Selecting type of 1/2 HP motor for quiet operation and reliability 7

[LowEnergyParticle]

I need to select a 1/2 HP motor. The basic service is 1740 rpm, single-speed, 120/240 VAC single-phase, runs 8 hours/day, lightly loaded at 20% capacity except for intermittant 1% duty cycle 2-second long requirements for up to 80% capacity. The motor will be face-mounted into a 30:1 worm gear right-angle gearbox. The environment is room air, ambient operating temperatures 5C - 55C.

Quiet operation is an important feature: unfortunately, instead of a number I've been told "as quiet as you can make it." Oh, joy.

Additionally, the motor will be extremely difficult to access for service, so long term reliability is important with MTBF > 10,000 hours.

I was thinking perhaps a split-phase motor in a TENV enclosure, possibly with a passive convection heat sink? TEFC is tempting, but I'd like to eliminate the fan noise if possible. What made me think of the split-phase motor was reliability: no brushes, and no capacitor. It would be great to eliminate the centrifugal switch, as well: has anyone experience with a start winding that does not need to be switched out, that is, that runs continuously while the motor is powered? Obviously, it would cost some efficency, and increase the heat output.

Lastly, several manufacturers have an option where they will dynamically balance the rotors of your motors to a finer tolerance than is normally done. Has anyone any experience with doing this, and did it appreciably reduce the motor noise?

I'd appreciate your advice very much on how to keep this motor quiet and reliable. I may be quite wrong about specifying a split-phase motor, it's just an idea.

Thank you!
David

 

[ScottyUK]

Consider a 3-phase motor as they are about as simple and reliable as you can get. Thye're smaller and have lower torque pulsation than a single phase machine too. If you only have 1-phase available then a small VFD will generate 3 phase from the single phase; small VFDs are muck cheap these days. The only possible issue is that some motors 'sing' at switching frequency with a VFD supply.


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If we learn from our mistakes I'm getting a great education!

 

[LowEnergyParticle]

Scotty,

You are certainly correct, thank you! I have only single-phase available, but I've used the trick of 50% derating a small VFD and using it as a single-to-three phase converter in the past with good results.

My fault: I neglected to include that I've been asked to have no silicon in the solution. The only controller is to be a full-voltage starter. This puts paid to my two favorite solutions: a 3-phase squirrel cage induction motor and a BLDC servo.

Thank you for looking at my problem.

David

 

[sreid]

Consider a Permanent Split Capacitor Induction Motor (PSC). Lower starting torque but lower starting inrush current. If possible, you could use a motor with a resiliant mounting base to isolate motor noise from the machine base.

 

[itsmoked]

Why on earth would someone have a "no silicon" requirement in an ambient room application?

They must really not be all that interested in "quiet".
They must actually not be all that interested in reliability nor interested in the obvious ease of automatic multi supply voltage handling or jerkless starting.

I'd challenge the 'no silicon' logic.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[btrueblood]

Are you sure they weren't asking for "no silicone ?

Many automotive suppliers/shops are trying to eliminate sources of silicone grease contamination, because it plays hell with paint adhesion.

 

[Compositepro]

It been my observation that explosion-proof motors are generall relatively quiet. They have heavy duty cases and are fully enclosed. You could also look at installing precision bearings in place of standard "motor duty".
All it takes in money.

 

[roydm]

You might also find a motor with sleeve bearings will be quieter than one with ball race.
If you can't find a split phase motor with enough torque consider a relay similar to that used in a sealed unit refrigerator (drops out the start winding when the main winding current drops to normal)
Regards
Roy

 

[jraef]

No silicone... sounds like a requirement for a girlfriend, not a motor control system. <rim shot> "Thank you very much, I'm here all week...

Seriously though, I agree about the PSC motor, especially given the operating conditions you describe. Sounds as though starting torque is not going to be an issue. Look at good quality HVAC motors, they tend to be made for quieter operation because they often end up used inside of air handles where any noise is conducted with the air into office spaces.

You also may want to look in to a permanent magnet synchronous motor, essentially of the type used in clockworks. they tend to be very quiet. Speed may be a problem for that though.

 

[LowEnergyParticle]

I'm sorry it has taken me so long to get back to you all on this question. I've spent a lot of time looking into your suggestions, but I really should have gotten back to you sooner. I'm sorry.

ScottyUK, itsmoked, and betrueblood:

We've just concluded a series of acrimonious meetings about the "no silicon" requirement. I lose. No silicon allowed. Thanks for the good thoughts, though. By the way, it was silicon as in chips, not silicone as in grease or enhancements!

CompositePro:

Excellent idea, thanks! It seems that motor noise is related to case mass, so the explosion-proof motors would be the heaviest and therefore quietest of the normal, non-custom motor cases.

RoyDMatson:

The sleeve bearing idea has been very fruitful, thank you! We ran a pair of 1/2 HP motors side-by-side, one with ball bearings and one with sleeves. The sleeve bearing is significantly quieter.

