DspEdu 2.1

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List of Articles
 

1. Introduction

2. Smart Motor Controllers

2.1 Features of a Commercially Available  Smart Motor Controller

3.Variable Frequency AC Drives

3.1 Basic Principles of Variable frequency Operation of Induction Motors

3.2 Slip Compensation

3.3 Starting a Motor on VFD

3.4 Stopping a Motor on VFD

3.5 Voltage Boost in VFDs

3.6 Features of a Commercially Available Adjustable Speed Drive
 

 [ This article was prepared for a three days' short-term course Department of Electrical Engineering, NIT Calicut offered to working professionals from  Industry  in 1999. This HTML version was uploaded on 20^th March 2004]

1. Introduction

Two types of Power Electronic Controllers are in use to modify/improve the performance characteristics of Induction Motor Drives in the Industry. They are (1) Smart Motor Controllers(SMCs) and (2) Adjustable Speed Drives or Variable Frequency Drives(VFDs).The basic control action involved in a SMC is control of magnitude of applied voltage at a fixed frequency to achieve the objectives of start/stop/braking control and energy efficient operation at part loading condition. The basic control action involved in a VFD is to apply a variable frequency variable magnitude a.c voltage to the motor to achieve the aims of variable speed operation either from the process requirement point of view or from energy conservation point of view or from direct torque control point of view.

2. Smart Motor Controllers (SMC)

In SMCs three anti-parallel connected SCRs are used in series with the motor and are phase controlled to vary the fundamental component of voltage continuously to achieve a set of objectives. The connection is shown in Fig. 1.

DOL starting or Star-Delta starting is the popular starting methods for starting squirrel cage induction motors in the Industry. Both involve prominent torque transients (high amplitude pulsation) at starting resulting in shock and vibration in motor and driven machinery. Also such a motor start causes stress on the supply system too due to large starting currents involved. SMCs ensure a smooth start with desired control on starting torque and starting current and virtually eliminate torque pulsation at start. This results in less stress in the driven machinery and motor bearings and translates as less downtime and lowered maintenance cost in the long run. There are SMCs specifically designed to exercise only starting control and then they are called Starting Torque Controllers 

(STC). STC s are cheap since the power semiconductors need to function only during starting condition and can be small in size and cost. SMCs also exert control during stopping in a similar manner i.e. by controlling the applied voltage on the motor by phase control of thyristors. They can provide a braking action by introducing D.C. currents into the stator or by controlled plugging (additional thyristors will be needed for this). Also they can provide a soft stop if so desired by gradually decreasing the voltage across the motor on receiving a stop command.

Most of the motors in Industry run underloaded most of the time. This may be the natural result of variation in load as demanded by the process or this may be simply due to oversizing of motors. Drives like pumps, fans, compressors etc run under part load for extended periods of time. The losses in an Induction Motor include two parts-core losses which is a function of applied voltage and copper loss or load loss, which depends on the currents. If the applied voltage is reduced, the core loss comes down and copper losses go up, since for same power output the motor will draw higher active component of current from a lower voltage. But the increase in active current is partially offset by decrease in reactive component of current due to lower voltage. Thus, it is found that, at part loads the losses in the motor come down significantly as the applied voltage is reduced. For every load there is an optimum value of applied voltage for which the total motor losses will be a minimum. This value of voltage can be on-line estimated if the motor parameters and the motor load are known. Or a ‘minimum searching algorithms’ which seeks out the optimum voltage to minimise losses in real time in a periodic manner can be used to adjust the motor voltage continuously to save energy. A sophisticated SMC will do this too.

Features of a very popular Smart Motor Controller – SMC PLUS manufactured by M/s Allen-Bradley Company are explained below as a sample study. These units are sold and serviced in Kerala by M/s Sandra Power Engineers,1^st Floor, White House Building, Pullepady Road,Ernakulam,Cochin-682 018.

2.1  Features of a Commercially Available Smart Motor Controller

SMC PLUS from M/s Allen-Bradley cover LT motors in the 1-1000Amp range with voltage in the range 200-600V.The MV SMC PLUS series covers 2400-4200V range and 20-800Amps current range. SMC PLUS uses a microcomputer inside for control intelligence and offers three modes of starting - Soft Start with selectable Kick Start, Current Limit Start and Full Voltage Start- along with Energy Saving function as standard options. In addition , Pump Control option, Smart Motor Braking option, Slow Speed with Braking option, Accurate Stop option, Pre-set Slow Speed option, Soft Stop option etc are available on specification.

