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Generators & Electric Motors General Discussion Antique Generators and Old Electric Motors: Questions and answers about restoring and showing old power generation systems.

Generators & Electric Motors General Discussion

Frequently Asked Generator Questions

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Old 04-18-2008, 11:13:28 PM
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Default Frequently Asked Generator Questions

This is a thread starter for the purpose of building a F.A.Q. page (frequently asked questions) on Generators and stand-by power. This thread will be edited and maintained as a F.A.Q. (Do not drift off topic.) Moderators will also edit posts in this thread for clarity.

This thread may also be used to link to the absolute best technical posts in this forum group of MEP, Kohler, Onan and ALL that include general information of value to everyone.

Rewind / Replace Coils:

The ONAN FAQ is located here:


If you have a SEARS generator, check here first:

Can I use this generator for my home/shop/business etc.?

Planning is the Key to Generator Operations.

Notes for asking Questions, about specific Gensets.

(Read on...)
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Old 05-07-2008, 12:20:49 AM
Jim Rankin Jim Rankin is offline
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Default Can I use this generator for my home/shop/business etc.

When looking at an old generator, you first need to know if it makes AC or DC electricity.

DC might be useful if you have a battery bank to charge and you run all your AC appliances off an inverter that is powered by the batteries. High voltage DC is more dangerous than similar voltage levels of AC power and can exceed the rating of switches and fuses, so be careful. DC power of the correct voltage can be used directly for a few AC rated things like incandescent lights and resistance heating elements and old fashoned "universal" motors like hand power tools have. Modern variable speed AC electric tools DO NOT work with DC power, only the old single speed things like grinders, drills and skill saws etc which have brushes in the motor.

If you have an AC generator, you might have one that generates at a voltage or frequency that is useless in your situation. The most common might be a 480 volt 3 phase set or how about one of the military surplus 400 hz sets that come up for sale. What can you do with 400 hz AC? About the same things as with DC power, except you can't charge batteries directly.

If your generator makes the wrong voltage AC power, it might be reconnectable to change the output voltage or you could use a transformer to change the voltage level of the generator output to what you need. Transformers do not change the frequency, only the voltage, they also do not work on DC power.

My generator is 3 phase, can I get single phase power from it? Yes you can. 3 phase equipment connects to all 3 lines from the generator output, while single phase may use either 2 lines or 1 line and the neutral.

For example a 120/208 3 phase generator can supply 120 volt circuits from any of the 3 lines and the neutral (L-N). If it is possible to balance the load across all 3 lines, that would be ideal, but it is seldom possible and unless the load is a large part of the rated load of the generator, any imbalance is not likely to cause problems.

The same 120/208 3 phase generator can supply single phase loads connected between any 2 lines (L-L). These loads will get 208 volts SINGLE PHASE power. (((((((((( *I know, I know! It uses 2 lines/phases, so it must be 2 phase. NO, it's single phase if it is connected to 2 lines of a 3 phase source. If it connects to all 3 lines, it's 3 phase. So what happened to 2 phase? It was the standard method of generating and delivering power early in the 20'th century, but was long ago replaced by 3 phase equipment in most of North America except in a few isolated situations. For all intents and purposes, 2 phase AC power doesn't exist in North America.))))))))))

If the equipment is labled for 208 volts ( sometimes tagged 200V) you're good to go, but if your stuff is only labled 230 volt, you might have problems because of the low voltage. A heavily loaded motor might not be able to start because it's starting current surge normally pulls the voltage down a lot and starting at 208 instead of 240 means you don't have as far to go before getting into difficulty. A stalled/unable to start motor may burn up if it's overload or a fuse in the supply doesn't operate to stop it quickly enough. Loads like electric resistance heating elements will heat slower on 208 than 240 volts, but will not be harmed. Again it is great to balance the load across all 3 phases A-B, B-C, C-A, but it can be impossible if your home is wired for single phase power. If you can only load 2 phases of the 3 phase generator with your single phase loads, you will not be able to use the full rated capacity of the 3 phase generator, but the imbalance will not likely cause a problem.

