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Electrical Wiring in the Home/switching from 220v to 110v on ac motor


I have a 3/4hp 1750rpm ac motor with no tag or markings at all but the hp and rpm. It's wired now for 220v and need to change it to 110v if I can. Looking straight into the wire compartment theres 5 posts to work with 4 have a threaded and flat post and 1 has just the flat post. All start with the 4 double post first, on the right side the top one had the white wire, straight bellow that was the black wire. On the left side top was a red wire, all those were from the power cord. The 4th double post is dead center and has a red wire from inside the motor to it. The 5th single post is right between the top right and dead center posts and has nothing to it. Thats all the info there is, thanks for any info Dan


This is going to be tough,  no make, no model, no data plate.

Single phase,  motors in general,   at 3/4 HP is going to be a cap start, or cap start, cap run type motor,  

For dual voltage dual direction motors,  it takes 6 external leads,     4 run leads and 2 starts.

On a single direct   CCW or CW fixed non reversible   DUAL VOLTAGE motor, you COULD GET BY with 5 external leads, terminals  whatever type of connections,   but this is a rare bird,     

The terminal board,  [although you did a terrific job of describing the layout]  and the colors probably mean nothing to the user, in other words, the NEMA colors are not applicable,

Start at page 8 OF THE PDF,   above 8 is three phase,  below the single phase connections is DC<

Another single phase motor connection diagram pdf  above

ABOVE  Century motors,  down a few paragraphs,  explains the issues with terminal board type single phase connections lacking nameplate or data plate information,      

I see you have looked for tags and markings,  and I am sure you have, but I just had a question the other day,  close, but he had leads with numbers,   said there was no connection diagram,  luckily  he had 6 leads marked 1-4  and 5, 8       1-4 is   run     1-2 half the run, 2-3 the other half,  ALLOWING FOR CONNECTING THE RUN EITHER PARALLEL OR SERIES, PARALLEL FOR LOW,  SERIES FOR HIGH,      START IS 5, 8       START IS NORMALLY ALWAYS THE LOWEST VOLTAGE RATING,   IN OTHER WORDS,   IN PARALLEL,  THE RUN WOULD BE PARALLEL    13 TO LINE   24 TO LINE,   [AND BE CAREFUL 3 IS NOT ALWAYS THE END OF 1 ALTHOUGH NEMA STANDARDS GIVE A SPECIFIC NUMBERING SYSTEM  ALWAYS CHECK WITH AN OHM METER WHAT END OF EACH LEAD GOES TO]

In that case with 6 leads and no P  [protector leads]  it was fairly easy to provide a connection for the motor, but as soon as I sent the connection he found the connection diagram under the connection box cover,  covered with dirt and grime,  so look again    they tend to almost hide the connection diagrams,  some are in such a small font, the connection is located in the lower corner of a nameplate,  which is obviously missing from your motor,     

Often although I understand exactly what you described,  [again a very nice job of telling me what you have]  you have dual options on your terminals,   use a spade, or use a nut,   however, there are thousands of terminal boards, and without even the small luxury of knowing the BRAND, we cannot even go to a parts seller and try to find an identical terminal board by photo,   

Also  many dual terminal boards are on specific duty motors,   in other words, it is likely not a generic general purpose motor,     it was most likely ordered for a specific application such as a pump, compressor,    often are fixed voltage,  fixed rotation,  or both.

Then why have the multiple terminals?    This is often the last question, and the answer is this.

Although the motor is likely specific duty,  specific rotation,   specific voltage or a combination of both,  they do use a  "semi generic"  terminal board, to reduce the amount of stocked boards.

Before we go any further,  going to 110   [which is either a generic term, or a very old motor if that voltage is spelled out somewhere]

110 voltage in the last few DECADES is gone,   most utilities, now supply at least 115 and more and more it is becoming  120/240,  do to the explosion of electric devices and the need for more voltage in general to a residence or business.

But I understand you are simply wanting to go to the low side, whatever the applied voltage is,  versus the high,    and I wonder why?

3/4 HP in a very very generic rating is around or close to 14 amps,  in a residential circuit that is real close to being able to operate on a standard "115" volt wall plug or circuit, even dedicated to just the motor.

With the inrush current,  3/4 HP is likely to cause a ton of nuisance tripping mainly on start.

