IEEE Power Electronics Magazine - June 2014 - 12

Operating Principles:
Interior PM Machines

saliency helps in extending the speed
range of the motor further beyond the
The new standards are
base speed. Higher core losses will
Interior PM synchronous machines
achievable effectively
be generated at high-speed operation,
(IPMSMs) are employed in most of
and, since the excitation torque genthe currently available hybrid and
with electrified
erated by PMs is reduced, the torque
electric vehicles on the market,
powertrains utilizing
ripple content will be higher.
including the Toyota Prius (motor
An IPMSM is a synchronous
60 kW, generator 42 kW), Chevy Volt
electric propulsion
motor, and, as its name implies, the
(motor 110 kW, generator 55 kW), and
motors.
airgap magnetic field and the rotor
Nissan Leaf (80 kW).
are synchronized to rotate at the
In PM machines (Figure 1), an indesame speed. In an ideal case, the airpendent source of magnetic flux is
gap magnetic field does not change in time relative to the
provided by the rotor magnets. The operating principles are
rotor. Therefore, no voltage or current is induced and rotor
similar to those of the conventional synchronous machine;
losses would be zero. However, nonideal conditions, such
however, the PMs do not introduce significant ohmic losses
as space harmonics due to the spatial distribution of the
as contrary to the field windings in wound rotor synchroconductors around the stator slots, slot harmonics due
nous machines. In IPMSMs, the magnets are embedded in
to the variation of the airgap permeance around the slot
the rotor. By applying a careful design for the location and
openings, and time harmonics due to the nonsinusoidal
arrangement of the magnets, a significant reluctance torque
current waveform supplied by the PWM inverter, cause
can be maintained, which increases the output power capadistortion in the airgap flux density waveform. The relability of the machine.
tive harmonics rotate at different speeds than the rotor,
When operating at high speeds, the back-electromotive
and, hence, they induce eddy currents in the rotor core and
force of a PM machine is likely to exceed the terminal voltthe magnets. These are the main reasons for rotor losses
age. Field weakening is imposed by the adjustment of the
in PM machines.
current excitation angle, so that the stator flux effectively
With the use of high-energy and high-coercivity rareopposes the magnet flux. For the same stator current vector,
earth magnets, PM machines can provide high torque denthis reduces the excitation torque from the PMs; however,
sity, competing well against other motor technologies. Howthe additional reluctance torque component from the rotor
ever, PMs are very sensitive to temperature. The coercivity
of neodymium-iron-boron (NdFeB) magnets drops rapidly
as the temperature increases. In addition, due to the high
prices associated with the rare-earth elements, a significant portion of the total cost of an IPMSM may be for the
magnet cost, even though its weight is a small portion of
the total weight.

Operating Principles: Induction Machines
fig 1 A typical interior PM machine.

fig 2 The typical rotor laminations of a squirrel-cage IM.
(Photo courtesy of Polaris Laser Laminations, LLC, Illinois.)

IEEE PowEr ElEctronIcs MagazInE

z June 2014

Induction machines (IMs) are the most widely used motors
in many different applications, including industrial drives.
As compared with the PM motors, IMs benefit from a
mature technology. With the emergence of inverters and
advanced motor control techniques, they are common in
variable-speed drive applications. The stator and winding
structure of an IM is very similar to that of a PM machine. In
traction applications, a squirrel-cage IM is commonly used,
which has a rotor with aluminum or copper conducting
bars, short-circuited with end rings. Figure 2 shows the laminations of a squirrel-cage IM.
The IM is an asynchronous machine in which the magnetic field in the airgap and the rotor rotate at different
speeds; hence, they have different frequencies. Torque production in IMs is based on this difference. When there is a
difference between the speeds of the rotor and the airgap
magnetic field, rotor conductors are exposed to a timevarying magnetic field, which induces voltage across them.
Since the conductors are short circuited with the end rings,

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - June 2014