I appreciate this being a civil discussion. Responding to a few points:
1. I graduated with an MSEE 40 years ago. Don't recall any motor discussion in EE classes, but at the same time I know I've forgotten very much of what I never used after school. Motors were covered some in 2nd year Physics. Power transmission was covered in EE.
2. For what I'm trying to accomplish, it's not an issue if an air conditioner is a good candidate for soft start or not. The only thing I'm challenging is the claim that a voltage dip at startup is abusing the motor, and pointing out that this very thing is routinely and widely done intentionally.
3. Without knowing the A-D algorithm of the meter and if it's frontended with a sample/hold, I couldn't begin to guess the direction and magnitude of an error from a changing input. Maybe the current is the issue and maybe it's not, but the abuse discussion is focused 100% on the voltage reading.
4. I finally recalled that every freezer I've ever owned will have the motor stall if power is removed and quickly restored while it's running. Window air conditioners, too. Just about everybody has heard it: Motor stalls for a few seconds, breaker trips, breaker resets, motor stalls for a few seconds, repeat for a few minutes. Is this motor abuse? It's certainly more stressful than a low voltage start, and is a normal occurrence.
Edit: Thinking more about the meter reading, my comment above was too simplistic. In addition to the A-D algorithm there's the issue of how the meter resolves an AC waveform to a voltage reading. Does it read peaks and assume a sine wave? Does it use some algorithm to attempt an RMS reading? When the voltage waveform has a discontinuity due to a current spike, does this trick the algorithm? With the significant voltage dip and non-sinusoidal waveform that changes every half-cycle, I think it's a safe bet that if you gave a scope trace to 10 different engineers and asked "what's the voltage", you'd get 10 different answers. (Same principle as 10 CPAs doing the same tax return). When I ran the same test using an analog meter I recorded that the voltage dipped to around 90 (and recovered more slowly), but this is subject to the same pitfalls I just ranted about.
* This post was
edited 04/10/12 08:55pm by Wayne Dohnal *
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1. The designers of RV air conditioners are probably not fools and the technology has had at least 40 years to mature. Therefore, I think you can assume RV AC design has long since been optimized for the purpose. Any practical and reliable fix for high initial starting current would have already been implemented.
2. Designs are always a compromise. In the case of RV air conditioners, I am quite certain that robustness has been sacrificed for the sake of low weight and cost. You probably can't alter the original electrical design much before you start compromising reliability.
3. Voltage is seldom the root cause of electro-mechanical device failure. The only way for voltage to do any damage is by insulation breakdown and that occurs due to high voltage, not sags. The only voltage that is likely to do damage in this case is (V = L*dI/dT) flyback which is current driven. Overcurrent is what causes the heat that breaks down the insulation on motor windings and causes shorts.
4. Designing data acquisition systems has been a regular occurrence in my career. You have identified the two approaches that I know of for measuring A/C voltage. One of them is peak detect with the assumption that you are dealing with a sine wave and the other more accurate and useful RMS technique is basically an averaging technique. It is all implemented in hardware in either case. Neither of these techniques will measure a voltage that is lower than the actual RMS voltage. Spike peaks will always cause the voltage to read high on peak detect meters. In either case, the meter is displaying a snapshot in time and the probability that the meter was able to catch and display the worst case low voltage cycle is very low. As I already stated, voltage measurements of this sort don't tell you what you need to know anyway.
5. Soft start motor controls are only appropriate for applications with low starting torque (like fans) because, by definition, soft start is low torque start. If you try to soft start a motor that is trying to overcome the resistance of a compressor, it will just stall and eventually burn out the windings. The reason is that the "locked rotor" current is much, much higher than running current. That is not what is going on when you install a hard start capacitor on a compressor motor anyway.
6. Discussions of this sort are kind of like the discussions on whether a tow vehicle will safely pull a trailer that is heavier than the weight for which it is rated. Yes, lots of people do it but they presume that they know more than the person who designed and specified the weight capacities in the first place. That is risky business in my opinion, which is why I have posted all these words.
