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15.2 ELECTRIC MOTORS
An electric motor is composed of a rotating center, called the rotor, and a stationary outside, called the stator. These motors use the attraction and repulsion of magnetic fields to induce forces, and hence motion. Typical electric motors use at least one electromagnetic coil, and sometimes permanent magnets to set up opposing fields. When a voltage is applied to these coils the result is a torque and rotation of an output shaft. There are a variety of motor configuration the yields motors suitable for different applications. Most notably, as the voltages supplied to the motors will vary the speeds and torques that they will provide.
AC motors - rotate with relatively constant speeds proportional to the frequency of the supply power
Contactors are used to switch motor power on/off
Drives can be used to vary motor speeds electrically. This can also be done with mechanical or hydraulic machines.
Variable Frequency Drives (VFD) - vary the frequency of the power delivered to the motor to vary speed.
Wound rotor AC motor controllers - low efficiency, uses variable resistors to adjust the winding currents
A control system is required when a motor is used for an application that requires continuous position or velocity. A typical controller is shown in Figure 15.1. In any controlled system a command generator is required to specify a desired position. The controller will compare the feedback from the encoder to the desired position or velocity to determine the system error. The controller will then generate an output, based on the system error. The output is then passed through a power amplifier, which in turn drives the motor. The encoder is connected directly to the motor shaft to provide feedback of position.
15.2.1 Basic Brushed DC Motors
In a DC motor there is normally a set of coils on the rotor that turn inside a stator populated with permanent magnets. Figure 15.2 shows a simplified model of a motor. The magnets provide a permanent magnetic field for the rotor to push against. When current is run through the wire loop it creates a magnetic field.
The power is delivered to the rotor using a commutator and brushes, as shown in Figure 15.3. In the figure the power is supplied to the rotor through graphite brushes rubbing against the commutator. The commutator is split so that every half revolution the polarity of the voltage on the rotor, and the induced magnetic field reverses to push against the permanent magnets.
The direction of rotation will be determined by the polarity of the applied voltage, and the speed is proportional to the voltage. A feedback controller is used with these motors to provide motor positioning and velocity control.
These motors are losing popularity to brushless motors. The brushes are subject to wear, which increases maintenance costs. In addition, the use of brushes increases resistance, and lowers the motors efficiency.
15.2.2 AC Motors
Power is normally generated as 3-phase AC, so using this increases the efficiency of electrical drives.
In AC motors the AC current is used to create changing fields in the motor.
Typically AC motors have windings on the stator with multiple poles. Each pole is a pair of windings. As the AC current reverses, the magnetic field in the rotor appears to rotate.
The number of windings (poles) can be an integer multiple of the number of phases of power. More poles results in a lower rotational speed of the motor.
Rotor types for induction motors are listed below. Their function is to intersect changing magnetic fields from the stator. The changing field induces currents in the rotor. These currents in turn set up magnetic fields that oppose fields from the stator, generating a torque.
Induction motors require slip. If the motor turns at the precise speed of the stator field, it will not see a changing magnetic field. The result would be a collapse of the rotor magnetic field. As a result an induction motor always turns slightly slower than the stator field. The difference is called the slip. This is typically a few percent. As the motor is loaded the slip will increase until the motor stalls.
An induction motor has the windings on the stator. The rotor is normally a squirrel cage design. The squirrel cage is a cast aluminum core that when exposed to a changing magnetic field will set up an opposing field. When an AC voltage is applied to the stator coils an AC magnetic field is created, the squirrel cage sets up an opposing magnetic field and the resulting torque causes the motor to turn.
The motor will turn at a frequency close to that of the applied voltage, but there is always some slip. It is possible to control the speed of the motor by controlling the frequency of the AC voltage. Synchronous motor drives control the speed of the motors by synthesizing a variable frequency AC waveform, as shown in Figure 15.9.
These drives should be used for applications that only require a single rotational direction. The torque speed curve for a typical induction motor is shown in Figure 15.10. When the motor is used with a fixed frequency AC source the synchronous speed of the motor will be the frequency of AC voltage divided by the number of poles in the motor. The motor actually has the maximum torque below the synchronous speed. For example a 2 pole motor might have a synchronous speed of (2*60*60/2) 3600 RPM, but be rated for 3520 RPM. When a feedback controller is used the issue of slip becomes insignificant.
Wound rotor induction motors use external resistors. varying the resistance allows the motors torque speed curve to vary. As the resistance value is increased the motor torque speed curve shifts from the Class A to Class D shapes.
The figure below shows the relationship between the motor speed and applied power, slip, and number of poles. An ideal motor with no load would have a slip of 0%.
