ECN Forum Forums General Topics of the Trade General Discussion Area Horse Power--Oversize of Not?

Thank you all for your previous responses to my fractional hp motor question. It seems the residential motor market may not deliver as advertized.

My new question: Is it inadvisable to run a 3/4 hp motor at 70% of it's deliverable torque in order to achieve 1/2 hp of power? Although a more costly motor, the 3/4 hp motor should last far longer than an under-rated 1/2 hp motor trying to deliver a true 1/2 hp of work.

The enemy of motors is heat. Running a motor at light load is not a problem.

Short cycling... frequent starts and stops ... is a real motor killer, though.

I'll second that Reno!! My current screening room install has three motorized blackout shades, two for doors and one for a window, as wellas a drop-down screen with two sets of drop down masking.

The two door shades lost their memorized stops (the up and down limits are electronic) because the installers of the house control system kept short-cycling them!! Luckily, the motors didn't burn out.....

I have written delays into the control program for the screening room to prevent short-cycling from my end of the system. (AMX control.)

I have also had a few projector motors release the magic smoke from operators who "bump" the power switch to advance the film.

If you are replacing fan motors it is usually best to replace the motor and fan blade as a matched set. I got that from the HVAC guys. If your blade is pitched too much it will burn up the motor and if it doesn't have enough pitch you are not getting the design airflow for the power you are using.

I agree as well Reno!
I have seen many a pool pump motor just cook itself from being out in the baking sun with their air vents clogged with debris. Same goes with the older style belt driven furnace motors. seen many of them fail from starting and stopping every 1/2 hour and being corked up with dust, not to mention being run without being oiled in years.

Yet, on the other hand our old General Electric furnace motor was the original from '67, ran hot and under a slight overload for years but was oiled and the vent slots vacuumed religiously. Also it was run 24/7 and was only off during power outages or for cleaning. It literally fell apart in '93 when I took it apart after we changed out the entire furnace. Pretty darn good use out of that motor if you ask me!

A.D

All things being equal, running a motor at only 70% of load will waste a little more energy because the magnetization current for the 3/4HP motor remains the same regardless of load, so a greater portion of the energy used is not going to be doing useful work for you. If the application doesn't involve long run times this may be insignificant, but if it does, those extra few watts will add up to a significant amount of operating cost.

That said, if the 1/2HP motor is not really putting out 1/2HP at rated continuous speed as discussed earlier, then this may be your better option.