PSC vs Capacitor Start Capacitor Run AC Motors 3

[LRME20]

I am a Mechanical Engineer, but I am the only engineer in the USA for my company, so I really need to get a better understanding of what the design differences are in the windings of the two motor types. My story may be long, but I would really appreciate any help!

My company manufactures motors. To date, we only design and manufacture PSC motors (one capacitor bank for starting and running the motor). I believe our motors are becoming too large to continue without designing CSCR motors (we are in ventilation and are turning some fairly large fans now). Of course, the problem is we have to use a much larger powered motor to turn the same RPM as a competitor's motor, with not as good efficiency. I believe the remedy for this is to design a CSCR motor to stay competitive.

The problem is convincing my European colleagues to do this. I need to be able to prove my point and make it in an intellectual manner (because that's what engineers do!).

So, the first question is more of a verification: They once tried to test a PSC motor with a manual switch to switch capacitor banks once the fan was running. I believe this is a flawed experiment, is this true? I believe the winding must be designed specifically for a CSCR motor, correct?

The design of a winding is beyond me as I am a ME... But to get a true test, we would have to design a motor with essentially two windings, offset from each other, if I understand my limited research correctly.

Could you explain maybe some industry standards for the maximum size for a PSC and when a CSCR is needed?

Could you explain how a PSC motor would perform with a smaller capacitor as compared to a true CSCR motor?

Will I be able to use a smaller CSCR motor in place of a PSC motor and achieve roughly the same performance at a better efficiency?

Thank you in advance!

 

[jraef]

The only significant difference between a PSC and a CSCR single phase motor of the same HP (kW) rating is (as far as I know) that CSCR will provide more starting torque, which you should know as an ME, has to do with your accelerating time. This should give you a reasonable comparison.

The down side to that is, the starting CURRENT profile will look comparatively similar, i.e. the PSC motor is going to take maybe 200% starting current, whereas the CSCR may take upward of 600%. That could end up causing one problem while solving another. Also, a CSCR motor must have the starting cap and winding switched out at roughly 80-90% speed, hence the centrifugal switch. That can be replaced externally with what is called a "potential relay" that looks at the back EMF, but that is more expensive. Hermetically sealed refrigeration compressors and submersible pumps use that method, but only because locating a centrifugal switch in their machines is worse, and they often need more starting torque than a PSC motor will give them. So what are PSC motors good for? Fans! Lower accelerating torque requirements (because the affinity law says that low speed means very low shaft power requirements) and not having to get on a man lift to repair centrifugal switches makes them more reliable. They are also less expensive to build.

Once up and running however, the power rating is the power rating, HP / kW (shaft) is basically just a shorthand notation of torque at a given speed. The final shaft speed is the synchronous speed, that of the rotating magnetic fields based on the power line frequency and number of poles in the motor, less the "slip" percentage, which is the lag between the magnetic field rotation in the stator, and that which is induced in the rotor. Slip is where the torque is produced, and thereby is influenced by the load attached to it. So if your fan is not turning as fast as a competitor's, is turning the same RPM with a smaller motor is basically just pointing to the fact that your fan must have more inertia or more load, meaning it moves more air. If it's more air, that's a feature! If it's more inertia, that's an MEs job to find out why they can get away with that. As you might expect, it's likely for a good reason, because "there is no free lunch" as they say.

Efficiency of the motor is a different issue, that has to do with the motor design for sure. Both types are roughly equal in efficiency and also can however be designed for roughly the same high (relative) efficiency, but the techniques of getting there may be different between them. If efficiency is your goal and you are not buying someone else' motor to where you just specify a higher eff rating, I would have someone in your company look into those efficiency improvement techniques before deciding to change motor types and potentially add issues that you are not now having to contend with.

"Will work for (the memory of) salami"

 

[waross]

Take a look at the Cowern Papers. Types of motors starts on .pdf page 11

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

The only difference should be the accelerating torque so there is no reason your motors would be larger and less efficient to generate the same running shaft torque and speed. The only reason they may be larger is to satisfy the fan starting requirements.

Both motor types have 2 windings. The PSC has a single capacitor permanently connected. The CSCR motor typically parallels the permanent capacitor with another capacitor during starting that is switched off around 75% or 8% speed using a centrifugal switch.

 

[LRME20]

I really appreciate the responses! Especially jraef for taking the time to write out that long answer. It has already been insightful!

Waross: I checked out the paper and I found a table that stated PSC is typically used up to 1HP while CSCR is used from 2-15HP. I'm sure that there is a grey-area in between 1 and 2 HP... especially that the CSCR motor I am comparing our 1.7HP motor to is a PSC. This is a good table!

Lionel... I appreciate the concise breakdown. What I gather from your input (Also from jraef) is that maybe the impellers have enough mass that starting torque is much higher and then inertia of the blades mean that running torque can be less. Therefore, the 1HP motor has better efficiency of the 1.7HP motor because 1.7HP is required in a PSC just to start the fan, while the high starting torque of the CSCR motor overcomes the blades easily.

BTW, the fan in question is 54" in diameter, so these aren't just small fans. Our blades are made of Poly material though. But I do believe that our motors are sized so large just for starting torque and they are over-sized to run the fan. Does this sound like the probable cause?

You have given me a lot to think about while I am going through the data again.

 

[LRME20]

OK then, here's the next question:

I have a 1HP PSC motor (1.5 SF) of our own that under-performs... Is it a flawed experiment to decrease the capacitor size on it and create a manual switch with a large capacitor to simulate a CSCR motor? Will this give me a good indication of what the motor should do or does the winding have to be designed specifically for a CSCR setup?

