Tip & How-To about Electrical Supplies

Generator Not Producing Power ? Stator Winding

There are 4 requirements that have to be met before a generator is able to produce power. Due to number of questions that have been asked, a Coleman PowerMate 1500-1850 with a Briggs and Stratton Engine will be used here.

1) The Engine has to be turning at correct speed
2) Field Winding (Rotor) on the alternator must be energized
3) AC Winding (Stator) must create a voltage in presence of magnetic field
4) Output passes through Safety Devices before it reaches an outlet.

This Tip will cover the Stator Winding.

The stator winding is the winding that is around the outside of the rotor. It doesn't usually move. Previously, the alternator design was called a revolving armature, in which the field was stationary (outside), and the stator (armature) was the rotating member. This quickly was replaced, eventually, by the revolvingfield configuration, since the brushes carry a relatively small DCcurrent, rather than a larger AC current with destructive arcing. Brushes were prone to failure due to high wear.

In the revolving field architecture,there are primarily 3 types of alternators: Single Output, MultiOutput, and MultiPhase.

The single output alternator is just that, it outputs a single voltage. Typically, it will be either120vac (60Hz), or a single phase 240vac (50Hz) depending on the country it was bought in. The stator winding of this kind of alternator has only a single winding, with 2 wires coming off of it. One of these wires goes to the neutral bus for the outlets, the other wire goes to the circuit breaker, and the other side of the circuit breaker goes to the outlet. Disconnect both of the wires, and use your multimeter to check the continuity between them. You should have continuity here. Also check each wire to the frame of the alternator, there should not be continuity here though. If you don't have continuity between the wires, or if you have continuity between the wire and alternator frame, then you have what is called a grounded winding. This will have to be addressed by a motor shop, or replace the stator in its entirety. It is generally more cost effective to replace the generator though. Remove and sell the copper wire, and use that money towards a replacement generator.

Multiple Output alternators are usually found in the next step up from "bargain" generators. These usually will have an output of 120/240vac, and likely a low voltage battery charging circuit as well (12vdc being most common). The stator for these is checked in the same way as a single output, with a caution on the low voltage winding. On the low voltage winding,there will be a single diode (called half wave rectification) or a full wave rectifier (2 diodes) connected to the windings. You must disconnect at least 1 wire from the diodes to check the windings, and both wires to check for continuity between the windings and the frame of the alternator. If you don't disconnect the wire(s), the 1 way nature of the diode will indicate no continuity when there should be,and may cause you to not see continuity when there actually is (to the frame).

If the low voltage winding is open, the generator is still usable for the 120/240vac features. You just won't have benefit of the low voltage battery charging circuit, which is actually a very poor charger to begin with.

On a 120/240vac alternator, there may also be a switch that turns the 240vac on and off at the receptacle. This switch is put here so that the 2 120vac windings can be put in parallel for greater current capacity if 240vac is not required. If you have 120vac, but not 240vac, and the circuit breakers are not tripped, this switch is likely the culprit. It is just a double pole double throw switch that is easily replaced with one of the same type, and same or higher current rating. Never switch from 120 to120/240vac or vice versa while the engine is running. To do so invites winding damage due to arc-over, and may damage any devices that are connected from the voltage spike.

MultiPhase alternators are nothing more than single output alternators with a twist. Rather than having a single winding, there will be 3 windings, with a 120 degree phase separation. The in phase neutral wires will all be tied together, but the phase outputs will be separated, and connected to the receptacle in a specific order. This is called a Wye (Y) configuration. An alternate configuration is when the windings are all connected to each other, end to end style. The junction of each connection is then brought out to the receptacle. This is called a Delta configuration. Three phase / multiphase alternators will not be discussed in depth here as a much higher technical knowledge will be required.

As far as the stator winding goes, that is it. If you have continuity where you should, and don't have continuity where you shouldn't, the stator winding is likely good. As mentioned, if the low voltage battery charge winding is open, but not grounded / shorted to the frame, the primary function of the alternator is still usable.

If your checks of the stator are good,time to move on to the next tip.
Output and Safety Devices

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