motor load and current draw question

[James Baker SD]

I apologize for posting a question not directly related to my workworking (indirectly related because I am building the cabinet stand for the tank) but a lot of people on this forum seem to understand motors better than I do. This post was placed on a marine reef keeping forum, but nobody has dared to answer it in 3 days. Hopefully some of the engineers with more direct knowledge than mine (I push nanoAmps around silicon) can answer the question. Basically I have a water pump that seems to draw much more current than I expect. I have seen similar discussions on dust collector impellar motors and hope the same principles apply. Below is the original post.

"I have an AmpMaster 3000 pump by Dophin. It is not the current model, but about 5 years old, but never installed. On the side of the motor, it states 1.3 Amps. The web page from the dealer I bought it from stated "average amps 0.9" (still same value for the similiar 2700/3200 model).

Problem is I hooked up an Ampmeter in series with the pump (I am a micro EE so I think I hooked it in correctly) and the meter shows 2.3 Amps if I let it pump a lot of water, if I scale it way back, it still draws 1.9 Amps, and if I close the outlet values completely just letting it stir the water inside the head, it still draws 1.6 Amps. I do not see how to get an average of 0.9 unless I include turning it off for 12 hours a day.

What is going on here? I am not a good an EE as I thought? My Fluke meter is no good? Very misleading advertising?"

Thanks and moderators feel free to delete if this is not appropriate.

James

[Tom Veatch]

Those amperages seem reasonable to me. The power factor of a motor is not a constant. It's much lower under low load conditions than under rated load.

2.3 full load/1.6 no load doesn't seem unreasonable for that small a motor. What looks out of sorts to me is a 1.6 no load amperage when the data plate says 1.3 FLA. That 1.3 a marketing decal or a dataplate value.

[Ronald Mancini]

After the motor has on "stir" for an hour feel the motor and see how hot it is. If it is warm to touch everything is probably OK, but if it is hot the motor is probably defective. Again, run it at full load for a while and feel it to see how hot it is. If you can grab it it should be OK, too hot to grab you need professional help from somebody who knows that kind of motor.

[Lee Schierer]

One possible answer. The motor current rating is probably for the motor alone and came from the manufacturer of the motor. Add the pump impellar and seals and you have some added load, which could account for the increased current draw. If you run the motor for a couple of hours and can still hold you hand on it it is probably fine.

[James Baker SD]

I let it run pumping a lot (2.3 AMP) and could still touch it after a couple of hours, so it is not burning itself up. The 1.3A label on the motor was definitely placed there by the motor manufacturer, not the company that used it for their pump.

Problem is web page for hobby store listed max draw 1.3 (from label on motor) and claimed it drew less power if throttled back for an average of 0.9A. I bought it based on that 24/7 power consumption added to my electric bill. A 2.3A power consumption 24/7 will drive my electric bill too high.

So what I was wondering was if something is wrong with my measurement technique and the motor is really drawing much less than the 2.3A. Or if I need to investigate smaller pumps in order to keep my electric bill reasonable.

Fear that the later option is the path I have to take. But it that case I do not trust any advertised power consumption numbers and do not know how to make the proper decision.

James

[Rod Sheridan]

James, the motor power consumption isn't equall to the product of the voltage and the current, it's equall to the product of the current, voltage and power factor.

Small motors often have poor power factor.

the motor power may only 100 watts or so.

regards, Rod.

[James Baker SD]

Rod,

Then it seems I need a watt meter instead of an amp meter to measure what I want to measure. Thanks for the info.

James

[Dan Friedrichs]

James,

I'd recommend a "Kill-A-Watt" meter - it will give you PF, VA, and Watts.

[John Davidson]

Looking at the Dolphin web site, these motors can be ran on 115 VAC or 230 VAC. Is it possable the nameplate information you are looking at is for the 230 volt operation? You measurments would be correct for a low voltage setup if the nameplate data was given at 230 volts.

[Matt Edwards]

You said you hooked your fluke in series with the pump, is it possible that your series connections with the meter are the culprit? Try a clamp type amp meter and see if the readings are a little closer.

Matt

[Michael Wetzel]

You said you hooked your fluke in series with the pump, is it possible that your series connections with the meter are the culprit? Try a clamp type amp meter and see if the readings are a little closer.

Matt

If you use a clampmeter make sure you are only checking the hot wire. If that is not possible, buy a male and female plug end and 2' of Black/White/Green THWN and make a test cord.

[Eric Gustafson]

If you use a clampmeter make sure you are only checking the hot wire. If that is not possible, buy a male and female plug end and 2' of Black/White/Green THWN and make a test cord.

Just curious why it would make any diffrence whether you clamped the hot or the neutral, wouldn't the current be the same?

[Dan Friedrichs]

Just curious why it would make any diffrence whether you clamped the hot or the neutral, wouldn't the current be the same?

Either works. Just not both at the same time

(clamping around both wires will read zero current)

[John Davidson]

The clamp on meter works my measuring the magnet field that biulds up around the wire as a result of current flow. If both wires are in the clamp at the same time the fields in each wire cancel the other and the meter will read zero