Power Electronic Drivesy DC Drives y AC Drives y Classification :y Induction motor drives y Synchronous motor drives

2

Advantages of Induction motor Drives

1. AC motor are less expensive 2. Ac motors have low maintenance 3. For the same rating, ac motors are higher in weight as compared to dc motors. 4. AC motors can work in hazardous areas like chemical, petrochemical etc. whereas dc motors are unsuitable for such environments because of commutator sparking.3

DisadvantagesPower converters for1. the control of ac motors are more complex. 2. ac motors are more expensive 3. ac motors generate harmonics in the supply system & load circuit. Hence AC motors gets derated

4

Torque Speed Characteristics1. Motoring (0> fr , then the adjacent pulses are very close to each

other, consequently, the output is approximately sinusoidal. p y RMS value of output voltage, Hm Vo ! V m !1 T y The fundamental component in the output phase voltage of a PWM inverter operating with sinusoidal PWM is given by where m is the modulation index. y For given harmonic content in the motor terminal voltage, the current harmonics are reduced when the motor has higher leakage reactance, this reduces derating and torque pulsations. hence

42

SPWM contd..y Sinusoidal modulation eliminates lower order harmonics. Dominant

harmonics are the order of (2p 1).y As lower order harmonics are eliminated and higher order harmonics

are easily filtered, output has very low harmonic content.y With over-modulation,(m>1, square wave) fundamental output

voltage increases but it causes more harmonics.y As p increases, order or significant harmonic increases and the

filtering requirements are minimized.y But higher values of p needs higher switching frequency and hence

switching loss increases and inverter efficiency decreases.

43

Closed loop speed control and converter rating for VSI and cycloconverter IM drives

Closed loop slip controlled PWM inverter drive with regenerative braking44

Closed loop control contd..y Inner slip (current) loop and outer speed loop y Slip speed ensures motor operation b/w y y y y y y yms

and Tmax at all

frequencies. PWM inverter has the capability to operate in all four quadrants. The drive is applicable to any VSI or cycloconverter. A step increase in speed A step decrease in speed Operation above base speed up to sm. Slip speed upto break down torque Motor current can be allowed to carry several times the rated current during transient operations. Converter and inverter should be capable to provide these currents.45

Variable Frequency Control from a current source

Equivalent circuit with current source

y Neglecting Stator impedance y y y Im= ? y

46

Rotor Voltage Control for Wound-Field Induction Motors..y In a wound-field induction motor the slip rings allow easy

recovery of the slip power which can be electronically controlled to control the speed of the motor. y The oldest and simplest technique to invoke this slip-power recovery induction motor speed control is to mechanically vary the rotor resistance. y Slip-power recovery drives are used in the following applications:y y y y y

Large-capacity pumps and fan drives Variable-speed wind energy systems Shipboard VSCF (variable-speed/constant frequency) systems Variable speed hydro-pumps/generators Utility system flywheel energy storage systems47

Rotor Voltage Control contd..speed torque curvem

Increasing Rr R1 Tcms

R2 R3 R4

R4 >R3>R2>R1

Tmax

Torque 0 Ts

Speed Torque curves of motor with variable rotor resistance

48

Rotor Voltage Control contd..

From this equation it is clear that the torque-slip curves are dependent on the rotor resistance Rr. The curves for different rotor resistances are shown on the next slide for four different rotor resistances (R1-R4) with R4>R3>R2>R1. Max torque is independent of rotor resistance Speed at which max. torque produced changes with rotor resistance. Motor torque capability is unaltered even at low speeds 49

Rotor Voltage Control contd..

50

Rotor Voltage Control contd..y Although this approach is very simple, it is also very inefficient because the slip energy is wasted in the rotor resistance. y Rotor resistor can be varied stepless y AC voltage is rectified and fed to a fixed resistance and the effective value of RAB can be varied by changing duty ratio y Ld reduces the ripple and discontinuity in the dc link current Id

51

Rotor Voltage Control contd..Ir120

Id 0 /6 5 /6

120

2Id

t

The rms rotor current is

52

Rotor Voltage Control contd..y Average value of resistance RAB = R(1- ) y Power consumed by RAB is PAB = Id2 RAB y Power consumed by RAB /phase = PAB/3 y = 0.5 R(1- ) Ir2

equation suggests that the rotor circuit resistance per phase is increased by 0.5 R(1- ). y The total circuit resistance /phase will be RrT = Rr + 0.5 R(1- ) y RrT can be varied from Rr to Rr + 0.5 R as is changed from 1 to 0.y The

