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Preface
Acknowledgements
Technical Support
Chapter 30
Introduction
Selection, testing, controls and protection of electric motors
Part II Switchgear assemblies and captive (emergency) power generation
Part 111 Voltage surges, overvoltages and grounding practices
Part IV Power capacitors: power factor improvement and system
Bus systems in including metal-enclosed non-isolated and
PART I
Selection, Testing, Controls and
Protection of Electric Motors
Theory,
Performance and Constructional
Induction Motors
- Introduction
- Brief theory of the operation of a polyphase motor
- Stator current
- Rotor current
- Motor output and torque
- Motor ratings and frame sizes
- Preferred ratings at different voltages
- Influence of service conditions on motor performance
- Voltage unbalance and system harmonics As standard practice, all motors are designed for a balanced
- Voltage and frequency variations
- No-load performance
- Effect of loading on motor performance
- Effect of steel of laminations on core losses
- Circle diagram
- Drawing the circle diagram (Figure 1.16)
- Inference from the circle diagram
The amount of slip will depend on the speed and torque characteristics of the motor and the supply voltage. The motor torque would vary approximately inversely as the square of the frequency.
Choice of voltage
Under normal conditions, the starting current is too high, while the corresponding starting torque is not so high. An attempt to limit the starting current by increasing the slip and the rotor resistance in a squirrel-cage motor can therefore compromise the motor's performance.
Mounting of motors
The higher the full load slip, the higher the rotor loss and rotor heat will be. The choice of starting current and rotor resistance is thus a compromise to achieve optimal performance.
1.1 3 Enclosures
Protected motors (degree of protection 1
It is used in a slip ring motor to control both Tst and Ist with suitable external resistors. Splashproof: As in (2), but unless the degree of protection is specified according to IEC 60034-5, this remains a vague term and is not in regular 3.
Totally enclosed fan-cooled motors (TEFC) (degree of protection
2 Screen protected, drip resistant (SPDP): As above, but with additional protection against dripping water Figure 1.
NEMA enclosures
1.1 5 Degree of protection
Cooling systems in large motors
5 above, except that the heat exchanger is not mounted externally nor is it an integral part of the machine. 0 above, except that instead of surface cooling, cooling is achieved by relative movement of the coolant over the machine.
The hot air circulating through the motor stator and rotor ducts passes through these heat exchangers and is cooled. 3 Air Cooled Closed Air Circuit (CACA) This cooling system is the same as CACW, except that instead of water, air flows through the top mounted heat exchangers.
1 The size of capacitors C, C or C2 will depend on the horsepower of the motor and the torque of the load. When the voltage is applied across the stator windings, a magnetic flux is developed throughout the pole, which cuts the copper ring placed at the tip of the pole.
StartYwinding
Capacitor start and capacitor run windings (d) Speed-torque characteristics of capacitor start and. e) Capacitor start or capacitor start-capacitor run 1-0 motor. Cl = Run capacitor C, = Start capacitor General arrangement (b2) High start and high run torques Figure 1.25 Capacitor start and capacitor run windings.
Motor Torque,
Load Torque and Selection of
Motors
- Motor speed-torque curve
- NEMA rotor designs
- Special designs of rotors
- Double squirrel cage motors
- Other designs of rotor cage Use of skin effect
- Effect of starting current on torque
- Selection of motors
- Motor heating during start-up
Since the stator current is a function of the rotor current, the motor torque is proportional to the square of the stator current. The power is proportional to the third power of the speed ( P = N 3 ) The power is proportional to the square of the speed ( P = Nz).
Heating during a no-load start-up
This expression, except for the mechanical design, is totally independent of the type of starting and the electrical design of the motor. So at no load, regardless of motor torque and gear type, the starting heat will remain the same.
Heating during an on-load start-up
In slip ring motors, most of the rotor heat is shared by the external resistance, a feature that makes it a better choice for frequent starting and stopping, and for driving loads that have high inertia. It has been seen that most of the stringent load requirements can also be met with high torque squirrel cage motors, manufactured with a judicious design of stator and rotor resistances, an efficient means of heat dissipation and a proper choice of active material.
Thermal withstand time
- Heating phenomenon in a motor during a stalled condition
Whenever the rotor locks in a region nearly corresponding to region I of the motor (Figure 2.17), this will indicate a stall condition. For safe stall conditions, t,e must be shorter than the thermal resistance time of the motor in the lockout or short circuit condition. i) 8 is called the permissible temperature rise in the closed state. ii).
