Created Date: July 15,2018

Created by: Electronics55

Hi Guys,

I'll start the topic by saying I am an engineer who is very familiar with variable frequency drives running asynchronous (Induction) motors. I'm familiar with many popular drives from ABB to Siemens, mitsubishi, schnieder.etc.

What I'm not so familiar with is the synchronous side of things.

So I currently have an ABB BSM50-375 Brushless servo motor.

Now bear in mind, I'm perfectly familiar with encoder and resolver feedback set-ups when in conjunction with VFDs for feedback and speed control.

So from basic research I 've learned that the biggest difference between a servo and induction motor is the permanent magnet inside the servo motor and that servo drives heavily rely on feedback for proper control of the magnetic field.

What I'd like to ask is... I have a lot of different VFD's, many with encoder/resolver inputs/outputs. i.e micromaster 440. Now can I hook up the encoder to one of my VFDs and somehow set parameters to run my servo motor? Or do I need to have a servo specific drive for this? I 've noticed the micromaster has a parameter for motor type. Synchronous and Asynchronous.

Does this mean I'm ok to go ahead and set parameters based on the nameplate data of this motor with the drive set for synchronous?

other than that I'm just struggling to see why a VFD that supports encoder feedback wouldn't run an ac servo motor regardless of having a setting for synchronous motors.

Any advice is truly appreciated!

cheers.

Created Date: July 15,2018

Created by: Adisharr

There's a fuzzy line between the different drives and their capabilities these days. Servo drives tend to be lower horsepower and multi-purpose controlling a motor with torque, velocity, and positions loops. They also tend to have better motor models to work with as they are ideally designed to work with the same manufacturers motors.

You'll almost always get better performance out of a motor when paired with he proper drive.

It looks like the Micromaster series is similar in capabilities to the Danfoss drives we tend to use. While they have some of the same capabilities as a servo, (encoder-less positioning, PID loops, etc..) they can't compete with the dynamic response of a servo drive.

This is just a really general purpose answer, perhaps someone else here will have a better answer and can go into more detail.

Created Date: July 16,2018

Created by: MikeN

Whether it is possible or not IDK. I would assuem it *could* be done as both meet the voltage, amperage, and type of power requirements. However the real question is: why would you want to? Even assuming you got a servo running smoothly from a VFD, you wouldnt have any of the control capabilities of a servo drive that it is designed for. It wouldnt be any different from running a regular induction motor.

I also believe that a synchronous setting in the VFD is simply meant for a synchronous AC motor, like an induction motor you are used to but built synchronous. Not specifically for running a servo designed synchronous motor.

Created Date: July 19,2018

Created by: Alexandru

A servo drive has three hardware components: drive, motor, encoder. It works in two modes: speed/position following (+ torque following as a third mode, never encountered by me). To work properly, the encoder provides accurate feedback on the position of rotor shaft, which the servo processes into speed or position, depending on run mode.

The resemblance between vfd and servo drives is that vfd appear to function in speed following mode, although in open loop. One observation: the vfd has a control signal (0-20mA, or 0-10V, or just potentiometer) meant to tell the drive what speed should provide to the motor. However, the drive will never know if the motor achieved that speed because there is no feedback from motor shaft towards vfd. All drive knows is that the power it supplies to the motor should produce a certain speed at the shaft, provided the motor reached synchronous speed.

The difference is that the servodrive works automatically to reach that speed or position, while the vfd just provides power but doesn’t check if the speed is reached.

All vfd provide an acceleration- deceleration scheme. Within that scheme, there is the possibility to adjust the control signal as to change the speed. Since the vfds also provide PID, it means that if the motor shaft speed could be backfed to the vfd as a control signal, the vfd will automatically adjust the speed of the motor.

in conclusion, turning a vfd into a servodrive is possible, within the limits of how fast the vfd can accept an update of the control signal and how well tuned the PID loop is. Even so, the vfd will not provide more than speed following. Now given the fact that speed following works well in open loop ( all conveyors, fans, etc), then turning a vfd into a servo system is impractical even for speed following.

As for PID, tuning is difficult even for heating processes, where usually the inertia never changes. I remember when a heater went awol because it ran out of glue and adding too much cold glue would disturb the heating. Or when starting the heaters on Sunday night, I had to be creative to prevent burning of the glue, without touching the PID. My point being that PID doesn’t react well to inertial changes and to employ this to moving parts is an added safety risk. This is why vfds never made it to servo drive status.

A remark on steppers: they are open loop drives too, however they resemble servo drives because they work in a mode similar to position following.

Now to answer your questions:

if your vfd accepts as little as quadrature encoder input, it most probably has limited servo drive capabilities. Turning this into a servodrive is a matter of reading the drive manual enough to understand what it can and can not do. However, if involving the PID is part of the process- and I’m sure it is, I would be concerned that exceeding the narrow range of PID can lead to unpredictable behaviour.

Created Date: July 19,2018

Created by: Adisharr

A servo drive has three hardware components: drive, motor, encoder. It works in two modes: speed/position following (+ torque following as a third mode, never encountered by me). To work properly, the encoder provides accurate feedback on the position of rotor shaft, which the servo processes into speed or position, depending on run mode.

The resemblance between vfd and servo drives is that vfd appear to function in speed following mode, although in open loop. One observation: the vfd has a control signal (0-20mA, or 0-10V, or just potentiometer) meant to tell the drive what speed should provide to the motor. However, the drive will never know if the motor achieved that speed because there is no feedback from motor shaft towards vfd. All drive knows is that the power it supplies to the motor should produce a certain speed at the shaft, provided the motor reached synchronous speed.

The difference is that the servodrive works automatically to reach that speed or position, while the vfd just provides power but doesn’t check if the speed is reached.

All vfd provide an acceleration- deceleration scheme. Within that scheme, there is the possibility to adjust the control signal as to change the speed. Since the vfds also provide PID, it means that if the motor shaft speed could be backfed to the vfd as a control signal, the vfd will automatically adjust the speed of the motor.

in conclusion, turning a vfd into a servodrive is possible, within the limits of how fast the vfd can accept an update of the control signal and how well tuned the PID loop is. Even so, the vfd will not provide more than speed following. Now given the fact that speed following works well in open loop ( all conveyors, fans, etc), then turning a vfd into a servo system is impractical even for speed following.

As for PID, tuning is difficult even for heating processes, where usually the inertia never changes. I remember when a heater went awol because it ran out of glue and adding too much cold glue would disturb the heating. Or when starting the heaters on Sunday night, I had to be creative to prevent burning of the glue, without touching the PID. My point being that PID doesn’t react well to inertial changes and to employ this to moving parts is an added safety risk. This is why vfds never made it to servo drive status.

A remark on steppers: they are open loop drives too, however they resemble servo drives because they work in a mode similar to position following.

Now to answer your questions:

if your vfd accepts as little as quadrature encoder input, it most probably has limited servo drive capabilities. Turning this into a servodrive is a matter of reading the drive manual enough to understand what it can and can not do. However, if involving the PID is part of the process- and I’m sure it is, I would be concerned that exceeding the narrow range of PID can lead to unpredictable behaviour.

Just an FYI, the Micromaster drive he's referencing is a full closed loop vector drive.