To answer these questions we’ll look at the main motor types: AC Induction, Permanent Magnet DC, Brushless DC and Universal motors to discuss why when an ideal speed verse torque curve is plotted does the plot change from one motor to the next.

(Note: All these motor types naturally experience drop in speed as torque is added to the shaft, however this is not the speed being referred to.) 

More speed variation can be expected in some motor types; typically the two types with the most variation are universal and PMDC brushed motors. Conversely, brushless PMDC and AC motors usually have less variation.

Brushless DC Motors

The brushless motor has less variation because it is used with a control system.  A control is needed for commutation in a brushless motor and can be used to control speed.  Actually, depending the control type, speed can be maintained through specific ranges in torque very accurately.

AC Induction motors

Less variation in the AC induction motors occurs due to synchronous speed; basically this means that the speed of the motor will synchronize to the frequency of the AC current wave.  However, in induction motors there is slip so the rotor speed and AC wave frequency will not be exactly synchronous, but the drop in speed is more predictable than brushed and UM motors.

Brushed Permanent Magnet DC and Universal Motors

Brushed PMDC and Universal motors have the most speed variation.  This is primarily because they do not necessarily need to be run with a speed control.  The electric circuit contains a number of variables that contribute to speed variation.  Those are winding resistance, magnetic strength, brush grade variation, brush friction, electrical connecting from brush to commutator and air gap to name the largest contributors.

In general if precision speed is needed then controls need to be used to regulate speed through feedback from some sort to speed signal on the motor, i.e encoders, hall effect sensors or tachometers.  If speed is critical in your application then work with your motor supplier to implement the technology and equipment necessary regulate speed.