Asynchronous Motors or DC Motors ¿!?

I can't answer your question - I'm just a dumb old ME. But, I think you would need to go beyond the motor itself, to the power source and include any conversion processes. If you are starting with a DC source, the reliability path will be different than if your source is AC. And what are the power requirement characteristics of the driven machine - variable speed, constant torque, etc? In my experience, a 3-phase squirrel-cage induction motor in the proper installation is the most reliable, but I couldn't quantify that. It's when you start adding VFDs, soft starts, etc. in order to match the machine's needs that reliability starts to suffer.

Hi Bigg, thanks for your comments. My concern is only regarding the motors itself, enabling a decison making based on studies like MTBF or others.

MTBF.......not having brushes to maintain has to give induction motors an advantage over DC motors on this count.

Brushless DC motors are not really DC motors. They're synchronous motors with integral inverters.

Everybody seems to focus on the brushes for reasons that baffle me. In my experience I've seen far more bearing and winding failures of all motor types than brush failures of DC servo, universal or synchronous motors combined.

The choice between motor types will depend far more on the available power and application than any MTBF statistic on the motor. I also expect that it may be difficult to obtain a true MTBF value on any motor design. The mechanical loads attaching to the motor will significantly alter the performance and lifetime of any motor. An unnoticed misalignment of shafts can quickly mangle a motor bearing. Unexpected mechanical resonances can chew up almost anything. Then there's the possibility of power grid fluctuations from lightning coupling to switched inductive loading that can zap insulation wiring at any time.

In most DC motors, the major differences extra to an AC motor are a more complicated build mechanically, extra maintenance, brush wear, commutator wear and the resulting dust that often fills the motor and can even damage certain bearings, as well as making working on such motors a really dirty business.....been there, done that and washed the T-shirt.

Most of the rest of the construction is basically the same, so it was hardly worth mentioning...most of us thought anyway....

There may even be even negative health aspects to consider from the carbon dust (you breathe and taste "sweet" for several days afterwards!!), maybe someone can comment on that possibility...

Dust will be in almost any motor, but carbon dust is horrible and is conductive, possibly causing flash-over......

Of course a brushless DC motor has far fewer of these bad aspects, but it must also have DC, which needs to be produced and maintained, whereas AC comes in through the wall.....both cheap in comparison (no Power Supply) and highly accurate frequency control.

I prefer AC motors myself....

Also, there a some tiny true brushless DC motors that use permanent magnets in the rotor assembly along with an encoder for commutation switching.

a vfd on a dc motor?

That wouldn't work? The point I was trying to make and I think SolarEagle was adding to down in #4 was that other factors besides motor reliability alone (and motor efficiency for that matter) have to be considered to make a reliable (and efficient) system. For example, most of my work in petrochem dealt with centrifugal pumps and fans and the vast majority of those were driven, and well served, by direct-on-line induction motor installations. However, that would not have made a goodreliable installation on the one extruder in the plant.

I'm pretty sure a DC frequency looks like------------------------------, not a lot to alter there!

I agree with you.

If the comparison DC motor has brushes (not all do!!), in my limited experience, they are less reliable in the long term and need extra maintenance on the brushes alone....

Thanks for the comments to everybody who joined this discussion.

My concern lies on the motors itself. The advantages/disadvantages of induction vs DC motors are well known. What I'm looking for is for an official study promoted by one recognized institution or industry association showing practical and measurable arguments on the use of such motors, like MTBF or others. Please refer to the attached figure for traction motors which shows the MTBF for each motor class. I'm looking for a similar study comparing the reability of induction motors vs DC motors.

Thanks in advance for your contributions.

AC motors have less wearing parts and tend to run without problem for many years. Many have already said it......but hopefully there is a study somewhere to confirm that for you.....

Have a look at this:Free DOE software supports motor and motor systems planning

If you're asking is one design inherently more dependable than the other....I don't think that can be answered in a definitive manner....I believe it would be totally dependent on the nature and characteristics of the power supply, the environment, the motor type, the application, and any other relevant factors that could effect the integrity and resilience of the entire system....including maintenance required, dependability of maintenance personnel, level of and quality of maintenance materials....How well the motor design fits the needed criteria for optimum performance....