Please feel free to add to this thread or contact me with any concerns and I will attempt to clarify/revise it.
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Old 05-07-2008, 03:37:57 AM
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Talking Planning is the Key to Generator Operations

It is always helpful to understand what your Loads are when planning to install a Backup Power Generator. Are you planning a "Whole House Feed" or just an "Essential Load Feed"? Are you going to make a permenant connection to your Loads, or just use extension cords, for connections? Is a Transfer Switch in your future? Are you going to generate 120/240 Vac Single Phase, 120 Vac only, or 120/208 Vac using two Phases of a Three Phase Genset. How big of Genset will you need to run the Loads, you plan to power. What is your choice of Fuel, Gasoline, Propane, Natural Gas, Diesel? Each fuel type has it's pluses, and minuses. What is your expected Backup Runtime, and how much fuel do you need to have on hand, and how is it to be stored? What will it take to replenish your fuel storage, if you exceed your Backup Runtime? What Spare Parts will you need for maintainance of your Genset? What consumables (LubeOil, Filters, Spark Plugs, Points, etc) will you need to keep on hand? What will you do when "the thing" just won't start, and Momma is sitting in the Cold/Heat, waiting for the Lights and Tv to come back on?
Bruce in alaska
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Old 05-07-2008, 03:56:00 AM
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Talking Notes for asking Questions, about specific Gensets

It is really helpful to our Resident SmartGuys, if you can include in your posts, as much information about the specific Generator, you have questions about. Often OEM's put DataPlates on their equipment, that list Model Codes, Voltage Codes, Spec Codes, Build Numbers, and Serial Numbers, as well as listing Maximum Ratings for Kw, KVA, Volts, Amps, Hz, Rpm, and other useful data. Listing as much as possible, gives us a better idea of Exactly what you have, and helps narrow down the answers to your questions. If you don't list very much information, about the Unit, our answers tend to be very general in nature, likely not what you really need, for a specific unit. Pictures are worth "A thousand Words". We have a very knowledgeable group of regulars on this Forum, and it's subForums. Many who are in the Genset Maintainance business, with YEARS of experience and expertise. Give us the information and we can help answer your questions. It is also likely that your question has come up before, so do a "Search" of the Archives with specific Keywords, and see if you can find your answers there, as well.
Bruce in alaska
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Old 05-14-2008, 06:20:37 AM
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Default Notes on Transfer Switching of Generators

FYI re transfer

Where backup power is required by law... a permanent system with automatic transfer is needed, get it professionally installed

Where you need a permanent backup system... a manual or automatic transfer system is also needed. This often involves a new utility company entrance with a bonded disconnect, also isolating the neutral buss in the main [now sub] panel to achieve proper bonding. a licensed electrician will likely be needed.

Connecting a temporary or portable generator to your house wiring requires a means of preventing inadvertent connection of the mains with the generator.

all require proper overload protection.

Last edited by BTPost; 05-14-2008 at 08:07:26 PM. Reason: More descriptive Title
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Old 12-18-2008, 09:51:44 PM
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Talking FAQ: How to Setup the Frequency & Voltage on a Genset

This was written by one of our Resident SmartGuys, (Gunny) specifically for folks, who are NOT Genset Mechanics, and are trying to understand the Basics, of how Gensets Work, and are Setup.

This is about AC (Alternating Current), different rules apply for DC (Direct Current). The difference is that with DC, one wire is positive the other is negative, with AC, they alternate. When they alternate 60 times a second, we have 60Hz power, the standard for the US. Occasionally we encounter something a little different, like 50 Hz or 400 Hz.

The frequency of a generators Output Power is directly dependent on the design of the generator rotor and engine speed. The most common generators we encounter are either 2 or 4 pole, the 2 pole generators must turn at 3600 rpm to generate a 60Hz output and a 4 pole machine turns at 1800 rpm.

Voltage is the force of the electrcity and can be compared to pressure in a water system. The usual voltages we encounter are single phase 120/240 Vac. This system has 4 wires, (2) hots, a neutral and a ground. Voltage measured between the hots should be 240 Vac and between either hot and the neutral, 120 Vac. Variations to this would be the three phase systems with an additional hot, in other words (5) wires, (3) hots, a neutral and a ground. Three common US phase combinations are usually 120/208 Vac, 120/240 Vac and 277/480 Vac, with other combinations possible when the frequency slows to 50 HZ.

