I want to trigger on the primary frequency (perhaps 60 Hz)+ this frequency is modulated with the motor frequency (perhaps 600 Hz) with an amplitude of perhaps a third of the primary frequency's amplitude. The scope will not adequately trigger on the motor current signal.

This signal creates Multiple zero crossings which makes the triggering unstable. Turn down the amplitude of the composite signal (ch1) so that the scope's built in hysteresis will swamp out the motor drive zero crossings. This channel will be used to trigger the scope, a second channel can then be used to view the composite waveform with greater sensitivity. The proceedure, Reduce amplitude on ch1 and then view the same wfm on ch2. (Channel 1 can be turned off but will still trigger channel 2.)