Motor Learning Center

See what is inside a three-phase induction motor, the kind of motor that does most of the work in industrial plants. Select a part to learn what it does, how it wears, and what a technician may look at.

Motor explorer

The example is a totally enclosed, fan-cooled (TEFC), foot-mounted three-phase squirrel-cage induction motor: a closed motor, cooled by a fan on the outside, that stands on mounting feet.

Simplified side view with the parts pulled apart, cut open to show the stator. Not to scale.

Tap a part, or a name below, to see what it does.

This is a simplified model made for this website to explain how a motor works. It is not a manufacturer’s design, it is not to scale, and it is not a guide to taking a motor apart or putting one together. Real motors differ in construction, cooling, bearings and terminal arrangements.

Explore by equipmentSee how a motor drives a surface water pump, a submersible drainage pump and an air-conditioner condenser fan.Compare motor typesThree-phase induction, single-phase induction and brushed DC motors side by side.

The parts, one by one

Each section explains what the part does, how it works with the rest of the motor, how it wears, and what a qualified technician may check. The signs listed are possibilities, not a diagnosis. Each section ends with the manufacturer and industry documents it is based on. This content has not yet been reviewed by the PERS technical team.

Frame (housing)

What it does

The frame is the outer shell of the motor. It holds the stator core (the fixed inner part), holds the end shields (the end covers that carry the bearings), and has the feet you bolt down. Heat from inside the motor also leaves through it. On a totally enclosed fan-cooled motor, a fan blows air along the outside of the frame, and the fins give that air more surface to cool.

Learn more: wear, warning signs and checks

How it works with other parts

The stator core sits inside the frame, so heat from the windings and core passes out through it. The end shields fit into each end of the frame, which keeps the rotor (the turning part) centred. The outside fan blows air along the fins.

How it wears

Dirt building up on the frame and in the air openings, so the motor cools less well.
Water from condensation collecting inside if the drain holes are blocked.
Problems at the feet and base: a weak or uneven foundation, or loose hold-down bolts.
Worn or damaged seats where the end shields fit. This can make the air gap (the small space between the rotor and the stator) uneven.

Signs you may notice

Running hotter than usual for the same load.
Shaking (vibration) or noise from a weak or loose mounting.
Cooling fins you can see are blocked.

What affects how long it lasts

How clean the motor and the air around it are kept.
Humidity and local weather, which affect how often a motor should be inspected.
A firm, level foundation.

What a qualified technician may check

General condition of the frame, fins and feet.
Whether the foundation and mounting bolts are sound.
How well the end shields fit into the frame when the motor is taken apart.

Sources:ABB Motor guide, IC 411 cooling, p. 23; ABB installation and maintenance manual, §4.4 p. 10; §4.8 p. 12; §7.1 p. 19; Table 10.1 p. 26.

Stator core

What it does

The stator is the part of the motor that stays still. Its core is a stack of thin steel sheets (laminations), each separated from the next by a very thin layer of insulation. Slots around the inside hold the stator windings. The insulation between the sheets limits eddy currents, which are small unwanted currents that waste energy as heat. If that insulation is damaged, eddy currents and heating increase.

Learn more: wear, warning signs and checks

How it works with other parts

Current in the windings magnetises the core, and the core carries the magnetic field across the small air gap into the rotor. The frame holds the outside of the core, and much of the core’s heat leaves the motor that way.

How it wears

Damage to the insulation between the steel sheets. This can happen when a stator is burned out at too high a temperature during a rewind, or from serious overheating.
Sheets fused together, or pieces missing, after a major electrical fault such as a short circuit inside the slots.
The rotor rubbing the core, often after a bearing fails. This smears the inside surface and raises losses.
Damaged teeth on the steel sheets when too much force is used to pull out old windings.

Signs you may notice

More wasted energy and a hotter-running motor after a winding fault or a poor rewind.
Visible score marks inside the stator where the rotor has rubbed.
Discoloured slot wedges or inner surfaces after serious overheating.

What affects how long it lasts

Avoiding serious overheating.
Bearing condition, which keeps the rotor from rubbing the core.
Care taken when old windings are removed during a rewind.

