EE 1403 - SOLID STATE DRIVES UNIT 1 - Fundamentals of Electric Drives

Electrical Drives Drives are systems employed for motion control Require prime movers Drives that employ electric motors as prime movers are known as Electrical Drives

Electrical Drives About 50% of electrical energy used for drives Can be either used for fixed speed or variable speed 75% - constant speed, 25% variable speed (expanding)

Example on VSD application Constant speed Variable Speed Drives motor pump valve Supply Power In Power loss Mainly in valve Power out

Example on VSD application Constant speed Variable Speed Drives motor pump valve Supply motor PEC pump Supply Power loss Mainly in valve Power out Power In Power In Power loss Power out

Example on VSD application Constant speed Variable Speed Drives motor pump valve Supply motor PEC pump Supply Power In Power loss Mainly in valve Power out Power loss Power In Power out

Conventional electric drives (variable speed) Bulky Inefficient inflexible

Modern electric drives (With power electronic converters) Small Efficient Flexible

BLOCK DIAGRAM OF ELECTRIC DRIVE

Components in electric drives Motors DC motors - permanent magnet – wound field AC motors – induction, synchronous (IPMSM, SMPSM), brushless DC Applications, cost, environment Power sources DC – batteries, fuel cell, photovoltaic - unregulated AC – Single- three- phase utility, wind generator - unregulated Power processor To provide a regulated power supply Combination of power electronic converters More efficient Flexible Compact AC-DC DC-DC DC-AC AC-AC

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Components in electric drives Control unit Complexity depends on performance requirement analog- noisy, inflexible, ideally has infinite bandwidth. digital – immune to noise, configurable, bandwidth is smaller than the analog controller’s DSP/microprocessor – flexible, lower bandwidth - DSPs perform faster operation than microprocessors (multiplication in single cycle), can perform complex estimations

AC-DC Converters or Rectifiers

AC-DC Converters or Rectifiers (Cont.)

AC Voltage Controller

VSI Controlled Inverter for IM Drive

CSI Controlled Drives for IM

DC – DC Converter (Chopper)

Overview of AC and DC drives Extracted from Boldea & Nasar

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Overview of AC and DC drives DC motors: Regular maintenance, heavy, expensive, speed limit Easy control, decouple control of torque and flux AC motors: Less maintenance, light, less expensive, high speed Coupling between torque and flux – variable spatial angle between rotor and stator flux

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Overview of AC and DC drives Before semiconductor devices were introduced (<1950) AC motors for fixed speed applications DC motors for variable speed applications After semiconductor devices were introduced (1950s) Variable frequency sources available – AC motors in variable speed applications Coupling between flux and torque control Application limited to medium performance applications – fans, blowers, compressors – scalar control High performance applications dominated by DC motors – tractions, elevators, servos, etc

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Overview of AC and DC drives After vector control drives were introduced (1980s) AC motors used in high performance applications – elevators, tractions, servos AC motors favorable than DC motors – however control is complex hence expensive Cost of microprocessor/semiconductors decreasing –predicted 30 years ago AC motors would take over DC motors

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Motor steady state torque-speed characteristic Synchronous mch Induction mch Separately / shunt DC mch Series DC SPEED TORQUE By using power electronic converters, the motor characteristics can be changed at will

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Load steady state torque-speed characteristic Frictional torque (passive load) Exist in all motor-load drive system simultaneously In most cases, only one or two are dominating Exists when there is motion SPEED T~ C Coulomb friction T~ 2 Friction due to turbulent flow T~  Viscous friction TORQUE

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Load steady state torque-speed characteristic Constant torque, e.g. gravitational torque (active load) SPEED TORQUE Gravitational torque  TL Te Vehicle drive FL gM TL = rFL = r g M sin 

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Load steady state torque-speed characteristic Hoist drive Speed Torque Gravitational torque

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Load and motor steady state torque At constant speed, Te= Tl Steady state speed is at point of intersection between Te and Tl of the steady state torque characteristics Tl Te Torque r2 r3 r1 Steady state speed r Speed

