ElectricityElectrical Machines II (year 6)

Motor winding

A burnt-out motor is not always a lost motor: with a sound core, a new winding that is correctly calculated, insulated and tested brings it back almost to factory condition. The craft comes down to three things: reading the old winding before destroying it, reproducing its layout without mistakes and proving with instruments that what was done is what was intended.

Slots and poles Coil pitch Form-wound coils Impregnation Tests

01The stator and its slots

The stator of an induction motor is a core of silicon-steel laminations 0.35 to 0.5 mm thick, insulated from each other by a surface film and stacked inside the frame. The laminations are punched with the slots already cut; stacking them aligns the slots into channels parallel to the shaft where the winding sits. Why the rotating field comes from three windings out of phase, and how the terminal box is wired, is covered in AC motors; here we look at how that winding is built, calculated and repaired.

Figure 1. Left, section of a 36-slot stator: each slot is 10° from the next and the coloured dots show which phase occupies it in the example of section 03. Right, one slot enlarged: insulation, conductors and wedge.

Part	What it is and what it does
Frame	Cast-iron or sheet-steel housing. It holds the core, the end shields with the bearings and the terminal box.
Yoke (back iron)	Ring of lamination behind the slots. It closes the magnetic path from one pole to the next.
Teeth	The lamination left between slots. They guide the flux to the air gap; if they burn or deform, the machine loses torque.
Slots	Channels that hold the coil sides. In small motors they are semi-closed (narrow opening) and the conductors are passed in one by one through the opening.
Slot insulation	Liner or paper that separates the copper from the iron. It sticks out of the slot at both ends so it is not cut by the edge of the laminations.
Wedge	Strip of insulating material that closes the opening and keeps the conductors in, clear of the rotor.
End windings	Lengths of conductor that leave one slot and enter another, at both ends of the core. They produce no torque but do produce heat and resistance.
Phase separators	Insulating strips between end windings of different phases, where the voltage differences are greatest.

The core is hard to repair: protect it

The winding is replaced; the core is not. If the iron overheated, the laminations weld together in the damaged zone, eddy currents appear and the motor draws more and runs hot even with a new winding. That is why all stripping work is done with one aim: do not damage the laminations: no blows, no cutting tools on the teeth and no excess temperature.

02Winding concepts

A three-phase winding is described by a handful of numbers. With them any layout can be calculated, drawn and reproduced, even if the motor is unknown.

Quantity	Symbol	What it is
Slots	$Q$	Number of stator slots. Counted on the core.
Poles	$2 p$	Number of poles; $p$ is the number of pairs. It sets the synchronous speed $n_{s} = 60 f / p$ .
Phases	$m$	3 in three-phase motors.
Slots per pole and phase	$q$	How many consecutive slots one phase occupies under one pole. It is the number of coils in a group.
Pole pitch	$τ$	Slots corresponding to one pole: from the middle of one pole to the middle of the next.
Coil pitch (span)	$y$	Slots between the two sides of a coil. With $y = τ$ the pitch is full; with $y$ less than $τ$ , short.
Electrical slot angle	$α$	Phase shift of the induced voltage between one slot and the next.

q = \frac{Q}{2 p m} τ = \frac{Q}{2 p} α = \frac{p \cdot 360 °}{Q}

q

must be an integer for ordinary windings. The angle

α

is in electrical degrees: one pole pair equals 360 electrical degrees, whatever the mechanical angle.

Belts and groups. Under each pole, a phase occupies a belt of $q$ adjacent slots. The coils of one belt, connected in series, form a group. The three phases share the 360 electrical degrees in six belts of 60°, in this order: $U^{+}$ , $W^{-}$ , $V^{+}$ , $U^{-}$ , $W^{+}$ , $V^{-}$ . Phase $V$ starts 120° after $U$ and $W$ 240° after; in slots, $120 ° / α$ and $240 ° / α$ .

Figure 2. How the slot is filled (top) and how the end windings are arranged (bottom). With one layer there is one coil side per slot; with two layers, two. The end windings can be concentric (nested coils of different spans) or lap (equal coils, each shifted one slot).

