Generate measurable reverse torque with DC motor as a dynamometer
If you want to measure torque there are two options.


Mount the motor housing between two pedestals supported by trunnion bearings. Attach a torque arm to the housing and measure the force between the torque arm and the base with a platform scales or a force transducer.





Couple the engine to the motor through a shaft torque transducer.


The motor armature current is proportional to torque. You can use that as an indication of torque, but you need to have accurate torque vs. current data for the motor for accurate results. You need data for the full range of operating speeds.

Regarding the comment:


  What happen to the current through the current sensor as I vary the control signal to the drive?


As a dynamometer control, the drive must act as a brake. It must provide torque in the direction that opposes the rotation of the engine. To do that the motor must generate current. The controller must limit the generated current to the value that satisfies the torque command. The direction of the current will be the reverse of the direction that would cause the motor to drive the engine to a higher speed. There needs to be some method to prevent the motor from actually driving the engine in the reverse direction since that is essentially what the motor is trying to do. Essentially, the motor controller must have a speed regulator with a zero speed reference that is overridden by a current regulator that has a reference that corresponds to the desired torque.