MACS Monthly Newsletter - 2021 - MAR7

3-Phase BLDC (Brushless DC) Motors (Blower Motors & Fuel Pumps)
BLDC design use a permanent magnet rotor (Figure 11)
attached to the motor's fan / pump's turbine, and three
coils tied together at each end acting as a fixed stator
(Figure 12). You may also see this design in other small
motors such as the one that moves the transmission park-

Wikipedia

Toyota

Car makers have for years been making brushless motors. The HD truck world has had brushless alternators
since the 1970's. How do they work? HVAC blower motors,
cooling fan motors and fuel pumps that are of the 3-phase

Figure 13: Presumptive 3-Phase BLDC Cooling Fan. Common
applications for 3-phase BLDC motors include engine
cooling fan motors, HVAC blower fan motors, engine
water pumps, sunroof motors, ABS motors, transmission
fluid pump motors, fuel pumps and EPS (Electric Power
Steering) motors. On this 2018 Toyota Camry's electric
cooling fan motor block diagram, we can clearly see that
there are three circuits going into the actual cooling fan
from the cooling fan ECU. The ECU is part of the overall
fan / motor / controller assembly. This makes direct access to the (presumed) 3-phase circuits improbable.

Figure 11: BLDC Motors. Brushless DC Motors (BLDC) are
used in some applications as HVAC fan motors and fuel
pumps. In all the HVAC fan motor and cooling fan applications we are aware of, the fan uses a module built on
to the fan assembly that communicates with the PCM via
a digital signal (PWM input or serial data) to control the
3-phase fan motor's speed.

mpoweruk.com

ing pawl into place when you press the " Park " button on
the dash. In the case of blower and cooling fan motors using this design (Figure 13) you usually can't tell the difference. The motor will have power, ground and some other
control circuits such as a LIN or CAN bus. From a technician's standpoint, you diagnose those circuits per your
normal routine. If the blower motor won't run, and there's
ample power and ground as far as OCV (Open Circuit
Voltage) measured to the non-functional motor, check for
any issues with the serial bus circuits. You should see a
square wave with your scope and find no " U-Code " DTCs
stored in any modules associated with the ECM or blower
/ HVAC controls. Next it would be wise to connect a substitute load that will draw 20 amps (known good simple
2-wire blower motor) across the BLDC motor's power and
ground feeds (disconnected from the OE motor) to see
if there is an excessive voltage drop with a known good
load. Beyond that, you're guessing the BLDC is faulty.

Figure 12: The control circuits for 3-phase BLDC (Brushless
DC) motors are far more complicated compared to simply
2-wire DC brushed style electric motors. Every 120 degrees, the motor drive module sends out a high frequency
pulse to the three stator windings inside the motor. Notice the three coils tie together at the center like the stator windings in a conventional alternator. A 3-phase BLDC
fuel pump's three wires will have very little resistance
when measuring between each other (just like an alternator's stator or 3-phase hybrid drive motor's connections).

March 2021

BLDC motors are VERY complicated devices compared

MACS Service Reports

http://www.mpoweruk.com

MACS Monthly Newsletter - 2021

Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2021