You could run a 20 amp dedicated feed, but if you are going to that much trouble,  breaker, wire so on,  why not just run a dedicated "220" circuit,  and if you notice, the labeling of off the cuff voltages is just that,    if the low side is 115 volts the high side cannot be 220  it has to be double the low or half the high,    so it would be 115/230.  or 110/220  but it is often referred to as low 115 and high 220,      

When you are talking HP in this rating,  a fraction of an amp could make all the difference in the world,    

But to answer your question the best possible, you will need to identify the motor leads that you have access to,     the run winding if dual voltage will have two halves both even in ohms and relatively low ohms,  say 1 or 2 ohms per half,  in series total 2 to 4,  where your start winding is going to be twice, three or four times the ohms,  up around 6  

I am throwing out arbitrary numbers, for talking points,   it is the DIFFERENCE in the run winding ohms versus the start winding ohms, not the empirical values,   for identification purpose only,     

With five options   it could be dual voltage,   because the start is on the low voltage side,   so in parallel     the start would be across line,  on high connection   one end of the start is tied to the series connection   right in the center,     example if    1-4 are the runs,  series would be connected   line to 1,   line to 4    start to either   and the remaining end of the start winding is  centered off the    2-3 series connection,       

So with five options,  it cannot reverse, or it cannot be dual voltage, one or the other or simply a fixed voltage fixed rotation specific purpose motor,  not built to be changed in voltage or rotational direction,     very common on what you described,   

You almost have to have some sort of capacitor setup,      either a start cap, or a start and run set of caps,        

To try to identify the windings from the terminals,  first jump out the caps,  by using a jumper across the cap terminals,   so you are not getting confusing ohm readings,    

If you identify two exact ohm values from four of the five available,  good chance you have a dual rated run,   but only one start left,  which means the other end of the start is internally connected to the run,         

Why do this?  They do that for pumps, fans, etc, that might have a threaded bore,  run in the wrong direction and not only does the pump not pump or the fan not stir air,  the impeller could and will spin right off the end        not good,    

It all depends on how much experience you have with an ohm meter,   second best is to find a local motor shop and have them tell you what you have,     

As far as expense,  the check out, we would do for nothing,   some shops have a minimal charge for just anything,     so ask,   up front,   

NOW because the motor might be specific duty,  and fixed for voltage or rotation,   it can be changed internally,   it is labor intensive, most shops are lacking single phase knowledge now days,  and can tie up a bunch of high priced time,       at that point maybe you need a new or used or new surplus motor,    instead of putting out cash to convert your existing motor,   

Wish I had better news or a better solution but it what it is,  and with the lack of data, no one knows,      

So move on to your reason for needing or wanting low voltage,     already explained the narrow window of 3/4 HP on a 15 amp residential circuit,    you are right on the ragged edge

Again if you have to run a dedicated 20 amp circuit,  why not run 220?  Smaller wire size, smaller breaker,  much easier to provide power,        

Your threaded and spaded connections are common, and most often an indicator that the motor is specific for some application,      if you know the history that might help,    but likely not much   we still need power for 3/4 HP,  

Next budget,   what are you going to build or design?  If you provide me the goal, maybe we can come up with some alternative,          maybe sell the existing,  use that for a generic,   if the budget has room,   and you want the best,     consider a three phase motor with a 115 [low] single phase input, to a 220 volt three phase output.

It is called a frequency drive,   1/2 HP is about the limit when boosting low to higher voltage out in three phase, but there are larger drives, that go up to over 1 HP,   

The beauty of a three phase motor is the simplicity, no caps, no switches, no nothing,    that covers the motor,   and the cost of HP in single phase motors  is multiples of a generic three phase motor,    so you can get a same HP motor in three phase for even less than half the cost of a single phase   due to the caps, switches and complications,       where a three phase motor is a stator, rotor, two housings, and two bearings.

I have worked this formula a million times, and if you do some shopping,  you can almost if not, purchase a three phase motor with drive,   for the cost of a single motor alone,  

A three phase frequency drive has it all,   you can choose chassis mount,  or you can purchase in a NEMA one box,  with keypad,  includes the motor starter, overload protection, reversing, braking, ramp up and ramp down,   plus around 46 user adjustable parameters,  which are NOT difficult to setup,  read the manual, and the control functions are very exacting,  but again it depends on the application and budget,     

Transformers, not a likely solution,   much expense to go from 220 down to low]  I would go for the drive,   you can google a million frequency drives,  from Vtech, Marathon,  and a whole host of Chinese drives which will work just fine,     

Cost  for 3/4 drive new,  $200?  New surplus just shop,   I recently purchased a 30 HP drive, normal cost,  $1000 plus plus,   got it new surplus,   2010 model  for $400 bucks,   

Bottom line,   no way to know even with a perfect description of the terminals, without ohming out the external connections         as far as we know, it could be a two speed motor,    with five terminals,     this is the problem with used,   or older motors,    hard to or impossible to identify with just terminals and colored wires,     

