The spare is stalling. DC motors can saturate at the poles due to armature reaction and different designs, shunt, stabilized shunt etc can react differently as discussed by DickDV. He is correct in that it is acting like a compound wound differentially connected series field opposing the main...
The motor requires constant volts/hz to maintain the stated torque at any given frequency. When the voltage stops rising, the flux in the rotor drops off as frequency increases. The motor load capacity is based on the product of the flux in the rotor and the flux in the stator. Stator flux is...
