs not Thyristors) to the AC mains. What specific things should we look out for?
The AC mains is an incredibly stressful place for a power converter. If you come from a switchmode design background ensure you take care of de-saturation protection for switches, over voltage levels, impulse withstand and input filter requirements especially if the converter is to be used with residual current detectors. At ELMG we generally consider that MOSFets are not rugged enough to be used directly connected to the AC mains.
Why do bearings fail sometimes in motors when they are used with motor drives?
Typically the failure is due to capacitive currents from the dv/dt of the switching flowing in the bearing causing its destruction. These currents are generally small but flow as impulses in the bearing and, without going into too much detail, each impulse removes metal from the bearing surface or ball. This causes surface degradation and eventual bearing failure.
How does a motor drive sometimes cause the failure of motor windings?
The winding insulation has inter-turn capacitance. The current that flows in this insulation can cause it to fail. It is also possible that the motor cable is acting as a transmission line to cause an overvoltage at the motor terminals.
What is important in using digital or microprocessor control of power electronics?
When power electronics is controlled by digital circuitry or from a microprocessor the timing resolution of the switching signals can be low. It is important in these cases to avoid producing low frequency components in the voltage as these will lead to inductor or motor saturation. Other considerations include start up signal scheduling and software failure mode analysis.
We are happy working on the design and construction of converters from 2W to 30MVA. The key thing is building large converters is managing the risk by taking more care. Please note that ELM G does not have a high voltage high current lab.
Our power electronic product has EMC problems: that is, it does not pass. Can you help us?
The answer to this is maybe.
Generally this is a stressful job with people wanting "miracles" in short time frames. We have done this but it is usually for clients who understand that we cannot always work "miracles". So give us a call and we'll let you know so long as you understand that we'll do our best and expect to be paid in full even if we cannot achieve the "miracle".
Often we'll take one look and recommend a total re-design.
That said we have had some good results, especially by getting the test lab to do the test correctly.
We have a power electronics product that passed EMC testing when it was tested for us here. We export it and the customer had it retested and says it fails. What should we do?
This is beginning to happen. It is a real concern. First thing is that when you test you should aim to pass by a large margin. Just passing is generally a recipe for failure in the field or in another test lab. Secondly ensure the installation instructions are clear in the manual and that the product is installed as instructed. Digital camera pictures of the foreign installation or test setup are useful to review.
However we have found that at times the failure notice is accompanied by an offer to fix the problem for a certain fixed sum. Be careful at this point as this may not be a technical problem but rather one of cultural clash or simply the cost of doing business. How you handle that is up to you.
We have at times provided written opinions on such matters having supervised re-testing the product. This has on occasion proved successful.
Final thing to note. When you do your initial tests insist that your test lab take pictures of the test setup that they used and include these in the test report. Some reports have a picture of the product without a picture of the entire test set up. We have found that sending this test setup picture can often fix the failure in the other country.
We are going to buy a pre-built stack from a supplier. What should we look for?
This is a risk management question. Make sure the stack suits your application. Generally low voltage < 1500V stacks are relatively simple to design. MV and high voltage stacks require considerably more effort especially if they are to survive rated impulse voltages.
What is the difference between an AC motor drive and a solid state softstarter?
The key difference is that the AC motor drive is typically a variable voltage variable frequency system. The softstarter is an inline connected controllable semiconductor switch that allows the motor voltage to be varied to limit the start current in the induction motor.
How important is knowing about the power electronic device?
Having a solid understanding of how the power electronic device operates at a semiconductor level is extremely important in designing power converters. It is typically the difference between having a functioning product and a very reliable one that can be sold in large numbers.
Get information on Power Electronics, DSP and Embedded Systems
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