Parts of a Starter Generator – Labeled Cutaway Diagram

All 16 parts on the diagram

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Frame, shaft and armature 10

Pulley A belt joins it to the engine. The unit turns the engine through the belt to start it, and the engine turns the unit through the same belt afterwards.
Drive end frame Closes the pulley end of the field frame. It holds one of the two bearings. Some manuals call it the drive end head.
Bearings One sealed ball bearing sits in each end frame. It carries the shaft.
Shaft The armature and the commutator are fixed to it. The pulley is on its end.
Armature Copper wire coils wound in the slots of an iron core. The core is made of thin sheets. As a starter, current in these coils makes it turn. As a generator, turning it makes current in them.
Commutator Insulated copper segments, each joined to armature coils. The brushes rub on it to carry current in and out of the spinning armature.
Field frame (yoke) Also called the yoke. It carries the poles and the end frames, and completes the magnetic circuit between the poles.
Commutator end frame It closes the commutator end and holds the other bearing. It comes off, after the through bolts, to reach the brushes.
Through bolts Two long bolts run the length of the unit. Taking them out lets the commutator end frame come off.
Terminals One is the armature terminal and one the field terminal. A maker's manual marks them A and F.

Poles and field coils 3

Pole shoes Hold the field coils in place and act as their iron core. Bolted inside the field frame. Also called pole pieces.
Shunt field coil Many turns of fine wire. It makes the magnetic field when the unit works as a generator.
Series field coil A few turns of heavy wire that carry the full starting current. It gives the strong field and the high torque needed to crank the engine.

Brushes 3

Brushes Usually made of carbon. They make the sliding contact between the spinning commutator and the fixed wiring, and they wear down in use.
Brush holders Each holder guides its brush. The brush is free to slide in it as it wears.
Brush springs Keep each brush pressed on the commutator as the brush wears shorter. One sits behind each brush.

How this list was compiled: Every name checked on 2026-09-30, a name kept only where two of these use it. (1) FAA, Aviation Maintenance Technician Handbook - General (FAA-H-8083-30), chapter 12, DC generators and the starter generator, https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/amtg_handbook.pdf : field frame (or yoke), pole shoes and field coils, armature on its shaft, commutator, brushes in brush holders, brush springs, drive end frame, commutator end frame, sealed ball bearings; series field coils of a few turns of large wire. (2) FAA, Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32), chapter 5, https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/amt_powerplant_handbook.pdf : the starter generator as a shunt generator with an added heavy series winding that gives high torque for starting. (3) US Army, TM 9-8000, Principles of Automotive Vehicles (1985), chapters 13 and 14 and figure 13-5, https://archive.org/details/tm-9-8000-principles-of-automotive-vehicles-1985 : armature, field frame, field coils arranged for shunt connection, pole shoes, commutator of insulated copper segments, carbon brushes, brush holders, brush spring, end housings with bearings, pulley, through bolt, armature terminal and field terminal; series-wound field coils of heavy copper wire in a cranking motor. (4) A garden tractor maker's service manual for a belt-driven motor-generator, https://archive.org/details/ih-cub-cadet-gss-1369-service-manual : cranking motor when the starter switch is closed and generator when it is open; brushes, brush spring, commutator end frame, two through bolts, A and F terminals, drive belt and pulleys. (5) Wikipedia, text only: Commutator (electric), Brush (electric), Armature (electrical), Field coil. Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; no maker's unit, nothing traced. Licence: own work

How it works

A starter generator is one unit that cranks the engine as a motor, then charges the battery as a generator once the engine runs. This is the belt-driven kind fitted to gasoline golf carts and garden tractors.

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The different parts of a starter generator, in three views

Cut lengthwise, numbers 1 to 10 run from the pulley to the terminals: two end frames with a bearing in each, the shaft, the armature and its commutator inside the field frame, and the through bolts. Cut across, 11 to 13 are the pole shoes and the two kinds of field coil. From the commutator end, 14 to 16 are the brushes, their holders and their springs.

Two field coils, two jobs

The series field coil is a few turns of heavy wire. It carries the starting current and gives the strong field that cranks the engine. The shunt field coil is many turns of fine wire, and makes the field when the unit is generating.

The commutator is the copper drum; the brushes are the carbon blocks that rub on it.
Drive end means the pulley end. The commutator end is the other one.

Questions and answers

What is a starter generator?

One electrical machine that does two jobs. It works as a motor to crank the engine, and once the engine is running the same machine is switched to work as a generator and charge the battery.

How does a starter generator work?

It is built like a shunt generator with an extra heavy series winding. For starting, battery current flows through the series winding and the armature, which turns the engine through the belt. After that the engine turns the armature and it generates current.

What are the main parts of a starter generator?

The field frame with its pole shoes and field coils, the armature on its shaft, the commutator, the brushes with their holders and springs, the two end frames with their bearings, the pulley and the terminals.

What do the brushes do in a starter generator?

They are carbon blocks that press on the turning commutator and carry current between it and the fixed wiring. A spring keeps each one in contact as it wears.