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Mechatronics Electric Motor Selection. Mechatronics Servosystem Selection Servo generally is used as a synonymous ofbrushless. Brushless motors are generally.

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Presentation on theme: "Mechatronics Electric Motor Selection. Mechatronics Servosystem Selection Servo generally is used as a synonymous ofbrushless. Brushless motors are generally."— Presentation transcript:

1 Mechatronics Electric Motor Selection

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

3 Mechatronics Motion Transmission Gearboxes: 1) Gearboxes: Moment of inertia to the motor shaft: J 1 = (n 1 /n 2 ) 2 J 2 J tot = J mot + J 1 Motor n 1 teeth n 2 teeth

4 Mechatronics Motion Transmission (contd) Belt: 2) Belt: J = m r 2 = m (v/ ) 2 Screw: 3) Screw: J = m (s/(2 )) 2 m v r m s

5 Mechatronics Motors 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 Mechatronics Motors Basic Equations Electrical Equations: e = fcem = K t [V] C = K t I [Nm] Mechanical Equations: P = dE/dt = C = dE/d [Nm/s = W] C = J d /dt [Nm] Speed [rad/s] Current [A] Angle [rad] Energy [J=Nm=Ws]

7 Mechatronics Field-weakening (Deflussaggio) Increasing velocity the DC bus limit is reached (e = fcem = K t ). For increasing furthermore the speed it is necessary to lower the statoric flux with 1/ and doing so also K t will be lowered and so also C = K t I q ). (The effect can be obtained changing the phase of i s beyond /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 = cost.

8 Mechatronics Field Weakening (contd) Torque fcem e velocity

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

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

11 Mechatronics AC induction motor Torque const. torque const. power with I s max B prop. to V/f = cost. V cost. Pull-out torque Torque follows pull-out torque Torque characteristics of the different motor types (contd) Torque Unstable zone Nominal Work area brake generator s = 0 s = 1 1/ 1/ 2

12 Mechatronics AC brushless motor Torque Velocity Peak torque Nominal torque Field weakening Nominal Work area Torque characteristics of the different motor types (contd)

13 Mechatronics Formulas Summary Rotational Case E = C = P t [Nm=Ws=Am 2 T] P = dE/dt = C = dE/d [Nm/s] C = F leverage = J d /dt [Nm] F = I B l ; B = / A [N] [T] = L I = MMF / R ; MMF =N I [wb] [A] Linear Case E = F s = P t [J=Nm] P = dE/dt = F v [N m/s] F = m a [N=Kgm/s 2 ]

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

15 Mechatronics Shannon Sampling Theorem 45Hz < signal bw 55Hz > signal bw 50Hz f sample = 100Hz = 2f signal f sample = 110Hz > 2f signal f sample = 90Hz < 2f signal

16 Mechatronics Servo digital control loop sampling time (tempo di campionamento): 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 Mechatronics lag error, following errorlag error, following error (Errore di inseguimento): (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. Servo digital control loop (contd)

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

19 Mechatronics Ironless Motors (DC motor) (With integrated gear) B i F1F1 i F2F2 B

20 Mechatronics Ironless Motors (brushless)

21 Mechatronics AC induction motors 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 Mechatronics AC induction motors (contd) D-Connection (Connessione a triangolo) Y-Connection (Connessione a stella) Squirrel Cage (gabbia di scoiattolo) 230VAC

23 Mechatronics The synchronous speed is the rotation speed of the magnetic field, generated in the field windings when supplied with a three-phase AC voltage: slip (scorrimento). 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). AC induction motors (contd)

24 Mechatronics 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. AC induction motors (contd)

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

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

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

28 Mechatronics Steppers Motor Types: 1.Variable Reluctance (iron teeth) 2.Permanent Magnets (PM teeth) 3.Hybrid (rotor iron teeth misaligned axially, PM inside the rotor with N-S axially spaced) 4.Direct Drive Variable Reluctance (ring-like rotor, double face stator) 5.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)

29 Mechatronics


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