Effects of frequency on SSR/PWM

i have been running a 100amp solid state relay to run a davies craig ewp150 for about 2 years. touch wood no issues. i have ran as low as 5% PWM at 150Hz.

recently i was told 50Hz is describable for the electric water pump.

My question is what effect does the frequency have on a brushed motor such as water pumps fans and fuel pumps?

does it effect heat generation of the motor? (or relay?)

not having any issues currently, just looking to understand the effects.

I haven't personally used a SSR for speed control but I would have thought 150 Hz was quite low? We use an audrino to simulate a 0-5v input on our engine dyno for the accelerator pedal position. To get a smooth voltage input we needed to run the PWM output at 3-4 khz. Maybe someone else on here will have more first hand experience.