Choosing a speed controller is an important part of building an electric motor drive application and greatly impacts the performance, cost, efficiency, and longevity of the project. While there are many different types of motors, speed controls can be broadly categorised as AC and DC, which operate on different fundamental principles.
AC Motor Speed Control
Because the speed of an AC motor is effectively determined by the frequency of the AC power supply, speed control is achieved by modifying this frequency. A device that does this is known as a Variable Frequency Drive or VFD. VFDs first convert the AC power supply to DC using a rectifier, and then back to AC at the desired frequency, using an inverter.
There are two main types of VFDs. The most common is referred to as a ‘V/Hz’ drive and operates by maintaining a consistent voltage to frequency (V/Hz) ratio to ensure consistent torque across a wide speed range. This type of controller provides good speed control above 5 Hz. Below this speed, because of the relationship between voltage, frequency and torque changes, simply keeping the V/Hz ratio consistent is not enough to control the motor, which typically begins to overheat due to inefficient conversion of power to torque. Therefore, this type of control is great for medium to high-speed applications requiring a narrow band of speed adjustment and is not sufficient for very low speed or no speed (holding torque) applications such as servo motors.
The second type of VFD is known as a ‘vector drive’ and enables control of the speed and torque of an AC motor even at very low speeds. It achieves this by separately controlling two different types of current within the motor, the magnetizing current and the torque-producing current. Using a complex algorithm, vector drives manipulate these currents to maintain optimum conversion of power to torque at very low speeds and reduce heat losses at low speeds. This means that vector drives typically provide higher starting torque and precise control across the full speed range, and are ideal for low speed, reversing and holding torque applications.
DC Motor Speed Control
Controlling the speed of a DC motor is achieved simply by controlling the voltage of the supply power (within the safe operating range for the motor) using a potentiometer, and DC motors maintain consistent torque across the entire speed range without the need for additional components. This makes controlling their speed considerably easier compared to AC motors, and they are well suited to applications requiring precise control at any speed.
However, there are further considerations depending on the requirements of the speed controller. DC controllers operating on AC power require conversion of the supply using a rectifier. Unlike AC motors, braking or reversing a DC motor requires additional components, typically a power resistor for braking, and a relay for switching the polarity of the supply power to reverse the motor. It is also necessary to ensure that the motor has stopped before reversing the polarity of the supply, which requires a means of sensing when the motor is at a standstill. This can add up to a significant additional cost, especially for larger applications.
Traditionally, for applications requiring a high level of speed control, choosing a DC drive was the only real option. Today, however, technological advancements have enabled AC drives to catch up in terms of capability, and modern AC vector drives can provide the range and precision of speed control required in even the most exacting applications, such as servo motors. In some cases, AC drives even provide an advantage, especially when frequent braking and reversing is required.
Generally, AC speed controllers are more expensive than DC controllers due to their greater complexity. However, because AC motors are typically cheaper, the cost of the controller/motor combination may be less than an equivalent DC drive, especially for applications over 2 HP horsepower. The cost of AC speed controllers is also declining, as rising demand drives improved manufacturing techniques and technological innovation. It is therefore important to compare cost in terms of the full scope of the application over its entire life cycle.
Because AC speed controllers are more complex, they typically require some amount of configuration and tuning during installation, whereas DC drives are relatively simple to connect and use. However, this enables them to offer a wider range of programmable failsafe protections, and modern software is improving the ease of installation of AC drives, such as enabling the transfer of configuration data between units to make replacement faster and easier. For automated control systems applications, AC speed controllers may be the better choice, because they often come with the hardware and software capabilities required for integration into a monitoring and control network.
For high precision speed control, both AC and DC applications require a speed sensor such as a tachometer or encoder to operate in a closed-loop configuration. This enables them to achieve extremely precise control in varying torque applications.
AC and DC motor speed controllers operate on different design principles, each with its own advantages and disadvantages. When selecting a motor speed controller, it is important to consider all the requirements of your application to make the best choice. eMotors Direct provides a wide range of electric motors and speed control solutions to suit every project, as well as online tools to help you select exactly the motor/drive combination you need.
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