50 hz Motor revisited

EtzRon

New member
I am in the process of replacing my Oneida with Clearview. I am going to buy the CV1800.

I live in Israel, and I am having trouble understanding about the difference between 220 V- 50 cycle(hz) current that we have here, and the 110 V- 60 cycle (hz) that is used in America. I am trying to figure out how many HP the motor I
purchase here should have. It seems that each person I ask has a different bit of info and it has all left me pretty confused.

A colleague here told me that motors sold in Israel are rated by the European Standard, and that therefore if I buy a 3HP motor, that is equivalent to a 5HP motor on the American standard. From my non engineering viewpoint what this means is that American motors are rated no load, and European with load. However, when I asked my motor supplier about this, since he deals with motors made in Australia (European Standard), he could not tell me if this was true.

I do not want to run a 60 cycle motor on 50 cycles. I AM going to purchase the bigger impeller (16"). I have 3 phase power.

The motor I have my eye on is made by CMG Australia, and here are the specs: 3 phase, build (frame type?) B5 (I was told this is a flange type), V5 (is strong enough to hang), K90(?), 3 HP, 3000 RPM,IP55 (?),380V, 50 Hz, insulation F and made of cast iron.

I can get the above motor in 4 HP, with K100 instead of K90, otherwise the same specs, or 5.5 HP with K112 and same specs.

Is 3000 enough RPM?

I want to purchase a motor that will move enough air without overkill. Which should I choose?

Thanks for your suggestions, Amy G
 
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dwdrury

New member
Horsepower Definitions

Horsepower Definitions

Amy,

Consulting my Pratt & Whitney pocket handbook, 1 HP = 745.7 watts. In the US, as I understand it, electrical motor manufacturers rate their motors by input power requirements, power draw. In other words, volts times amps = watts, then convert to HP.

For example, the CV motors run on 220v, 1ph (at least here, though CV may be able to supply you with a 3ph motor, though it probably will be 60 Hz). If they draw 17 amps, then you have 220 * 17 / 745.7 = 5.02 HP, near enough to call 5 HP.

However, reaching way back to those three EE courses they made this ME take in college, I seem to recall most AC motors have an efficiency of about 80%, meaning 80% of the power input gets delivered to the shaft, 20% is converted to heat. In the example, then, the electrical power input is 5 HP, but the power delivered to the shaft would be 4 HP.

It is possible Europe and Israel define power by the shaft output rating. I've asked an IAI friend in Tel Aviv to please clarify how power is defined "over there", and hope he will get back to me shortly. When I hear, you'll hear.

On to your RPM question. From memory, which may be faulty, think I remember the US spec motors at 60Hz turn at 3450 RPM. While slowing it down to 3000 RPM doesn't seem like all that much, I will point out that Erik, from Belgium, noted that running a 60 Hz motor at 50 Hz, which would slow down the impeller to 2875 RPM, would result in a large loss of airflow. The number escapes me at present, but Erik wrote about it in these boards. If you search for his name under the member list (he's the only Erik with a K), then review his posts, you can find it. As I recall, and am even more hazy on this number, think it was 35 or 36% loss of airflow, but don't quote me on that. If you have trouble finding it, holler and I'll go hunt it up. (Edit: Found it, here Scroll down to post#4)

But bottom line, I think, is that if you're going to use CV's fan, you're probably better off trying to find a motor that'll do 3450 RPM. +/- 50 RPM, I'd guess.

Another alternative, also courtesy Erik, is to use a US spec motor and run it through a power inverter, though he also warns that if you're not comfortable playing with electricity, might be a good idea to consult with a qualified electrician.

Matt and / or Ed may have some better ideas.

Regards,
DWD
 
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dwdrury

New member
More Info

More Info

Amy,

My IAI friend passed the question along to another friend of his, one with both mechanical and electrical degrees. The edited highlights of this gentleman's response are as follows:

Apparently I wasn't too far off the mark with my speculation that in Europe and Israel, motor power is defined by what the motor delivers to the output shaft, not the input electrical power as seems to be the US standard. So per the example in my earlier post, the motor you'd be looking for would have a European / Israeli / Australian rating of around 4 HP. You can confirm this by comparing the current draw to the rating spec. If your math of the power supply converts to, oh, 5 HP but the published spec says 4, then my explanation is correct.

But more interestingly, it seems the difference in frequency will make your search for a matching motor impossible. My friend's friend reminded me that electric motors are comprised of electromagnetic poles, the number of which are integers. At 60 HZ, a two pole motor would theoretically turn at 60 cycles per second times 60 seconds per minute = 3600 RPM (though in practice it seems they rotate at 3450 RPM. Similarly, at 50 Hz, one gets 3000 RPM, hence the rating on the motor you found. A four pole motor would turn at half those speeds. And there isn't an integer combination that will yield the correct RPM at 50 Hz.

Which leaves you with some unpleasant options if you want the design airflow and suction: (1) An inverter as Erik used, to convert your 50 Hz power to 60 Hz. (2) A gearbox (Yikes!) or pulley and V belt arrangement. (3) A different fan designed to supply that airflow and suction at 50 Hz. (4) Accepting the 36% airflow reduction due to the reduced RPM produced by 50 Hz.

For option (3), my friend's friend points out that if the rest of CV's impeller design remains the same, one way to provide the required flow would be to increase the impeller's diameter by 6/5. Thus a 16" fan would become 19.2" in diameter.

Sorry, but hope that clarifies your choices.

Regards,
DWD
 
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