Single phase AC motors can run in either direction. To compensate for this a shading pole is used on the stator windings. It basically acts as an inductor to one side of the field which slows the field buildup and collapse. The result is that the field strength seems to naturally rotate.
Thermal protection is normally used in motors to prevent overheating.
Universal motors were presented earlier for DC applications, but they can also be used for AC power sources. This is because the field polarity in the rotor and stator both reverse as the AC current reverses.
Synchronous motors are different from induction motors in that they are designed to rotate at the frequency of the fields, in other words there is no slip.
Synchronous motors use generated fields in the rotor to oppose the stators field.
Starting AC motors can be hard because of the low torque at low speeds. To deal with this a switching arrangement is often used. At low speeds other coils or capacitors are connected into the circuits. At higher speeds centrifugal switches disconnect these and the motor behavior switches.
Single phase induction motors are typically used for loads under 1HP. Various types (based upon their starting and running modes) are,
- split phase - there are two windings on the motor. A starting winding is used to provide torque at lower speeds.
- shaded pole - these motors use a small offset coil (such as a single copper winding) to encourage the field buildup to occur asymmetrically. These motors are for low torque applications much less than 1HP.
15.2.3 Brushless DC Motors
Brushless motors use a permanent magnet on the rotor, and use windings on the stator. Therefore there is no need to use brushes and a commutator to switch the polarity of the voltage on the coil. The lack of brushes means that these motors require less maintenance than the brushed DC motors.
A typical Brushless DC motor could have three poles, each corresponding to one power input, as shown in Figure 15.14. Each of coils is separately controlled. The coils are switched on to attract or repel the permanent magnet rotor.
To continuously rotate these motors the current in the stator coils must alternate continuously. If the power supplied to the coils was a 3-phase AC sinusoidal waveform, the motor will rotate continuously. The applied voltage can also be trapezoidal, which will give a similar effect. The changing waveforms are controller using position feedback from the motor to select switching times. The speed of the motor is proportional to the frequency of the signal.
A typical torque speed curve for a brushless motor is shown in Figure 15.15.
15.2.4 Stepper Motors
Stepper motors are designed for positioning. They move one step at a time with a typical step size of 1.8 degrees giving 200 steps per revolution. Other motors are designed for step sizes of 1.8, 2.0, 2.5, 5, 15 and 30 degrees.
There are two basic types of stepper motors, unipolar and bipolar, as shown in Figure 15.16. The unipolar uses center tapped windings and can use a single power supply. The bipolar motor is simpler but requires a positive and negative supply and more complex switching circuitry.
The motors are turned by applying different voltages at the motor terminals. The voltage change patterns for a unipolar motor are shown in Figure 15.17. For example, when the motor is turned on we might apply the voltages as shown in line 1. To rotate the motor we would then output the voltages on line 2, then 3, then 4, then 1, etc. Reversing the sequence causes the motor to turn in the opposite direction. The dynamics of the motor and load limit the maximum speed of switching, this is normally a few thousand steps per second. When not turning the output voltages are held to keep the motor in position.
Stepper motors do not require feedback except when used in high reliability applications and when the dynamic conditions could lead to slip. A stepper motor slips when the holding torque is overcome, or it is accelerated too fast. When the motor slips it will move a number of degrees from the current position. The slip cannot be detected without position feedback.
Stepper motors are relatively weak compared to other motor types. The torque speed curve for the motors is shown in Figure 15.18. In addition they have different static and dynamic holding torques. These motors are also prone to resonant conditions because of the stepped motion control.
The motors are used with controllers that perform many of the basic control functions. At the minimum a translator controller will take care of switching the coil voltages. A more sophisticated indexing controller will accept motion parameters, such as distance, and convert them to individual steps. Other types of controllers also provide finer step resolutions with a process known as microstepping. This effectively divides the logical steps described in Figure 15.17 and converts them to sinusoidal steps.
15.2.5 Wound Field Motors
Uses DC power on the rotor and stator to generate the magnetic field (i.e., no permanent magnets)
- when the load on these motors is reduced the current flow increases slightly, increasing the field, and slowing the motor.
- these motors have a relatively small variation in speed as they are varied, and are considered to have a relatively constant speed.
- the speed of the motor can be controlled by changing the supply voltage, or by putting a rheostat/resistor in series with the stator windings.
- as the motor speed increases the current increases, the motor can theoretically accelerate to infinite speeds if unloaded. This makes the dangerous when used in applications where they are potentially unloaded.
- differential compound motors have the shunt and series winding field aligned so that they oppose each other.
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