MEMS1049 Mechatronics Chapter 10 Actuators 10-3
Electric Motor
DC Motor
DC Motor
DC Motor
DC Motor
DC Motor
Motor terminology
Motor field current interaction
Motor commutator It consists of a ring of alternating conductive and insulating materials connected to the rotor windings; The brush slide on the surface of the commutator as it rotates
Field-field interaction to produce torque The right brush contact with the commutator segment A, and the left brush contact with segment B
DC Motor
Motor Construction
DC Motors DC Motors can be classified into 4 categories: DC permanent magnet motor DC shunt wound motor DC series motor DC compound motor Starting Toque T s : the maximum torque the motor can produce, at zero speed No-load speed ω max: is the maximum sustained speed the motor can attain.
Motor torque-speed curve
DC Motors DC permanent magnet motor schematic and torque-speed curve I L is the load current V is the DC voltage supply PM motor is lighter and smaller than others; the actuator is linear. Used in servomotor.
Permanent magnet DC motor characteristics
DC Motors DC shunt wound schematic and torque-speed curve I A is the rotor (armature ) current I F is the current in the stator (field) winding I L is the total load current The armature and field windings are parallel, and powered by the same source. Low starting torque, constant speed
DC Motors DC series schematic and torque-speed curve I A is the rotor (armature ) current I F is the current in the stator (field) winding I L is the total load current The armature and field windings are series, so the armature and field currents are equal. High starting torque; High speed when load is small
DC Motors DC series schematic and torque-speed curve I A is the rotor (armature ) current I F is the current in the stator (field) winding I L is the total load current Including both shunt and series field windings, resulting in combined characteristics of both shunt and series motor.
DC Motors Motor Armature equivalent circuit
DC Motors DC Motor Armature dynamic equation As the conducting armature begins to rotate in the magnetic field produced by the stator, a back emf V emf is induced in the armature windings opposing the applied voltage. V emf = K e ω K e is the electrical constant of the motor R is the resistance of windings R L is the resistive loss in magnetic circuit, and an order of magnitude larger than R. V in is the input voltage to armature I in is the current di Dt RI in V in = L + in + Keω
DC Motors Permanent Magnet DC motor Dynamic Equation The torque generated by a PM motor is proportional to the current T = K t I in K t is the torque constant When load is connected to the motor, the torque is given as dω T = + dt ( J ) a + J L + Tf TL Where J a and J L are polar moments of inertia of armature and the attached load T f is the frictional torque opposing armature rotation, T L is the torque dissipated by the load,
DC Motors At steady state condition diin V in = L + RIin + Keω = RIin + Keω Dt R V in = T + k t Keω T T = Solving the Torque kt R V in kek R t ω 1 ω ω ( ω) = T s max
DC Motors Comparing the above two equations, we have kt Ts R Ts = V ω in max = R kekt The power delivered to the load ( ) ω P ω = Tω = ωts 1 ω max The maximum power output occurs at speed 2ω = T s 1 = ω ωmax dp d The best speed to run a permanent magnet motor to achieve maximum power is half the no-load speed. 0 1 ω = ω 2 max
DC Motors The stall Current In addition to electrical and torque constants, armature resistance R is also an important parameter The stall current can be found in terms of the armature resistance and supply voltage I s = V R in This equation is only valid when the motor is not turning; otherwise the current is affected by the back emf induced in the rotor windings The stall current is the maximum current through the motor for a given supply voltage
AC Motor: As in the DC motor case, a current is passed through the coil, generating a torque on the coil. Since the current is alternating, the motor will run smoothly only at the frequency of the sine wave. It is called a synchronous motor. More common is the induction motor, where electric current is induced in the rotating coils rather than supplied to them directly.
AC Motor: Induction Motor Action Induction motors use shorted wire loops on a rotating armature and obtain their torque from currents induced in these loops by the changing magnetic field produced in the stator (stationary) coils. At the moment illustrated, the current in the stator coil is in the direction shown and increasing. The induced voltage in the coil shown drives current and results in a clockwise torque. Note that this simplified motor will turn once it is started in motion, but has no starting torque. Various techniques are used to produce some asymmetry in the fields to give the motor a starting torque
Simple AC Induction Motor A large percentage of small AC motors are classed as induction motors. This implies that there is no current supplied to the rotating coils. These coils are closed loops which have large currents induced in them because of their low resistance. An induction motor must achieve a rotating magnetic field to continue to exert a torque on the armature coils. In this example, the rotating field is achieved by the extra coils on the pole pieces.
AC induction motor