Type	Number of coils and groups	Where it is used and its advantage
One layer, concentric	$Q / 2$ coils; $m p$ groups	Small and medium series-built motors. Inserted by machine and the slot fills well. Average pitch is fixed, it cannot be shortened.
One layer, lap	$Q / 2$ equal coils; $m p$ groups	Widely used in hand rewinding: every coil is made on the same form.
Two layers, lap	$Q$ equal coils; $2 m p$ groups	Medium and large motors. It allows short pitch, which improves the waveform and shortens the end windings. The standard for medium power.

How the groups are connected. With one layer there are $p$ groups per phase, all traversed in the same direction: each group produces one pole and the next appears “by consequence” (consequent poles). The end of one group is joined to the start of the next. With two layers there are $2 p$ groups per phase, one per pole, and neighbouring groups are traversed in opposite directions: end to end and start to start. In both cases parallel branches can be formed to share the current: with two branches each has half the groups, so to keep the voltage per branch the turns per coil are doubled and the wire section is halved.

Factors that correct the flux. The coils of a group are not in the same slot, and with short pitch the two sides are not one pole pitch apart. In each case the induced voltage is a little lower than if everything were concentrated:

k_{d} = \frac{\sin (q α / 2)}{q \sin (α / 2)} k_{p} = \sin (\frac{y}{τ} \cdot 90 °) k_{w} = k_{d} k_{p}

Distribution factor, pitch factor and winding factor. They equal 1 in the ideal case and lie between 0.90 and 0.96 in real windings.

03Calculating a three-phase winding: a complete example

You find a three-phase stator with 36 slots and 4 poles, no nameplate and no winding. The winding has to be decided, slot by slot. We choose one layer, concentric, the type used in most small motors.

Example · A 36-slot, 4-pole winding, step by step

1 · Slots per pole and phase. $q$ $= \frac{Q}{2 p m}$ $= \frac{36}{4 \cdot 3}$ $= 3$ . Each group has 3 coils.

2 · Pole pitch. $τ = \frac{Q}{2 p} = \frac{36}{4} = 9$ slots. Full pitch is 9 slots: a coil with one side in slot 1 has the other in slot 10.

3 · Angle and phase shift. $α = \frac{2 \cdot 360 °}{36} = 20 °$ electrical between slots. Between one phase and the next there are $\frac{120 °}{20 °} = 6$ slots: $U$ starts in slot 1, $V$ in slot 7 and $W$ in slot 13.

4 · Number of coils and groups. With one layer, $\frac{Q}{2} = 18$ coils, in $m p = 3 \cdot 2 = 6$ groups of 3: two groups per phase.

5 · Slot allocation. The six belts of 3 slots repeat twice around the stator (next table). Each slot holds a single coil side.

Phase	Outgoing belt ( $+$ )	Return belt ( $-$ )	Coils, innermost to outermost
U, group 1	1 – 3	10 – 12	3-10 · 2-11 · 1-12
U, group 2	19 – 21	28 – 30	21-28 · 20-29 · 19-30
V, group 1	7 – 9	16 – 18	9-16 · 8-17 · 7-18
V, group 2	25 – 27	34 – 36	27-34 · 26-35 · 25-36
W, group 1	13 – 15	22 – 24	15-22 · 14-23 · 13-24
W, group 2	31 – 33	4 – 6 (wraps around)	33-4 · 32-5 · 31-6

Example (continued) · Coil spans and connection of the groups

6 · Spans. The three coils of a group are concentric: the innermost spans 7 slots (3-10), the middle one 9 (2-11) and the outermost 11 (1-12). The average, 9, is the pole pitch. No single coil needs to have the exact pitch: what matters is that the coil sides land in the right belts.

7 · Connecting the groups. With one layer, the two groups of each phase are traversed in the same direction and joined in series: end of group 1 to start of group 2. Phase U: enters through $U_{1}$ (slot 3), runs through coils 3-10, 2-11 and 1-12, goes to slot 21 and runs through 21-28, 20-29 and 19-30, and leaves through $U_{2}$ (slot 30). $V$ and $W$ are identical, shifted 6 and 12 slots.