Amperage is the amount of current passing a particular point, how many electrons go by in that wire. Can be compared to flow or volume in a hydraulic system. Amps are an indication of how much electricity you are using.

Generators have (2) devices that directly affect the output, the voltage regulator controls the output voltage, if your machine has one and not all of them do. The other device is the engine governor. This controls engine speed and thus the output frequency.

It is very important that the engine be running at the correct speed with and without load as the voltage regulating system only works well within a narrow window, too slow and voltage is too low, too high and voltage will probably be too high, either case causing equipment damage. Best to first adjust frequency, use a good quality meter such as any of the Fluke brand, if a known good meter is not avalible a tachometer can be used to measure the engine speed. You can check a frequency meter by measuring normal power from your power company, the meter should read either 59.99 or 60.00, no other reading is acceptable.

Governors are either mechanical or electronic. On a mechanical governor, spring tension is what adjusts the speed. An electronic governor will have an adjustment labeled speed. As there are a lot of different designs, best to get a service manual for your particular machine and follow the proceedure.

Voltage regulators regulate the genset output voltage by controlling the level of magnetism in the rotor. Several ways this is done through an optical coupling, rotating exciter, slip rings & brushes but the point is there should be an adjustment or two, sometimes more, on a voltage regulator. One adjustment is the voltage level, the other stability. A large number of voltage regulators will read generator frequency and if too low, it will back off the excitation voltage. This is done this way for a couple of reasons, one is to allow the engine to get back to proper speed should a large load be applied that is causing the engine to loose speed.

Generators without voltage regulators are not as common, but there are still a lot of them around. They use some of the ouput voltage, pass it through a rectifier and use it for excitation. Simple, cheap and reliable IF you never shut the engine down while powering a load. Most of the cheaper and smaller Gensets fall in to this group.

Generator output. Most of the machines we deal with here are either 4 wire or 12 wire output, there are 10 lead machines but rarely encountered anymore. This refers to the number of load wires coming out of the stator windings. A 4 wire is what it is, no changing it. A 12 wire can be reconfigured to any of the common voltage schemes, 120/240 Vac, 277/480 Vac, whatever suits you. These output leads should be marked 1 through 12, some English stuff (Newage comes to mind) might be different and use letters. A reconnectable generator usually has a drawing, decal or a printing showing all the possible combinations. This drawing will also show any other changes that may be needed such as repositioning of jumpers on the voltage regulator.

Onan model/spec numbers are very important when inquiring about a particular machine. A voltage code will tell us if your machine is reconfigurable/what it left the factory as. Particularly desireable Onans will have a voltage code of -18R, a -4X is less useful should you want to change voltage, there are other codes, just used these for an example. There is a FAQ in the Onan Forum that gives most of the Onan Voltage Codes.

It is always an excellent idea to get a service manual for your generator. Read and understand any proceedure before taking action. Follow accepted practise and safety proceedure. You are dealing with electricity, rotating machinery, hot parts, and flammable fuels.

Thanks to all our Resident SmartGuys, who helped in writing the Forums FAQ's.
Bruce in alaska
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Old 12-20-2008, 01:00:09 PM
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Elden DuRand Elden DuRand is offline
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Default Re: FAQ: How to Setup the Frequency & Voltage on a Genset



Since the North American power grid runs at a very accurate 60 Hz frequency, it is about the best standard you can get for checking your generator frequency.

Here's how you do it. First and -very- important, connect the neutral of the genset to the neutral of your commercial power source. Do the same with the grounds.

Now, take a couple of identical (say, 60 Watt) 120 volt light bulbs and hook them in series so you have, in effect, a 240 volt light bulb. The reason you need 240 volts worth of bulbs is that, when the generator output sine wave is exactly opposite the sine wave from the commercial power, there will be twice the line voltage across the lamps.

