How to turn an AC induction motor into a DC motor

“The field of high-performance motor control has been dominated by synchronous DC motors. This group of motors includes brushed, brushless, magnetic and permanent magnet varieties. The simple reason for the dominance is that DC motors are easier to control. This is especially true if the application requires good control of motor torque, speed or position.

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This document was written by Steve Bowling, an application segment engineer at Microchip Technology, and discusses the working principles of electric motors and describes AC induction motors, variable speed ACIM control, and FOC. This document also explains how to turn an AC induction motor into a DC motor.

The field of high-performance motor control has been dominated by synchronous DC motors. This group of motors includes brushed, brushless, magnetic and permanent magnet varieties. The simple reason for the dominance is that DC motors are easier to control. This is especially true if the application requires good control of motor torque, speed or position. The electromechanical model of the DC motor shows that the motor torque is an approximate linear function of the input current within the limit range. Therefore, it is a relatively easy task to use a proportional integral derivative (PID) controller to obtain reliable performance from a DC motor.

In the “real design world”, the process of selecting a motor to be used in an application can be complicated. It is not possible to select a specific motor based solely on the ease of control. There are many other system-related variables that need to be dealt with, such as:

How easy is it to maintain the motor?

What will the system do when the motor fails? (Ie short-circuit winding)

What is the working environment?

How will the motor cool down?

How much does the motor cost?

The list of considerations can continue to…

AC induction motors (ACIM) have obvious advantages over other types of motors, and are usually used when a robust fixed-speed solution is required. The development of microcontrollers (MCUs) and power electronics has made ACIM’s inexpensive variable speed control possible. However, the basic control method cannot match the performance of the DC motor. This article will explore the topic of Field Oriented Control (FOC) and how it can be used to improve the control of ACIM using a Digital Signal Controller (DSC). FOC allows you to use DC control technology for AC motors and can delete one of the variables for the next design during the motor selection process.

How the motor works

When current flows through the magnetic field, the motor generates mechanical force. The synchronous motor has a magnetic field source. The magnetic field can be provided by permanent magnets or windings powered by a current source. Within the limits, the torque response of the motor is a linear function of current and magnetic field strength. The linear response makes these motors easy to control in high-performance applications. The PID controller can be used to control the motor current and the generated motor torque. If necessary, an auxiliary PID controller can be used to control position or speed.

So it seems that we have solved the problem! We will only use synchronous motors with field windings or permanent magnets to obtain good control performance. Well, “wait a minute,” you might say. “In my application, I need a high-power motor. I can use a motor with a rotor and stator windings. However, I will have to worry about replacing the brushes and keeping the rotor cool. I can use a motor with permanent magnets. Brush the motor, but the cost of the magnet will make the cost of the motor too high.”

AC induction motor

In this case, ACIM can really help. ACIM has windings on the outside of the motor, so it is easy to provide cooling. The rotor is a simple steel cage, so it is durable and can withstand high temperatures. ACIM will wear out without brushes. Well, so far, everything is fine. Now, let’s look at how the motor works.

Because AC power is widely available, the specific line voltage and frequency are usually considered when designing an ACIM. In this discussion, let’s take a look at the nameplate of a typical ACIM. The parameters shown on our example nameplate are as follows:

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

Among them, the nameplate specifies the rated power, working voltage, working frequency and working RPM of the motor. The stator windings of the motor are arranged in such a way that when energized with alternating current, a rotating magnetic field is generated.

The rotor of ACIM must rotate at a lower speed than the rotating magnetic field. The difference between the excitation speed and the rotor speed is called slip. Slip can be expressed as a ratio or frequency, but it is helpful to consider slip frequency. For this example motor, the rotating magnetic field speed will be 60 revolutions per second or 3,600 RPM. However, you will notice that the nameplate RPM under load is only 3,450 RPM, which is 57.5 revolutions per second. Therefore, the slip frequency is 60 Hz C 57.5 Hz or 2.5 Hz.

In this example, you can think of the 2.5 Hz slip frequency as an AC power source that provides energy to the rotor through transformer coupling. Alternating current energizes the rotor, thereby generating the rotor magnetic field, so that the motor generates torque. ACIM slip allows the motor to self-adjust its speed to a certain extent. As the motor load increases, the rotor speed will decrease. Then, the slip frequency will increase, which will increase the rotor current and motor torque.

Variable speed ACIM control

By changing the frequency and voltage supplied to the motor, ACIM can operate at different speeds and torque levels. Suppose you want to run the example motor at 1/2 rated speed. To do this, you can reduce the frequency input to the motor by 1/2 or 30 Hz. If we want to run the motor at 1/4 speed, the frequency will be reduced to 15 Hz.

You also want to keep the stator magnetic field relatively constant by keeping the stator current constant. ACIM motors are inductive, and the stator current will increase as the input frequency decreases. Therefore, when the frequency is reduced, the input voltage needs to be reduced proportionally. A constant V/Hz curve is usually used to provide ACIM variable speed operation. The V/Hz constant of our example motor can be calculated by dividing the operating frequency by the operating voltage.

K = V / Hz = 230/60 = 3.83

Now, for a given input frequency selection, we can calculate the required drive voltage for that input frequency:

Voltage = K * Frequency

Figure 1 Typical V/Hz curve for variable speed ACIM applications

The result is called the “Volt-Hertz” curve and can be plotted as shown in Figure 1. There is no fixed rule that the drive voltage must maintain a fixed linear relationship with the frequency. In fact, the shape of the V/Hz curve is usually changed in a specific frequency range to optimize the drive performance in a specific speed range. For example, the contour shape shown in Figure 1 has been adjusted to provide a higher voltage in the low frequency range. When the motor is started from a standstill, this modification can increase the motor torque to help overcome load friction and inertia. Within the mechanical limit of the motor, the drive frequency can also be increased beyond the nameplate value to achieve higher speeds. However, the available voltage may be limited, so the motor torque will also be reduced.

Figure 2 A typical system block diagram for variable-speed ACIM control

For applications that do not require frequent changes in speed or load, the V/Hz method used to control ACIM works well. This is especially true when using a control loop to adjust speed or motor current. Figure 2 shows a typical system block diagram that can be used in V/Hz applications. The MCU has a dedicated PWM peripheral to drive the 6-transistor inverter circuit. The MCU measures the frequency of the motor tachometer, calculates the speed error, and uses the PID control loop to generate drive demand. The V/Hz profile is used to convert the driver requirements into the required voltage and frequency, and the PWM modulation code changes the duty cycle over time to generate a sinusoidal drive signal with appropriate amplitude and frequency.