DC drives are known for their ability to provide tight speed control and full torque at any speed, whereas traditional AC drives had a more narrow speed range and limited torque control. But newer AC drives using vector control, also known as field oriented control (FOC), have performance similar to DC motor and drive systems.

Through the independent control of both components of the stator current (the magnetizing current and the torque-producing current), an AC vector drive is able to control torque as well as speed. AC motor-drive systems are also free of the maintenance issues that have historically plagued DC motors—namely, brush wear. So, with the advantages gained by using an AC vector drive, when does it make sense to choose a DC motor and drive system over an AC vector drive system?

Standard AC drives, also referred to as variable frequency drives (VFDs), control motor speed by varying the frequency of the electrical supply to the motor. AC vector drives use a complex algorithm to control the torque-producing and magnetizing components of the stator current independently. This allows better speed control over the entire speed range, as well as better torque control, especially at low speeds. 

Simplicity

First, there’s the issue of availability. Not of the components themselves, but of the power required for the drive. VFDs typically use 3-phase AC supply voltage, which isn’t always available. If the application only has access to single-phase power, then a DC drive is a better option, since the VFD would have to be de-rated in order to ensure that its components could handle the higher current associated with the single-phase input.

Also, an AC vector drive requires an encoder or feedback device in order to operate in true closed-loop mode, which adds cost and complexity to the system. A DC drive, on the other hand, can operate via internal armature feedback, foregoing the need for an external encoder.

The need for commissioning and tuning, according to the motor parameters and application, are additional examples of the complexity of AC vector drives. Conversely, DC drives are simple to start up, troubleshoot and maintain. Even DC motor brushes have become more robust and are less likely to require maintenance or replacement than they once were.

In general, when faced with a choice between two solutions that both meet the application requirements, the more desirable option will be the one that is less complex, which, in this case, is the DC motor-drive system.

Startup torque

Traditional AC motors cannot produce torque at low or zero speed due to slip, which is the difference between the speed of the rotating magnetic field and the speed of the rotor. Slip is essentially energy loss, which is converted to heat that can damage motor and cable insulation. Because of this heat, the motor cannot produce full torque at low (or zero) speed in continuous operation. However, closed-loop vector control of VFDs solves this problem, by allowing the controller to adjust the torque through control of the flux (magnetizing) current. This enables the drive to provide good torque control regardless of speed, including down to zero speed.

So why wouldn’t AC vector drives be preferred over DC motor-drive systems for applications that require high startup torque or holding torque? The primary reason is simple: cost. Vector drives are complex, and thus, more expensive than DC drives. And for true, closed-loop operation of an AC vector drive, the need for an additional encoder further drives up the cost.

With simple startup, good torque and speed regulation, and an overall lower cost, DC motor-drive combinations are, in many cases, the preferred choice for applications requiring high startup torque or zero- or low-speed holding torque.

Feature image credit: Sprint Electric Limited