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The PM motors specially BLDC or PMSM they get their armature energy from the permanent magnets on their rotors unlike the Induction Motors in which the armature gets its energy from the Stator rotating flux. So we can say that PM require less current or energy or power as compared to IM for same work.

Does it means that when energy of PM is used to do the work then at the same time the magnetic flux or field or strength of the PM gradually reduce as it supply the energy to do the work in the motor action?

So if the magnets keep losing their energy then after certain amount of hours-of- working they will de-magnetize and a PM will not run. Please correct me if I am wrong in this understanding.

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There are two magnetic fields, a stator field and a rotor field. Torque is developed due to the attraction and repulsion forces between the two fields. The energy is supplied through electric current in the stator field. The stator is designed so that its field does not cause the rotor field to deteriorate. If that field is allowed to become too strong, the rotor can be partially demagnetized, but a proper controller design will prevent that from happening. Excess temperature is also a risk to the rotor magnets, so that also must be considered in the design. Be assured that a well designed motor and controller can operate for a very long time without significant deteriorating of the rotor magnets.

Understanding the Basic Physics

Remember that if you have a force without any motion, no work is being done, so no energy is transferred or dissipated. In an electric motor, there are two magnets with a force between them, but they don’t move with respect to each other. The distance between them may change a little, but one is pulling the other almost like scrap is lifted with a magnet in a scrap yard. The force moving the magnets does the work, not by the force holding then together.

In an AC motor, electric power makes the stator magnet rotate around the stator. The stator magnet exerts a force on the rotor magnet that holds it close. Holding the two together requires no energy. Only the electric current that is moving the stator magnet does work.

It may also be helpful to compare magnets to springs. When you compress a spring, work is done and energy is stored. When you release the spring it does work, but it doesn’t lose its strength to resist compression.

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In normal specified use, the permanent magnets in a PM motor retain 100% of their field.

There are several ways to reduce the field of the magnets, all involve abusing the motor beyond its specifications.

The first is to deliver excess current. The current in the armature generates a field in a direction that opposes the field of the permanent magnets. Magnets are made from a magnetically 'hard' material, that resists this effect. The specifications of the motor are written such that when the motor is taking its maximum current, at stall, from the maximum terminal voltage, the magnets are hard enough to completely resist the demagnetising effect. A large expensive motor will often have an explicit 'max current before demagnetisation' specification. A smaller cheaper motor will usually just specify a maximum voltage, below which you should be safe even at stall.

If you power a motor from a higher voltage in the hope of getting MOAR POWA, then you might just ruin the magnets. The smart motor overdriver (I've run 24v motors from 36v in a robot competition) actively limits the current to avoid this.

A second way is to raise the motor to too high a temperature. Above their Curie temperature, all magnets lose their magnetisation. Most have a CT well above the point at which insulation will start smoking, so you're unlikely to ruin a motor this way without ruining it another way first.

Another way is to mechanically shock the motor. However, the amount of shock needed to reduce the magnet field strength is likely to mechanically damage the motor anyway.

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  • \$\begingroup\$ From high school physics in which we were taught that energy comes from somewhere when you get it and its total sum remains unchanged.. so how is it possible that we run a 1000's of Watt of motor for 100's of hours so that amounts to a lots of Joules of energy that we are using for the rotor magnetic field in the motor.. if we are using that energy then the magnets should demagnetize at the same time also.. how can we equate that energy budget equation? Or is it the case that the magnets have extra ordinary high energy and this small amount that is consumed in motor action is not significant? \$\endgroup\$ – scico111 Jan 13 '19 at 10:39
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    \$\begingroup\$ @scico111 Yes, I'm puzzled by magnets too sometimes, but then I just have to remember that it's what happens, and so have to live with it. Physics is very bad at saying 'why', good at saying 'what' and 'how much'. Superconductivity and magnetism are two examples of persistent things that you need quantum mechanics to understand at a deeper level. Once started, each has something 'keep going' for, as far as we can tell, ever. A motor stator is typically assembled with non-magnetised material, and then 'charged' with a large current pulse in a dummy rotor. Look up coercivity and remnance. \$\endgroup\$ – Neil_UK Jan 13 '19 at 20:07
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    \$\begingroup\$ @scico111 Because the energy that makes the motor spin doesn't come from the rotor magnetic field, of course. The magnetic field has to be there. But it's not where the energy comes from. \$\endgroup\$ – user253751 Jan 15 '19 at 2:18

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