I'm not sure I undestand the requirements exactly.
Does the speed have to be exactly constant or a few percent variation acceptable (when load varies).  Is there a specific speed that you need to drive the load at, or can you choose among approx 3600, 1800, 1200,900 etc.

I assume when you say constant torque you mean that the load torque doesn't change with speed (is that right?).

How many horsepower?

With respect to recommending what Design motor to use for your load, I would assume that a constant torque, constant speed application would be one that:

- load requires constant torque regardless of speed with horsepower requirement of course proportional to torque and speed.
- load operates at constant speed meaning that the normal operating speed will be nameplate or full frequency speed of the motor, ie. not operated on a VF drive.  

Unfortunately, the most important piece of information needed to determine the best Design class of motor for this appication is missing. The inertia of the load is what will determine how much power is needed to start it moving and to accelerate it to (any) speed.

Assuming that you are starting the motor across the line, the speed-torque curve of the motor will be critical for determining how (if?) the load will start and accelerate up to (near) synchronous speed. For 90% of the applications that fall under your description a Design B motor will be suitable. However, the other 10% are characterized as high inertial loads such as crushers, ball mills, bucket elevators, etc. For these loads, a Design C or D motor may be required to provide sufficient starting toque for the load.

Please provide more info on the type of load you are interested in and I am sure that someone can help you with the right info. (I won't be back for a week or so...)

Suggestion to the original posting:
dhritiman (Electrical) Jul 5, 2001
what is the best motor AC
///Does AC stand for Alternating Current or have you meant a motor AC drive?\\\
 for a constant speed constant torque application?

the torque requirement is very unimportant and very low 5-10Nm as compared to the speed which is 50,000rpm.
starting torque should not be more than 110%
i am so late in replying because of internet connection problem.
sorry folks

the load is fan type, space is 1.5m/0.4m/0.3m,
available source is a 798cc IC engine.
What is the better motor-generator set than the alternator & compound dc motor that can be used allowing for smooth speed control by the use of a stick lever?

Suggestion: Nowadays, variable speed drives based on variable frequency principle tend to replace dc motors since the dc motors have problems with commutators. However, the ac motor for the high frequency, produced by the variable frequency drive, has to be designed for the high frequency or rpm.