2 Engineering
2.9 Three-phase motors
DB1 variable frequency drive
11/20 MN040031EN
Eaton.com
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2.9.3 Permanent magnet motor (PM motor)
PM motors are three-phase motors that are excited by permanent magnets
and have a speed that is directly proportional to the supply frequency.
Together with a high-pole-count, three-phase stator winding, the permanent
magnets on the rotor make it possible to produce large torques at low
speeds, which in turn makes it possible to forgo the use of a gearbox in
many applications.
By combining high efficiency and good power factor characteristics with a
lightweight and compact construction, PM motors make for a compelling
choice when compared to asynchronous motors. Accordingly, these high-
efficiency motors are primarily found in roller and press drives, agitator and
mill drives, drives for extruder screws, and drives used by the crane industry
for a variety of applications.
2.9.4 Brushless DC motor (BLDC motor)
Contrary to what their name might seem to imply, brushless DC motors
(BLDC, also referred to as “EC motors”) do not have the same configuration
as a DC motor, but are instead put together the same way as three-phase
synchronous motors. The three-phase AC field coil in brushless DC motors
generates a rotating magnetic field that pulls the permanently excited rotor
along.
The rotor position is determined during sensorless vector control by way of
the counter-voltage (counter electromotive force) generated in the stator
coils. This means that the variable frequency drive’s output voltage must
always be live in all three phases (block voltage control), even when the rotor
is stationary. If this condition is met, short current pulses will be generated
when the system is stationary – these pulses will not move the motor, but
they will have an effect on the rotor’s magnetic field.
The control response for BLDC motors is to a large extent the same as that
for a shunt DC motor. BLDC motors are primarily used in drive systems for
machine tools, servo drives in conveyor systems, and compressors and
metering pumps.
￫
In order to use vector control with permanent magnet motors,
the values for parameters P-60, P-61, and P-62 on DB1 variable
frequency drive need to be changed:
•
Change the value for P-60 to 2 (“PM motor speed control”).
•
Change the value for P-61 to 1 (“Motor identification”).
Automatic autotune to determine the motor parameters
when the motor is stationary.
•
P-62 (“MSC gain”). Adjust the gain factor for the speed
controller.
