Electric Motor

Topics:

Introduction
Operating principle of electric motors
DC electric motor with brushes
DC electric motor as shunt motor (parallel) or series motor
– Calculating starting torque
– Calculating counter-EMF, load torque and equilibrium speed
– DC motor with two positive brushes for low and high speed
DC electric motor without brushes

Introduction:
Electric motors are being used in more and more places in the car. In an electric motor, an electric current is converted into motion and heat. We find an electric motor in the mirror and seat adjustment, but also as a wiper motor in the wiper mechanism or as a starter motor. These electric motors operate at a voltage of 12 to 14 volts. On this page we restrict ourselves to the electric motors in the interior and exterior.

Electric motors also provide the (partly) electric drive in hybrid and fully electric vehicles. This type of electric motor is discussed on the page: HV electric motors.

The DC electric motors can be divided into:

Electric motors with brushes (electromagnetic field and armature)
Series electric motors;
Parallel electric motors;
Brushless electric motors.

Operating principle of electric motors:
In an electric motor, an electric current is converted into a rotating motion. The motion is created because two magnetic poles attract or repel each other:

A north pole and south pole attract each other;
Two north poles repel each other;
Two south poles repel each other.

A magnet has both a north and a south pole with opposite charges. When that magnet is broken in half, you do not suddenly have two separate poles, but two new magnets, each again with a north and a south pole.

Several magnetic poles (north and south) are fixed to the housing. A magnetic field exists between the north and south pole. The output shaft (the armature) rotates as a result of changes in the magnetic field.

In an electric motor, using (usually) permanent magnets, or otherwise electromagnets, two like poles are constantly placed opposite each other. Because like poles repel each other, motion is created.

DC electric motor with brushes:
Almost all electric motors in automotive engineering are designed as DC motors with permanent magnets and brushes. In this type of electric motor we find the following magnets:

Permanent magnets (one north pole and one south pole): a stationary magnetic field exists between them;
Coils: an electromagnetic field is generated in these. The rotating electromagnetic field is generated in the coils.

The permanent magnets are located to the left and right of the rotor and consist of one north pole and one south pole. Between this north and south pole there is a stationary magnetic field that does not change when the electric motor is running or stationary.

In the coils a rotating electromagnetic field is generated as soon as current flows through them. The current is supplied to and removed from the coils via the brushes through the commutator.

Reversing the direction of current takes place by means of commutation: two brushes slide over the commutator, which consists of a positive and a negative side. The brush on the positive side conducts the current to the conductor (green arrows in the image). The current leaves the conductor via the brush on the negative side. The current flowing through the conductor creates an electromagnetic field.

Between the generated magnetism in the armature (the conductor) and the field (the permanent magnets) a force arises (red arrows in the image). This force causes the armature and commutator to rotate about their axis. The brushes then contact the other part of the commutator, reversing the direction of current in the armature. The magnetic field and the force are built up in the same direction, so that the armature again rotates about its axis.

We can change the direction of rotation of the electric motor (i.e. the armature) by reversing the positive and negative of the brushes.

Reversing positive and negative can be obtained by commutation (as explained above), or can be achieved by means of an H-bridge in an ECU circuit.

The ECU (1) controls two of the four transistors or FETs (4) at the same time;
The FETs (2) supply the electric motor (3) with positive and ground. Depending on which two FETs are turned on, the upper brush is positive and the lower is ground, or vice versa;
The potentiometer next to the electric motor registers the position and direction of rotation. Not all electric motors are equipped with a potentiometer.

See the page H-bridge for the possible designs and switching methods of the H-bridge.

DC electric motor as shunt motor (parallel) or series motor:
Electric motors are used in vehicles in different ways, depending on the requirements for torque, speed and efficiency. We distinguish two types of DC motors: the shunt motor and the series motor. These are explained below.

Shunt motor (parallel motor):
In a shunt motor, also called a parallel motor, the field windings are connected in parallel with the armature winding. As a result, the current through the field winding remains virtually constant, regardless of the load on the motor. This ensures an almost constant magnetic flux, so that the speed changes very little with varying load. The motor torque mainly depends on the current through the armature winding, which gives the motor a stable and well-controllable speed.

Shunt motors are used in drives where an almost constant speed is important, such as conveyor belts, fans, machine tools and other industrial installations.

In modern vehicles, comparable applications usually use permanent magnet DC motors (PMDC motors). With these, the field winding is replaced by permanent magnets, so that the magnetic flux also remains virtually constant. Therefore, PMDC motors have similar characteristics to a shunt motor and are widely used as wiper motor, interior blower, electric fuel pump and electric water pump.

Series motor:
In a series motor, the field windings are connected in series with the armature winding, so the same current flows through both windings. The greater the current flowing through the motor, the stronger the magnetic field becomes. This means that the magnetic flux increases as the load increases, which leads to higher torque. This makes a series motor perform particularly well at high loads. However, the speed of the motor decreases as the load increases, because the additional magnetic flux increases the torque but at the same time reduces the speed.

