Servo motor and drive · Lesson 1 of 12

What is a servo?

A servo is the motor that goes exactly where you want, at exactly the speed you want, and corrects itself if something pushes it. In this lesson you will understand the secret: the closed loop. You will try it yourself against a stepper and learn to choose between a servo, a stepper and a VFD.

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1Servo = motor + drive + encoder

The word servo comes from "servant": something that carries out the command exactly. A servo is not a motor on its own, it is a system of 3 parts working together:

The PLC tells the drive "go to 250.00 mm at 500 mm/s". The drive sends current to the motor, the encoder tells the drive every instant "this is where the motor is now", and the drive corrects until it gets there exactly. This happens thousands of times per second.

Where will you find servos? Packaging and filling machines, CNC machines, robots, cut-to-length machines, printing, winding, and anywhere that needs exact position + high speed + strong acceleration.

2Open loop and closed loop

Open loop: the controller sends the command and hopes the motor carried it out. Like closing your eyes and walking 10 steps: if someone pushes you, you will not know. A normal stepper and a VFD without an encoder work this way.

Closed loop: there is a sensor (the encoder) that returns the real position, and the drive compares the target with the actual and corrects the difference. Like walking with your eyes open. That is a servo.

Try it: stepper vs servo

From the field: a labelling machine with a stepper kept putting labels on crooked every so often. The cause: when the reel got heavy, the stepper lost steps and nobody knew. After it was replaced with a servo, any error was corrected automatically, and if the error is very large the drive raises an alarm instead of carrying on wrong.

3Servo, stepper or VFD?

	Servo	Stepper	VFD + induction motor
Knows its position?	Yes (encoder)	Usually not (counts steps)	No (unless you add an encoder)
Accuracy	Very high	Good if the load is constant	Speed only
Torque at high speed	Almost constant up to rated speed	Drops quickly	Good
Acceleration	Very strong (×3 peak torque)	Limited	Medium
If the load suddenly increases	Raises the current and corrects	Loses steps without knowing	Speed drops a little (slip)
Price and complexity	Most expensive, needs tuning	Cheap and simple	Medium and simple
Use	Position + speed + acceleration	Light, simple moves	Pumps, fans, conveyors

Note: there are steppers with encoders (closed-loop steppers) and VFDs that do position control with an encoder. But when someone in a plant says "servo", they usually mean a PMSM motor + a servo drive + a high-resolution encoder.

Drill: choose the right motor

?Lesson quiz

Answer at least 5 of 6 correctly so the lesson is marked complete.

The numbers and values in this lesson (voltages, frequencies, gains, limits) are approximate training values based on general field experience, and they differ from brand to brand and model to model: your drive's manual is the reference before any setup on a real machine. Alarm and parameter names also differ from one manufacturer to another. Your progress is saved on this device.
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