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View Full Version : Do PSC blower motors use less power at lower fan speeds?

briank101
07-24-2009, 04:48 PM
The reason I ask this is that I've heard only variable speed brushless reduce power consumption at lower speeds. My understanding is that the PSC fan is a big electrical usage hog and all the wattage it uses is eventually is added as heat.

I have a Carrier 58MCB-120 with a 4 speed PSC blower and I wonder if I'm reducing electrical usage at the lower fan speeds and if so by how much.

Would anyone know the wattage used at each of the speeds?

Thanks.
I

elec2hvac
07-24-2009, 04:54 PM
Amps x Voltage will give the Power being used. If you want to take an amp reading at each speed then multiply by the voltage you will see the differences in power usage. Or maybe the amps are already given at each speed on the motor specs?

Ed Janowiak
07-24-2009, 05:05 PM
Would anyone know the wattage used at each of the speeds?

You'd be looking at 10% or less between speeds.

I'd would look at setting the blower to accomplish it's task properly before I would look at it's power consumption.

WhoIsThat?
07-24-2009, 05:44 PM
http://www.fanhandler.com/Main%20Frame%20Pages/Technical%20Stuff/index.htm?Technical%20Stuff.htm~main

tecman
07-24-2009, 06:39 PM
Amps x Voltage will give the Power being used.

Actually to find the power used, you would also need to know the power factor, Volts x Amps x P.F. = Actual power. For the motor PF will be a definite factor. You would need a true wattmeter to know what the actual number is.

Paul

briank101
07-24-2009, 11:00 PM
I don't have an ammeter that measures higher than 500mA AC. I guess I could buy one of those Kill A Watt devices along with a little improvisation to measure it. My google search has not produced any useful results on this, surprising really considering in many homes it is in the top 5 devices in wattage in these days of greater energy awareness.

I'm not sure if the fanhandler.com site gives me the answer, if this was the case halfing the speed would result in one fourth of the power consumption, which I believe is not the case for PSC motors. Hopefully someone will post some info or charts such as typical PSC motor power consumption versus RPM.

DanW13
07-24-2009, 11:12 PM
From what I have read regarding ECM motor vs. PCS motors is that when you cut the speed of a PCS motor down you increase the power consumption, where it's just the oposite for ECM motors.

WhoIsThat?
07-25-2009, 12:04 PM
I don't have an ammeter that measures higher than 500mA AC.
7.7' of #14 AWG copper will read ~200 mV across it when 10A flows through it.

wildbill99
07-25-2009, 01:06 PM
7.7' of #14 AWG copper will read ~200 mV across it when 10A flows through it.

I would not recommend that to some one that is not very, very experienced. It can be dangerous in several ways.

Really need a clamp on. And you can get clamp on adapter that put out so many millivolts per amp of current so that you can use the existing meter.

But you really need a true RMS meter to meaningfull results with a ECM motor.

And that still ignored power factor, which depending on the characteristics of the ECM controller may or may not be a big factor.

But if comparing that to a PSC it will be.

While not always the easiest thing to do the I think that his idea using the Kill-a-watt is probably the best.

There are wattmeters that clamp on for the current and clips for the voltage measurements. But I think that they start about \$150.

bmathews
07-25-2009, 07:48 PM
Why are you asking? Neither one is going to be a huge difference in power usage of the blower motor. Maybe a few cups of 7-11 coffee difference a month in cost. Most of the difference in is comfort level and your cost savings will be that you're condenser runs less because you are generally able to keep your thermostat 2-3 degrees higher because you will have lower humidity levels in your house. Lower humidity makes higher temps. seem colder than they really are.

arcticbreeze
07-25-2009, 07:56 PM
Not to mention any gain achieved at the lower speed may be consumed by capacity loss. As airflow reduces you change you sensible to latent ratio also.

WhoIsThat?
07-25-2009, 08:18 PM
This may or may not work for your application.

dan sw fl
07-25-2009, 08:34 PM
The reason I ask this is that I've heard only variable speed brushless reduce power consumption at lower speeds. My understanding is that the PSC fan is a big electrical usage hog and all the wattage it uses is eventually is added as heat.

I have a Carrier 58MCB-120 with a 4 speed PSC blower and I wonder if I'm reducing electrical usage at the lower fan speeds and if so by how much.

Would anyone know the wattage used at each of the speeds?

Thanks.
I

How much savings can one realize when the 58MCB motor is only 1/3 hp?

at half power, ...
1/6 hp (~125 watts ) running for 2000 hours per year will save 248 kW or about \$ 25

jrbenny
07-25-2009, 09:08 PM
Wow. Big savings there.

briank101
07-27-2009, 09:08 AM
It's actually a 3/4 hp blower (562 watts)
Well lower speed, means somewhat less watts (over 300 watts less) added as heat during cooling, less stress on motor startup (motor may last a few years longer), better dehumidification (so I set the temp a degree or 2 higher) and leaving it on constant fan during heating season means more even distribution of heat. My average electrical usage over 24 hours during the non cooling season is 600W which is about \$60 per month, if that is bought down to 500W average load by going from Med-Lo to Lo (maybe a 100W reduction) that's a 20% savings (\$10/mo) i.e. total \$50/mo for a 3000 sq ft home.
During cooling, by blower goes to high speed if supply temp goes below 50F or if outside temp is above 85F or humidity is below 45%RH due to the custom setup I installed.

wraujr
07-27-2009, 10:49 AM
The problem with your logic is you seem to be assuming that the current fan setting is somehow too high and you have wiggle room. If the fan is set properly, then your system is operating at its "sweet" spot and is giving you the best efficiency.

Lowering the blower speed may place higher load on compressor thereby shortening its life by a few years and its more expensize to replace than a blower motor. You may also reduce cooling output which increases run time of compressor which uses more watts than your blower motor.

In proper system analysis you can't look at one item in isolation, YOU MUST look at the overall system as any changes in one will impact other aspects.....

I seriously doubt you have the necessary equipment to properly analyze the full impact of changing blower speed, so if setting matches manufacturer specs.. leave it alone.