Why would motor be inefficient as generator?

A colleague chose a motor based on its efficiency (90% efficiency specified by vendor). We are running it as a generator in a test, and the measured efficiency is much lower (70% efficiency). Is that unexpected? My EE colleagues (I'm a lowly programmer) say that means there is a measurement error since the motoring and generating efficiency should be much closer, but I want to make sure: Could the same device to be an efficient motor but an inefficient generator? If so, why? Perhaps control or windings optimized for motoring would not be optimal for generating?

Hi,

Motor and Generator efficiency are two different things. The efficiency varies upon the application it is manufactured. Although theoretically, we can say that they can work vice versa. Mostly, generators create energy but are wasted as heat causing less efficiency. Motor efficiency on the other hand are affected by the power it consumes to do the work.

Energy losses in generator is more than motor..

What processes or characteristics cause there to be greater energy loss in a generator than a motor, if the same device is used as a motor and a generator?

The high efficiency of the transfer of energy from a generator to a motor finds widespread application in the operation of diesel-electric locomotives. It is difficult to mechanically transfer energy from the powerful diesel engines to the wheels of the locomotive which have low-friction metal-to-metal contact with the steel rails. So the power of the diesel engines is used to run a generator, and the generated electric power is used to operate electric traction motors to drive the wheels. The generated electric power can be very slowly and smoothly transferred to the wheels by the electric motors which drive the axles. You don't "pop the clutch" on a big locomotive with steel-on-steel wheels! You would just sit and spin the wheels until you melted the rails.