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Unavoidable power losses causes temperature increase Insulation used in the windings are classified based on the temperature it can withstand. Motors must be operated within the allowable maximum temperature Sources of power losses (hence temperature increase): - Conductor heat losses (i2R) - Core losses – hysteresis and eddy current - Friction losses – bearings, brush windage

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Electrical machines can be overloaded as long their temperature does not exceed the temperature limit Accurate prediction of temperature distribution in machines is complex – hetrogeneous materials, complex geometrical shapes Simplified assuming machine as homogeneous body Ambient temperature, To p1 Thermal capacity, C (Ws/oC) Surface A, (m2) Surface temperature, T (oC) p2 Emitted heat power (convection) Input heat power (losses)

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Power balance: Heat transfer by convection: , where  is the coefficient of heat transfer Which gives: With T(0) = 0 and p1 = ph = constant , , where

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Heating transient  t Cooling transient t 

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations The duration of overloading depends on the modes of operation: Continuous duty Load torque is constant over extended period multiple Steady state temperature reached Nominal output power chosen equals or exceeds continuous load Continuous duty Short time intermittent duty Periodic intermittent duty Losses due to continuous load t p1n 

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Short time intermittent duty Operation considerably less than time constant,  Motor allowed to cool before next cycle Motor can be overloaded until maximum temperature reached

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Short time intermittent duty p1s p1 p1n  t1 t

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Short time intermittent duty  t1 t

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Periodic intermittent duty Load cycles are repeated periodically Motors are not allowed to completely cooled Fluctuations in temperature until steady state temperature is reached

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Periodic intermittent duty p1 heating coolling t

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Periodic intermittent duty Example of a simple case – p1 rectangular periodic pattern pn = 100kW, nominal power M = 800kg = 0.92, nominal efficiency T= 50oC, steady state temperature rise due to pn Also, If we assume motor is solid iron of specific heat cFE=0.48 kWs/kgoC, thermal capacity C is given by C = cFE M = 0.48 (800) = 384 kWs/oC Finally , thermal time constant = 384000/180 = 35 minutes

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Thermal considerations Periodic intermittent duty Example of a simple case – p1 rectangular periodic pattern For a duty cycle of 30% (period of 20 mins), heat losses of twice the nominal,

Type of Loads Load torque can be of two types Active load torque:- Active torques continues to act in the same direction irrespective of the direction of the drive. e.g. gravitational force or deformation in elastic bodies. Passive load torque:- the sense of the load torque changes with the change in the direction of motion of drive. e. g. torques due to friction, due to shear and deformation of inelastic bodies

Type of Loads (Cont.) It is a passive load to the motor. Load torque is independent of the speed of the motor. Characterized by the requirement of an extra torque at very near zero speed. It is also known as break away torque or stiction.

Type of Loads (Cont.) Viscous Friction Load Torque is directly proportional to the speed. Calendaring machines, eddy current brakes and separately excited dc generators feeding fixed resistance loads have such characteristics. Viscous Friction Load

Type of Loads (Cont.)

Type of Loads (Cont.) Fan type Load Load torque magnitude is proportional to some power of speed. Centrifugal pumps, propeller in ships or aeroplanes, fan or blower type of load has such characteristics. For fan, Fan type Load

Types of Load (Cont.) Constant Power Load Hyperbolic speed-torque characteristics, where load torque is inversely proportional to speed or load power is constant. Certain type of lathes, boring machines, milling machines, steel mill coilers etc are having this type of load characteristics. Constant Power Load

Types of Load (Cont.) Load torques that vary with time Load variation with time can be periodic and repetitive in certain applications. One cycle of the load variation is called a duty cycle. The variation of load torque with time has a greater importance in the selection of a suitable motor. Classification of loads that vary with time: (a) Continuous, constant loads: Centrifugal pumps or fans operating for a long time under the same conditions, paper making machines etc. (b) Continuous, variable loads: Metal cutting lathes, hoisting winches, conveyors etc.