8 · Connecting the phases. The three starts, $U_{1}$ , $V_{1}$ and $W_{1}$ , go to the terminal box. The three ends are tied together to make the star, or taken to the terminal box so that star or delta can be chosen (see the six-terminal box). The names $U_{1}$ , $V_{1}$ , $W_{1}$ and $U_{2}$ , $V_{2}$ , $W_{2}$ are those of IEC 60034-8.

9 · Winding factors. With $α = 20 °$ and $q = 3$ : $k_{d}$ $= \frac{\sin 30 °}{3 \sin 10 °}$ $= \frac{0.5}{0.5209}$ $= 0.9598$ . A one-layer concentric winding has full average pitch, so $k_{w} = k_{d} = 0.960$ . If the same slots were wound in two layers with the pitch shortened to 8 slots: $k_{p}$ $= \sin (\frac{8}{9} \cdot 90 °)$ $= 0.9848$ and $k_{w} = 0.9598 \cdot 0.9848 = 0.945$ ; with 7 slots, $k_{p} = 0.9397$ and $k_{w} = 0.902$ .

10 · Check. Adding up the currents of the 36 slots at a given instant gives a distribution with four zero crossings: four poles. If $U$ , $V$ and $W$ are connected to $L_{1}$ , $L_{2}$ and $L_{3}$ , the field moves toward the higher slot numbers (with $U$ at its peak, the poles are centred between slots 6 and 7, 15 and 16, 24 and 25, 33 and 34).

Figure 3. Developed winding diagram of the example, animated: phase U, phase V, phase W and the three together with the pole positions. Each phase is the previous one shifted 6 slots (120° electrical). The dotted columns repeat slots 1 to 6 so that the group of W that wraps around can be drawn.

One single reversal is enough to ruin the field

If one group is connected backwards, the outgoing and return belts swap direction, the poles of that phase are no longer correctly alternated and the motor draws unbalanced currents, vibrates and delivers no torque. It is checked with a compass or with the rotating-field test (section 06) before impregnating. The same mistake when joining the phases gives a motor that runs backwards or does not start.

Turns, wire and slot

The layout says where each coil side goes; it does not say how much copper. To rewind a motor with its old winding, the method is to count. Count the turns of one coil, measure the wire diameter and the number of parallel strands, and copy them. Calculating from scratch is done only to check, or when a figure changes (the voltage, for example). The voltage of each phase fixes the flux:

V_{f} \approx 4.44 f N k_{w} Φ

V_{f}

is the phase voltage,

f

the frequency,

N

the series turns per phase,

k_{w}

the winding factor and

Φ

the flux per pole. Neglecting the drop in the copper, if the turns change the flux changes in inverse proportion.

Example · A 4 kW motor: turns, wire and resistance

A 4 kW, 380 V star, 8.7 A motor with the winding of the example (36 slots, 4 poles, one layer). On stripping it 24 turns per coil were counted, with 2 parallel strands of 0.95 mm, and a mean turn length of 0.70 m.

Turns per phase. 6 coils in series per phase: $N = 6 \cdot 24 = 144$ .

Flux. $V_{f} = \frac{380}{\sqrt{3}} = 220 V$ and $Φ$ $= \frac{220}{4.44 \cdot 50 \cdot 144 \cdot 0.9598}$ $= 7.17 mWb$ : a reasonable order of magnitude for this power.

Section and density. Each strand: $S_{1}$ $= \frac{π}{4} {0.95}^{2}$ $= 0.709 mm²$ ; two strands, 1.418 mm². Density: $J = \frac{8.7}{1.418} = 6.14 A/mm²$ .

Slot fill. Each slot carries 24 turns of 2 strands: $48 \cdot 0.709 = 34.0 mm²$ of copper. If the free slot area, after the insulation, is 85 mm², the fill is $\frac{34.0}{85} = 0.40$ : 40 %, reasonable for inserting by hand or by machine.

