"Conventional" Stepper Motor

An extremely simple stepper motor circuit is shown to the right; it consists of a bar shaped permanent magnet which can rotate about its center, a "C" shaped piece of soft iron, a length of insulated copper wire (wrapped around the piece of soft iron), a switch, and a battery.

Fig 1: Conventional Stepper Motor Circuit (1 Of 7)

Courtesy of BMW OF NORTH AMERICA, INC.

With the switch open, the bar magnet will rotate to position itself with the soft iron as shown and it will stay there.

When the switch is closed, current flows through the wire creating a magnetic field. With the wire wrapped around the soft iron in the direction shown here, the magnetic field North pole will be located at the top.

Fig 2: Conventional Stepper Motor Circuit (2 Of 7)

Courtesy of BMW OF NORTH AMERICA, INC.

Since "like" poles repel and opposites attract, the permanent magnet rotor will turn 180 degrees to align itself with the new magnetic field and then it will stop there.

NOTE: In this simple motor, the rotor can turn in either direction to reach its new position.

Opening the switch will cause the magnetic field to collapse, but the rotor will remain in its new position.

Fig 3: Conventional Stepper Motor Circuit (3 Of 7)

Courtesy of BMW OF NORTH AMERICA, INC.

By reversing battery polarity, though, and closing the switch, a new magnetic field is created. One that has its North pole located at the bottom of the "C." The rotor moves again, this time back to its original position.

Using this information, we can again redraw the parts of the circuit in a slightly different way to get a stepper motor that is closer to the ones found on BMW vehicles.

All we've done is wrap two coils around the soft iron core (one wound in each direction to give opposite polarities), and use two switches so each coil can be energized independently .

Fig 4: Conventional Stepper Motor Circuit (4 Of 7)

Courtesy of BMW OF NORTH AMERICA, INC.

Rearranging some parts of this circuit just one more time puts the beginning of both coils at the middle of the "C" (for manufacturing reasons) so we have to wind the top coil in the other direction to have its polarity stay the same.

The bottom coil is unchanged.

Fig 5: Conventional Stepper Motor Circuit (5 Of 7)

Courtesy of BMW OF NORTH AMERICA, INC.

Now, momentarily closing switch "B" energizes the bottom coil, a magnetic field forms with its North pole at the top, and the rotor turns 180 degrees to align with the field.

Fig 6: Conventional Stepper Motor Circuit (6 Of 7)

Courtesy of BMW OF NORTH AMERICA, INC.

The rotor then stops there.

Momentarily closing switch "A" energizes the top coil, a magnetic field forms with its North pole at the bottom, and the rotor turns 180 degrees again to align with the new field.

Fig 7: Conventional Stepper Motor Circuit (7 Of 7)

Courtesy of BMW OF NORTH AMERICA, INC.

Alternately closing switches "A" and "B" toggles the magnetic field polarity, causing the rotor to rotate in 180 degree steps

Compare our "generic" 90° stepper motor to a BMW climate control system stepper motor as shown in an Electrical Troubleshooting Manual (ETM) and note the similarities.

Fig 8: BMW Climate System Stepper Motor Circuit (1 Of 2)

Courtesy of BMW OF NORTH AMERICA, INC.

Fig 9: BMW Climate System Stepper Motor Circuit (2 Of 2)

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They both:

have 4 coils arranged around a permanent magnet rotor
have the 4 coils receiving power directly from the battery
energize the coils individually using ground switches (transistors inside the control module for the ETM version)

Another important difference for the actual BMW part is that it is mounted on a high reduction gearbox. In fact, on the typical stepper motor, the stepper motor rotor must rotate about 300 revolutions (that's 7200 separate rotor steps) for the gearbox output shaft to rotate through just a single revolution.

Fig 10: Identifying Coils Arranged Around A Permanent Magnet Rotor

Courtesy of BMW OF NORTH AMERICA, INC.

This high-reduction gear box allows a relatively low power motor to operate the flaps against the substantial suction and pressure forces generated by the blower motor.

Taking another look at an "actual" BMW stepper motor, notice that it requires 5 connections:

power (to all 4 coils)
ground (control) to coil "S1"
ground (control) to coil "S2"
ground (control) to coil "S3"
ground (control) to coil "S4"

These connections are important for diagnostic purposes, since it is impractical to run a stepper motor once it has been disconnected from the control module.

One way to test for a faulty stepper motor is to measure coil resistance.

Fig 11: All Except Fresh Air Flap Motor Circuit

Courtesy of BMW OF NORTH AMERICA, INC.

On E31, E32, and E34 stepper motors except the fresh air flap motor, each of the motor's 4 field coils has a resistance of about 85 ohms .

For the fresh air flap motor only on E31, E32, and E34 vehicles, each of the 4 field coils has a resistance of about 26 ohms .

The fresh air flap motor is designed to operate faster in the "close flap" direction than in the "open flap" direction, so it has different characteristics. It's easy to distinguish the fresh air flap stepper motor from the other motors because it has a different color identification label; typically red or blue, depending on production date.

Fig 12: Identifying Fresh Air Flap Motor Circuit

Courtesy of BMW OF NORTH AMERICA, INC.