List of formulae used
1 In the tables of relevant Standards in this book, while the latest editions of the standards are provided, it is possible that revised editions have become available. The year recorded against each standard may also refer to the year of its last amendment and not necessarily the year of publication.
Further reading
It is therefore advisable that readers should consult the relevant organizations for the latest version of a standard for more authentic references.
Duties of Induction Motors
- Duty cycles
- Continuous duty (CMR) (SI)
- Periodic duties
- Short-time duty (S,)
- Intermittent periodic duty ( S , )
- Intermittent periodic duty with start (S,) This is a sequence of identical duty cycles. each consisting
- Intermittent periodic duty with start and This is a sequence of identical duty cycles, each consisting
- Continuous duty with intermittent periodic loading (S,)
- Continuous duty with start and brake ( S , ) This is a sequence of identical duty cycles, each consisting
- Continuous duty with periodic speed This is a sequence of identical duty cycles, each consisting
- Non-periodic duty (S,)
- Duty with discrete constant loads (Sl0) This is a type of duty consisting of a number of varying
- Factor of inertia (FI)
- Heating and cooling characteristic curves
- Time constants
- Heating curves
- Drawing the thermal curves
- From cold conditions ( a ) For I , I 2 0 0 % I ,
- From hot conditions
- Rating of short-time motors
- Equivalent output of short-time duties
- Shock loading and use of a flywheel
- Size of flywheel
- Energy stored by the flywheel
The temperature rise corresponding to the rated current of the machine can be expressed exponentially with. For a closer setting of the relay, you can draw several points in the desired area.
List of formulae used
In this book, I table\ the relevant standards, while the latest editions of the standards are prohibited. Note that each standard may also refer to the year of its last amendment and not necessarily to the year of publication 7.
Starting of Squirrel Cage Induction
Direct on-line starting (DOL)
For heavy rotating masses, with large moments of inertia, this is the ideal switching method. With this in mind, even local electricity authorities sometimes restrict the use of DOL starting above a certain rating, for example I O h.p., for small installations.
- Reduced voltage starting
- Autotransformer (AIT) starting
- Soft starting
In addition, the start-up time must also be shorter than the thermal resistance time of the motor. The resistance of the electrolyte changes smoothly and helps the engine to start smoothly.
Starting and
Control of Slip-ring Induction Motors
- Important features of a slip-ring motor
- Starting of slip-ring motors
- Selection of rotor resistance
- Determining external resistance and Consider Figure 5.5 with six steps (rotor resistance unit
- Number of steps
- Duty cycle and duty rating of
- Temperature rise limits
- Hypothetical procedure to calculate the rotor resistance
- Calculation of time between each step
- Speed control of slip-ring motors
- Resistance for speed control
- Moving electrode electrolyte starters and controllers
- As a rotor resistance for slip-ring motors These are similar to stator resistance starters, as discussed
- Automatic speed control of slip-ring By making the electrodes move through a geared motor,
The slip ring motor speed can be varied up to 25% of rated speed. Also, since the torque is proportional to the rotor current equation (1. 1), the developed torque will be greater in this case.
Static Controls and Braking of Motors
Speed control in squirrel cage motors
- One winding
- Two windings
Due to limitation in motor size and flux distribution, winding groups of more than two are not recommended. The h g l e winding can be connected in delta/double star (NYV) to give two pole combinations in the ratio 2:I.
Speed control through solid-state technology
- Theory of application
- Effects of variable supply parameters on the performance of an induction motor
With the advent of static technology, which will be discussed later, it is now possible to use squirrel cage motors with the same ease and accuracy of speed control, even better than DC motors. Speed control of slip ring motors using solid state technology is popularly known as d ip recuperative systems as the slip power can also be fed back to the power source via a solid state feedback converter bridge which will be discussed later.
Vlfcontrol (speed control at constant torque)
The starting current can also be reduced to only 100-150% of the rated current or as desired, to the extent possible, by varying the Vlf appropriately. If more than 150% is required, the drive's rating is worked out based on the starting current requirement and its duration.
Phasor (vector) control
- Single phasor (vector) control
- Field-oriented control (FOC)
- Direct torque control (DTC)
In absolute terms, they can be represented by (6.2), where 8 represents the electric position of the rotor field in space relative to the stator. With this technology (any of the three methods mentioned above) it is now possible to achieve high machine performance ie.
