Embarrassing question for me: Will a bad start cap on a cheap 1 phase motor (air mover on an AC unit) allow the motor to run long -term (one week) at less than rated RPM with no overload problems?
The only experience I have had with a faulty start capacitor resulted in the motor not starting.
If the capacitor is there only for start, then the running speed is not effected by it. This is from my electrical engineering 101 course taken long ago.
My largest experience with starting capacitors on fractional HP motors is the condenser fan on AC units. Over the years, I have dealt with 3 or 4 instances where the compressor would turn on, but the fan wouldn’t run. I found that if I could spin the fan when it tried to start, it would run. If I didn’t get it running within a minute of so of it trying to start, a thermal overload switch in the motor would activate and I would have to wait a while for it to cool off
If I did manage to get the fan going (I generally used a bent coat hanger through the grill at the top), the fan motor would still run hot. Apparently, for some of the units I dealt with, there would be enough cooling from the air pushed by the fan to keep the motor cool enough to keep running. On others, I’d have to rig up a water hose to drip water on the fan motor (hey, it’s designed to run in the rain, so I figure it wouldn’t hurt), which would keep it cool. I only operated said units for a day or two, until I could get a replacement capacitor.
So, my experience says, “I don’t know, but I doubt it”. One thing I am curious about is how you are going to run the motor “at less than rated RPM”? In general, 1 phase induction motors will get very hot when run at less than the rated RPM. Air handler motors will often have multiple windings so that the motor speed can be changed depending on how the windings are connected (changing the number of poles in the stator), but they will still run at (or near) the “rated RPM” for the number of poles wired.
The no slip speed of an electric motor is given by 1200f/n where f is the frequency of the grid and n is the number of poles.
For the US, the above simplifies to 72,000/n. So for a 2 pole motor (common ones), the speed will be about 36,000 rpm. The lower the speed (slip) from this number, the higher the current it will consume.
Sorry, that is just wrong. Wherever did you get that formula?
No slip speed of an ac motor is (freq * time in sec / 2 ) / no of poles.
A typical 2 pole motor at 60 Hz is 3600 rpm with no slip. 4 pole is 1800.
You are correct - I added an extra zero. It is 120f/n.
If this is a capacitor start motor then it is likely that the motor will simply stall and buzz on attempted startup.
Are you completely sure that it is the capacitor that is faulty, its far more common for the centrifugal switch to go faulty.
If this is a capacitor start/run motor then it might well start with reduced torque but it will also run more slowly - this in turn will increase the slip speed, and in turn will induce greater current in the windings, its highly likely the motor will overheat.
This might not be a huge overheating effect however it will significantly reduce the life of the insulation - it depends upon how heavy the motor is loaded.
If its an ultra cheap motor such as a shaded pole motor then it will be already very inefficient and a little bit more slip will probably be hardly noticeable
As ironic as it might seem, you are both incorrect.
am77494 misplaced a decimal point. Not all that horrible of a mistake, but it did leave me wondering what the air handler was going to do at 36,000 RPM.
Khendrask divided when he was supposed to multiply (and left me wondering, for a while, what “time in sec” was the time for). Turns out that the time is number of seconds in a minute, to convert revolutions per second to revolutions per minute. The “/ 2” in his formula should be a “* 2”, since you will get two revolutions per cycle, divided by the number of poles. Of course, you will have some slip, depending on motor design and power requirements.
am77494 made another mistake by agreeing that Khendrask was correct.
Now, I have reviewed, spell-checked, and re-reviewed this post. Despite all that, I wouldn’t be surprised to find that it, also, contains an error…
I know, I know…what you and others have said makes perfect sense. Unfortunately, my OP was a red herring. Here’s the real deal: The motor was in fact running @ low rpms, and the service guy did replace a motor capacitor that in turn, solved the problem. The problem is, I am stupid.:smack:
When the tech said I had a bad cap, I asked “what did it read?” he said it was about 3mf and that it should be 10mf. My ASSSumption was, it was a start cap, when IN FACT it was a run cap. All the symptoms were there… The clues just didn’t click in my head.
Sorry for wasting everyone’s time.
am77494: check your math!:eek:
For some reason had my reply up all day without submitting. When I did, I noticed the ninja on my math comment… wasn’t intending to be redundant.
I need a nap.