Hi,
most modern AC servos have a motor and a servo drive control box. If you look closely you will see that like most servos they have two cables,
one, a low power cable, is for the encoder and the other carries the three power wires for the servo motor.
There ar a lot of really cheap Chinese made servos on AliExpress but the manufacturers are little known and the quality and backup on their servos
is questionable. Two brands, Delta (Taiwanese manufactured in China) and DMM ( Canadian manufactured in China) are good quality with good support
and won't break the bank, but they are more than the Chinese bargain basement units but still much cheaper than Yaskawa, Seimens and Fanuc.
There are some servos that have the control electronics built into the motor. Clearpath's are the most common example. They are made by Technics
and are good quality but they are expensive for what they are. You could buy a Delta servo, drive and cables for less and be twice or three times more
powerful. Clearpath advertise that their servos are a drop in replacement for steppers, and they do fill that role well.
There are lots of closed loop steppers available, cheaper than servos and to listen to the bulls****t that the manufacturers spout you swear they were
as good as servos.....THEY AREN'T.
ALL steppers lose torque the faster they go, having a closed loop driver does not change that. If a closed loop stepper is marginally overloaded and loses
a step the drive will insert an extra step to catch up.....but guess what...the extra step suffers the same fate as the ones gone before it...it gets missed
because the stepper doesn't have the POWER to catch up.
Closed loop steppers do have a few advantages but if you want genuine closed loop performance and decent overload tolerance get AC servos,
don't muck around with 'Mr. In Between'
Craig