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Anonymous Poster

Power and Speed Change with Respect to Voltage Drop

05/09/2008 8:40 AM

I have a fan motor which runs at 230VAC @ 850 rpm. The power measured is 320. What happens if the voltage drops to 200VAC? Is there an equation to estimate the new rpm and power consumption (assuming the speed-Torque curve data is available at 230VAC). Obviuosly, the fan blade is the same.

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#1

Re: Power and Speed Change with Respect to Voltage Drop

05/09/2008 10:00 AM

One would have to make the assumption that your fan motor is a type of induction motor because that is generally the least expensive type. Within the family of induction motors there are different types, such as shaded pole which is useful in forcing the motor to operate only in one direction. But without knowing more about the specific motor, it is just guess work. Since you already measured the power once, why not measure it again at 200VAC. If you don't have a 240 VAC variac sitting around you might have a low voltage transformer that can be connected as a bucking transformer in series with the fan. If this sounds unfamiliar, do a search for boost-buck transformer circuits on the net. There are lots of resources out there.

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#2

Re: Power and Speed Change with Respect to Voltage Drop

05/09/2008 10:45 AM

Thanks. I did have a variac and changed it to 200 VAC. The power dropped from 320w to 285w and the rpm dropped from 852rpm to 837rpm. I actually have the speed-torque curve for 230AC and 208VAC. Is it prudent to extrapolate the 200VAC speed-torque curve for the motor and superimpose the fan load curve on to get the point of intersection for new rpm and watts? I am trying to do this on a bunch of units and trying to avoid testing all of them as it is very time consuming. Thanks.

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#3
In reply to #2

Re: Power and Speed Change with Respect to Voltage Drop

05/09/2008 11:36 AM

The prudence for extrapolating really depends on reason you are making this effort. There are a lot of variables involved such as fan blade pitch, surface area, compound curve effects, number of blades, diameter and probably a few I forgot to mention, that all effect your loading of the various motors. I would want to determine how many combinations you had to deal with (i.e. motor size, mfgr., type) and take a hard look at the blades.

I would also guess that you are concerned about the quantity of air these fans are no longer pushing, but I don't like to guess. I do, however, approximate and then over-design a little bit quite frequently. So again, it depends on why you are taking these measurements. Since you mentioned RPM, I have to ask, are you considering changes in elevation (i.e. density of the air)? There will be less loading in thinner air so the RPM will change with changes in elevation.

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#4
In reply to #3

Re: Power and Speed Change with Respect to Voltage Drop

05/10/2008 2:25 AM

That last point is an important one.

Varying only the voltage on a (assumed) shaded pole motor does not directly change the speed. The lower voltage reduces the output torque by the square of the applied voltage. So a 240V motor with 200V applied means the voltage is 83% of normal voltage, so the output torque of the motor becomes .83 x .83 or 69% of normal. With only 69% torque and the fan blade pitch fixed, the same "load" on the fan, meaning the air resistance, will cause the fan to slow down. So the speed reduction becomes a factor of the reduction in the motor's ability to move air. If the air density changes, so too will the speed.

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#5

Re: Power and Speed Change with Respect to Voltage Drop

05/10/2008 5:18 AM

Some motors, depending upon load etc, will just draw more current and eventually overheat.

This did not appear to be the problem you were having I must admit, but having installed US equipment many times in Europe, I found there was a need to use transformer tappings that were seemingly "too high" eg., but it was the only way to get the correct voltage out to allow correct speed and other running characteristics.

f memory serves me well, they were single phase capacitor start motors.....

I even got a small prize for sorting that out in the early 70's.....from Univac USA....

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#6

Re: Power and Speed Change with Respect to Voltage Drop

05/10/2008 11:01 AM

I'm showing my ignorance here, but I thought most fan motors, particularly ceiling fans, were synchronous motors whose speed was determined by the cycles per second of the line current. You can increase the voltage until the wiring melts or reduce it to the point that it won't have enough power to turn the fan, but as long as the fan is turning, its speed is pretty close to whatever the coils are set up to deliver--60 rpm in the us 50 rmp in europe--or some whole fraction of that. Minus whatever loss there is to air resistance. Any loss--any rpm less than the whole fraction of the AC cycles--will result in loss in heat from the coils known as hysteresis. Reducing the voltage to reduce the fan speed will result in increased loss in the form of heat, not in a significant reduction in the speed of the fan. I am here to learn. Please tell me if I'm way off base.

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#7
In reply to #6

Re: Power and Speed Change with Respect to Voltage Drop

05/10/2008 4:30 PM

In my limited experience in this area, you are right!!!

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#8
In reply to #6

Re: Power and Speed Change with Respect to Voltage Drop

05/10/2008 10:04 PM

No, most fan motors are shaded pole. They are far far cheaper to mass produce than synchronous motors.

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#9
In reply to #8

Re: Power and Speed Change with Respect to Voltage Drop

05/11/2008 6:06 AM

Most "SMALL" (one could even say "Tiny") fans are shaded pole would be a much better and more accurate statement here.

I looked on the web and the largest shaded pole motor that I could find was a 50 watt unit. I am not saying that there are none bigger (I simply did not find them if true!), but generally speaking, shaded pole motors tend to be only in fractional horse power versions.

1 Horse power is if my memory serves me correctly about 746 watts.....

Larger ones are always (proper!) induction motors in the AC world. There are several different types of induction motor of course.......

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#10

Re: Power and Speed Change with Respect to Voltage Drop

05/11/2008 7:12 AM

boys, boys, don't fight. There is a kind of motor that, without a load, will accelerate until it flies apart. I understand that this kind of motor is generally used in fans. Is that the shaded pole motor?

I guess the answer to guest's question is up in the air until the type of motor can be established.

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#11
In reply to #10

Re: Power and Speed Change with Respect to Voltage Drop

05/11/2008 10:59 AM

In the DC world you are correct, its a DC Motor with the field in series with the commutator. Look at diagram C. below....

D. C. below are compound DC motors where special fields hold the lines of force straight and stop them bending in relationship to the brushes, which would cause sparking. A. is the normal shunt.

The fan blade is the only thing that slows it down to a reasonable speed.....no fan blade, no stopping the continued acceleration.

In the AC world, a series motor of this type would still work and the same holds true if the fan blade were missing.

Most DC motors have the field connected in parallel to the commutator by the way....

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