Type of Loads (Cont.) (c) Pulsating loads: Reciprocating pumps and compressors, frame saws, textile looms and generally all machines having crank shaft. (d) Impact loads: Apparent, regular and repetitive load peaks or pulses which occurs in rolling mills, presses, shearing machines, forging hammers etc. Drives for such machines will have heavy fly wheels. (e) Short time intermittent loads: Almost all forms of cranes and hoisting mechanisms, excavators, roll trains etc. (f) Short time loads: Motor generator sets for charging batteries, servo motors used for remote control of clamping rods of drilling machines. Loads of the machines like stone crushers and ball mills are characterized by frequent impact of small peaks so they are classified as continuous variable loads rather than the impact loads

Types of Load (Cont.) One and the same machine can be represented by a load torque which either varies with the speed or with the time. For example, a fan load whose load torque is proportional to the square of the speed, is also a continuous, constant load. Load torque of a crane is independent of the speed and also short time intermittent nature. Rocking pumps for petroleum have a load which vary with angular position of the shaft, but also be classified as a pulsating load.

Type of Load (Cont. High speed Hoist Traction Load (Constant torque; but with viscous friction)

Power requirement for different Loads Mine Hoist Polishing Machine

Power requirement for different Loads (Cont.) Sheering machine for cutting Textile loom

Power requirement for different Loads (Cont.) Planing Machine

Power requirement for different Loads (Cont.) Drilling Machine Grinding Machine

Dynamics of Motor-Load Combination The motor and the load that it drives are represented by the rotational system. The basic equation of the motor-load system is,

Dynamics of Motor-Load Combination where is motor and load torque respectively in Nm, J is the moment of inertia and is the angular velocity in rad/sec. Motor torque is the applied torque and load torque is the resisting torque. Different states at which an electric drive causing rotational motion are (i) :- The drive will be accelerating, in particular, picking up speed to reach rated speed. (ii) :- The drive will be decelerating and particularly, coming to rest. (iii) :- The motor will continue to run at the same speed, if it were running or continue to be at rest, if it were running.

Quadrant diagram of Speed-Torque Characteristics The speed is assumed to be positive if the direction of rotation is anticlockwise or in such a way to cause an ‘upward’ or forward motion of the drive. For reversible drive positive direction of the speed can be assumed arbitrarily either clockwise or anticlockwise. The motor torque is positive if it produces increase in speed in the positive sense. The load torque is assigned the positive sign when it is directed against the motor torque. Plot of speed torque characteristics of the load/ motor for all four quadrant of operation is known as quadrantal diagram.

FOUR QUADRANT OPERATION

Four Quadrant Operation Motor is driving a hoist consisting of a cage with or without load, a rope wound on to a drum to hoist the cage and a balance weight of magnitude greater than that of the empty cage but less than that of the loaded cage. The arrow in the figure indicates the actual directions of the motor torque, load torque and motion in four quadrants. The load torque of the hoisting mechanism is of active type and assumed to be constant due to negligible friction and windage for low speed hoist. Speed torque curve of the hoist is represented by vertical line passing through two quadrants. Loaded hoist characteristics in first and fourth and unloaded in second and third quadrants. In the first quadrant the load torque acts in the opposite direction to that of rotation. Hence to drive the loaded hoist up, the motor developed torque must be in the direction of the rotation or must be positive. The power will also be positive so, this quadrant is known as ‘forward motoring quadrant’.

