20% over nameplate

[GOTWW]

I have a 24Vdc 1/4hp motor PM-4 pole, that runs max efficiency at 10A. I have installed it is an evaporative cooler driving and propeller blower. I currently have 2-pully options, the first draws 9.5A, the second draws 12A. Now that is 2*24= ~50W over nameplate, maybe not optimum for the motor, But it sure seems to be better from a "system", btu-cfm standpoint.

By the way, I have hear (JB) the term "motors are constant KVA devices" This surely does not apply to DC motors with variable supply,does it? These seem to increase speed with voltage.

Sofar I donot have a problem with the extra 20%, nameplates were not etched by god, only some tech in the factory. besides, it resides, inside a cooler.

any comments?

 

[ScottyUK]

Motors aren't constant kVA devices, unless they happen to have constant load and constant supply voltage. Induction motors might, to a first approximation, be considered constant kVAr devices if they operate on a constant supply voltage because the majority of the reactive load is due to the magnetising current which is relatively constant. Maybe this is what was meant by the post you referred to?

And no, kVA and kVAr obviously don't apply to a DC motor because they are AC quantities.

The nameplates might have been etched by a technician, but I bet the numbers he etches onto the plate were determined by the motor design engineer. Whether you heed the nameplate depends on whether you know more than the designer does about the motor and the application it is going in to!



-----------------------------------

Ask a silly question and you are laughed at for a few moments.

Don't ask the question and you might be laughed at for eternity.

 

[edison123]

The motor is nameplated for designed winding temp rise, which is the ultimate factor for determining its output. If you can refrigerate (? !) the motor, then probably you could load still further because of better rate of heat removal. The fact that your motor is inside a cooler could justify your overloading.

 

[aolalde]

The nameplate or nominal data are the guaranteed operating parameters of a motor. These include the power supply, load, ambient, cycling, etc. The manufacturer develops intense design and prototype testing to arrive to those parameters.
If your particular application can remove more heat from the motor, probably you could overload it. You must monitor the temperatures until the motor reaches a steady rise.
As Skosgurra mentioned, no motor is a constant VA machine, including DC motors, the electric power input (VA) changes mainly with the load and the voltage too since the efficiency is affected.

 

[busbar]

At 24 volts, the rules are probably different, but how will water-saturated air affect winding-insulation life?

 

[GOTWW]

http://www.imperialelectric.com/cgi...l.html?item=145&lvl1=PMM&lvl2=42D&lvl3=&lvl4=

Is the unit (TENV). The humidity factor in the evaporative cooler does not seem to be an issue. I have had this motor running in a squirrel cage evaporative cooler for at least 5, maybe 7 years.

Funny thing, I never bothered before to acurately measure the current in the old set-up. The ~10A is an aqward current to measure with a dvm. I am using a current shunt now.

I have the eff/hp/rpm/amp vrs. tourque curves for the motor that indicate the efficiency holds flat at around 76%, the Hp goes up linearly with the current. Since I am not losing efficiency and the unit is located in a region on airflow ~4-5000 cfm, I will just run it for a while to see hot hot it gets, I have a new spare, just in case

 

[jbartos]

Comment: All electrical motors that have increasing current with decreasing voltage and decreasing current with increasing voltage, while having constant shaft load, are approximately considered constant power load apart from resistive loads that have decreasing current with decreasing voltage and increasing current with increasing voltage.
Various electrical power system modeling software actually demonstrate the above statements.

 

[GOTWW]

I would have to agree with JB somewhat, as I an earlier motor I was using got it's brushes worn to the springs, training a new wife on the frugality and limitations of the power supply I have, it was often run at the lower end of the life support 22.8 V, and I assume that it was hogging more current than it should, (no overloads were in the circuit). I quoted JB once, but had second thoughts later, As for this instance, when the voltage increases the DC motor RPM increases, and therefor the (fan-load) HP increases (the BUSS here runs ~23-28V, not quite ANSI standard). I am a little worried about this aspect of the thermally on the motor. But when the DC voltage is at the low end, the rpm does not seem to drop off much, I only have data for 1 DC voltage. I will try and monitor it.

 

[jbartos]

Comment: Since the original posting does not address a type of DC motor at a question, it is necessary revisit the types of DC motors. There are various types of DC motors: E.g.
1. Shunt
2. Series
3. Compound
4. Separately Excited

The DC Series motor (e.g. compensated series motor operating on alternating current) phasor diagram showing current locus will reflect the terminal current increasing with voltage increasing and speed decreasing up to certain culmination point. This point determines how much of power can be processed by the motor magnetic circuit design. If the motor is designed physically for 10HP max, there is not much more the magnetic circuit (iron) can process HP-wise. There is no such thing that one would purchase 10HP motor and run 20HP load (would not this be nice for Wall Street investors?).
Once, the Series motor reaches the culmination point in Terminal Voltage, Et, terminal current, It, plane, then further increase of the motor current will cause reduction of speed and reduction of Et up to a point where the motor speed becomes equal to zero, the motor will draw maximum current and there will some smaller terminal voltage Et.

Other types of DC motors Et, It loci are left for exercises since the original posting does not address a specific type of DC motors.

 

[GOTWW]

All of the above, have wound armatures, As far as I remember. This motor is a simple 4-pole-4-brush comutator-rotor, with a perminant magnetic aramature. Not much else I can say (low as they get 1/4 Hp). Anyway, the exercise of this couple weeks is somewhat of a bust. As for striving for the maximum, possible effeciency, another animal bites you in the butt. That would be noise. Prop blowers are better at CFM/Watt, And the DC motor at 76% efficiency seems to blow the doors off the single phase induction heater/motors available (considering starting torque), (please correct me if I am wrong). But in this case the prop just put out to much noise. They will have to live with it until an alternate solution is arrived at.

 

[jbartos]

Suggestion: Visit

http://zone.ni.com/devzone/conceptd.nsf/webmain/FFB03CC27F0408FC0625683F00762B2B

for: DC motor details

http://www.ecmweb.com/ops/electric_direct_current_motor/

for: Basics

http://www.pdma.com/DCmotor.html

for: DC motor analysis and troubleshooting
etc. for more info

 

[GOTWW]

Thanks for the nonrelevent tutorial. I am not a motor man, but did take two of the ten probs on the PE in mototrs.

 

[jbartos]

Suggestion to the previous posting: It would be very helpful if the motor nameplate data were posted including the motor type/model, manufacturer, etc.

 

[GOTWW]

live by the sword, die by the sword.