When the motor is switched on, a strong magnetic flux is generated because the current through both the field and armature windings is high. This provides a high starting torque. This makes the series motor very suitable as a starter motor, especially under heavy conditions, such as with a cold engine or low battery voltage. Other applications of series motors are: electric winches, drive of forklifts.

Calculating starting torque:
A series motor has a higher starting torque than a shunt motor for the following reasons:

the starting torque becomes directly high when the electric motor is still at a standstill;
no back EMF has yet been generated from standstill,
the current is at its maximum at the moment of activation.

The starting torque of a shunt motor (parallel) and a series motor can be compared. The formula for calculating the starting torque of an electric motor is:

T = torque [Nm]
F = Lorentz force [N]
B = magnetic flux in Tesla [wb/m2]
I = current [Ampere]
l = length of copper wire [m]
d = diameter of armature [m]

To enter real values into the formula , we can make assumptions based on typical values for shunt and series motors in vehicles. A series motor initially has a higher current (I) and therefore a higher magnetic flux (B) than a shunt motor. When the other data (length of the copper wire and the diameter of the armature) are the same, we can compare the generated torque between the two motors.
First, the table with the properties of both motors is shown.

In the table above we can see that the I (current) for both motors is high, but that the B (magnetic flux) is high for the series motor during switch-on, while it remains constant for the shunt motor.

If we reason this in a formula, and use 1 for constant and 2 for high, we see that the generated torque of the series motor is higher:

If you prefer to calculate with numbers instead of reasoning with a 1 or 2 for constant or high, you can look at the formulas filled in below, in which the shunt and series motors are compared. Here, the magnetic flux (B) and the starting torque (I) are variables, and l and d are equal. In this case, the torque of the series motor is also significantly higher.

Calculating back EMF, load torque and equilibrium speed
When a DC motor is switched on, the armature starts to rotate. The armature windings move through the magnetic field of the stator. According to Faraday’s law of induction, this induces a voltage that is opposite to the applied supply voltage. This voltage is called the back EMF (counter-electromotive force).

At standstill, the back EMF is equal to 0 V. The full supply voltage is then across the armature winding, causing the armature current to be at its maximum and the motor to deliver its highest starting torque. As the speed increases, the back EMF becomes larger. In a DC motor with a constant magnetic flux (such as a shunt motor or permanent magnet motor), the back EMF increases directly proportional to the speed:

$E$ = back EMF (V)
= motor constant
$Φ ()$ = magnetic flux (Wb)
= angular velocity (rad/s)

Because the back EMF is opposite to the supply voltage, the effective voltage across the armature winding becomes smaller. As a result, the armature current decreases according to:

= supply voltage (V)
= back EMF (V)
$_{a}$ = armature resistance (Ω)

The electromagnetic motor torque is directly proportional to the armature current and is determined by:

$T_{motor}$ = electromagnetic motor torque (Nm)
= motor constant
= magnetic flux (Wb)
= armature current (A)

Because the armature current decreases, the electromagnetic motor torque also gradually decreases as the speed increases.

While running, the motor is subjected to a load torque (also called counter-torque). This is the mechanical torque that opposes the rotation of the rotor. The load torque is caused, among other things, by:

the load of the driven machine;
bearing friction;
gear and drivetrain friction;
the resistance of the driven components, such as windscreen wipers on the windshield.

The motor continues to accelerate as long as the electromagnetic motor torque is greater than the load torque.

Due to the increasing back EMF, the armature current decreases further, which also reduces the electromagnetic motor torque. Eventually, the motor torque becomes exactly equal to the load torque.

This point is called the equilibrium speed. At that moment the motor no longer accelerates or decelerates and runs at a constant speed.

When the load increases, for example because the wipers move over a dry windshield, the motor slows down. As a result, the back EMF decreases, causing the armature current to increase again. The motor torque then increases until a new equilibrium is reached between motor torque and load torque.
If the load decreases, the speed rises. The back EMF therefore increases, causing the armature current and the motor torque to decrease until a new equilibrium speed is reached.

DC motor with two positive brushes:
The previous paragraph explained that a DC motor reaches an equilibrium speed when the electromagnetic motor torque equals the load torque.

In a windscreen wiper motor, this principle is used to achieve two different rotational speeds.
The wiper motor for the windshield has two positive brushes for this purpose. By switching the supply voltage to a different positive brush, the point at which the current through the armature windings is commutated changes. This changes the rotor position at which the Lorentz force acts, and therefore also changes the effective lever arm.

Low speed:
When the positive brushes are positioned directly opposite each other, the current is commutated at the most favourable moment.
The Lorentz force then acts almost perpendicular to the armature conductors, resulting in the greatest leverage on the rotor. According to the previously derived formula, the large diameter “ provides a large electromagnetic motor torque (T).

For a DC motor, the same relationship can be described using the motor equation alongside. At low speed, the motor constant $k$ is utilised to the maximum. As a result, with the same armature current the motor delivers a large electromagnetic motor torque and a high starting torque is obtained. Because sufficient motor torque is available, the equilibrium between motor torque and load torque is already reached at a relatively low speed.