Sreid:

The Permanent Split Capacitor (PSC) motor you've suggested seems excellent; many thanks! If I understand it correctly, the PSC is an ordinary 3-phase squirrel cage induction motor with a capacitor permanently across T2 & T3, and the single-phase input power connected to T1 & T2. The one capacitor serves to both start and run the motor, and is never switched out of the circuit. I have a couple questions on this, if that's alright?

1. Is my description of the connections correct?

2. Starting caps are generally much larger values than Run caps: there is only the 1 cap in a PSC, how do you calculate it's value?

3. My understanding is that for continuous operation the best selection for the type of capacitor would be an oil-filled steel case cap. Other than using a higher-than-necessary voltage rating (e.g. 570 volt cap), is there any other way to increase the reliability of the capacitor?

Thank you again for the excellent idea of using a PSC!

jraef:

You really piqued my interest with your offhand mention of PMSCs as used in clock works! As you can imagine, this technology is no longer in common commercial usage for clocks. I did some searches, though, and found this interesting website about Westclox's AC clock motors:

http://clockhistory.com/westclox/products/electric/motors/index.html

Thanks again to all of you!
Dave

 

[sreid]

Low Energy Particle,

You ask some very good questions most of which I can't answer.

I believe that most PSC motor are simply three phase motors with a capacitor connected to the third phase. I have no Idea how the capacitor is sized. I've always used the capacitor supplied by the manufacturer. Often these would not appear to be of the highest quality. Perhaps other forum members could recomend high reliability types.

 

[electricpete]

I think you guys have misunderstood what a permanent split capacitor motor is.

It is a single phase motor. It does not start as a 3-phase winding.

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[electricpete]

By the way I was not disagreeing with sreid. Only the two posts before mine. Here is description of a PSC motor:

http://www.iprocessmart.com/leeson/leeson_singlephase_article.htm

Now, there are obscure ways to make a 3-phase motor run on single phase using capacitors. I have only heard of this used by hobbyists, not in manufactured applications for industrial, retail, commercial etc. I don't know much about them except I think the caps in conjunction with motor set up a complex voltage divider that gives approx the correct voltage. As far as whether this arrangement is quiet - I'm not sure. I'm guessing it can only be balanced at one load level (after all the motor impedance varies but the cap values don't) and if the 3-phase motor input is not balanced in both magnitude and phase, it's noise will increase.

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[itsmoked]

I agree that a PSC motor holds no resemblance to a three phase motor. (Well they are both cylindrical!)

Curious as to what the prohibition on silicon was for?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[sreid]

Certainly many PSC motor windings are optimized for that application. But it is common in small fractional HP motors to use a three phase motor with a phase shifting capacitor on single phase.

 

[itsmoked]

sreid; that seems really rare. Just try to find a sub HP three phase motor. They appear to be fairly rare to me. So I have a hard time believing manufacturers would be reaching for them instead of an efficiently designed PSC.

Can you give a few examples?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

I am with itsmoked. I have never heard of a single phase motor intentionally manufactured from a balanced 3-phase winding.

(again, there are people who will try to convert a 3-phase motor to run on single phase... but that is not the intended application when it was manufactured).

I will believe it when I see it.

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[sreid]

The small FHP PSC motors I have used come from Oriental Motors.

http://www.orientalmotor.com/products/ac-motors/index.htm

I don't know for a fact that the PSC and 3 phase motor windings are the same. I Ohmed the windings once on both motors and the resistance was the same.

 

[LowEnergyParticle]

Some of the manufacturer's literature is pretty unclear on this point. During my looking around, I found several references to PSC's with 3-phase windings. This is the only one I can lay hands on right now: see page 10 of this document:

http://www.filcap.de/pdf/General Information.pdf

Anyway, it's good to know that a purpose-built PSC stator does not have 3-phase balanced windings!

More good news: Friday afternoon I got a change order that specifies the motor must be able to start against 80% of it's mechanical load. Nothing like a little change-of-heart to make your day brighter! So, I'm considering one of the configurations that uses a centrifugal switch on a start winding or a start cap. My understanding is that the integral centrifugal switches break often. Since I'm already breaking the budget on this, does anyone have any experience with an external centrifugal switch that has better reliability?

Also, in terms of reliability, I'm thinking that I'd be better off with a start winding rather than a start capacitor. Is this reasonable? Looking for input!

Thanks for all the help with this; I sure appreciate it!

Dave

 

[sreid]

Considering all of your requirements (single phase, no silicon, quite, cheap, reliable), you may be stuck using a capacitor start motor. And this may be OK.

Capacitor start motors may have a partially undeserved reputation for unreliability. There are two basic failure modes;

Capacitor failure; as you have seen, there are grades of start capacitors. Get a motor with a high reliability capacitor.

Start contacts; Contacts wear out related to the number of starts.

Your application appears to be "Start and run for a long time." If this is the case, neither the start capacitor aor contacts will be used much.