U of I Seminar Page 1 TAK 9 / 16 / 2016 Inductions Motors Overview of history, physical design, basic theory, and performance with emphasis on aircraft applications Typical large induction motor circa 1894 100 HP 8 pole 600 RPM must be 40 hertz Drip proof, self vent Formed coil / chain wound / single coil per slot (consequent pole) Wound rotor with spider construction Slip rings outboard of bearing Sleeve bearing
U of I Seminar Page 2 TAK 9 / 16 / 2016 1. Description, configuration, features Other names are asynchronous or squirrel cage motor Only one of the windings is excited with electrical input as a requirement Does it even have a winding on the rotor? It s like a transformer with an air gap and the secondary is shorted out and spins It s like a clutch being ridden all the time It s simple and robust It s synonymous with 3 phase, polyphase, and rotary magnetic field One of the 10 best discoveries in the last 130 years. It s not just a motor it s a brake or a generator Tesla s first motor 2 phase (4 wire) 2 pole Concentrated or concentric pole wound Wound rotor shorted
U of I Seminar Page 3 TAK 9 / 16 / 2016 The second Tesla motor Gramme ring wound stator Slotted 6 pole? Solid rotor 2 phase (4 wire) Induction motors comprise of the following electrical / magnetic elements: Stator the only part normally connected to external electrical power. Most commonly on the exterior and almost always stationary. Stator winding are three phase or single phase today and carry full power of the motor. Stators have laminated cores and most have slots for the coils either formed coils or random wound coils. It s hard to tell induction motor and traditional synchronous stators apart. Early machines quickly moved to the modern stator configuration.
U of I Seminar Page 4 TAK 9 / 16 / 2016 Rotor the rotor need not receive external power or excitation other than from the stator. Most rotors are squirrel cage consisting of bars that are shorted together on each end to end rings. Cage rotors are cast or fabricated of aluminum, copper, or other metals. Most rotors have no insulation. Wound rotors have winding schemes similar to stators typically wound for 3 phases and with a pole count matching the stator. Slip rings are used to connect a wound rotor to external resistance to change the motor speed / torque characteristic. Early Tesla / Westinghouse 2 phase wound rotor motor 8 concentric coils on pole pieces 2 phase 4 poles (2 coils per pole) 2 slip rings
U of I Seminar Page 5 TAK 9 / 16 / 2016 3 phase formed coil stator and matching rotor Transition to 3 phase and many slotted distributed winding This machine is formed coil, concentric, consequent, and chain wound 15 coil groups, 5 per phase note 1 group has a special cross over 10 pole, 120 slot, 4 slots per pole per phase Small slot opening to reduce slot leakage but coils inserted from ends Circa 1900 Matching squirrel cage rotor 2 radial vents rotor and stator) Fabricated construction Note open slot tops 90 slots (originally wound?) How many phases or poles??
U of I Seminar Page 6 TAK 9 / 16 / 2016 Core configuration Barrel slotted, Gramme ring, or concentrated pole wound Slot type Stator Winding Basics Parallel side formed coil Semi closed ( parallel tooth) random winding Coil shape Loop wound or wave wound
U of I Seminar Page 7 TAK 9 / 16 / 2016 Layers 2 layer lap 3 groups per pole if 60 degree belts or 3 groups per pole pair if 120 degree belts (low speed of 2 speed motors) 1 layer 3 groups per pole pair since half as many groups can be called consequent pole Loop shape Knuckled lap coils Concentric / chain coils Winding repetition Integral winding slot per pole per phase ( s/p/ph) is an integer all groups have same # of coils in series no restrictions in parallels Fractional winding s/p/ph is a fraction parallels limited