Soft Start provides smooth , stepless motor acceleration while minimising damage to gears, couplings and belts by reducing torque surges.(Fig.2).

Current Limit Starting increases productivity by limiting line disturbances caused by high inrush currents and also reduces starting torque. The current limit can be set from 50% of full load current upto 500% of full load current.(Fig.3).

In Full Voltage Mode the SMC PLUS acts like a solid state contactor. Full inrush current and locked rotor torque is the result.(Fig.3)

Soft Start provides Kick Start which can be activated if needed. Kick-start provides a pulse of torque to overcome stiction in high friction loads. Soft start reduces high starting torque.(Fig.5)

The Pump Control Option reduces surges caused by uncontrolled acceleration and decceleration of centrifugal pumps. The controller’s interactive algorithm provides controlled acceleration and decceleration of the pump motor without feedback devices.(Fig.6)

The Slow Speed with Braking Option combines the benefits of smart braking and slow speed options for applications that require slow set-up speeds and braking to a stop.(Fig.8)

The Accurate-Stop Option provides rapid braking to a slow speed and then braking to stop , facilitating cost-effective positioning control.(Fig.9)

The Slow Speed Option provides two pre-set slow speeds available in both the forward and reverse direction that facilitate process set-up and alignment.(Fig.10)

The main disadvantage of the SMCs is the introduction of large amount of harmonics in the a.c line due to phase control switching of thyristors. This is especially true if the energy saver function is always active. Sometimes the increase in motor losses due to harmonic currents in the motor more or less offsets the reduction in losses obtained by voltage reduction. A thorough study of the controller and motor will be needed to evaluate the net energy savings possible in this context.
 

3. Variable Frequency A.C Drives

Variable Frequency Induction Motor Drives are widely used in various Industries such as 

A wide range of power ratings ranging from 100W to megawatts is available in the Indian market with a host of additional features in addition to the basic variable speed function.

Both Voltage Source Inverters and Current Source Inverters are used to drive the Induction Motor in VFDs.However Voltage Source Inverter(VSI) have been more popular and only VSI based systems are addressed in this lecture.

The power circuit of a VFD consists of a AC/DC converter as the front end, a three phase VSI and the Induction Motor.The AC/DC converter may be a simple uncontrolled three phase bridge rectifier with capacitor filter or a three phase controlled thyristor converter with L-C filter. It transforms the line into a DC voltage (usually in the 650-750VDC range) and this DC Voltage is the input for the three phase VSI.The VSI output is fed to the Induction Motor.

Two kinds of VSIs have been popular. They are six-step square wave VSI and Sinusoidal Pulse Width Modulated (SPWM) Sine Wave output VSI. In the Six-Step Inverter, the switching of power devices is simple and reliable and the inverter tends to be more rugged; efficient and reliable. However the output waveshape is far from sinusoidal and will result in a harmonic-rich motor current. Increased motor losses due to harmonic currents will require derating of the motor by about 5-10%. Also, at low frequencies, i.e. at low speeds, the harmonics in currents will cause torque pulsation, which will get translated as speed pulsation of driven machinery. With simple square wave switching in the inverter, any voltage magnitude control desired at the motor input will have to be done at the D.C bus and hence the AC/DC converter has to be a controlled one i.e. a three phase thyristor bridge with µ control. The magnitude of applied voltage to the motor is kept proportional to the frequency in VFDs and hence when the motor is required to run at low speeds the applied frequency as well as the applied voltage will be kept low. This is possible in a square wave inverter only by lowering the DC bus value i.e. only by using large values of µ in the front-end converter. With a large value of µ the front-end converter will draw power from AC line at a very low power factor. Thus, the advantages of six-step square wave inverter in a VFD are simplicity, ruggedness, reliability, efficiency and low cost. The disadvantages are derating of Induction Motor and very low line power factor and high harmonic content in the a.c line at low speeds.

