|Forum topic by TopamaxSurvivor||posted 01-24-2011 10:53 AM||1753 views||5 times favorited||17 replies|
01-24-2011 10:53 AM
Seems this is a constant subject of debate here on LJ. I thought I would add a little to what I posted on a motor thread and post it with a lot of search tags.
I have been doing mostly motor and control work for 43 years. I don’t know much about plugs & lighting and really don’t much care about it. Anybody can install outlets or hang light fixtures all day. The little bit of that type of work I have done was enough to last me a lifetime. ;-)) If you have more than 3 % voltage drop when starting your motors, you will see it flickering incandescent lights. If it is 5% or more, you will probably see it in fluorescent lights. Depending on how far you are from the transformer that serves you, some slight flickering may be unavoidable when motors start.
The bottom line on single phase motors; @ 1 hp & above consider wiring 220, 230, 240 (what ever you want to call it, your preference, its all the same thing).
If there is a noticeable lag in starting the motor of any size or voltage, it will eventually be damaged by that starting pause. It may take many years for it to show, but it will be damaged. The current goes up from 5 times to infinity during that starting pause. It is already at lest 5 times the running current during starting unless you have a soft start. High current means more heat; more heat means insulation break down; insulation break down means short circuiting in the motors winding; that, of course, means the end of the motor’s useful life.
There is really no reason for any properly made electric motor to ever fail with the exception of the capacitor or starting switch on single phase motors or the bearings on any motor. Properly started and loaded, they will, literally, last forever.
A watt of power is a watt of power whether it is at 120 single phase or 480 3 phase. From a power bill standpoint, it does not matter which voltage you use.
The little bump or bumps on your single phase motor is the capacitor(s). One for capacitor start and 2 for capacitor start capacitor run. It just gives the motor a little push in the right direction to get it started.
Capacitors can be checked with a multi-meter (VOM) or, a small DC current such as a 6 volt battery series. They definitely will hold a charge. You discharge them by shorting across the connections with a screwdriver. What ever happened is ok ;-) Even if you jump, it is still ok ;-)) Most capacitors these days, have a built in resistor to take care of any remaining charge, but don’t rely on it.
To check a capacitor you charge it with a voltage from a VOM or a few batteries. Watch for a faint little spark when you short it if you used 3 or 4 batteries to charge it. It is good if you see a spark. With a VOM, place the leads on the terminals with the meter on the ohms setting. Leave it for a minute or 2. Watching the meter, reverse the leads, if the meter shows little blip, the cap is good; if not, no good. Watch close, because you can easily miss it. Old analog meters with a needle movement are much easier to see than the new fancy dancy digital meters. Matter of fact, they are better for most trouble shooting applications of power and motors. Digitals have a habit of misleading you if you are not aware of how they operate. Most cheap digital meters will not show the capacitor as being good because they do not react fast enough. Meters like Fluke which have an analog bar on the display will show you what you what you want to know.
If there are oil caps on the bearings, put a few drops of Rislone in there once a year with a few drops of light machine oil. About the only thing you can to a motor to destroy it is over load it, over lube the bearings or over stress the bearing by running things too tight or at an odd angle. Remember just don’t over do it and it will last longer than you will. Few = < 5!!!
Reminds me of a burned up motor I found in a steel plant. The motor had zerks on the motor bearings. This was an old 25 hp +/- from the 60s or earlier. They greased the machine every shift, including the motor ;-(( Grease must be a very good heat conductor because it was packed full and no air could circulate to cool it ;-))
Standard disclaimer: This is not intended to make the novice an electrician. The intention is to inform you enough to know if the electrician you are using knows as much as you do now and enough about motors to be trouble shooting them.
-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence