There are VFD and motor combinations which will produce the correct amount of output torque for most of today’s applications, and they do provide energy cost savings, but they are not the answer to every application and do have some drawbacks. The DC drive also has a good output torque at a cost-effective ratio but is shunned because of the brushes and higher maintenance. Soft starters are also an option for applications where full speed of the motor is required constantly, but there is a poor energy saving based on consumed energy between full load and no-load conditions.
The VFD (Variable Frequency Drive) Industry has been erroneously leading people to believe that there is no thought necessary for the application and installation of a VFD. Anyone who has argued with this has been accused of simply not having a good VFD. This is not the case, as can be read in this US Department of Energy "Minimize Adverse Motor & VFD Interactions".
DC drives and motors generate considerably less heat than their AC counterparts.
In today’s AC Drive world we see a lot of companies wanting to convert their DC Drives over to AC Drives and motors. While this can be done, please keep in mind that a in a DC Drive, 7% of the total power that passes through the Drive is converted into heat, and that in an AC Drive, that percentage increases to 25% heat conversion. Since heat is the main killer of drives and motors in the plastics extrusion industry, it is usually much better to use DC drives and motors.
Through the independent control of both components of the stator current (the magnetizing current and the torque-producing current), an AC vector drive can control torque, as well as speed. AC motor-drive system are also free of the maintenance issues that have historically plagued DC motors
Complexity
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.
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.
The main advantage of AC motor over DC motor is that the speed - torque characteristic of AC motor is very close to the ideal characteristic. Torque of AC motor is constant up to a certain speed and then decreases, but in case of DC motor torque linearly decreases as speed increases.
AC or DC Drive?
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.
For further information see:
US Department of Energy "When Should Inverter Duty Motors be Specified?"