Windings for Electric Motors

motor windings
Wind­ings for elec­tric motors can be divid­ed into con­cen­trat­ed wind­ing and dis­trib­uted wind­ing. In the case of dis­trib­uted wind­ing, the wind­ing is always wound over at least two sta­tor teeth. In the case of con­cen­trat­ed wind­ing, on the oth­er hand, only one sta­tor tooth is wound. Both types of wind­ing have dif­fer­ent advan­tages and dis­ad­van­tages. When to use a con­cen­trat­ed wind­ing and when to use a dis­tri­b­u­tion wind­ing depends on the dimen­sions of the motor and its appli­ca­tion. But also the num­ber of units and how the motor is to be man­u­fac­tured has an influ­ence on the choice of wind­ing type. Oth­er dis­tin­guish­ing fea­tures, which are impor­tant for the design of the wind­ing, are the frac­tion­al slot wind­ing and the inte­ger slot wind­ing, as well as sin­gle lay­er wind­ing and dou­ble lay­er winding. 
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Concentrated Winding

concentrated moto winding

With a con­cen­trat­ed wind­ing, the sta­tor is always wound exact­ly over one tooth. This has sig­nif­i­cant advan­tages in the man­u­fac­ture of the sta­tor. For exam­ple, already fin­ished wind­ings can sim­ply be pushed onto the sta­tor. How­ev­er, the design of the sta­tor’s teeth must be designed for this. A major advan­tage of a con­cen­trat­ed wind­ing is the very small wind­ing head at the top and bot­tom of the motor. In a dis­trib­uted wind­ing, this wind­ing head is larg­er because of the over­lap of the wind­ings. Due to the small­er wind­ing head of con­cen­trat­ed wind­ings, the ohmic loss­es in the low­er speed range, are low­er all for a dis­trib­uted wind­ing. When using a clas­si­cal design of an elec­tric motor with a con­cen­trat­ed devel­op­ment, the back EMF is trape­zoidal. As a result, high torque can be gen­er­at­ed with this type of wind­ing, espe­cial­ly in the low­er range. How­ev­er, this does not mean that it also results in high effi­cien­cy. This is because the main dis­ad­van­tage of a con­cen­trat­ed wind­ing is the har­mon­ics that are gen­er­at­ed, which can lead to high loss­es, espe­cial­ly at high speeds. These loss­es can occur in the wind­ings in the lam­i­na­tions as well as in per­ma­nent mag­nets. Con­cen­trat­ed wind­ings are pri­mar­i­ly used for motors that are short and have a large diam­e­ter. An exam­ple of this is wheel hub dri­ves for elec­tric bicy­cles with­out gears. Con­cen­trat­ed wind­ings are also used for dri­ves that require high dynam­ics and effi­cien­cy is not so crit­i­cal. Anoth­er dis­ad­van­tage of con­cen­trat­ed wind­ings is a not insignif­i­cant torque rip­ple. This can be com­pen­sat­ed, for exam­ple, by mov­ing the mag­nets on the rotor rel­a­tive to each oth­er or by shap­ing the mag­nets accordingly.

Distributed Winding

With a dis­trib­uted wind­ing, at least two teeth of the sta­tor are always wound, as in this exam­ple on the pic­ture. How many teeth are wound over is called coil pitch or step size and of course you can also wind over 3, 4, 5 or more teeth. In a dis­trib­uted wind­ing, the wind­ings over­lap at the top and bot­tom of the elec­tric motor, this area of the motor is also called the wind­ing head. Because of the over­lap, the wind­ing head is larg­er in a dis­trib­uted wind­ing than in a con­cen­trat­ed wind­ing. For very short elec­tric motors, there­fore, a con­cen­trat­ed wind­ing is usu­al­ly used rather than a dis­trib­uted wind­ing. This allows the ohmic loss­es from the wind­ing head to be reduced. For longer elec­tric motors, the influ­ence of the loss­es from the wind­ing head is not so great in rela­tion to the total loss­es. A very impor­tant advan­tage of a dis­trib­uted wind­ing for an elec­tric motor is that the result­ing back EMF has a smooth most­ly sinu­soidal char­ac­ter­is­tic. This means that the pro­por­tion of har­mon­ics is very low and so are the loss­es in the lam­i­na­tions of the sta­tor and the wind­ings. Dis­trib­uted wind­ings are used wher­ev­er high effi­cien­cy is required, such as in elec­tric vehi­cles. Anoth­er advan­tage of dis­trib­uted wind­ings is a high syn­chro­nism, which means that the torque rip­ple and there­fore also the noise of the motor are very low. For this rea­son, elec­tric motors with dis­trib­uted wind­ings are main­ly used for rotary spin­dles of machine tools, oth­er­wise the torque oscil­la­tions would have a neg­a­tive effect on the qual­i­ty of the work­piece being machined.

distributed winding

Motor Winding Diagram

In a con­cen­trat­ed wind­ing, one tooth of the sta­tor car­ries exact­ly one wind­ing. The dif­fer­ent col­ors stand for the three phas­es U, V and W. The two wind­ings of one phase are con­nect­ed to each oth­er and the three phas­es can then be con­nect­ed in a star or a delta. In a dis­trib­uted wind­ing, at least two teeth are wound, as in this exam­ple. Over how many teeth is wound is called coil span and of course you can wind over 3, 4, 5 or more teeth. The coil span depends on the num­ber of teeth of the sta­tor and the num­ber of pole pairs of the rotor.

Winding Diagram Electric Motor

Distributed Winding vs Concentrated Winding

What is the tech­ni­cal dif­fer­ence between a dis­trib­uted wind­ing and a con­cen­trat­ed wind­ing. For this we will look at the volt­age of the back EMF. This is the volt­age that can be mea­sured at the ter­mi­nals of an elec­tric motor with per­ma­nent mag­nets when the motor is rotat­ed fast enough by hand or with a load machine.

back-emf electric motor

In the dia­gram you can see that the volt­age of the elec­tric motor with a dis­trib­uted wind­ing is nice­ly sine shaped. On the oth­er hand, the back EMF of the con­cen­trat­ed wind­ing is rather in the shape of a trape­zoid. What does this mean for the per­for­mance of the motor? With a motor with a con­cen­trat­ed wind­ing you can gen­er­ate more torque due to the trape­zoidal back EMF. How­ev­er, the loss­es in the cop­per and lam­i­na­tions are greater because there are more and high­er har­mon­ics. With a dis­trib­uted wind­ing, you can achieve a high­er effi­cien­cy due to the sinu­soidal back-EMF and this is par­tic­u­lar­ly impor­tant for elec­tric vehi­cles. How­ev­er, it does not direct­ly mean that if an elec­tric motor has a sine-shaped back EMF, that this motor has a high effi­cien­cy. And even with con­cen­trat­ed wind­ings, a sine-wave char­ac­ter­is­tic of the back EMF can be achieved by adapt­ing the rotor design.