AQ: Control Servo motor with a variable frequency drive
Looking at those AC drives they recommend an Induction motor. A servo motor with permanent magnets which is not quite an induction motor. So, if a servo with permanent magnets can be used instead an induction with these kinds of AC drives.
Actually, the term “Servo” makes a reference about “feedback”, it means, whether we need a control loop, we are talking in terms about Servo, in this case, we have, or we know, the “feedback” by an encoder. Typical variable frequency drive doesn’t have a input for an encoder, so, if you want to control a Servo Motor with a VFD, you can move the motor, but you can’t control it.
A servo motor can be an induction servo, a brushless servo, a reluctance servo a dc servo – each of these can be either linear or rotary and can come with a variety of feedback such as tachometer, resolver of various pole counts, incremental or absolute encoders discreet or serial interface with different bus options, laser feedback, halls etc.
Then you come to the term variable frequency drive. Brushless servo amplifiers are also vfds. Do standard inverters have proper control of induction, and brushless motors. Some allow for a software switch, some allow for a firmware download, some don’t. Will inverters accept feedback – some have it built in, most that allow it do so by option cards, many do not.
Normal input in a variable frequency drive is, digital to start or stop, and we could have an analogic input to control by potentiometer.
Using AC Drives for the servo application is quite possible, provided the application is less demanding in critical positioning purpose.
There are number of makes that showcases pinpoint positioning of motor shaft being driven by AC Drives like Hitachi SJ700 / Emerson Uni drive SP / Danfoss FC etc.
Its beneficial to opt for the AC Drives as it supports SLVC [ VFD gives almost servo-like torque at low rpms if you give it encoder feedback ], multiple motors can be accessed, torque requirement can be met if required, power dips can be sustained using VFD’s.