U of I Seminar Page 8 TAK 9 / 16 / 2016 Integral example 6 pole 54 slots (60 degree belts, 2 layer) 333333333333333333 = 54 s/p/ph = 3 3 3 3 3 3 3 18 coil groups all the same 3 3 3 3 3 3 6 coil groups per phase 3 3 3 3 3 3 6, 3, 2, or 1 parallels available Fractional example 8 pole 90 slots (60 degree belts, 2 layer) 433343334333433343334333 = 90 s/p/ph = 3.75 4 3 3 3 4 3 3 3 24 coil groups not all the same 3 4 3 3 3 4 3 3 8 coil groups per phase 3 3 4 3 3 3 4 3 2, or 1 parallels available Always 4 groups in series 3 + 3 + 3 + 4 = 11 = 3.75 x 4 Winding factors Pitch factor Kp: Coil embrace, pitch, or throw what part of pole pitch is covered. Sine of 90 degrees x embrace. Distribution factor Kd phase effect of adjacent coils in a group in series never less than 3/π (.955) for 60 degree belts. Connection parallel groups, series groups, wye or delta. Example 36 slot / 4 pole / 2 layer lap winding s/p/ph = 36 / 4 / 3 = 3 3 turn coils / 2 parallel Y / 1 to 7 throw pole pitch = 36 / 4 = 9 coil pitch = (7 1) / 9 =.67 kp= sin ( 90 x.67) =.866 kd =.960 4 groups per phase since 2 parallel there are two groups in series Series turns = 4 turn x 3 coils per group x 2 groups in series x.866 x.96 = 19.95
U of I Seminar Page 9 TAK 9 / 16 / 2016 Partially wound formed coil stator Lap wound 2 coil sides per slot Bar wound = 1 turn = 1 knuckle Multi turn 2 knuckle Multi turn formed coils with corona gradient paint in core area. Stator coil at loop stage and after spreading 2 four turn coils in slot, without and with side ways stranding
U of I Seminar Page 10 TAK 9 / 16 / 2016 Random wound stators in process Aircraft hydraulic pump motor Random lap wound Semi closed slots Parallel teeth Note phase papers
U of I Seminar Page 11 TAK 9 / 16 / 2016 Rotor Construction Wound rotor Bar wound Maybe 4 pole because coil end turn crosses about 1/8 th of circumference. Bar to bar connection will be soldered using clips. First motor I ever wound was like this in 1977. Took me 3 weeks. Traditional fabricated bar rotor are used for: 1. High efficiency (copper bar) 2. Large size Copper trade groups tout the advantages of copper rotors aluminum rotors have bad efficiency Fabricated rotor have brazed or soldered connection and potentially loose bars. They are most prone to rotor problems Is efficiency always a good thing? Rotor slot combo crib sheetblack art at its best. Refer to Gabriel Kron, AIEE, 1931
U of I Seminar Page 12 TAK 9 / 16 / 2016 24 HP rotor 6 pole machine Die cast aluminum ½ slot stator skew Open slot tops Double tear drop design 65 slot (13 x 5) 56 slot stator (6 pole x 3 phase x 3 slots/pole/ph = 54) 65 / 54 = 1.20 (greater than 1.2 or less than.8) R S = 9 (not 6, 6+/ 1, or 6+/ 2) Comparison of moderate and deep bar designs Slot opening carter factor for effective air gap Slot leakage (reactance) Various depth over width ratios Skin effect Deep bar effect Rotor reactance and resistance varies with slip / speed
U of I Seminar Page 13 TAK 9 / 16 / 2016 2. Theory and some history The transformer analogy Rotary magnetic field Force mechanism Logic, slip, and the equivalent circuit
U of I Seminar Page 14 TAK 9 / 16 / 2016 The rotating magnetic field The first AC machines were alternators (synchronous machines) but these machines were conceived as an AC replacement for DC dynamos. They were 2 wire machines that essentially replaced the commutator of a DC machine with slip rings. Then the ideal came to place this new type AC winding on the stator. The alternator was the necessary companion to the transformer for long distance transmission. Early alternator with dynamo exciter disc wound design Field stationary Armature rotating with slip rings single ckt First Commercial 3 phase alternator, claw tooth rotor Lauffen to Frankfort system 110 miles 32 pole, 40 hertz, 2000 volt, 50 volt, 1400 amp 93 hertz at 400 RPM, weighs 9 tons To date no one had tried to operate these machines as motors nor had they been paralleled. Frequency was not important because nothing operated off frequency. Early machines were single phase there was no concept of multiple or polyphases.