Now, connect one end of the series string to the 120 volt hot lead of your generator and hook the other end of the series string to the 120 volt hot lead of your commercial power. With the generator -not- running, the light bulbs should be at half brightness.

Now, start the generator. The light bulbs will vary from full on (240 volts) to full off (zero volts) at exactly the difference frequency between your generator and commercial power.

If your generator speed is close, the bulbs will flicker. To find out if you're fast or slow, nudge the governor a little fast and see if the flicker rate increases or decreases.

- If the flicker rate increases, you are running fast and need to adjust the governor a little slower until, without any other load, the lights go on and off about twice a second. This will mean that you're running at about 62 Hz.

- If the flicker rate slows down when you nudge the speed upward, you need to adjust the governor up until the lights are -very- slowly changing brightness then increase some more until the lights flicker about twice a second.

Now, if you have a way to load your generator to near full load, do so, leaving the bulbs connected. The flicker rate should be about 2 cycles -below- the 60 Hz of commercial power or about 58 Hz.

I've used this method of adjusting genset speed several times and it works well.

Take care - Elden
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Old 12-20-2008, 02:24:16 PM
Jim Rankin Jim Rankin is offline
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Default KW, kVA and PF

What dos this alphabet soup of electrical values you found on the tag of your generator tell you?

kVA tells you what the generator end can do (volts X amps or "apparent" power) This value will not be found on a DC generator (unless it's exactly the same as the kW value, because DC equipment always has a PF of exactly 1.0

kW tells how much the engine can do ("real" power for doing work)

PF is the ratio of kW to kVA of the generator set.

These values can tell you things like the maximum amperage you can draw even if it's not on the tag.

To calculate the amperage of a 3 phase set ......
***Not kW***, but kVA x 1000 = VA divided by 1.732 divided by line-line voltage = amperage.
example 100kVA X 1000=100,000 VA/1.732/208 volts= 277 amps

Or for the same 120/208 3 ph generator, kVA X 1000 = VA divided by 3phases divided by line-Neutral voltage = amperage.
example 100kVA x 1000=100,000 VA/3=33,333 VA/120 volts=277amps.

Single phase calculation-kVA X 1000 = VA divided by line-line voltage = amps per line.
example - 5kVA x 1000 = 5000 VA/120=41.66 amps if you have only 120 volt output (not 120/240)
another case - 5kVA X 1000 = 5000 VA/240 volts= 20.8 amps. In this case, it could either be the rated amps on each 120 volt line out of the generator if it's a 120/240 generator or the rated amps on the two 240 volt lines out.

Why not use kW in these calculations?
kW is the measure of engine POWER, and determines electrical REAL POWER capacity but not amperage. The amperage required to deliver that electrical power depends on the power factor of the load. It may be an "easy", low power factor load like a lightly loaded motor, or a "hard", high power factor load like a bunch of electric strip heaters or a fully loaded high power factor motor. In either case, the same amperage limit derived from the kVA rating applies because, amps or current is what causes the heating in the generator windings.

kW or watts or real power is what causes the heating in the engine driving the generator, so kW and kVA are two different things that look alike and sometimes are equal numbers, but tell you 2 different things about a generator set.

kW divided by kVA is power factor. Usually generators are built with PF of .8, but sometimes it's 1.0 especially on smaller sets where the same engine is used on a couple of sizes of generators with a power factor of .8 with the larger generator end and 1.0 on the next size down.

Power factor of the electrical load gets more complicated, but it's still basically the same thing. First, it's not stamped on your generator's tag. The power factor of the generator set is what it is, and won't be changed unless you can safely get more power out of the engine or more amps out of the generator. The power factor of the power it delivers depends entirely on the loads connected to it.

The difference between kVA and KW is "reactive" power or "magnetizing current" which is necessary for things like AC motors and transformers to operate. It travels over the wires along with the "real" power, so it "takes up space", requiring the wires to be sized to handle it along with the real power. But reactive power requires very little/no engine power to produce it. It's conserved and moves back and forth between the generator and load along with the real power.