What a qualified technician may check

Looking at the core and its inner surface for rub marks, damage and discolouration.
A core loss test before the old winding is burned out, and again after the core is stripped and cleaned, to see whether losses have gone up.

Sources:EASA Good Practice Guide, pp. 2, 4, 7; §2.7.2 p. 14; §3.2 p. 16; ABB Motor guide, §5.1 p. 59.

Stator windings

What it does

The stator windings are coils of insulated copper wire laid in the stator slots and connected into three phases. Current in the windings creates a magnetic field that turns around the inside of the stator, and this field drives the rotor. Each coil runs along one slot, loops round outside the core, and comes back along another slot. The looped parts that stick out beyond each end of the core are the coil extensions, or end turns. The distance between a coil’s two slots is its pitch.

Learn more: wear, warning signs and checks

How it works with other parts

The coils sit in the stator slots, kept off the steel by slot insulation. Their leads run to the terminal box, where the supply cable is connected. Their heat passes into the core and frame, so clean cooling surfaces and a working fan directly affect how hot the windings get.

How it wears

The insulation ageing from heat. A hotter motor wears out its insulation sooner: even running 10 °C cooler makes a noticeable difference.
Dampness after the motor has stood idle or been stored for a long time, which lowers insulation resistance (how well the insulation stops current leaking).
Dirt packed into the winding, which may mean the motor’s enclosure does not suit the place it works in.
Damage from overload, a missing phase, or uneven voltage between the phases.
Extra voltage stress when the motor is run from some frequency converters (speed controllers). Manufacturers deal with this using reinforced insulation or filters.

Signs you may notice

Overheating, a burnt smell or discoloured windings.
Blown fuses or protection tripping.
Humming without starting.
Low insulation resistance found by a qualified person.

What affects how long it lasts

How hot the motor runs compared with the temperature its insulation is rated for (its thermal class).
Load compared with the motor’s rating.
Supply quality: correct voltage, with all three phases present and balanced.
Cleanliness, humidity and how the motor is stored when idle.

What a qualified technician may check

Insulation resistance measured at the motor terminals with the supply cables disconnected, and compared over time.
Looking at the windings for discolouration and the pattern of any failure.
Whether a damp winding can be dried out, or whether it needs to be rewound.
Recording all the original winding details (turns, coil pitch, connection, wire size, coil extension) before any rewind.

Sources:ABB Motor guide, §4.1 p. 33; §4.2 p. 34; ABB installation and maintenance manual, §4.3 p. 10; §6.2 and Table 6.1 pp. 15–16; Table 10.1 pp. 25–26; EASA Good Practice Guide, §2.7.3 p. 14; §2.9 p. 15; §3.1 p. 15; §4 p. 20.

Rotor (squirrel cage)

What it does

The rotor is the part that turns. It is a stack of thin steel sheets with metal bars running along its length, joined at each end by a ring. The bars and rings look like a cage, which is why it is called a squirrel cage. The stator’s turning magnetic field sets up current in the bars, and the two fields together produce torque (turning force). The rotor always turns slightly slower than the field. This difference is called slip, and it gets bigger as the load gets heavier.

Learn more: wear, warning signs and checks

How it works with other parts

The rotor is fixed to the shaft and held by the two bearings, centred inside the stator with a small, even air gap. Worn bearing or end shield seats make the air gap uneven, which wastes energy and can cause noise.

How it wears

Broken rotor bars or a loose rotor, often seen as cracks near the end rings.
Loss of balance, for example from balancing weights that have shifted.
Rubbing against the stator if a bearing fails.

Signs you may notice

Not reaching full speed.
Taking too long to speed up, or drawing high current.
Vibration from imbalance.

What affects how long it lasts

Bearing condition, which keeps the air gap even.
Balance of the rotor and of the coupling or pulley fitted to the shaft.
Load compared with the motor’s rating.

What a qualified technician may check

Looking at the bars and end rings for cracks near the rings.
Balance checks.
Whether a damaged rotor should be replaced. ABB’s troubleshooting chart notes that a new rotor may be needed, because repairs are usually temporary.

