As you might be aware, Speed control of DC motors is easy as compared to AC motors.
Speed control of D C motors has two parts. 1) Constant torque control 2) Constant HP control
In constant torque control, the armature voltage is varied from 0 to rated voltage to get speed from 0 to base speed. In this region of operation the torque remains constant but the hp varies.
In constant hp control, the field current of Shunt winding is decreased to get speeds above base speed. The max. speed that can be reached is limited from sparking at brushes and safe mech. speed of armature.
Speed control of AC motors as you might know is not so easy for which you need adjustable speed drive. The cost of the ASD is very high. Also, Induction motor working on ASD needs to be designed specially(insulation & cooling system) to take care of voltage surges and most of the cases the machines are derated.
The motor required does not need to be high speed. It is basically a unwinder for a roll about 250kg, about 600mm diameter. I need to maintain constant tensional force of the material. It is pulled by a machine, which stops and shears the material. My proposed design is to control the speed of the material via a variable voltage from an axial potentiometer on the pivot of a dancer roller. What is the best and most cost-effective implementation for this?
Best is to use the DC motor with DC drive. Generally for unwinder application the motors with speed ratio of 1: 3 is used. At the start of the unwind, the macine operates at base speed, giving max. torque(you have to calculate) and as the dia. of the roll reduces, the machine operates in field control mode(torque reduces and speed increases)reaching its max. field weakening speed.
For more details, you can consult any DC motor/drive manufacturer.
and type Motors: AC under Product / Services which will return 286 companies to inquire about your AC motor needs
and type Motors: DC under Product / Services which will return 361 companies to inquire about your DC motor needs