Four Quadrant Operation (Cont.) Speed torque curve of the hoist is represented by vertical line passing through two quadrants. Loaded hoist characteristics in first and fourth and unloaded in second and third quadrants. In the first quadrant the load torque acts in the opposite direction to that of rotation. Hence to drive the loaded hoist up, the motor developed torque must be in the direction of the rotation or must be positive. The power will also be positive so, this quadrant is known as ‘forward motoring quadrant’. The hoisting up of the unloaded cage is represented in the second quadrant. As the counterweight is heavier than the empty cage, the speed at which hoist moves upwards may reach a very high value. To avoid this, the motor torque must act in the opposite direction of rotation or motor torque must be negative. The power will be negative though the speed is positive, so this quadrant is known as ‘forward braking quadrant’. The third quadrant represents the downward motion of the empty cage. Downward journey will be opposed by torque due to counterweight and friction at the transmitting parts, move cage downwards the motor torque should must be in the direction of the rotation. Electric machine acts as a motor but in the reverse direction compared to first quadrant. The torque is negative as speed is increased I the negative direction, but the power is positive, this quadrant is known as ‘Reverse motoring quadrant’.

Four Quadrant Operation (Cont.) Fourth quadrant has the downward motion of the loaded cage. As loaded cage has more weight than the balanced weight to limit the speed of the motion, motor torque must have opposite polarity with respect to rotation and acts as a brake. The motor torque sign is positive, but as speed has negative direction; the power will be negative, this quadrant is designated as ‘Reverse braking quadrant’

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Torque-speed quadrant of operation  1 2 T -ve +ve Pm -ve T +ve +ve Pm +ve T 3 4 T -ve -ve Pm +ve T +ve -ve Pm -ve

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 4-quadrant operation  Direction of positive (forward) speed is arbitrary chosen Te m m Te Direction of positive torque will produce positive (forward) speed Quadrant 2 Forward braking Quadrant 1 Forward motoring T Quadrant 3 Reverse motoring Quadrant 4 Reverse braking Te Te m m

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Ratings of converters and motors Torque Power limit for transient torque Transient torque limit Continuous torque limit Power limit for continuous torque Maximum speed limit Speed

Steady State Stability of an Electric Drive The drive is said to be in equilibrium if the torque developed by the motor is exactly equal to the load torque. If the drive comes out of the state of equilibrium due to some disturbance, it comes back to steady state for stable equilibrium but for unstable equilibrium the speed of the drive increases uncontrollably or decreases to zero. When the drive coming out of the state of equilibrium preserves it steady state at different speed (lying in small range), it is said to be in neutral range. The stability of the motor load combination is defined as the capacity of the system which enables it to develop forces of such a nature as to restore equilibrium after any small departure therefore. Equilibrium state of the drive mainly disturbs because of the following two types of disturbances, 1.Changes from the state of equilibrium takes place slowly and the effect of either the inertia or the inductance is insignificant – Steady state stability. 2.Sudden and fast changes from the equilibrium state so effect of both inertia and inductance can not be neglected- Dynamic or transient stability

Steady State Stability of an Electric Drive (Cont.) Criteria for steady state stability:- Let the equilibrium of the torques and speed is and the small deviations are After the displacement from the equilibrium state the torque equation becomes,

Steady State Stability of an Electric Drive (Cont.) Considering the small deviation, changes can be expressed as a linear function of change in speed, From the torque equation, where all quantities are expressed in terms of their deviations from the equilibrium,

Steady State Stability of an Electric Drive (Cont.) Solution is, Where, is the initial value of the deviation in speed. For the stable system the exponent must be negative, so speed increment will disappear with time. The exponent will always be negative if,

Steady State Stability of an Electric Drive (Cont.) Criteria for the steady state stability is for a decrease in the speed the motor torque must exceeds the load torque and for increase in speed the motor torque must be less than the load torque. Load torque results in a stable equilibrium point, and the load torque results in an unstable situation.

Steady State Stability of an Electric Drive (Cont.) To check the stability at an operating point of the motor, if an increase in speed brings greater increase in load torque than the motor torque, the speed will tend to decrease and return to its original value, so operating point will be a stable point else operating point will be an unstable point. Cases (a), (b) and (c) represents stable operation of drive. Cases (d), (e) and (f) represents unstable operation of drive. Case (g) represents indeterminate condition. Various Speed and Torque Curves of Motor and Load

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1 Steady-state stability

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