Sources:ABB Motor guide, §4.1 p. 33; §5.1 p. 59; ABB installation and maintenance manual, Table 10.1 pp. 25–26; EASA Good Practice Guide, p. 7; §2.7.2 p. 14.

Shaft

What it does

The shaft carries the rotor and passes its power to the driven machine at the drive end (D-end) through a coupling, pulley or gear. A slot at the drive end (the keyway) holds a key that locks the coupling or pulley in place. Motors are commonly balanced with a half key, so couplings and pulleys must be balanced the same way.

Learn more: wear, warning signs and checks

How it works with other parts

The shaft turns in the two bearings, which sit in the end shields. Anything fitted to the drive end puts load on the shaft and bearings, so alignment, belt tension and pulley position all matter.

How it wears

A bent shaft.
Damage from hammering on a coupling or pulley, which can also damage the bearings.
Too much load from belts pulled too tight, a pulley mounted too far from the shaft shoulder, or a pulley that is too small.

Signs you may notice

Hot bearings.
Vibration.

What affects how long it lasts

Alignment between the motor and the driven machine.
Belt tension within the belt supplier’s and the manufacturer’s limits.
Correct tools and methods for fitting couplings and pulleys.

What a qualified technician may check

Whether the shaft is straight.
Alignment of the motor with the driven machine.
Belt tension, pulley size and position.

Sources:ABB Motor guide, §5.1 p. 59; ABB installation and maintenance manual, §4.5–4.7 p. 11; Table 10.1 p. 26.

Bearings

What it does

The bearings hold up the turning shaft and keep the rotor centred inside the stator. There is usually one at the drive end and one at the non-drive end. Most bearings outlast the machine they are fitted in. Of those that fail early, SKF puts about one third down to lubrication problems, about one third to contamination, and about a quarter to how they were chosen or fitted.

Learn more: wear, warning signs and checks

How it works with other parts

Each bearing sits on the shaft and inside the centre of an end shield. Their condition decides how even the air gap is. Loads from belts, couplings and poor alignment pass through them.

How it wears

Lubrication problems: the wrong grease or oil, the wrong amount (too little or too much), or greasing at the wrong times.
Dirt or water getting in past seals that do not work well.
Too much load from poor alignment, side or end thrust, or belts pulled too tight.
Incorrect fitting.
Electrical damage when current passes through the bearing. For motors run from frequency converters, manufacturers specify measures such as insulated bearings to stop harmful bearing currents.

Signs you may notice

Unusual noise.
Rising vibration.
A hotter bearing.
Changes in the look of used grease.

What affects how long it lasts

Lubrication: the right type, amount and timing.
Cleanliness and sealing.
Alignment, belt tension and load.
Correct storage and fitting.

What a qualified technician may check

Listening for unusual noise, and measuring vibration and bearing temperature.
Checking used grease.
Replacing bearings with the same type as originally fitted, removed with pullers and fitted by heating or with special tools.
Examining a failed bearing to find the root cause.

Sources:SKF Bearing damage and failure analysis, pp. 6–8, 28; §5.4.2 p. 41; ABB installation and maintenance manual, §6.4 p. 16; §7.1 p. 19; §8 p. 23; Table 10.1 p. 26.

End shields

What it does

The end shields (also called end brackets or end bells) close each end of the frame and hold the bearings. They fit closely into the frame, so they keep the shaft and rotor centred inside the stator.

Learn more: wear, warning signs and checks

How it works with other parts

They connect the frame to the bearings. The drive-end shield takes the load from the coupling or pulley through its bearing. At the non-drive end, the fan sits just outside the end shield, inside the fan cover.

How it wears

Worn end shield or bearing seats, which make the air gap uneven.
Bearings out of line.

Signs you may notice

Noise from an uneven air gap.
Vibration.
A scraping noise if the fan rubs on the end shield.

What affects how long it lasts

Bearing condition. A failed bearing can damage its seat.
Careful taking apart and putting back together.

What a qualified technician may check

How well the end shield and bearing seats fit.
Alignment of the bearings.
Visible damage.

