1. Electric
Motor
Selection

2. Servosystem Selection
“Servo” generally is used as a synonymous of “brushless”.
Brushless motors are generally defined in terms of torque, not power, since the torque is available from zero to nominal speed, while P = C * w
Velocity
Torque
servo
servo
Induction vector controlled
Induction vector controlled
induction
V/F cost.
induction
V/F cost.
time
velocity

3. Motion Transmission 1) Gearboxes: J1 = (n1/n2)2 J2 Jtot = Jmot + J1
Moment of inertia
to the motor shaft:
J1 = (n1/n2)2 J2
Jtot = Jmot + J1
Motor
n1 teeth
n2 teeth

4. Motion Transmission (cont’d)v r w
2) Belt:
J = m r2 = m (v/w) 2
3) Screw:
J = m (s/(2p))2 m s

5. Motor’s poles number
Brushless can technically be built with any pole pair number.
A high pole pair number generally
gives high torques.
The limit given by permanent
magnets distance on the rotor and
from the diameter of the motor.
AHR190J8 rotor
with NdFeBo magnets

6. Motors Basic Equations
Speed [rad/s]
Electrical Equations:
e = fcem = Kt w [V]
C = Kt I [Nm]
Mechanical Equations:
P = dE/dt = C w = w dE/dq [Nm/s = W]
C = J dw/dt [Nm]
Current [A]
Energy [J=Nm=Ws]
Angle [rad]

7. Field-weakening (Deflussaggio)
Increasing velocity the DC bus limit is reached
(e = fcem = Kt w).
For increasing furthermore the speed it is necessary to lower the statoric flux with 1/w
(and doing so also Kt will be lowered and so also C = Kt Iq).
(The effect can be obtained changing the phase of is beyond p/2 with respect to the rotor position; the current thus staying maximum and thus avoiding quantization effects due to small digital vectors).
We thus have P = C w = cost.

8. Field Weakening (cont’d)
Torque
fcem “e”
velocity w

9. Torque characteristics of the different motor types
DC brush motor
Torque
Torque
Peak torque
Universal Motors
(motori serie)
Nominal torque
Nominal
Work area
Velocity
Field weakening
Velocity

10. Torque characteristics of the different motor types (cont’d)
Stepper motor
Torque
pull-out torque
Max speed possible to put as
set point
at speed zero
Resonance
zone
Nominal
Work area
Load
inertia
Velocity
(steps frequency)
pull-in
rate

11. Torque characteristics of the different motor types (cont’d)
AC induction motor
Pull-out torque
Torque
Torque
brake
Unstable
zone
const. power
with Is max
const. torque
Torque follows pull-out torque
Nominal
Work area w
1/w
1/ w2
s = 1
s = 0
generator w
B prop. to V/f = cost.
V cost.

12. Torque characteristics of the different motor types (cont’d)
AC brushless motor
Torque
Peak torque
Nominal torque
Nominal
Work area
Field weakening
Velocity

13. Formulas Summary Rotational Case Linear Case
E = C Da = P Dt [Nm=Ws=Am2T]
P = dE/dt = C w = w dE/dq [Nm/s]
C = F leverage = J dw/dt [Nm]
F = I L B l ; B = F/ A [N] [T]
F = LI = MMF/R ;MMF=NI [wb] [A]
Linear Case
E = F Ds = P Dt [J=Nm]
P = dE/dt = F v [N m/s]
F = m a [N=Kgm/s2]

14. Rewinding (Riavvolgimento cave statoriche)
For increasing Kt with the same motor is sufficient to rewind stator slots with smaller section cable so to make more windings:
Kt = F Srot N / A will be thus increased.
With the same motor, I will thus have more torque C = Kt I with the same current I,
But with a smaller max. speed since e = Kt w
prop. to N I: a little bigger due to
Better slot filling
It increases proportionally to
number of windings

15. Shannon Sampling Theorem
45Hz < signal bw
55Hz > signal bw
50Hz
fsample = 100Hz = 2fsignal
fsample = 110Hz > 2fsignal
fsample = 90Hz < 2fsignal

16. Servo digital control loop
sampling time (tempo di campionamento): to avoid z-transform analysis (that would mean to work at the control system limits) it is necessary to sample 5-10 times faster than Shannon theorem says.
Generally we have:
Load Response Bandwidth = 10-50Hz
Sample&Update Rate > 1KHz

17. Servo digital control loop (cont’d)
lag error, following error
(Errore di inseguimento): each control block introduces a delay (integral action plays an important role in this respect) that leads to a lag error naturally different from zero. To minimize it the feed-forward could be useful: it bypasses closed loops regulating blocks (and thus it does not load the integral actions). The feed-forward action it is dependent from: velocity, inertia, acceleration, viscous friction, that thus have to be known with good accuracy.

18. DC Motors (motori in continua o motori a spazzole o motori a collettore)
Simple drive electronics
Cheap
Possible problems with commutator and brushes

19. Ironless Motors (DC motor)B i F1 F2
(With
integrated gear)

20. Ironless Motors (brushless)

21. AC induction motors (a induzione o in alternata o a gabbia di scoiattolo)
Frequency-controlled asynchronous (induction) motors are mostly used for simple drive functions, without feed-back. For example to regulate the speed. The motor is a squirrel-cage asynchronous motor, and the control unit a frequency converter.
The squirrel-cage asynchronous motor is the absolutely most commonly used AC induction motor:
• it is CHEAP,
• it is VERY RELIABLE,
• it is a STANDARD PRODUCT within the IEC std.

22. AC induction motors (cont’d)
230VAC
230VAC
Squirrel Cage
(gabbia di scoiattolo)
D-Connection
(Connessione
a triangolo)
Y-Connection
(Connessione
a stella)

23. AC induction motors (cont’d)
The synchronous speed is the rotation speed of the magnetic field, generated in the field windings when supplied with a three-phase AC voltage:
The actual, true, speed of the rotor is determined also by how great a load the motor is driving. This speed is called the asynchronous speed, and the difference between the two is termed slip (scorrimento).

24. AC induction motors (cont’d)
Note that the AC induction motor (asynchronous) has always a physiological slip (in speed), while the AC brushless motor (synchronous) has always a physiological lag error (in position).
From a construction point of view the stator of an AC induction motor and the one of an AC brushless are quite similar (both has a winding lay-out so to obtain a single sinusoidal rotating field from 3 sinusoidal pulsating fields).
Often an AC brushless drive can also control (with Vector Control techniquies) an induction motor.

25. Steppers (motori passo o passo-passo)
2 phases,
4 poles
6 rotor teeth
Small loads
No feed-back
Cheap
2 phases,
8 poles
50 rotor teeth

26. Steppers Half Stepping: 1-phase-ON (FullStep):
4 poles * 6 teeth = 24 steps
1-phase-ON (FullStep):
4 poles * 6 teeth / 2 = 12 steps

27. Distribuzione del campo magnetico al traferro di un motore passo-passo

28. Steppers Motor Types: Variable Reluctance (iron teeth)
Permanent Magnets (PM teeth)
Hybrid (rotor iron teeth misaligned axially,
PM inside the rotor with N-S axially spaced)
Direct Drive Variable Reluctance (ring-like rotor, double face stator)
Multi-Stack (rotor divided axially in 3 parts with teeth misalingned of 1/3; stator also divided in 3 parts each energized in sequence: only 1/3 of Fe used at the same time)