Section 2
Rotor current oriented regulation is more popular among various manufacturers to achieve high speed regulation accuracy in induction machine. the rated torque (T,) at zero speed (during start-up) should be able to take most loads smoothly and smoothly.
Section 3
Switching block controls the switching of the inverter unit to regulate its output to the preset reference line quantities. Pulse Codes - For feedback of the actual speed of the motor and the angular position of the rotor relative to the stator at a particular time.
Section 4
Any errors in these two data (reference and actual data) are compared by these comparators at extremely high speed, as mentioned above, and provide Tand @error signals to the inverter unit switching logistics. 3 Drive performance would also depend on the accuracy of the motor mathematical model used for phasor control.
Section 7
- Use of phasor control for flux braking
- Control and feedback devices
- Speed sensors
- Application of solid-state tech no1 ogy
- Power diodes
- The power transistor family
- The thyristor family
- Conduction and commutation
- Circuit configurations of semiconductor devices
- Converter or rectifier unit
- Inverter unit
- Voltage source inverter (VSI) using IGBTs
- Current source inverter (CSI) (to vary 1, a n d f )
- Cyclo converters (frequency converters) In addition to the above inverter systems there is one
- The regenerative schemes
- Smoothing ripples in the d.c. link
- Providing a constant d.c. voltage source
- Providing a constant current source
- Generation of harmonics and switching surges in a static
- Suppressing the harmonics (in phase- Phase-controlled rectifier circuits generate excessive odd
- Protection of semiconductor devices and motors
- Overvoltages and voltage surges caused by disturbances in an LT system
- Energy conservation through solid-state technology
- Illustration of energy conservation In an industry there may be many drives that may not be
- Computation of energy saving
- Application of static drives .1 Soft starting (Vlf control)
- Soft stopping
- Slip recovery system (to control wound As discussed earlier, the motor speed-torque
- Application of solid-state technology in the operation of a process plant
- Other applications
- Speed variation through
L e transformers and cables can be provided at the entrance of static circuits. The output of the inverter unit being in the form of a non-sinusoidal voltage waveform also adds switching transients.
Static drive versus fluid coupling
Thermal power plant accessories such as flow control of primary air fan, ID fan and forced draft fans, boiler feed pumps circulating water pumps and condensate pumps, coal handling plant (eg ball mill, wagon tipper and stacker recycler).
F ASS
- D.C. drives
- Braking
- Types of braking
- Induction generators
- Number of starts and stops
- Inching or jogging
They grip a brake drum or disc, rigidly connected to the NDE of the motor shaft. This braking force will depend on the size of the car and the time of braking.
List of formulae used
State agencies in India for micro siting of windmills
Special-Pu rpose Motors
- Textile motors
- Loom motors (IS 2972 Part I) Electrical features
- Card motors (IS 2972 Part 11)
- Determining the size of motor
- Crane motors
- Sugar centrifuge motors
- Motors for deep-well pumps
- Deep-well turbine or a vertical wet Use of vertical hollow shaft motors
- Submersible pumps using submersible motors A more economical alternative is found in a submersible
- Motors for agricultural application
- Torque motors or actuator motors
- Surface-cooled motors
- Vibration and noise level
As for 'Cop Bottom Build' and 'Nose shape', the frame should be run at a lower speed to reduce end breakage, while the rest of the yarn can be run at a higher speed. The pump shaft passes through the motor shaft to the top of the motor and is bolted there.
Note In higher speed ranges and for HT motors, these levels of shaft vibration are generally of the same order or slightly better than prescribed in IEC 60034-14, which corresponds to Table 11.3. By providing a unidirectional axial flow fan By providing a sound-absorbing fan shroud on the non-drive side, as shown in Figure 7.12 By transforming the inlet axial airflow to a radial airflow, as illustrated in Figure 7.13, thereby significantly reducing friction and thus suction noise.
7.1 1 Motors for hazardous locations
- Classification of hazardous locations It is important to identify areas in accordance with the
- Classification of gases, chemical vapour Based on the ignition temperature of these inflammable
- Specification of motors for Zone 0 locations
- Specification of motors for Zone 1 locations
- Flame- or explosion-proof motors,
- Pressurized enclosures, type Ex. 'p' These may be standard TEFC motors suitable for operating
The maximum permissible temperature at the outer surface of the machine must also be limited so that it does not ignite the flammable substances at the installation. The limiting temperature must be less than or equal to the ignition temperature of the prevailing atmosphere shown in Table 7.4, otherwise the limiting temperature will become the same as the ignition temperature, according to Table 7.4.