In SPWM Inverter the Inverter switches are switched according to a firing logic derived by comparing a set of low level three phase sinusoidal reference voltages of desired output frequency with a high frequency triangular wave. The switching will be at a high frequency and the switched inverter output will contain the desired output sine wave along with high frequency components. These high frequency components get filtered effectively by the motor leakage inductances and the motor current will be almost pure sine wave. Also, controlling the amplitude of reference sine waves that decide the gating of switches can easily control the amplitude of fundamental sine wave contained in the inverter output at an electronic level. Therefore this kind of inverter needs only an uncontrolled diode bridge as the front-end converter. The displacement power factor i.e. the fundamental power factor of current drawn by an uncontrolled bridge is close to unity though there will be harmonics injected in the line-predominantly fifth harmonic. This is the favoured front-end/Inverter combination in the present day VFDs. The switches employed currently are IGBTs. There are definite relations to be maintained between the desired output frequency and the frequency of triangular waveform used to decide the h.f switching in this kind of inverter as the desired frequency changes. Commercially available ICs take care of all these constraints in choosing the triangle frequency.

During regenerative braking or overhauling of the Induction Motor by the load, energy is returned to the DC bus by the motor. The front-end which is a unilateral converter (diodes or thyristors) cannot send this energy back into the supply and hence the capacitor across D.C. bus will try to absorb this energy. This will lead to a growth of voltage across D.C. bus capacitor and eventual over voltage and damage unless checked. A resistor is usually switched on across the D.C. side capacitor under these modes to control the bus voltage and dissipate the energy returned by the motor. Greater control of Induction Motor during braking, stopping, load overhauling etc. and greater energy conservation will be possible if the front-end converter is replaced by a Switched Mode Bilateral AC-DC Converter. Also, with such a converter at front-end, it will be possible to maintain a constant DC bus voltage even when the line is varying – all the time drawing a pure sinusoidal current at unity power factor from the line. Thus a Switched Mode Bilateral AC-DC converter as the front-end of the drive will solve the problems of poor power factor and harmonics in line current, problems associated with regenerative braking and load overhauling and problems in inverter control due to a varying D.C Bus voltage in a single stroke. This is the current state of technology in the front-end converter and many reputed VFD manufacturers in India have already adopted this technology.

3.1. Basic Principles of Variable Frequency Operation of Induction Motors

The basic principles involved in speed control of Induction Motors by stator frequency control are enunciated below. The stator resistance and stator leakage reactance is assumed to be negligible for simplicity at this stage. The effect of these parameters will be dealt with separately later.
 

(1) The electromagnetic torque developed in an Induction Motor is proportional to the magnitude of air gap flux and the amplitude of rotor current. In a VFD, the Induction Motor is run at various speeds by applying suitable frequency to the stator. The motor must be able to develop full load torque at all speeds i.e. at all stator frequencies to be an effective drive. Hence the air gap flux magnitude must be kept constant at all applied frequencies to maintain the full load torque capability of the motor at all speeds. The torque control must be by rotor current control and not by air gap flux control. Of course it is possible to increase the electromagnetic torque capability of the motor by increasing the air gap flux; but this is not favoured due to a possibility of magnetic circuit saturation in the machine. To sum up, the air gap flux must be kept constant when the drive frequency is varied to obtain variable speed operation.

(2) With negligible stator resistance and leakage inductance, the applied voltage is always equal to the stator-induced e.m.f in the motor. The induced e.m.f in the stator winding is proportional to the air gap flux magnitude and frequency of operation. Hence, to keep the magnitude of air gap flux in the motor constant at all frequencies of operation it is necessary to vary applied voltage magnitude in such a way that the ratio (V/f) is maintained constant i.e. V is kept proportional to frequency. This is called V/f control.

(3) At any applied frequency the rotor of the motor slips with respect to the air gap flux. The magnitude of e.m.f induced in the rotor is proportional to air gap flux and the slip frequency and is at slip frequency in the rotor. For example a motor with synchronous speed of 1500rpm at 50Hz will have 2Hz voltage in its rotor if it is running at 1440rpm and its slip frequency is 2Hz and slip speed is 60rpm.At such low values of frequency in the rotor the rotor leakage inductance does not offer meaningful impedance to current flow and hence rotor current amplitude is essentially decided by rotor resistance and will be almost in phase with rotor voltage. But the same air gap flux is inducing voltages in stator and rotor and the stator-induced e.m.f will be in-line with applied voltage. Hence rotor current will be in-line with applied voltage.