U of I Seminar Page 15 TAK 9 / 16 / 2016 Galileo Ferraris and Tesla theorized that AC current with a fixed phase relationship (polyphase) could be placed about a machine geometrically such that these pulsating MMF s would result in a wave that rotates about the machine and allow inducted excitation of a rotor across the air gap. Phase A MMF in a machine Phase B MMF in a machine 120 degrees away from A Phase C MMF in a machine 120 degrees away from B Resultant MMF wave moving or travelling
U of I Seminar Page 16 TAK 9 / 16 / 2016 The result of this idea lead to 3 advancements: 1. The idea that polyphases would allow for a traveling MMF that could be put to use. Mikhail Dolivo Dobrovolsky showed three phases (Drehstrom) was more practical than two phase. AC was already identified as having a separate advantage because it allowed high voltage distribution via transformers. 2. The invention of the induction motor nearly simultaneously by Galileo Ferraris and Tesla. (Later Mikhail Dolivo Dobrovolsky simplified the motor with the squirrel cage rotor.) 3. The understanding that the alternator could also operate as a synchronous motor. Tesla or Ferraris and who else? Walter Bailey 1879 demonstrated a 2 phase device powered by dry cells and a hand crank commutator. Ferraris demonstrated polyphase and rotating magnetic fields in 1885. He built induction type motor prior to Tesla. Tesla received a US patent on May 1 st 1888. He made great prototypes and presented the motor like a showman at technical gatherings. Dolivo Dobrovolsky added a lot to practicality 3 phase theory and rotor construction. Behrend and De La Tour published practical theory and the circle diagram in 1901. Steinmetz provided the equivalent circuit we use today replacing the more complete Heyland circle diagram. By 1911, Bailey s book The induction Motor summarizes modern theory.
U of I Seminar Page 17 TAK 9 / 16 / 2016 Bailey s Motor 1879 Commutating sequence (partial): Ferrasis s first 4 motors 1885 and 1886 All two phase The last one has a magnetic core
U of I Seminar Page 18 TAK 9 / 16 / 2016 Force Mechanism Flux created by MMF Refer to Direct Current Machines by Michael Liwschitz (Second Edition 1956) Chapter 14 Tangential Forces in Electric Machines for a detailed presentation of force theory.
U of I Seminar Page 19 TAK 9 / 16 / 2016 Logic, Slip and the Equivalent Circuit The MMF and flux wave of an induction motor travel the same way that they do in a synchronous machine thus: RPM (synchronous) = 120 x Frequency / poles If the rotor of an induction motor travel exactly in step with the statorcreated field, then the flux looks to be at stand still when viewed from the rotor. If this is the case then no voltage / current can be induced in the rotor and it can produce no torque to keep itself spinning in lock step with the field wave. Induction motors never operate at synchronous speed under their own power. When motoring, induction motor run below synchronous speed or are slipping relative to synchronous speed. Slip ( S ) = (synchronous speed actual speed) synchronous speed
U of I Seminar Page 20 TAK 9 / 16 / 2016 At synchronous speed slip = 0 or 0% At stall / locked rotor slip = 1.0 or 100% Example: a 4 pole 60 hertz motor runs at 1740 RPM RPM synchronous = 120 x 60 / 4 = 1800 RPM Slip = ( 1800 1740) / 1800 = 60 / 1800 =.033 = 3.3% Rotor electrical frequency = slip x stator frequency At synchronous speed Slip = 0, rotor frequency = 0, (No frequency, no induction) At stall slip =1, rotor frequency = line frequency For our example above rotor frequency =.033 x 60 hz = 2 hertz Rotors can be made of lower grade steel since operating frequency is low (and losses are low). It easy to observe that as load is increased in an induction motor that it slows down a bit just like DC motors. The induction motor equivalent circuit is a combination of the transformer equivalent circuit and the idea of slip. It was developed by Steinmetz. Prior to the equivalent circuit, motor performance was calculated using the circle diagram.
U of I Seminar Page 21 TAK 9 / 16 / 2016 Remember that the induction motor was described as being like a clutch?
