Servo motor and drive · Lesson 2 of 12

Inside the servo motor

We will open the servo motor and look inside: windings, permanent magnets, an encoder on the back, and sometimes a brake. At the end you will read the nameplate like a technician and calculate the power from torque and speed.

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1A permanent-magnet motor (PMSM)

The AC servo motor you find in plants is usually a PMSM (Permanent Magnet Synchronous Motor): the stator has 3-phase windings like any motor, but the rotor is not aluminium bars like an induction motor, it carries strong permanent magnets.

Synchronous: the rotor turns exactly with the field, with no slip. That is why the drive must know the magnet angle every instant (from the encoder) to send current into the right winding.
Torque = current: every ampere gives a known torque (the torque constant Kt in N·m/A). This makes torque control easy and accurate.
Small, high torque and no rotor current: meaning less heat in the rotor and a faster response.

Never connect a servo motor straight to the mains! This motor will not start on its own on 220/380 V like an induction motor; it will burn out or shake. It needs its own servo drive.

2Open the motor

Cross-section of a servo motor

From the field: if you turn a disconnected servo shaft by hand you will feel a slight "ticking" (cogging) from the magnets. And if the U V W leads are shorted together it will be very stiff, because the motor works as a generator. This is not a fault.

3The brake: for holding, not stopping

The servo holding brake: a spring presses on a disc and holds the shaft when power is removed, and when it gets 24 V DC the magnet pulls it and it releases.

It is needed on vertical axes (Z axis, hoist, lift) so the load does not fall when power is lost or the servo switches off.
It is not made to stop a running motor except in an emergency, and every such stop wears it down.
The drive has a BRK output (names differ) that drives the brake relay: it releases the brake shortly after Servo ON and applies it shortly before Servo OFF, so the load does not drop and the motor does not pull against it.

Safety: before you remove the motor of a vertical axis, support the load mechanically. The brake may be worn or oily and not holding.

4Reading the nameplate

On the nameplate	Meaning	Why it matters
RATED OUTPUT	Rated power	To choose the right drive (usually the same power)
RATED TORQUE	Continuous torque	The normal load must stay below it
MAX TORQUE	Peak torque (about ×3)	For acceleration and braking only
RATED / MAX SPEED	Rated and maximum speed	Above rated speed the torque drops
RATED CURRENT	Rated current	You compare it with the drive current and the load monitor
ENCODER	Encoder type and resolution	The drive must support it
BRAKE	Brake and its voltage	Usually 24 V DC

P = T × 2π × n ÷ 60

Power in watts = torque (N·m) × speed (rad/s). Example: 2.39 × 2π × 3000 ÷ 60 ≈ 751 W, so a 750 W motor.

Drill: read the nameplate

The motor inertia: it is also given in the catalogue. There are low inertia models (fast, for short and frequent moves) and medium/high inertia models (for heavy loads like rotary tables). You will see why it matters in the tuning lesson.

?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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