Servo motor as lathe spindle motor

[Muzzer]

It's being described as "DC" injection because the machine is assumed stationary initially. When you want to generate a torque in either type of machine, you need to set up a magnetic field to interact with the rotor. It's only when that field is established and it starts to move relative to the rotor that torque can be generated. I don't imagine it speeds up the turn-on time much.

It's a bit confusing for them to be using the words "DC injection" in this context, as this normally refers to driving a DC (stationary) current in the stator of an IM machine. As you can imagine, a strong but stationary magnetic field will generate a fairly large torque on a spinning rotor. That's a simple method of generating a relatively large braking torque without requiring a braking resistor but the downside of this method is that it dumps the kinetic energy as heat in the rotor which isn't ideal.

IIRC, my Yaskawa VFDs allow both braking resistor AND DC current injection to be set up simultaneously. I think the DC injection only comes in once the speed is relatively low.