I thought this might assist some on the general topic..http://www.groschopp.com/synchronous-vs-asynchronous/

I don't know of any meaningful (by that I mean non-biased) study done which makes the direct comparison you are looking for. The reason i believe that is the case, is because the APPLICATION issues are too different. For someone to do an in depth study, one would need to apply the two different motors to identical applications, and then remove all influences of how they are powered. That is immediately the first problem. We don't distribute DC and convert it to AC, and we cannot run a DC motor from an AC source without rectification somewhere. Nobody in their right mind would ever even just use a simple rectifier for a DC motor, because making it into a DC drive is only going to cost a small amount more. If you do create a distributed DC power system in a machine or facility, how can you ignore all the mainenance and conversion issues that entails? Then how do you remove the difference in maintenance issues in the two motors from any and all influence of the application? Is it fair to compare them in an explosive environment for example? How about a wet application? How about on a remote rock crusher that is making giant clouds if silica dust around the machine?

Yet if we go back to the power issues, we CAN use an AC motor without a VFD. That alone gives an edge to AC, but it is related to the POWER SOURCE, not simply the motor itself. Then if you compare an AC motor driven by a VFD to a DC motor driven by a DC drive, your comparison then gets back into performance issues, which are as much related to the drives as they are the motors. It used to be that if I wanted full torque at zero speed, I was FORCED by that application to use DC. But now with a good VFD and an encoder feedback, I have no problem doing that with an AC motor and I eliminate the maintenance headaches with having a DC motor and drive. But without a VFD? No way. But if I only need 2HP for a simple shop winch? A DC motor and drive will be FAR less expensive and complicated than an AC motor and FVC drive.

So in your query, you are insisting on divorcing the power source and maintenance issues from the motors themselves, and I believe that is an erroneous concept. They are not married, they are conjoined!

Here is an example I am working on right now. I am bidding on a project that involves a series of pump stations built in the 1940s that used 600HP 500VDC motors, because at that time, that was the best way to handle variable flow. There are 40 of these drives involved. The DC drives they used are no longer supported, the mfr went out of business long ago and although they have been scavenging to keep them running, even the board repair people are having difficulty finding good components now, because nobody does things that way any more. So all of my competitors are coming in and IMMEDIATELY proposing AC replacement solutions, because all of the "conventional wisdom" and the consulting engineer they hired, states tha AC is better than DC. However I took a look at their maintenance logs and determined that these guys are VERY fastidious and adept at maintaining their DC motors, and that has persisted through almost 3 generations of maintenance techs, so it is systemic. I am proposing that they LEAVE the motors as DC, and just replace the aging analog DC drives with new modern digital DC drives. As you might imagine, my proposal is incredibly less expensive, which has touched off a 54!7 storm of controversy between the end user and the consultant they hired who told them they must swap out all the motors to medium voltage AC with VFDs.

GA

Hi, herewith is a particular case. I have an inverter switch, which can be operated by a DC motor or an induction motor, but the supply and command are AC. Thus I'm looking for arguments to convince the customer to accept an induction motor instead of an DC motor, which is his preference, avoiding change the supply and command.
The advantages/disadvantages of induction vs DC motors are well known, but I'm looking for additional arguments based, per example, on statistical results like MTBF.

Thanks in advance for your contributions.

Why does the customer want a DC motor?

Does he have a perceived benefit in mind?

Is this perceived benefit so great that only a reputable citation will swing him in a rational (IMHO) direction?

Hang on. Are you sure you meant "but the supply and command are AC"?

If the supply and command are DC, which is inferred by you saying "avoiding change the supply and command", then, I can see where the customer is coming from.

This is a motorised switch, yes?

Hi Wal,

Yes, this a motorised inverter switch for HV application.
My design supply and command is AC, thus I'm looking for arguments - beyond the well known advantages/disadvantages of induction vs DC motors - to convince the customer to accept an induction motor instead of an DC motor, which is his preference (God knows why)…

Tell him it will be cheaper to go with the AC motor.

If he still insists then quote him accordingly and get on with making him happy.