Losses in Electric Motors

electric motor losses

As seen here in the dia­gram, elec­tric motor loss­es can be divid­ed into ohmic loss­es, iron loss­es, stray loss­es, and mechan­i­cal loss­es. Ohmic loss­es are also called cop­per loss­es and iron loss­es, often also core loss­es. Elec­tric motor loss­es occur in the sta­tor lam­i­na­tions, rotor lam­i­na­tions and wind­ings, and also in the per­ma­nent mag­nets. The low­er the loss­es in the elec­tric motor, the high­er its effi­cien­cy, of course, more details about this in teh fol­low­ing video.

Play Video about elec­tric motor loss­es video

Ohmic Losses

Ohmic loss­es occur pri­mar­i­ly in the wind­ings of the sta­tor and are depen­dent on the resis­tance and the cur­rent. The ohmic loss­es can be divid­ed into fre­quen­cy depen­dent and fre­quen­cy inde­pen­dent. The fre­quen­cy inde­pen­dent loss­es depend on the dimen­sions, i.e. the length and the cross-sec­tion­al area of the wire, as well as the mate­r­i­al, for exam­ple cop­per. A big increase of the motor tem­per­a­ture also increas­es the resis­tance and there­fore also the loss­es. You should always make sure that the elec­tric motor is well cooled, oth­er­wise the tem­per­a­ture of the wind­ings will rise quick­ly. The fre­quen­cy depen­dent ohmic loss­es increase with increas­ing fre­quen­cy, due to the skin effect. The skin effect reduces the area through which cur­rent can flow. To reduce this effect, wires with a large diam­e­ter are divid­ed into sev­er­al sep­a­rate wires. How­ev­er, divid­ing them into too many wires also increas­es the resis­tance due to the prox­im­i­ty effect. So it is not easy to find the best num­ber of par­al­lel wires. Induc­tion motors also have high ohmic loss­es in the rotor, so the rotor gets hot quick­ly. It is very dif­fi­cult to remove the heat from the rotor, so Tes­la uses liq­uid cool­ing of the shaft in the induc­tion motors.

Iron Losses

Iron loss­es can be divid­ed into hys­tere­sis loss­es, eddy cur­rent loss­es and addi­tion­al loss­es. Iron loss­es are giv­en per weight and depend on the fre­quen­cy and the max­i­mum flux den­si­ty. This means that the faster the motor rotates, the high­er the iron loss­es. And the small­er the elec­tric motor is designed, the less space is left for the mag­net­ic flux and the high­er the flux den­si­ty becomes. The pro­por­tion­al­i­ty con­stants C depend on the mate­r­i­al and its man­u­fac­tur­ing. Iron loss­es occur most­ly in the sta­tor and rotor, but eddy cur­rent loss­es can also occur in per­ma­nent mag­nets. Loss­es in mag­nets are usu­al­ly small, but crit­i­cal because mag­nets usu­al­ly do not have good tem­per­a­ture resistance.

motor hysteresis losses

Hysteresis Losses

Mag­net­ic mate­ri­als are divid­ed into many small domains, each with a dif­fer­ent mag­net­ic ori­en­ta­tion. When the mag­net­ic ori­en­ta­tion of the domains changes, loss­es occur. These remag­ne­ti­za­tion loss­es are called hys­tere­sis loss­es because the mate­r­i­al pass­es through hys­tere­sis dur­ing mag­ne­ti­za­tion. The loss­es depend on the area of hys­tere­sis the mate­r­i­al pass­es through dur­ing remag­ne­ti­za­tion. To keep the loss­es low, soft mag­net­ic mate­ri­als such as elec­tri­cal sheets with a small hys­tere­sis curve are used.

Eddy Current Losses

Eddy cur­rents occur when the mag­net­ic flux in the sta­tor changes. The eddy cur­rents gen­er­ate loss­es in the sta­tor and heat it up. To reduce the loss­es, the sta­tor is divid­ed into sep­a­rate lam­i­na­tions that are insu­lat­ed from each oth­er. This sig­nif­i­cant­ly reduces the eddy cur­rent loss­es. The thin­ner the sheets are made, the low­er the eddy cur­rent loss­es in the sheet.

Additional Losses

As men­tioned above, mag­net­ic mate­ri­als con­sist of areas sep­a­rat­ed by walls. A change in the mag­net­ic field can cause a dis­place­ment of the walls, which leads to loss­es. These loss­es are called addi­tion­al loss­es or excess losses.

formula iron losses

Stray losses

Stray loss­es are the scat­ter­ing of com­po­nents and mate­ri­als. But also scat­ter­ing in the pro­duc­tion process and very small devi­a­tions from the design. The stray loss­es are there­fore dif­fi­cult to esti­mate and can be around 1% for elec­tric motors at peak load. 

Mechanical Losses

Mechan­i­cal loss­es can be divid­ed into fric­tion loss­es and ven­ti­la­tion loss­es. Fric­tion­al loss­es depend on the speed and occur, for exam­ple, in the bear­ings. To keep fric­tion­al loss­es as low as pos­si­ble, the bear­ings should always be prop­er­ly lubri­cat­ed. The bear­ings must there­fore not become too hot, oth­er­wise the lubri­cant will dis­si­pate. Fric­tion loss­es also occur in the brush­es of sep­a­rate­ly excit­ed syn­chro­nous motors and com­mu­ta­tors of DC motors. Ven­ti­la­tion loss­es occur in elec­tric motors with non-round rotors and also depend on the speed. For exam­ple, sr-motors or sep­a­rate­ly excit­ed syn­chro­nous motors have rotors that are not round. By cast­ing the sta­tor and band­ing the rotor, the ven­ti­la­tion can be reduced. If cast with the right mate­r­i­al, this can also improve heat dis­si­pa­tion. Ven­ti­la­tion in elec­tric motors can also be used to move heat out of the cen­ter of the elec­tric motor. This dis­trib­utes the heat bet­ter in the elec­tric motor and reduces hotspots.

Power Loss Flow Chart

The visu­al­iza­tion of the con­ver­sion of the pow­er of elec­tric motors and gen­er­a­tors is illus­trat­ed with the help of the pow­er flow dia­gram. In elec­tri­cal motors, the dia­gram shows how much elec­tri­cal input pow­er is sup­plied and how much mechan­i­cal out­put pow­er remains after the pow­er loss­es have been dis­si­pat­ed. In a gen­er­a­tor, the flow of pow­er dis­si­pa­tion revers­es. This means that if you have too much fric­tion in the bear­ings, there is less pow­er left to con­vert into elec­tri­cal power.

losses motor power flow