53

Closed loop Control

54

Rotor Voltage Control closed loop contd..y It consists of inner current loop and outer speed loop. y Rotor current Ir and hence Id has contant value at max torque point both during motoring and plugging. y If the current limiter is made to saturate at this current the drive will accelerate and decelerate at max. torque, giving very fast transient response. y Adv : y 1.Smooth & stepless speed control. y 2.Fast response y 3.Less maintenance y 4.Compact size y 5.Rotor resistance remains balanced b/w the three phase for all operating points55

Slip Power Recovery Scheme

y WRIM operation with an injected voltage in the rotor. y Red arrow represents the polarity of Vr when Pr is

ve. y Blue arrow represents the polarity of Vr when Pr is +ve. y Assume stator to Rotor turns ratio is unity. y When Pcu is neglected , Pm= Pg- Pr. Where Pr is the power absorbed by the source Vr.56

Slip Power Recovery Scheme contd..y When Pr = 0, motor runs at natural y y y y y

-T chars. When Pr = +ve, then Pm will decrease, hence motor runs at reduced speed for the same torque. When Pr = Pg, then Pm = 0, m=0 Thus Pr varies from 0 to Pg, speed varies from ms to zero. When Pr = -ve, then Pm >Pg hence motor runs at higher speed than ms.m

57

Slip Power Recovery Scheme contd..y Instead of wasting the slip power in the rotor

circuit resistance, a better approach is to convert it to ac line power and return it back to the line. y Two types of converter provide this approach: 1) Static Kramer Drive - only allows operation at sub-synchronous speed. 2) Static Scherbius Drive allows operation above and below synchronous speed.

58

Static Kramer Drive (contd)A schematic of the static Kramer drive is shown below:

Static Kramer Drive (contd)y The slip power from rotor is rectified by dc

bridge. y Inductor Ld smoothens the ripples in the rectified voltage. y DC voltage is converted to ac voltage at line frquency by line commutated inverter. y As the power flow is from rotor circuit to supply SKD offers constant torque. y Speed control is below wms only (approx. of wms).

60

Static Scherbius DriveTransformer source side to converter side turns ratio m : 1

Stator to rotor turns ratio n : 1

Vd1 Vd2

61

Static Scher ius Drive (contd)

62

Static Scherbius Drive contd..y y

The static Scherbius drive overcomes the forward motoring only limitation of the static Kramer drive.

Regenerative mode operation requires the slip power in the rotor to flow in the reverse direction. This can be achieved by replacing the diode bridge rectifier with a thyristor bridge. This is the basic topology change for the static Scherbius drive from the static Kramer drive. y A portion of rotor ac power is converted into dc by a Controlled Rectifier (diode bridge speed below ms only) .63

SPRS contd.. Analysisy Power fed back Pr can be controlled by controlling inverter counter emf Vd2 y The dc link inductor is provided to reduce ripple in dc link current Id. y Since slip power is fed back to the source , the drive has high efficiency. y To make analysis easy, assume the firing angle of the controlled rectifier is zero ,so Vd1 isy y y y

Where

is firing angle64

SPRS contd.. Analysisy Neglecting the drop across the inductor y Vd1 + Vd2 =0. y s = -n/m cos = -a cos where a=n/m. y Max. value

is restricted to 165 for safe commutation of inverter. y Slip s varies from 0 to 0.966a when varies from 90 to 165. y By appropriate choice of a required speed range can be obtained. y Transformer is used to match the voltages Vd1 to Vd2.65

SPRS contd.. Analysisy At the lowest speed required from the drive Vd1 will

have the max. value Vd1m=V smax /n where smax is the value of slip at lowest speed. y If is restricted to 165, m is chosen such that the inv. voltage has a value Vd1m when =165.

y ie m = 0.966 n/ smax y Such a choice of m ensures inv. Operation at highest firing angle at lowest motor speed, giving highest pf and lowest reactive power. y This improves the drive pf and reduces the reactive power at all speeds.

66

SPRS contd.. Analysisy Operation : y Drive is started by resistance control by keeping the

rectifier circuit open. y When the speed is within the control range of the drive the external resistance is removed and the rectifier circuit is connected.

67

SPRS contd.. Analysisy Power consumed by RAB /phase = PAB/3 = 0.5

R(1- ) Ir2 y When referred to dc link, resistance (sRs +Rr) will be 2(sRs +Rr). y The approx. dc equivalent circuit of the drive is Rd be the resistance of dc link inductor

68

SPRS contd.. Analysisy If the rotor copper loss is neglected.

sPg = |Vd2| Id . y Pg = |Vd2| Id/ s y T =Pg/ ms = |Vd2| Id/ (s ms) y This provides constant torque control .The speed torque characteristics are shown below.m

= 901 2

90