Resistance per phase at 20 °C. Copper length $= 144 \cdot 0.70 = 100.8$ m, and $R_{20}$ $= ρ \frac{ℓ}{S}$ $= 0.0172 \cdot \frac{100.8}{1.418}$ $= 1.223 Ω$ (with $ρ = 0.0172 Ω·mm²/m$ ). In a workshop at 28 °C, $R_{28}$ $= 1.223 \cdot \frac{235 + 28}{235 + 20}$ $= 1.261 Ω$ . That is the value each phase must give when finished. At $I_{n}$ , the copper losses are $3 \cdot {8.7}^{2} \cdot 1.223 = 278 W$ .

Example · Changing the winding voltage

The motor above has an original winding of 22 turns per coil with 0.75 mm wire and runs at 380 V star; it is to be wound for 220 V star (a three-phase 220 V line-to-line supply). For the flux to stay the same, the turns change in proportion to the voltage and the section in inverse proportion:

$N^{'}$ $= 22 \cdot \frac{220}{380}$ $= 12.7 \to 13$ turns. $S^{'}$ $= \frac{π}{4} {0.75}^{2} \cdot \frac{380}{220}$ $= 0.763 mm²$ , that is a wire of $d$ $= \sqrt{\frac{4 \cdot 0.763}{π}}$ $= 0.986 \to 1.00$ mm.

Rounding to 13 turns lowers the flux by 2 %, which the motor tolerates. The total copper in the slot does not change: section is gained and turns are lost. Any other voltage change is solved the same way.

04Data collection and stripping

Before cutting a single turn, everything the old winding knows and the new one must repeat has to be written down on a form. Once stripped, a figure that was not recorded cannot be recovered.

Data	How to get it
Nameplate	Power, voltages and connection, current, speed, frequency, insulation class, duty, protection rating. Photograph it.
Slots and poles	Count the slots of the core. The poles, from the nameplate speed or from the number of groups of the old winding.
Winding type	One or two layers; concentric or lap; number of groups per phase; series or parallel connection; star or delta.
Coil pitch	Slots between the sides of each coil, in each layer. Draw the layout.
Turns per coil	Count on one coil, before cutting it and without deforming it. Confirm on another.
Wire	Copper diameter (without enamel) with a micrometer; number of parallel strands; thermal class of the enamel if it can be read from a sample.
Mean turn	Length of one turn, measured with a string or on a sound coil, to calculate the expected resistance.
Slot insulation	Material, thickness, width and how far it protrudes. Keep a sample. Position of wedges and separators.
Leads	Number of cables, their colours or marks, which group each comes from, and where the star point is. Photograph the end windings before cutting.
Previous condition	Resistance per phase if it can still be measured, insulation with a megohmmeter, and type of fault (section 06). The core: colour, damage, rubbing.

First the diagram, then the knife

Draw the old winding on a developed diagram like the one in Figure 3 and compare it with the calculated one. If they match, the stator has a factory winding and it can be reproduced with confidence. If they do not match, a previous poorly done rewind is involved, and that is where it should be corrected, not copied.

Stripping

There are two methods, and the choice depends on the insulation and on the equipment available:

Method	How it is done	What to watch
Mechanical	One end winding is cut off with shears or a saw, and the sides are pulled out of the slots with an extractor, pulling on the opposite end winding.	Do not cut flush with the core or lever on the teeth. Hardened varnish makes pulling harder.
Thermal	The stator goes into a burn-off oven that burns the varnish and insulation, and the conductors come out with no effort.	Controlled temperature, around 350 °C in the core and no more: above that, the insulation between laminations is damaged. A controlled oven with gas treatment is needed.

Smoke, heat and dust

Old varnishes and enamels give off irritating gases when heated: the burn-off oven must have a gas outlet and the workshop must be ventilated. A stator fresh out of the oven is at hundreds of degrees. When cutting and extracting copper, wear safety glasses, gloves and safety footwear. The workshop is governed by Law 19.587 and Decree 351/79 on occupational health and safety; varnish and solvent waste does not go in the ordinary rubbish. More in Occupational health and safety.

After stripping, the core is cleaned (bristle brush, dry air with no moisture, no scraping), the laminations are checked for damage or burnt spots and the slots are verified with a gauge or template. A core with melted zones is discarded or evaluated with a core test before rewinding. How the extracted copper and insulation are replaced is the subject of the next section.