High speed:
At the second positive brush, the brush is rotated by a few degrees. This causes the current through the armature windings to be commutated at a different moment. The Lorentz force therefore acts at a different rotor position, which reduces the effective lever arm “d”. According to the previously derived formula, this also reduces the electromagnetic motor torque T:

Although the motor constant “ is a design characteristic of the motor, it is utilised less effectively due to the changed commutation with the brush positioned at an angle. As a result, with the same armature current the motor delivers less electromagnetic motor torque.

Because the available electromagnetic motor torque is smaller, the point at which the electromagnetic motor torque equals the load torque shifts. This means that equilibrium is only reached at a higher speed.

The higher equilibrium speed has a direct effect on the back EMF (). Because the angled brush has a smaller motor constant (), the motor must rotate at a higher speed to generate nearly the same back EMF (approximately 12 V) as with the low-speed brush. This is one of the main reasons why the angled brush provides a higher motor speed. The magnetic flux () remains constant; the higher rotational speed compensates for the smaller motor constant.

Due to the higher back EMF, the effective voltage across the armature winding becomes smaller. As a result, the armature current (Ia) decreases, which according to the motor equation also reduces the electromagnetic motor torque.

The armature current decreases until the electromagnetic motor torque again equals the load torque.
The wiper motor therefore reaches a second equilibrium speed that is higher than with the first positive brush.

Low speed:	High speed:
Positive brush opposite the negative brush is active	Angled positive brush is active
Large effective lever arm	Small effective lever arm
Motor constant k is used optimally	Motor constant k is used less effectively
High electromagnetic motor torque	Low electromagnetic motor torque
High starting torque	Low starting torque
Low back EMF	High back EMF
Low equilibrium speed	High equilibrium speed

Brushless DC electric motor:
The brushless DC motor is a synchronous motor. The electronic control has taken the place of the brushes. This type of electric motor closely resembles the synchronous AC motor with permanent magnets, as used in the drivetrain of electric vehicles. The main difference between the two motors is the control: the AC motor is driven by a modulated sinusoidal AC voltage and the DC motor by a block-shaped voltage.

The stator often contains three or six coils (U, V and W) and the rotor is a permanent magnet. The image below shows the schematic construction of the DC motor with, next to it, the voltage pattern through the three coils. In reality, several Hall sensors are placed between the poles to determine the rotor position.
Based on the rotor position, the control unit determines which coils it must energise.

In the next image, the U+ coil is energised. The way the coil is wound around the pole determines whether it becomes a north or south pole. In this example, U+ is the north pole and U- is the south pole.

The rotor is implemented as a permanent magnet. As described in the previous paragraphs, the rotor positions or rotates as a result of a varying magnetic field generated by the coils.

To rotate the rotor to the left from the position shown in the previous image, the V coils are energised.

The V+ becomes the north pole, V- the south pole. The permanent magnet rotor rotates;
the north and south poles attract each other, as do the south and north poles on the other side of the magnet.

Now the W coils are energised to rotate the rotor a further 60 degrees.

The W+ coil becomes a north pole and W- the south pole. The rotor rotates and takes up its new position.

In the next image, the rotor has rotated 180 degrees since the first situation; in the first image the south pole was facing upwards; now it is the north pole.

The polarity of the U+ coil and the U- coil has been reversed, causing the current to flow through the coils in the opposite direction. This makes U+ a south pole and U- a north pole.

The permanent magnet rotor is further rotated by the change in the magnetic field.

To rotate the rotor another 60 degrees, V- is made a north pole and V+ a south pole. The rotor takes up the new position.

Once again the rotor rotates 60 degrees as a result of the change in the magnetic field in the coils:

The W- coil is the north pole and W+ the south pole.

In the six situations described above, two coils are constantly energized at the same time. We also often find brushless DC motors with three coils instead of six. With three coils, the U, V and W coils are also energized one after another, but there is no change in polarity.

The brushless DC motor is a powerful motor that is suitable for applications where a high torque is required for starting, medium speed and high speeds. The brushless DC motor and the stepper motor are often confused with each other. This is understandable, because the operation and control of the motors have many similarities: both motors are driven by creating a magnetic field between the coils and the rotor with permanent magnets. Nevertheless, apart from the terminology, the two motors have essential differences mainly in their application and, therefore, in material selection.

The stepper motor is in fact a brushless DC motor, but it is used in a different field. Whereas the DC motor is mainly used for continuous running at high speeds, we see the stepper motor in applications where adjustment to an exact position is the most important factor.

The control of the DC motor shown takes place every 60° rotation of the rotor. This could possibly be reduced to 30° if we energize four coils at the same time between each control action, thus creating an intermediate position. However, a stepper motor is capable of adjusting in steps of 1.8° down to as little as 0.9°. This further illustrates that the stepper motor is suitable for very precise positioning.

You can find the different versions, the control methods by the ECU and the applications on the stepper motor page.