Do AC motors overcome DC series motors?

AC induction motors typically have low starting torque. They reach their maximum torque at just short of synchronous speed. While it is possible through varying the rotor design to produce higher starting torques, an line fed induction motor torque-speed curve will never look like that of a series wound DC motor. this article has a nice description of induction motor torque-speed characteristics for different rotor designs.

In contrast, DC series motors attain their maximum torque at zero speed. Their torque falls off quickly as speed increases.

To get a similar torque characteristic from an AC induction motor, you would need to use a slip-control or vector-control adjustable speed drive. These drives essentially keep the input frequency of the motor set so that the motor is always running in its optimal torque region, just below synchronous speed.

Hope this helps.

Steve

Let's not forget PMSM (brushless dc) motors. High torque, great efficiency, but high cost and sometimes difficult to control.

Can a comparitive torque speed curve be creatred for AC and DC series motors.

I have been asked to do a comparisons on tractive effort of a DC serries vs a AC vfd (VVVF) conrolled induction mortor.

High torques at low speed are usually not a problem as long as the proper motor is selected. Motor data (design) curves will let you know if the selected AC motor can produce the required torque for your application's speed range.
An AC induction wound motor, connected to a 'true' vector control drive, can replace DC motors in most applications.
High torque at 'zero' speed is slightly more difficult to regulate (with AC), paticulary under quickly changing load requirements.
Also note that the AC motor may not have the same horsepower(HP) rating as the DC motor, thus choosing based on HP alone is usually a mistake.