In unpolorized terms (for us lay mortals), low voltage typically leads to high current...High current causes motor winding failures...Motor winding failures causes costly equipment repairs.
The number of AC motors that "stall" (it's called rotor lockup) for no reason at all is what .00002%??
The number of AC motors that go into locked rotor because of undervoltage is great enough to cause suppliers to think about vacationing in Mallorca, not "Semantics" wherever that is.
What the hell?
The ROOT CAUSE of motor burnout IS undervoltage. Causing VARS to shift. Causing over amperage. Causing overheating. Causing catastrophic destruction of wire insulation, causing shorting within the winding, causing me to autopsy the winding, causing me to observe the damage, causing me to to arrive at a conclusion.
mexbungalows wrote: What the hell?
The ROOT CAUSE of motor burnout IS undervoltage. Causing VARS to shift. Causing over amperage. Causing overheating. Causing catastrophic destruction of wire insulation, causing shorting within the winding, causing me to autopsy the winding, causing me to observe the damage, causing me to to arrive at a conclusion.
High current generates heat. High voltage does not. You can argue semantics all you want but what I said is absolutely true. If you stall an electric motor, you are going to burn it up whether the voltage is low or nominal. The reason is obviously overcurrent. Whether the current and voltage are in phase is irrelevant.
I do understand what you are getting at. In this particular discussion, I agree that low voltage is causing high current but low voltage isn't doing the damage.
I think there's pretty wide agreement that low voltage operation damages motors, but that statement in itself is not addressing the issue in this thread, which is low starting voltage that lasts a second or two, with the motor successfully starting. If somebody finds a document or can give expert testimony that the low starting voltage for a short duration causes damage, that's what I want to hear. Since this exact situation is intentionally created in lots of motor situations, it would help me to also know why it hurts an RV air conditioner motor while being neutral or beneficial with other motors. I know that the reduced starting voltage technology is used almost exclusively with larger motors, but as I noted earlier the technology is working its way downward, an example being consumer central vac motors that are spun up over an approximate 5 second interval using an itty-bitty triac.
mexbungalows wrote: The number of AC motors that "stall" (it's called rotor lockup) for no reason at all is what .00002%??
The number of AC motors that go into locked rotor because of undervoltage is great enough to cause suppliers to think about vacationing in Mallorca, not "Semantics" wherever that is.
I don't mean to offend, but if you want to argue or discuss technical issues with someone, you need to develop a sufficient command of the terminology to know when you may be misunderstood. That isn't my problem; it is yours.
I have worked as an electrical engineer for 30 years and I have worked for or consulted with some of the largest companies in the US. The term "locked rotor" is typically used in conjunction with the measurement of the resistance/reactance of a rotor winding and implies that the rotor shaft is purposely fixed in position for test purposes. In all my years of working, I have never heard of an electric motor spontaneously "going into" a locked rotor condition. Perhaps you have heard the term used that way. However, I can say with some confidence that it is not commonly used that way throughout the US.
"Stall" is a generic term used to describe a motor that has stopped turning due to either an electrical or mechanical fault condition. In either case, the motor will burn up and it is due to current, not voltage.
Wayne brought up the use of low voltage soft start electronics and that obviously is not applicable to air conditioner motors. That is the context in which I was discussing the fact that it is current rather than under-voltage that burns motor windings. If we are talking electric motors in the general sense, then far more motors are burned up due to a mechanical stall or electrical fault than due to under-voltage.
I plan to look up the location you call "Mallorca" and I will invite you to look up the definition of "semantics", which is not a place at all. Rather, it is a linguistics term very commonly used to describe what you are arguing about.
Well it turns out that reduced voltage starting is starting to be used with single phase air conditioners. This device: The first Single Phase, 208/240 VAC Soft Start is required to be used in the Maui area to limit inrush current to 100 amps. The approval letter from Rheem (available from the web page) says that testing with a Copeland compressor showed no damage to the compressor or the motor. The Rheem approval is for units in the 1.5 to 5 ton range. This is getting pretty close to RV air conditioners which are generally over one ton, and makes me even more skeptical that a low voltage start harms an air conditioner motor in any way whatsoever.
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