U of I Seminar Page 22 TAK 9 / 16 / 2016 Is Ir or I2 Im The equivalent circuit as a tool No Load S 0 (near synchronous speed) R2 /S Rotor (secondary) nearly open circuited Almost no induced voltage Locked rotor (stall / standstill) S= 1 R2 / S = R2 Xm and Rc >> other parameters so they can be ignored Starting torque ~ I^2 x R2
U of I Seminar Page 23 TAK 9 / 16 / 2016 Plot of Equivalent circuit performance
U of I Seminar Page 24 TAK 9 / 16 / 2016 The derivation of the equivalent circuit is quite complex but the result is quite simple. All inductive parameters are treated with stator / line frequency though X2 physically experiences variable frequency. R2 is subject to skin effect and deep bar effect as slip increases. As a result R2 is greater at stall than running thereby increasing motor torque. (R2start > 1.5 x R2run). X2 is subject to saturation of the slot leakage due to frequency and high current at stall. As a result X2 is lower at stall than running. This increased locked current and thereby increases torque. (X2start < 1.5x X2run). With these effects varying with slip, the calculated motor performance looks more like actual performance of the motor as follows:
U of I Seminar Page 25 TAK 9 / 16 / 2016 FLT (full load torque) FLA (full load amps) BDT Locked PF LRT LRA With this enhance modeling: Locked current ( LRA) increases from 69 amps to 92 amps ( X1 + X2 reduced to 75% ) Locked torque (LRT) increase from 21 to 79 in # ( R2 increased to 210% of running value) Locked power factor increase from 19% to 38% Breakdown torque ( BDT) increases from 97 to 111 in # Some motors exhibit a belly in the torque curve called pull up torque (PUT). It s hard to predict or model.
U of I Seminar Page 26 TAK 9 / 16 / 2016 Running performance / Load saturation
U of I Seminar Page 27 TAK 9 / 16 / 2016 3. Motor Design and Application: The equivalent circuit, magnetics, and D^2xL The previous discussion of the equivalent circuit suggest that motor performance boils down to designing something that has the equivalent circuit parameters that will provide performance desired. How is this tied to magnetics and size or volume of the motor? What about thermal capability? The key parameter in motor performance / capability are X1, X2 and R2. All three are controlled by slot dimension, the number of turns (squared), and the size of the coil. (Slot permeance is a detailed subject of its own.) The parameter Xm, R1 are secondary parameters whose effects we wish to minimize. Ideally we want Xm to be infinite (to improve power factor) and R1 to be zero. Both of these parameters are strongly influenced by the size of the machine and the number of turns in the coils / winding. R1 and Rc in large part determine the machine losses and have no positive effect. R2 controls rotor loss but is special in that it controls locked torque and the slope of the torque curve in the running section. When flux loading of a machine is higher it can have less turns in the primary, having the big effect of reducing the equivalent circuit parameters and increasing machine capability. On the
U of I Seminar Page 28 TAK 9 / 16 / 2016 down side increasing flux loading decreases Xm and Rc increasing magnetic losses. (N= turns per phase) Φ= total flux per pole in lines The idea of D^2xL relates torque capability of a machine to volume measured at the air gap. Sizing constants (torque per air gap volume) consider magnetics, losses, heating and experience.
U of I Seminar Page 29 TAK 9 / 16 / 2016 No load saturation Xm, and Rc If the rotor or secondary current is 38 amps at unity power factor then: I = (21^2+38^2) = 43.4 amps PF = 38 / 43.4 =.876
U of I Seminar Page 30 TAK 9 / 16 / 2016 Is low rotor resistance a good thing? Consider if the rotor of the RJ motor was copper instead of aluminum. Customer says I want a motor with high efficiency a copper rotor but it needs to have good starting performance and a decent power factor when starting.??#??+
U of I Seminar Page 31 TAK 9 / 16 / 2016 Shaping the speed / torque curve
U of I Seminar Page 32 TAK 9 / 16 / 2016 How does voltage affect the motor? Torque profile proportional to V^2 ( 85 / 111.5)^2 =.58 Current proportional to V (85 / 111.5) =.76 Power factor unchanged Motor slows down torque margin eroded. How about at load conditions? Speed will reduce with lower voltage Simple load model Depends on load type (constant HP, constant torque, cube law) Current and PF depend on load change and dominance of magnetizing versus working current.
