Lecture 11: Other AC Motor Types

1 / 28 Lecture 11: Other AC Motor Types ELEC-E8405 Electric Drives (5 ECTS) Marko Hinkkanen Aalto University School of Electrical Engineering Autumn 2015

2 / 28 Learning Outcomes After this lecture you will be able to: Name common AC motor types and describe their typical properties Explain the operating principle of a synchronous reluctance motor

3 / 28 Introduction We started this course by studying DC motors Pedagogically reasonable starting point Seldom used in modern applications During the last lectures, surface-mounted permanent-magnet synchronous motors were considered Models and control methods of other AC motor types are similar This motor type is commonly used in servo applications Brief overview of other AC motor types will be given today

4 / 28 Common AC Motor Types Synchronous motors Surface-mounted permanent-magnet synchronous motor (SPM) Interior permanent-magnet synchronous motor (IPM) Synchronous reluctance motor (SyRM) Excited-rotor synchronous motor Asynchronous motors Induction motor with squirrel-cage rotor Wound-rotor induction motor

5 / 28 Outline Surface-Mounted Permanent-Magnet Synchronous Motor (SPM) Interior Permanent-Magnet Synchronous Motor (IPM) Synchronous Reluctance Motor (SyRM) Induction Motors Application Examples: Motors for Electric Vehicle

Surface-Mounted Permanent-Magnet Synchronous Motor (SPM) 3-phase stator winding Distributed sinusoidally along the air gap Produces rotating magnetic field Permanent magnets (NdFeB or SmCo) mounted at the rotor surface Benefits Very high efficiency (or power density) No magnetising supply needed Drawbacks Price of the magnets and manufacturing Limited field-weakening range F q β d ϑ m α F In SPMs, a concentrated stator winding is also possible instead of a distributed winding. 6 / 28

7 / 28 SPM Model in Rotor Coordinates Stator voltage Stator flux linkage u s = R s i s + dψ s dt + jω m ψ s ψ s = L s i s + ψ f Torque is proportional to the q component of the current T M = 3p { } 2 Im i s ψ = 3p s 2 ψ fi q

9 / 28 Rotor Concepts q q d d SPM (L s = L d = L q ) IPM (L d < L q ) Permeability of the magnets is almost the same as for air (µ r 1.05)

10 / 28 IPM Model in Rotor Coordinates Stator voltage Stator flux linkage u s = R s i s + dψ s dt + jω m ψ s Torque consists of two terms ψ s = L d i d + jl q i q + ψ f T M = 3p { } 2 Im i s ψ = 3p s 2 [ψ fi q + (L d L q )i d i q ] where the last term is the reluctance torque If L d = L q = L s, the IPM model reduces to the SPM model

12 / 28 Synchronous Reluctance Motor (SyRM) i s i s Two poles (p = 1) Four poles (p = 2) Note: no-load condition is illustrated in the figures for simplicity

Structure and Operating Principle Distributed 3-phase stator winding Rotating magnetic field produced by the stator currents Torque-production principle: Rotor tries to find its way to the position that minimizes the magnetic field energy More efficient than induction motors Cheaper than permanent-magnet motors Pump and fan applications Figure: ABB 13 / 28

14 / 28 SyRM Model in Rotor Coordinates Stator voltage Stator flux linkage Only the reluctance torque u s = R s i s + dψ s dt ψ s = L d i d + jl q i q + jω m ψ s T M = 3p { } 2 Im i s ψ = 3p s 2 (L d L q )i d i q High difference between L d and L q is necessary

15 / 28 Permanent-Magnet SyRM (PM-SyRM) SyRM designs can be improved by placing permanent magnets inside the flux barriers of the rotor Resulting motor is basically an IPM having high reluctance torque These kind of motors are often called permanent-magnet SyRMs (PM-SyRMs) or hybrid synchronous motors (HSMs) What is the reluctance torque in the figure? i s

Compared to the SyRM, the permanent magnets improve the power factor and contribute to the torque Low-cost ferrite magnets can be used Good field-weakening properties Minor risk of overvoltages due to the low back-emf induced by the permanent magnets Figure: http://www.abb.com/cawp/seitp202/ad26393b09a61275c1257caf00217a16.aspx 16 / 28

Cage-Induction Motor: Structure Figure: http://www.ctiautomation.net/about-motors.htm 18 / 28

19 / 28 Induction Motor Most common motor in industrial applications Advantages Robust, inexpensive, durable Can be started by direct connection to the mains (unlike other standard AC motors) Low-performance control with the frequency converter is very simple (open-loop scalar control aka volts-per-hertz control) Disadvantages High-performance control is complicated Less efficient than synchronous machines (but more efficient than DC machines)

Operating Principle Magnetised from the stator If the motor rotates synchronously, no current is induced in the rotor Once the motor is loaded mechanically, the rotor starts lagging behind the synchronous rotation of the flux Current is induced in the short-circuited rotor winding Torque counteracting the lagging effect is produced Animation: https://en.wikipedia.org/wiki/induction motor#/media/file:asynchronmotor animation.gif 20 / 28

21 / 28 Open-Loop Scalar Control Voltage u s,ref is proportional to the frequency Supply frequency corresponds to the desired rotor speed Some speed error due to the slip (can be partly compensated for) Slow or oscillating dynamics Torque cannot be controlled For pumps, fans, etc. ψ s,ref = constant ω s,ref Scalar control u s s,ref PWM Voltage vector is simply rotated at ω s,ref : u s,ref = ω s,ref ψ s,ref (+R s i s compensation) ϑ s = ω s,ref dt u s s,ref = u s,refe jϑs M

More Advanced Control Methods Resembles field-oriented control of PMSMs Based on the dynamic motor model Field-oriented control, direct torque control (DTC)... ω M,ref Speed controller T M,ref Fieldoriented controller Torque can be controlled High accuracy and fast dynamics i a, i b, i c For elevators, servo drives, etc. ω M M Speed is often estimated, rotor position is not needed 22 / 28

24 / 28 Motors for Electric Vehicle Applications: Typical Features Typical rated power 50... 300 kw PM-SyRM, IPM, or induction motor due to their wide constant power range Lot of poles in order to reduce the iron in the stator yoke High max speed and frequency (e.g. 12 000 r/min @ 1 000 Hz, 10 poles) High power and torque density, liquid cooling Integrated resolver Sometimes form-wounded copper stator winding

Example PM-SyRM For truck and bus applications Low magnetic material Speed: 4 400 rpm (nom), 12 000 rpm (max) Torque: 270 Nm (S1), 460 Nm (max) Power: 145 kw (S1), 220 kw (max) DC-bus voltage: 400 V (also 750 V available) Weight: 76 kg Figure: http://www.brusa.eu/en/products/drive/motors-400-v/hsm1-101822.html 25 / 28

Example PM-SyRM Figure: http://www.brusa.eu/en/products/drive/motors-400-v/hsm1-101822.html 26 / 28

Example IPM: BMW i3 Speed: 4 500 r/min (nom), 11 400 r/min (max) Torque: 250 Nm (max) Poles: 12 Voltage: 250... 400 V Weight: 65 kg Figure: http://hybridfordonscentrum.se/wp-content/uploads/2014/05/20140404 BMW.pdf 27 / 28

Example IPM: GM Figures: http://blog.caranddriver.com/we-build-the-chevy-spark-ev%e2%80%99s-ac-permanent-magnet-motor/ 28 / 28