NOW make sure in the links I provided, you do look at the color coding charts,   NEMA does use standards for colors,   but again,  we have no idea if this is specific purpose or just what,   

Write me at my shop,    MEAR Services,    Kansas City Missouri,  e-amil for MEAR is,  no charge to help from that email,  MEAR phone,  816-650-4030  and MEAR fax number  816-650-4061

Or you can write to my personal e-mail,    or reply back through allexperts,        a photo of the motor from the side,   the shaft[s]  and a good photo of the terminal board, and once more double check for a connection covered up in the connection box cover,          from there we can look and see if we recognize the board,    then help guide you to figure out the best way to convert this motor,  or go another route,      

William Babbitt  
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This site answers questions related to home electrical wiring, home wiring, general electrical help,and other electrical questions related to aleternating current (AC). You can find help on the National Electical Code, home electrical issues, wiring electrical outlets, installing lighting, electrical grounding, and general electrical help for do-it-yourself projects not require an electrician. If you do not see your home electrical wiring question answered in this area then please ask your electrical wiring question here

Electrical Wiring in the Home

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Will Babbitt


Electrical issues of all types, wiring, control, appliances, components, specialty in Electric MOTOR or APPARATUS trouble shooting, electric motors, electrical problems, single phase, three phase, DC, capacitors, elevator MG-Sets, modifications, reverse engineering, VFD Drives, single to three phase convertors. Repair of most any electrical/mechanical/electronic apparatus, OEM, AC, DC, Industrial Applications, Three phase, single phase failure mode, determining the root cause of the equipment failure BEFORE failure repeats, Antique appliances, electric motors, fans, ceiling fans, base mount fans, poor equipment performance, Modifications, habitual failures, vibration, redesign, obsolete issues, collectible items restored, rewinding. Owner of EMR Repair Inc More information on electric motors, under Engineering/Motors Ask about any electrical or mechanical problems in the home, office, or even at your work. B2B or business to business CONSULTING I can do but it would be a much more complex issue, that would require a significant amount of time. from there we can discuss and begin work on about anything. Industrial electric motors, controls, troubleshooting, vibration, alignment, any type of industrial or commercial electric/electrcical equipment For the home owner, renter, apartment if you have odd things going on, describe to me, photos are normally very helpful, appliances, heating and air conditioning issues, light switches, installation, DIY PROJECTS FOR THOSE NOT EDUCATED IN ELECTRICAL, we can do some things, but for safety and sanity reasons, I may refer you to a local electrician, and help you find one, help you with pricing and even speak directly with anyone you hire if you have issues,


Over thirty years with a major repair and sales company, VP of Operations, and former owner of my own specialty repair shops, MEAR Services local to Kansas City Missouri EMR Repair Inc., located in Kansas City Missouri, 2014 sold both businesses to JCI Industries, Lee's Summit Missouri Evaluation and repair of electrical/mechanical apparatus. Electrical and mechanical repairs, trouble shooting including, vibration and balance issues. provides this service, and the huge costs to provide help on about any subject. They provide the servers, the people to vet the experts, costs that would be the same as operating a good sized business, They should be appreciated for what they do, the huge costs and the great help they provide for free, Electric motor questions, can be answered here but there is a dedicated category that has other experts to help and add to a solution. Other common questions, are noises in the walls, breakers tripping, devices not starting or shutting down for no apparent reason, smoke alarms buzzing, thermostat change outs, or sizing wire or needing information for a new project, but anything can be resolved, anything, with enough effort and patience, DIY, with electrical is possible, but in most cases the use of a simple ohm meter or volt meter is needed, they can be purchased at any hardware store for a few bucks. If you have smoke, or smell smoke, don't be writing me, call 911 the Fire Department has no problem looking for smoke smells, better then spraying water on a blazing fire. DO NOT GET CAUGHT INVESTIGATING AND TRAPPED IN A BASEMENT LOOKING FOR A BLOWER FAILING< As to the tip jar, it is up to you, it is appreciated, every expert spends out of pocket that I know, we spend on IP time, computer wear, printing documents, books, and of course our time, but if we prevent a $400 service call, we did well,

Former IBEW, EASA, IEEE, UL, ..............


10 plus years, various Technical Schools, followed by Industrial College Credit and non Credit Courses, electronics courses, experience in most any electrical apparatus, electric motors and generators, AC OR DC, fractional to above 5000HP, other electrical apparatus, slip rings, sleeve [plain] bearings, lubrication, identification-no data plate, control components, service and sales. I have continued night schools for decades, the list is extensive. I have been in the business of repairing most anything with wires or mechanical parts for decades, I have some helpful hints and directions you won't have to dig up. Or hopefully not have to pay a huge service call invoice to find.

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