Because there are varying requirements for reactive power among loads you may connect to you AC generator, all kVA isn't the same.........

some is high power factor (harder for the engine-more kW and less reactive) electric heater elements and light bulbs for example
some is low power factor (easier for the engine-less kW more reactive) lightly loaded motors or older fluorescent fixtures for example
and most generators see a mix depending on what's connected to it, so it may average out (or not). If you're running a battery of incandescent light bulbs or quartz halogen spotlights, your generator's engine may not be able to power the amps indicated by the kVA number. (you have a very high power factor load mix). On the other hand if you are running a collection of lightly loaded motors and older fluorescent and HID light fixtures, you may exceed the amperage rating of your generator without ever hearing the engine labor excessively (you have a low power factor load).
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Old 12-23-2008, 04:11:38 PM
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Default Can I reconnect my generator for another voltage etc?

Many generators are reconnectable. If it is designed to be reconnectable, you can usually find a schematic to follow. These are frequently inside the connection box on the generator, so that's the quickest place to check.

A single phase generator can frequently be changed from 120 only to 120/240 or 240 only by reconnecting 4 leads out of the stator in parallel or series connnection.

A 3 phase generator with 3 or 4 leads out can not be reconnected without major surgery into the lead end of the stator windings to find and bring additional leads out into the connection box.

A 6 lead stator can be reconnected from 3 ph to single phase but whatever the voltage is, it will stay in the same range. For example if it's 480 3 ph, it will be ~480 single phase if reconnected in the zigzag method. Not possible to change from high voltage to low voltage or vice versa. Instead a transformer is needed.

A 9 or 10 lead stator can be reconnected for high (series wye) and low voltage (parallel wye), but the stator arrangement will always be wye (Y or star). This means that it will be 120/208, 277/480 etc where the line-N voltage is 57% of the line-line voltage. ALWAYS, no exceptions. If one voltage is raised, the other goes with it. These 9 lead stators are not reconnectable for single phase because there is a Y connection buried in the stator where the ends of 3 stator circuits are connected together. This is usually brought out as the neutral N or sometimes designated L0 or T10.

A 12 lead stator can be reconnected for both single to 3 phase reconnections as well as high to low voltage connections, so voltages from 120 to 480 are usually available.

Reconnecting the stator leads in the correct pattern is usually only part of the reconnection process. There is frequently other reconnections to be made in the exciter/regulator system to allow it to function at the new voltage/phase arrangement. Failure to get this right can ruin your generator in short order, so some caution upon first startup after reconnection is always a good idea.

If the generator panel has a field breaker, perhaps start up with it off and only bump it on while watching the voltage and listening to the sound of the engine and generator for abnormal humming in the generator or loading of the engine. Alternatively you may be able to run the engine slow (NO LOAD!) and watch the voltage come up as you let it speed up to synchronous rpm.

Reconnecting the leads can be as easy as moving jumpers on a connection board, or require splicing stranded cable. If you have to deal with stranded cable without terminations, I would suggest either getting the proper ring terminals to crimp on the leads so you can use bolts or use split bolt connectors to make the splices securely. Any splices should be covered with layers of rubber splicing tape to cover the sharp corners of the bolts etc and then that first layer covered with layers of vinyl electrical tape. Simply using vinyl tape can allow sharp corners to chafe through and short out if the splice is against the side of the box etc.

Here are some links to stator reconnection diagrams
Single phase
3 phase
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Old 01-04-2009, 10:24:12 PM
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Default Parts of the Generator

I get confused myself and I am trying to get this all straight in my mind, so here goes...........
The simplest terms for the main parts of a generator are the
rotor - the part that rotates in the middle
stator- the stationary outer casing

The confusion comes in when we use the other more technical terms

field-the part of the generator which has a magnetic field to induce voltage/current in the other part called the
armature-the part which has voltage induced in it's windings by the action of the field moving by it

So for the most common type of AC generator, the rotor would be the field and the stator would be the armature. Power would flow into the rotor/field from the regulator and exciter and AC power would be produced from the stator leads. (THERE ARE VARIATIONS ON THIS PLAN, some smaller generators have a stationary field and the AC power is taken off the rotor/armature by brushes on sliprings).

And for the most common type of DC generator, the stator would be the field and the rotor would be the armature.
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