05Insulation, coils, insertion and impregnation

Insulation and its thermal classes

Each insulating material withstands a maximum service temperature. IEC 60085 groups them into thermal classes. The class of the motor is on the nameplate, and the rewind must not use materials of a lower class: it may use a higher one, which gives margin.

Class	Max. service temperature	Typical materials (indicative; the manufacturer’s data sheet governs)
A	105 °C	Impregnated papers and pressboard, cotton; old enamels.
E	120 °C	Polyester films, polyester enamels.
B	130 °C	Polyester and glass-fibre laminates, mica with organic binders.
F	155 °C	Polyester laminates with better resins, polyester-imide enamelled wire, class F varnishes. The most used in current motors.
H	180 °C	Aramid paper (Nomex type), silicones, polyamide-imide enamels.

Each layer of the system may be of a different class, but the assembly is only as good as its weakest link. The enamelled wire, slot liner, wedge, separators, tape and varnish must be compatible and of the same or a higher class than the motor. The properties of copper, enamels and resins are in Electrical materials.

From coil to slot

Step	What is done	Quality criterion
1	Insulate the slots. Liners are cut to the core length plus the overhang at each end, folded into a U and placed, with the folded edges turned inward.	No wrinkles or cuts. The overhang covers the sharp edge of the laminations at both ends.
2	Make the coils. On a winding machine with a turn counter and a form the size of the mean turn. Concentric: a stepped form, with the three spans at once.	Neat turns, no crossings or bare enamel. A guide thread helps, and the coil should be tied before it comes off the form.
3	Insert. The conductors are passed through the opening a few at a time, with a blade, until the slot is full; then the end winding is shaped with a rubber mallet or spatula.	No conductor trapped between insulation and lamination. Enamel intact at the end.
4	Close with wedges. A wedge is fitted under the opening of each slot, pushed in carefully.	Snug but without pulling the conductors. None sticks out toward the rotor.
5	Separate the phases. Insulation is placed between end windings of different phases.	Full coverage wherever neighbouring end windings belong to different phases.
6	Connect the groups. Ends and starts are soldered (or joined with a crimp terminal) as per the layout, and covered with insulating sleeve of the motor’s class.	Mechanically firm joint with no enamel underneath. Each lead marked with its name: $U_{1}$ , $V_{2}$ ...
7	Lace the end windings. With cord or tape the whole set of end windings is tied, shaping them so they do not touch the end shield.	Compact end windings, not rubbing the frame and not blocking ventilation.

Impregnation and baking

The freshly inserted winding is loose: the conductors can move and vibrate, and the air between them insulates poorly and removes heat poorly. The impregnating varnish fills the voids, bonds the conductors to each other and to the slot, seals against moisture and improves heat transfer. The process:

Preheat the stator to remove moisture and let the varnish penetrate; the value comes from the varnish data sheet.
Impregnate by dipping, or by trickling while rotating, until it stops bubbling and dripping.
Drain the excess, clean the bore and the seating faces with solvent.
Bake at the manufacturer’s temperature and time, until the varnish cures and hardens. Curing is a chemical reaction: rushing it with more temperature spoils the varnish.

A second impregnation is usual in humid environments or submersible motors. Nothing is varnished before testing the resistance, the insulation and the rotating field: after curing there is no way back.

06Repair: when to rewind and what to test afterwards

What the burnt winding tells you

Before dismantling, the look of the winding points to the cause. If it is rewound without correcting the cause, the motor burns out again.

What is seen	Probable cause	What to correct
Whole winding darkened and brittle, evenly	Sustained overload or blocked ventilation.	The mechanical load, the thermal relay setting, the condition of the fan and fins.
Two phases burnt and one sound (star)	Single phasing: a fuse blows or a contact opens.	Fuse or contactor; phase-loss protection.
One phase burnt (delta)	Single phasing.	Same as above.
One coil or one group burnt, the rest sound	Turn-to-turn short, from damaged enamel or an overvoltage.	Supply and voltage spikes; that the new enamel was not scraped bare during insertion.
Burn in one slot, with the iron marked	Ground fault.	Slot insulation and moisture.
End windings burnt between two phases	Phase-to-phase fault in the end windings, often for lack of separators.	Phase separators.
Rotor rubbing the stator, core marked	Worn bearings or bent shaft.	Bearings and alignment; the marked core may not be usable.

