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Stator Test

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#1 iron Mike OFFLINE  

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Posted August 13, 2011 - 12:38 AM

This is going to sound stupid but it's the only way I know of to learn anything. Can anyone tell me how to test a stator on my Onan 16 twin? Battery is not charging. Thanks,Mike

#2 EricFromPa OFFLINE  



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Posted August 13, 2011 - 02:14 AM

Found this a while ago but it's very helpful.

Three types flywheel alternator systems used in garden tractors: the nonregulated alternator, the dual circuit alternator, and the regulated alternator. Nonregulated alternator consists of stator, rectifier, and circuit protecting device. This type alternator has low amperage (about 1.25 amps), and is used for applications where starter the only load. A dual circuit alternator has two independent sets stator windings and two separate leads. One lead directed through a rectifier to battery, the other lead directed to lighting circuit. A regulated alternator contains a regulator and is for higher amperage output (between 15 and 30 amps).

Testing procedure for these three alternator systems basically same as those for belt driven alternators. Be sure to use caution when testing these systems, and don’t let stator leads (AC) touch together or short. Could permanently damage stator. Battery with at least 75% charge should be in machine being tested.
When checking flywheel alternator system, first step to check fuse. Then check all wires and connections for bare spots in insulation, open circuits, corrosion, and loose connections. Next test output of stator. This can be done by removing surface charge from battery, and testing battery voltage before and after running engine. Basic connections for testing flywheel alternator system are: connect DC voltmeter between negative (-) battery terminal and output lead of rectifier-regulator; connect ammeter between positive (+) battery terminal and output lead of rectifier-regulator. During this test, engine should be running at rated rpm. Depending on system, battery voltage should rise after running the system. In a 1.25 to 3 amp system, battery voltage should rise about 0.5 volts, while voltage in a higher amp system should rise between 0.5 and 2.0 volts.

If you detect problem in charging system, check stator leads for continuity and make sure they’re not grounded. Most stator windings have very little resistance, so check them for continuity only. If you obtain correct AC voltage during test, rectifier or rectifier-regulator is bad.
Next check the diodes in rectifier or rectifier-regulator with an ohmmeter or diode tester. Be aware that some components can’t be checked, only replaced.
To check duel circuit flywheel alternator system, disconnect AC lead from the alternator to the lights. With engine running at rated rpm, measure AC voltage from this lead to ground. A load light may also be used here (check the light bulb, wires, and connections first). If get reading of zero volts (or less volts than specified number), check stator lead for continuity to ground. Should be some resistance. If no resistance, stator shorted to ground. If infinite resistance, stator is open.

One other possibility that could prevent alternator from producing the proper current is lack of magnetism in flywheel magnets. If flywheel is exposed to excessive heat, is struck or dropped, or is very dirty or corroded, the magnets may fail. To check magnets, hold a medium sized screwdriver approximately one inch away from each magnet. Magnets should attract the screwdriver. If not, have lost their power.

Below is a troubleshooting chart can refer to when testing flywheel alternator charging systems:

NOTE: zero ohmmeters on each scale to ensure accurate readings. Voltage tests should be made with engine running at 3,600 rpm without load. Battery must be fully charged.

TEST 1: trace B+ lead from rectifier-regulator to key switch, or other accessible connection. Disconnect it from switch or connection. Connect an ammeter from loose end of B+ lead to positive terminal of battery. Connect DC voltmeter from loose end of B+ lead to negative terminal of battery. With engine running at 3,600 rpm, read voltage on voltmeter.
If voltage is 13.8 volts or more, place a minimum load of 5 amps* on battery to reduce voltage. Observe ammeter.
*NOTE: turn on lights, if 60 watts or more. Or place a 2.5 ohm, 100 watt resistor across battery terminals.
CONCLUSION 1: if voltage is 13.8 to 14.7 and charge rate increases when load is applied, the charging system is okay and battery was fully charged.
If voltage is less than 13.8 or charge rate does not increase when load is applied, test stator (test 2, 3, and 4 below).

TEST 2:Remove connector from rectifier-regulator. With engine running at 3,600 rpm, measure AC voltage across stator leads using an AC voltmeter.
CONCLUSION 2: if voltage is 28 volts or more, stator is okay. Rectifier-regulator is faulty. Replace the rectifier-regulator.
If voltage is less than 28 volts, stator is probably faulty and should be replaced. Test stator further using an ohmmeter (tests 3 & 4 below).

TEST 3: with engine stopped, measure the resistance across stator leads using an ohmmeter.
CONCLUSION 3: if resistance is .064/0.2 ohms, stator is okay.
If resistance is infinite ohms, stator is open. Replace stator.

TEST 4: with engine stopped, measure the resistance from each stator lead to ground using an ohmmeter.
CONCLUSION 4: if resistance is infinite ohms (meaning that there is no continuity), the stator is okay (not shorted to ground).
If resistance (or continuity) is measured, the stator leads are shorted to ground. Replace stator.
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#3 iron Mike OFFLINE  

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Posted August 15, 2011 - 10:20 AM

Thanks. This will be very useful.

#4 ducky OFFLINE  


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Posted August 15, 2011 - 10:30 AM

Very useful info Eric. Thanks.

#5 olcowhand ONLINE  


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Posted August 15, 2011 - 10:43 AM

Unplug your stator from the voltage regulator. Put your volt meter set to AC onto the two stator leads. At full rpm, the stator should put out 24 to high 30's VAC. If it's within this range, the stator is working fine.

#6 iron Mike OFFLINE  

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Posted August 16, 2011 - 01:32 AM

Thank you olcowhand. This will also be helpful.