After reading your answers, I can't help but think why our 1HP motor performs so much worse than their 1HP motor... Using the same pulley, we get ~25 blade RPM less than their motor, so I assume the slip is designed differently. Is this a parameter of the PSC vs CSCR type?

 

[LionelHutz]

The rotor design has a lot to do with the motor slip. Your motor design engineers should know the answers to all of these questions.

 

[LRME20]

The problem is they have never designed a CSCR motor, so they wouldn't be able to tell me the difference. I'm more curious about the experimental implications of making a PSC motor into a CSCR motor... is this a flawed method?

I don't need to know too much about the slip, but I do assume that the slip is less on a CSCR motor.

 

[John2025]

1.5SF seems awfully high, although I know very little about single phase motors. Is your competition using that service factor? Are you comparing apples to oranges?

 

[jraef]

Slip is what you design the slip to be when designing the motor, regardless of how the motor is started. It has, I believe, mostly to do with winding styles, slot geometry, flux penetration, things like that. I'm not a motor designer either, just a motor user, but I've read several papers on energy efficient motor designs in order to understand some of the ramifications that relate to my work, and in those they discuss where slip and torque come from. There are other members here who know a lot more about that than me, hopefully some may read this and chime in. But to be honest if they see the thread title, they may not be terribly interested, single phase motors get little respect...

But the comment on apples to oranges I think is a very valid issue. You are comparing RPMs only, but have you evaluated their efficieny vs yours? Are they moving the same amount of air? For example if their blade design spills off more air from the blade surfaces than yours, their motor is not as loaded and can spin faster, but the comparison is invalid if your fan is moving more air, or even the SAME air at a lower speed. A truly valid comparison could only be absorbed kW per unit of air volume. Start there before pushing for changes, because maybe the only change needs to be in marketing.

But as to "changing" a PSC into CSCR after the fact, I don't think that is viable. It's not that simple.

"Will work for (the memory of) salami"

 

[LionelHutz]

Yes, it's flawed. I suppose you could add an extra capacitor for starting and you might get more torque. But, you still have a motor designed for a certain size of permanently attached capacitor so changing that capacitor size would affect the motor running characteristics.

Most CSCR motors have the same winding configuration as a PSC motor with an extra capacitor added in parallel during acceleration. There are 2 design cases to balance - the motor starting characteristics with more capacitance and the motor running characteristics with less capacitance. An engineer that can design a PSC motor should have no trouble designing a CSCR motor. If they can't, maybe you need a new engineer.

 

[LRME20]

Thanks again,

John, yes SF of 1.5 is high, but these are European engineers who don't try that hard to Americanize the motors that we sell here in the State.... Part of why I'm having this discussion. If you ask the engineer over there, he will tell you that it is a 1.5HP motor, but we sell it as a 1.0HP motor with a 1.5SF. All of this is out of my control as of now. Hopefully once I get some more clout in the company, I can start to Americanize our nameplates.

jraef... I have tested our motor on their blade, their motor on our blade, and of course each motor on the respective blades. (All in two different housings). So I have a good comparison of what each motor will do on each blade. Their blade has more mass (meaning more inertia) and ours is Poly, so not a lot of respective inertia. In the data that I am seeing, their motor doesn't perform as well on our blade... I can only assume because less inertia = more work needed to keep the blade spinning under pressure. If this is the case, then maybe our problem lies almost entirely with our blade, because our startup torque isn't that bad and we don't need the start capacitor.

I really appreciate all the input from everyone! If anyone has any more to input, please do!

 

[waross]

Inertia is only important when the speed is changing. A fan with more inertia will take longer to come up to speed, but once at speed, the inertia is unimportant.
If you need more starting torque from a PSC motor, you can add another capacitor in parallel and switch the second capacitor out of the circuit at about 80% speed. Your PSC motor is now a CSCR motor. The running performance will be the same as before, but the starting torque will have been increased.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[zeusfaber]

When you say "perform as well", what are you actually measuring?

A.

 

[waross]

If you add a capacitor for starting and then switch the added capacitor out once the motor is up to speed, then the there will be no change in the motor's running performance. The motor will generate more starting torque.
I don't have to measure anything to state that the running performance will be the same. It is the same motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[zeusfaber]

Bill,

When I said "you" I didn't mean you! I was trying to understand the OP's last post.

A.

 

[LionelHutz]

If you motor performs better with your blade then what is the problem? Sounds like the total design works well together.

Lighter blades means less inertia and easier to start hence the CSCR motor may not be necessary.

 

[waross]

Sorry zeusfaber!

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LRME20]

zeus - Yes I am measuring CFM, amps, CFM/W, at each static pressure point in increments of 0.05" w.g. up to about 0.3". But our fan does not reach the efficiency (CFM/W) that we would like. That's the main issue. I'm just comparing the performance of all of the motor/blade combinations to try to draw a conclusion of where our problem lies. I'm getting more convinced that it is in our blade, but I'm not 100% sure on that.

Lionel - our motor/blade combination > their motor/our blade combination. BUT their motor/blade > our motor/blade. AND our motor/their blade ~= their motor/their blade.

I hope that clarifies that for you.

waross - But won't inertia tend to keep a blade moving at a certain speed? I think that a heavier blade would perform better at pressure than a light blade, because it would tend to "plow" through the pressure with less load on the motor than a lighter blade. Please let me know if I am thinking incorrectly though.

 

[davidbeach]

At steady state load inertia does nothing for you.

The motor has to supply all of the energy used to move the air. The inertia could be thought of as an energy bank; with fluctuating load you will draw upon the banked reserve and then recharge it. At a constant work rate there is no call on the reserve.