Sources:ABB installation and maintenance manual, Table 10.1 p. 26.

Terminal box

What it does

The terminal box is where the supply cable connects to the winding leads. On standard single-speed motors it normally has six winding terminals and at least one earth terminal. The windings are connected here in star or delta, two standard wiring patterns, to suit the supply. It may also hold connections for thermistors (temperature sensors), heaters or other extra devices.

Learn more: wear, warning signs and checks

How it works with other parts

It joins the outside supply to the stator windings. The connection must match the supply and the rating plate. Cable glands (the sealed cable entries) and the box seals help keep moisture and dirt away from the connections.

How it wears

Loose or faulty connections and leads.
Moisture or dirt getting in where glands or seals are not fitted correctly.

Signs you may notice

Overheating, or the motor running on and off.
One phase open (disconnected), or uneven voltage between the phases.
Blown fuses or protection tripping.

What affects how long it lasts

Correct cable lugs and connections when installed.
Suitable cable glands and seals.
Regular checks of connections.

What a qualified technician may check

Condition of terminals, leads and the earth connection, with the motor disconnected and made safe.
Whether the connection matches the supply and the rating plate.
Sealing of the box and cable entries.

Sources:ABB installation and maintenance manual, §3.1 p. 7; §4.9 pp. 12–13; §7.1 p. 19; Table 10.1 pp. 25–26.

Cooling fan

What it does

On a totally enclosed fan-cooled (TEFC) motor, the outside fan is fixed to the non-drive end of the shaft and turns with it. It pushes air along the outside of the frame to carry heat away. Because the shaft drives it, it turns at the motor’s speed. For motors run at different speeds on frequency converters, manufacturers publish loadability curves, which show how much load the motor can safely carry at each speed.

Learn more: wear, warning signs and checks

How it works with other parts

It works with the fan cover, which directs its air over the frame. It cools everything inside the frame, especially the windings.

How it wears

Damage to the fan.
A fan in the wrong position, or not fixed firmly.
Dirt build-up that reduces airflow.

Signs you may notice

Overheating.
A scraping noise if the fan rubs on the end shield or fan cover.

What affects how long it lasts

Clean surroundings, and regular cleaning in dusty places.
Correct fan mounting.

What a qualified technician may check

Fan condition and mounting, with the motor stopped and made safe.
Space between the fan, the cover and the end shield.
Whether cooling is enough for the work the motor does, especially at reduced speed.

Sources:ABB Motor guide, IC 411, p. 23; ABB installation and maintenance manual, §6.7 p. 17; §7.1 p. 19; Table 10.1 p. 26; EASA Good Practice Guide, §2.8 p. 15.

Fan cover

What it does

The fan cover (cowl) guards the turning fan and guides the air, so that air drawn in through its grille flows along the outside of the frame.

Learn more: wear, warning signs and checks

How it works with other parts

It is fixed at the non-drive end and surrounds the fan. Air enters through its grille and leaves along the frame.

How it wears

Damage that pushes the cover into the fan.
A grille blocked by dirt.

Signs you may notice

Overheating because air cannot flow freely.
A scraping noise if the cover touches the fan.

What affects how long it lasts

Space at the air inlet. ABB recommends a gap between the fan cover and a wall of at least half the air intake of the fan cover.
Cleaning in dusty places.

What a qualified technician may check

Condition, fit and clearance of the cover.
Whether the motor has enough room around it for air to flow.

Sources:ABB installation and maintenance manual, §4 p. 11; §7.1 p. 19; Table 10.1 p. 26; EASA Good Practice Guide, §2.8 p. 15.

Safety first

Motors and their supplies can carry dangerous voltages. Only qualified people should test a motor, open its terminal box or take it apart.
If a motor is not working normally, switch it off and keep it isolated (disconnected) from the supply until it has been checked.
Do not keep resetting a breaker or other protective device that has tripped. Never bypass or defeat protection to keep equipment running.
Keep clear of turning shafts, couplings, belts and fans. A motor that is controlled remotely can start by itself.
This page is general education. It does not diagnose your equipment or replace an inspection.