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Mechatronics Electric Motor Selection

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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.

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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

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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

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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

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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]

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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.

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Mechatronics Field Weakening (contd) Torque fcem e velocity

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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

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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

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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

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

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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 ]

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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

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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

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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

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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)

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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

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

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Mechatronics Ironless Motors (brushless)

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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.

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

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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)

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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)

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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

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

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

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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)

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Mechatronics

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