und-field and permanent-
magnet varieties. The simple reason for this domination is that DC motors are easier to
control. This is especially true if the application requires good control of motor torque,
velocity, or position. The electromechanical model of a DC motor shows that motor
torque, within limits, is an approximately linear function of the input current. So, it is a
relatively easy task to derive solid performance out of a DC motor with proportional-
integral-derivative (PID) controllers.
In the real design world, the selection process for a type of motor to use in an
application can be complex. A particular motor cant be chosen based solely on how
easy it is to control. There are many other system-related variables to juggle, such as:

How easy is it to maintain the motor?
What happens to the system when the motor fails? (i.e. a shorted winding)
What will be the operating environment?
How will the motor be cooled?
What is the cost of the motor?

The list of considerations can go on and on.

AC induction motors (ACIM) have distinct advantages over other types of motors, and
have typically been used when a robust, fixed-speed solution is desired. The evolution of
microcontroller (MCU) and power electronic devices has made inexpensive variable-
speed control of an ACIM possible. However, the performance of a DC motor cannot be
matched using basic control methods. This article will explore the topic of field-oriented
control (FOC) and how it can be used to improve the control of an ACIM using a Digital
Signal Controller (DSC). FOC lets you use DC control techniques for an AC motor, and
can remove one of the variables in the motor-selection process for your next design.

How a Motor Works
An electric motor produces a mechanical force, when current flows in proximity to a
magnetic field. A synchronous motor has a source of magnetic field. This field can be
provided by permanent magnets or by windings that are energized with a source of
current. Within limits, the torque response of the motor is a linear function of the current
and the magnetic field strength. The linear response makes these motors easy to control
in high-performance applications. A PID controller can be used to control the motor
current and resulting motor torque. If needed, secondary PID controllers can be used to
control position or velocity.

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So, it looks like weve got the problem solved! Well just use a synchronous motor with
field windings or permanent magnets to get good control performance. Well, Wait a
minute, you might say. I need a high-power motor in my application. I could use a
motor with rotor and stator windings. But, Ill have to worry about replacing brushes and
keeping the rotor cool. I could use a brushless motor with permanent magnets, but the
cost of the magnets would make the cost of the motor too high.

The AC Induction Motor
An ACIM can really help out in this situation. The ACIM has windings on the outside of
the motor, which makes it easy to provide cooling. The rotor is a simple steel cage, so it
is durable and can withstand high temperatures. The ACIM has no brushes to wear out.
OK then so far, so good. Now, take a look at how the motor operates.

Since AC power is widely available, the ACIM is usually designed with a specific line
voltage and frequency in mind. For this discussion, lets take a look at the nameplate of a
typical ACIM. The parameters shown in our example nameplate are shown below:

Voltage: 230
VAC
Frequency: 60
Hz
FLA:
1.4A
HP:
1/3
RPM:
3450

Among other things, the nameplate specifies a rated power for the motor, operating
voltage, operating frequency and operating RPM. The stator windings of the motor are
arranged so that a rotating magnetic field is created when energized with AC currents.

The rotor of an ACIM must turn at a lower speed than the rotating field. The difference
between the field speed and the rotor speed is called slip. Slip can be expressed as a ratio
or a frequency, but it is helpful to consider the slip frequency. For this example motor,
the rotating field speed would be 60 rev/s, or 3,600 RPM. But, youll notice that the
nameplate RPM under load is only 3,450 RPM, or 57.5 rev/s. So the slip frequency is 60
Hz 57.5 Hz, or 2.5 Hz.

In this example, you can consider the 2.5 Hz slip frequency as a source of AC power that
supplies energy to the rotor via transformer coupling. The rotor becomes energized with
AC currents that produce a rotor magnetic field, allowing the motor to produce torque.
The ACIM slip gives the motor the ability to self-regulate its own speed, to a certain
extent. As the motor load is increased, the rotor speed will decrease. The slip frequency
will then increase, which increases the rotor currents and the motor torque.

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Variable-Speed ACIM Control
An ACIM can be operated at different speeds and torque levels by varying the frequency
and voltage supplied to the motor. Lets suppose that you want to operate our example
motor at ½ the rated speed. To accomplish this, you would reduce the frequency input to
the motor by a factor of 1/2, or 30 Hz. If we wanted to operate the motor at ¼ speed,
then the frequency would be reduced to 15 Hz.

Youll also want to keep the stator field relatively constant by keeping the stator currents
constant. The ACIM motor is inductive and the stator currents will increase as the input
frequency is decreased. Therefore, you also need to reduce the input voltage by a
proportionate amount when the frequency is decreased. A constant V/Hz profile is often
used to provide variable-speed operation of an ACIM. The V/Hz constant for our
example motor can be calculated by dividing the operating frequency into the operating
voltage.

K = V/Hz = 230/60 = 3.83

Now, for a given choice of input frequency we can compute the desired drive voltage for
that input frequency:

Voltage = K* Frequency

The result is called the Volts-Hertz profile and can be plotted as shown in Figure 1.
There is no fixed rule that says the drive voltage has to maintain a fixed linear
relationship to frequency. In fact, the shape of the V/Hz profile is often altered in
specific frequency ranges to optimize the drive performance in a particular speed range.
For example, the shape of the profile shown in Figure 1 has been adjusted to provide
higher voltages in the low frequency range. This modification provides a boost to the
motor torque when the motor starts from rest to help overcome load friction and inertia.
Within the mechanical limits of the motor, you can also increase the drive frequency
beyond the nameplate value to achieve a higher speed. However, the available voltage
may be limited, so motor torque will also be lower.

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Figure 1: Typical V/Hz Profile for a Variable Speed ACIM Application

40
80
120
160
200
240
20
40
60
Boost
Region
Constant V/F
Region
Frequency (Hz)
Vo
lt
s RMS
High Amplitude
Limit
Low Frequency
Cutoff


For applications that do not require frequent speed or load variations, the V/Hz method
for controlling an ACIM works well. This is especially true when control loops are used
to regulate speed or motor current. A typical system block diagram that you can use for a
V/Hz application is shown in Figure 2. The MCU has a specialized PWM peripheral to
drive a 6-transistor inverter circuit. The MCU measures the frequency of the motor
tachometer, calculates the speed error, and generates a drive demand using a PID control
loop. The drive demand is translated into a required voltage and frequency using the
V/Hz profile. Finally, the PWM modulation code varies the duty cycle over time to
generate sinusoidal drive signals with the proper amplitude and frequency.

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Figure 2: Typical System Block Diagram Used for Variable Speed ACIM Control



For applications that require fast dynamic response, the V/Hz control method will give
sluggish response. Furthermore, the motor currents will be very high during load or
speed changes. The sluggish response occurs because the components of stator current
that control motor torque and the rotor field cannot be separated. A change in drive
voltage or frequency will cause a change in both torque and rotor currents.

Ideally, we would like to use an algorithm that lets us control motor torque independently
of other motor variables. The FOC algorithm lets us accomplish this goal. FOC controls
the voltage, frequency and instantaneous phase of the motor voltage to produce the
desired stator currents. (The V/Hz control method does not control the phase.) FOC will
obtain the best motor efficiency and dynamic response for a given application.

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FOC - A Matter of Perspective
If the motor is observed electrically from the perspective of the input terminals, all
signals inside the motor will appear sinusoidal. Sinusoidal signals can be difficult to
process in software, especially if we want to use PID controllers to regulate motor
currents. If we change the point of reference used in our calculations, then the signals
inside the AC motor can be made to look mathematically like DC values under steady
state conditions.

Specifically, FOC measures the AC motor currents. In a stationary refer