U of I Seminar Page 33 TAK 9 / 16 / 2016 How does frequency affect the motor? Torque inversely proportional to (frequency)^2 T@ 650 hz = (400/650)^2 of 400 hz = 38% Current inversely proportional to frequency I @ 650 hz = (400/650) of 400 hz = 62% For both V and F T ~ ( V / F)^2 I (starting) ~ (V / F) V / F = flux, magnetic loading / strength At low frequency R1 and R2 become more dominant and erode torque and current.
U of I Seminar Page 34 TAK 9 / 16 / 2016 Constant V over F operation Torque fade at locked rotor
U of I Seminar Page 35 TAK 9 / 16 / 2016 Wound rotor and high resistance rotors Typical applications: Speed Control ( historic, inefficient) Punch presses, crushers, grinders Amusement park rides
U of I Seminar Page 36 TAK 9 / 16 / 2016 Brine resistors for rotor circuits can be 100 gallons or more!
U of I Seminar Page 37 TAK 9 / 16 / 2016 4. Motoring, Generating, and Braking / Plugging So far we have talked about operation between standstill and synchronous speed (1 to 0 slip). But what about speeds above synchronous speed or if the motor is turning backwards (wind milling) when it is started? The model is equally valid for these situations: Above synchronous speed the motor is in the generating regime. Slip is negative and the power of R2 becomes negative. At negative speed slip is greater than 1 and the rotor resistance and torque is less than that at stand still. R2 / S < R2 if S > 1 Induction generators must be connected to voltage for excitation. Vehicle induction motor drives braking (AC traction locomotives and road vehicles). Windmills with low frequency power fed into the wound rotor. Small hillside hydropower units.
U of I Seminar Page 38 TAK 9 / 16 / 2016
U of I Seminar Page 39 TAK 9 / 16 / 2016 5. Specifications NEMA and Aircraft NEMA National Electrical Manufacturers Association NEMA MG 1 specifies many of the details of commercially available motor (AC / DC, induction, synchronous, etc.). Induction motors are the biggest impact area. Key specification areas include: Lead / terminal marking requirements dual voltage, starting modes, direction of rotation, and field winding for DC motor. Frame dimensions motor frame sizes are describe by NEMA. 48 frame, 56 frame, 184TD, 324U.. C flange, D flange Integral frame induction motor certain horsepower and speed combinations are assigned to specific frame sizes. Induction motor types type B torque curve. Type D etc. Type B is standard BDT = 200%, LRT = 130%, PUT = 100% for a 25 HP motor. Locked rotor Code locked KVA per horsepower. Code G = 5.6 to 6.3 locked KVA per HP Code F = 5 to 5.6 locked KVA / HP Temperature rise requirements for different classes of insulation Enclosure requirements open drip proof, TEFC ( totally enclose fan cooled), TENV ( totally enclosed non ventilated) Efficiency standards big area today, high efficiency, premium efficiency.
U of I Seminar Page 40 TAK 9 / 16 / 2016 Aircraft Electrical Specifications: Standards: MIL STD 704 Aircraft Electrical Power Characteristic MIL STD M 7969 General Specification for Aircraft Alternating Current Motors, 400 Cycle 115 / 200 Volt System RTCA DO 160 Environmental Conditions and Test Procedures for Airborne Equipment Systems: 115 / 200 VAC (most commercial aircraft) 230 / 400 VAC ( Airbus A350 and Boeing 787) 28 VDC ( 24 volt battery) 270 VDC ( 1.35 x 200 VLL) CF (constant frequency) 400 hertz ( constant speed drive) NF (narrow frequency) 360 to 650 hertz ( tied to engine speed) WF (wide / wild frequency) 360 to 800 hertz Key Terms: POR point of regulation UET utilization equipment terminals Normal / Abnormal / Emergency Voltage and Frequency Essential Bus / Non Essential Bus RAT Ram Air Turbine PMP Pump Motor package ACMP / DCMP ( AC or DC motor pump)
U of I Seminar Page 41 TAK 9 / 16 / 2016 Key ACMP Attributes and Requirements Rotary Piston Pumps Regulated via hanger angle and hydraulic control system Regulation curve to work in parallel with other pumps Max electrical load defined in amps Maximum weight specifications Temperature 65F to 225 F Altitudes to 51,000 ft. Explosion proofness