Is rewinding worthwhile?

Rewinding is a technical and economic decision. It makes sense if the core is sound, if a workshop can guarantee the work with materials of the right class and if the cost, over the life the motor has left, is lower than a new one. At small powers, a new motor often costs little more than the rewind. At medium and large powers, rewinding almost always pays. The method to decide, with equivalent annual costs, is in Maintenance management. A well-done rewind keeps the motor’s efficiency: the EASA and AEMT studies (2003) on good-practice rewinds found no appreciable loss; a bad one, with an overheated core or less copper, can lose one or more points and waste more energy every year.

Tests after rewinding

They are done in this order, and none is skipped: each detects a fault the others do not see. How each quantity is measured is covered in Motor and transformer tests.

Test	How it is done	What it detects and criterion
1 · Resistance per phase	With a bridge or micro-ohmmeter, in DC and at low current, before impregnating. The temperature is recorded.	Too many or too few turns, loose connections, wrongly joined groups. The three phases must match (difference of a few percent, 2 % at most) and match the calculated value.
2 · Insulation to ground	500 V DC megohmmeter between each phase and the frame, with the other two grounded, for 1 minute.	Damaged or damp insulation. In a new, dry winding hundreds of MΩ are expected; the minimum for one in service is 5 MΩ (IEEE 43), and that is only the floor.
3 · Polarization index	Ratio of the 10-minute reading to the 1-minute reading, with the same megohmmeter.	Moisture and dirt in the insulation. At least 2 is required for classes B, F and H (IEEE 43).
*4 · Turn to turn (impulse or surge)*	An instrument applies high-voltage pulses to each phase and shows the oscillatory waveform on a screen; those of two phases are superimposed.	Shorted turns. The waveforms must coincide; if one shifts and is damped, there are shorted turns. Done by the workshop with the proper equipment and trained staff.
5 · Rotating field	With the stator alone and very low voltage, raised from zero with a variac while watching the current, a steel ball or a compass is placed in the bore.	The ball must roll round without sticking and in the expected phase order. A compass reveals the alternation of poles. An inverted pole or group shows up here.
6 · No-load run	With the motor assembled, at rated voltage, for about 30 minutes. The three currents, temperature, noise and vibration are measured.	Balanced currents (a few percent difference), 40 to 60 % of rated for a 4-pole of this power; if very high, turns are missing or wrong; if very low, there are too many. Normal direction of rotation and noise.

Example · Resistances of the rewound 4 kW motor

With the motor of section 03, $R_{20} = 1.223 Ω$ per phase; in the workshop, at 28 °C, $R_{28} = 1.261 Ω$ is expected. Each phase is measured separately, before joining the leads, and gives 1.26 – 1.25 – 1.27 Ω: the average is 1.26 and the largest deviation is $\frac{0.01}{1.26} = 0.8 %$ . All in order.

If phase $W$ gave 1.19 Ω, the difference from the expected value would be $\frac{1.26 - 1.19}{1.26} = 5.6 %$ , which equals $\frac{8}{144} = 5.6 %$ of the turns: 8 turns are missing, a third of a coil. It cannot be impregnated: it must be opened up, the faulty coil found and corrected. If it were a whole coil, the difference would be $\frac{24}{144} = 16.7 %$ .

An insulation value of 800 MΩ at 1 minute and 2000 MΩ at 10 minutes gives a polarization index of $\frac{2000}{800} = 2.5$ : it meets the minimum of 2.

The tests are also a hazard

The megohmmeter delivers 500 V or more: after measuring, the winding is discharged by grounding it for a few minutes. The impulse test applies pulses of thousands of volts and is done with the manufacturer’s equipment and protection, only if the procedure is known. For the rotating field and the no-load run, the motor is secured to the bench and the shaft is guarded, and connections are made with the supply disconnected.

