Since the turbine pace lags on a regular basis driving the impeller (pump) pace, the velocity ratio is under 1. Therefore You can find friction from the transmission fluid which ends up in electrical power losses. The torque converter clutch solves this by locking the turbine into the converter’s housing, https://torqueconverterclutch64186.ltfblog.com/32938344/torque-converter-clutch-an-overview