Due to the built-in permanent magnets, permanently excited motors do not need a separate
excitation winding. This results in reduced losses in the motor and in increased efficiency. However,
they do need an electronic controller that generates a rotating field. Operation directly from the
mains is generally not possible, or results in reduced efficiency.
A basic difference with permanently excited motors is the waveform of the back EMF (Electro Motive
Force). When operating as a generator, a motor with permanent magnets produces a voltage known
as the back EMF. To ensure optimum control of this type of motor, the controller must match the
waveform of the supply voltage as closely as possible to the waveform of the back EMF. In the
case of BLDC motors, manufacturers use square-wave commutation due to the trapezoidal voltage
waveform.
Permanently excited synchronous motors (PMSMs) have a sinusoidal back EMF and therefore
operate with a sinusoidal voltage (sinusoidal commutation). A further distinction is made between
sinusoidally commutated motors, depending on whether the magnets are glued onto the rotor
(SPMSM) or are integrated in the rotor laminations (IPMSM). Due to these somewhat unwieldy
abbreviations, the term ”PM motor“ is often used in practice to refer to motors with sinusoidal
commutation.
PMAC = Permanent Magnet AC; BLDC = Brushless DC; PMSM =
Permanent Magnet Synchronous Motor; IPMSM = Interior PMSM
(embedded magnets); SPMSM = Surface PMSM (magnets mounted
on rotor)
As with all technologies, each type of permanent magnet motor has its own specific advantages and
disadvantages. Sinusoidally commutated PM motors are easier to implement from a structural
perspective, but they have more complex control circuitry. The opposite is true of EC motors which
are popular in HVAC: Producing a square-wave back EMF is more difficult, but the structure of the
control circuitry is simpler.
However, torque ripple is worse with EC technology due to square-wave commutation, as are higher
iron losses. In addition, the current is 1.22 times as large as with PM motors because it is distributed
over two phases instead of three.
EC and PM motor systems (electronics plus motor) with comparable configurations (mains supply,
EMC filter, etc.) have similar efficiency levels. These are higher than IE2 and even IE3 level.
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The reputation of EC motors as very high efficiency motors is primary based on its comparison with
commonly used shaded-pole and single-phase induction motors, in the power range of a few hundred
watts. Three-phase induction motors are typically used for higher rated power in excess of 750
watts. Compared with these motors, the efficiency advantage is significantly less, and it decreases
as the power level increases.
However, do you pay your energy bill for components or for your system? Investing in efficient
components only makes sense when it does not affect the overall system efficiency.
More info Optimum System Efficiency