07In the workshop

Practice 1 · Survey a winding

On a three-phase stator stripped in halves or with one end winding opened: count the slots, calculate $q$ , $τ$ and $α$ , fill in the data form of section 04 and draw the developed diagram, with one colour per phase. Criterion: the diagram must give four poles and the three phases shifted 120° electrical.

Practice 2 · Make and test a coil

With 0.6 to 1 mm enamelled wire, a form and a winding machine with counter, make three identical coils of 20 turns. Measure the resistance of each with a bridge. Criterion: differences under 2 % between coils, no bare enamel and the coil tied.

Practice 3 · Rewind a small stator

Materials: a 24- or 36-slot stator, slot liner of the specified class, wedges, wire, tape, insulating sleeve and varnish. Insulate, insert the groups as per the layout, close with wedges and separate the phases. Measure the resistance of the three phases before impregnating. Criterion: difference between phases under 2 % and insulation above 100 MΩ.

Practice 4 · Test the field and start up

With the rewound stator and before impregnating, supply it with reduced voltage through a variac and check with the ball that the field rotates. After varnish and baking, assemble the motor and run it at no load: measure the three currents, the noise and the temperature. Record everything and compare with the expected values.

08Common mistakes

Mistake	Consequence
Cutting the winding without drawing the layout or counting the turns	The data is lost. The winding must be redesigned or guessed.
Exceeding the temperature in the burn-off oven	The insulation between laminations is damaged: the core loses quality and the motor draws more and runs hot even with a new winding.
Using insulation of a lower class than the motor’s	It degrades with heat and the motor burns out sooner than expected.
Wire with a smaller section than the original	Higher current density, more heating and shorter life.
Wire with a larger section than the original	It does not fit the slot, or the enamel is scraped off when forcing it.
One group inverted when connecting	Poles badly alternated: unbalanced current, vibration and no torque.
Varnishing without testing first	Any error can no longer be fixed without removing the winding.
Rewinding without finding the cause of the failure	The motor burns out again for the same reason.
Measuring insulation without discharging afterwards	The winding stays charged and can give a shock.

09Self-assessment

A three-phase stator has 48 slots and 4 poles. How many slots per pole and phase does it have, and what is the pole pitch?

$q = \frac{48}{4 \cdot 3} = 4$ slots per pole and phase, and $τ = \frac{48}{4} = 12$ slots. The electrical angle between slots is $α = \frac{2 \cdot 360}{48} = 15 °$ and the phases are shifted $\frac{120}{15} = 8$ slots.

How many coils and groups does a one-layer, 36-slot, 4-pole three-phase winding have? And a two-layer one?

One layer: 18 coils in 6 groups of 3. Two layers: 36 coils in 12 groups of 3. Each group has $q = 3$ coils.

What is the difference between a concentric and a lap winding?

In the concentric the coils of a group are nested and have different spans; the average is the pole pitch. In the lap all coils are equal and each is shifted one slot from the previous one. The lap uses a single form; the concentric is inserted by machine.

How are the groups of a phase connected in a one-layer and in a two-layer winding?

In one layer there are $p$ groups per phase, all traversed in the same direction: end of one group to start of the next. In two layers there are $2 p$ groups and neighbours go in opposite directions: end to end and start to start.

A 380 V star motor has 22 turns per coil of 0.75 mm wire. What figures does it take for 220 V star?

The turns drop in proportion to the voltage: $22 \cdot \frac{220}{380} = 12.7$ , that is 13. The section rises in inverse proportion: $0.442 \cdot \frac{380}{220}$ $= 0.763 mm²$ , a 1.0 mm wire. The total copper in the slot is preserved.

After rewinding, the three phases read 1.26 – 1.25 – 1.19 Ω and 1.26 Ω was expected. What is happening and what is done?

The third phase has $\frac{1.26 - 1.19}{1.26} = 5.6 %$ less resistance: it is missing turns. It is not impregnated: that phase is checked, the badly made coil (counted short) is found and redone.

Why can the insulation material of the rewind not be of a lower thermal class than the motor’s?

Because at rated load the winding reaches the temperature of its class. A material of a lower class degrades with that heat, loses dielectric strength and the motor fails by insulation sooner than expected. It may be of a higher class.

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