As injectors are just damb power supplies, they power capability can't be known to AP. AP can just try to draw as much is needed and check that voltage is OK. So I suppose the best way with injectors is always set correct power setting in AP configuration and avoid any auto-detection, which is potential for problems. PoE switches are different, they really negotiate how much power they provide, and anything over 802.3at requires LLDP to work fro my understanding (on both switch and AP). I had in one case huge problem with R730 connected to Cisco Catalyst switches with 802.3at ports -- AP (out of the box) were unable to work, as switch dropped power after a few seconds, and it was not possible to get to AP to configure 802.3at power mode. There was no any other power source on site, it was at night during preparation for next day conference, and I had to drive back to office to bring on-site Alcatel switch and reconfigure APs before installation on network.
Even so R750 is just 4x4 and requires less power than R730 (802.3at is enough for it), it is still better to manually configure power mode. Many injectors on market are not providing what is stated in datasheet, but in this case I suppose this is auto-detection problem, not real power deficit.
Hope it helps.