(4) From the above it is clear that whatever be the stator frequency the magnitude of induced voltage and current in the rotor depend only upon the slip speed and slip frequency. Thus to produce same magnitude of current in the rotor at different values of stator frequency, the rotor frequency will have to be the same. Consider a 5HP, 400V, 5.2A, 50Hz, 1440rpm motor .Its full load slip is 4% and hence its rotor frequency at full load torque is 2Hz.Suppose the stator frequency is changed to 25Hz and applied voltage is correspondingly changed to 200V.Now the air gap flux in the motor is same as before. If it has to produce same full load electromagnetic torque its rotor current must have same value as before. For that the rotor must slip by the same slip speed i.e. by 60rpm.Then the voltage and frequency induced in the rotor will be same as before and frequency in the rotor will be 2Hz.Rotor will have same current and hence motor will develop same full load torque. But, note that now full load slip is 8% not 4%. That is, the current in the rotor and hence the rotor losses in the motor remain the same as the motor is run at different speeds by varying the stator frequency with the load torque kept constant; but the running slip (not the slip speed) will keep increasing with decreasing stator frequency. The actual values will depend on the value of load torque and will be the nominal full load values if the load torque demand is at rated torque value. For a constant torque load the actual running speed of the motor will deviate more and more from synchronous speed as the stator frequency is reduced i.e. the relation between running speed and frequency will be less than proportional with maximum deviation at frequencies close to zero.

(5) However, the ability of the motor to dissipate the losses will come down with speed and this may result in derating of machine at low speeds unless the cooling fan of the motor is supplied separately. Also, though the machine develops full load torque at all speeds, the mechanical power output is lower at lower speeds and hence the full load efficiency of the motor goes down at lower speeds (i.e. at lower stator frequencies). But if the driven machinery is a pump or fan or blower this does not matter since the load torque demand by these loads come down with decreasing speed.

Suppose the load torque on the motor is constant at full load value and corresponding slip speed is 2Hz and running speed is 1440Hz. Now if the motor is to be run at 750rpm with the same torque what is the frequency to be applied? If 25 Hz is applied the motor runs 23Hz electrically (because slip speed has to be 2Hz) i.e. it runs with a slip of 8%. The mismatch between the synchronous speed and running speed widens at lower speeds as explained already. This may not be tolerable in some cases. Taking a speed feedback and using a closed loop system with all the associated control complexities is one solution. Slip Compensation is a less costly solution with acceptable performance. In slip compensation, the motor current is monitored and the stator frequency is adjusted to be above the commanded value by a quantity equal to slip speed (approximately). Thus in a drive with slip compensation, setting 25Hz frequency will result in motor running at 750rpm whatever be the load. If the load is at full load electromagnetic torque value, the slip compensation supplies 2Hz value and the actual frequency given to the stator will be 27Hz and the motor will run at 750rpm corresponding to 25Hz.

3.3 Starting a Motor on VFD

A VFD fed motor does not a separate starter. The current during start up is to be kept at rated value in the rotor to avoid rotor overheating. But the rotor current is directly decided by slip speed if air gap flux is kept constant. Say we want to keep the rotor current limited to full load value at starting. This means that the slip speed at starting must be 2Hz in the case of example motor above. But slip frequency at start is same as stator frequency since motor is just being started. So 2Hz , 16Volt supply should be given at the beginning of start and frequency should be gradually increased such that the rotor current remains constant at full load value ; delivering rated electromagnetic torque throughout the starting period. But it not possible to monitor rotor current and stator current is monitored and kept constant at its initial value throughout the starting period. If a higher starting torque is needed it can be obtained by applying higher voltage magnitude than what is permitted by V/f law provided the machine does not saturate. Or short-time over-current rating of rotor may be utilised for this purpose by applying higher slip speeds to the rotor. It is also possible to program the starting mode in such a way that the motor accelerates as per a pre-programmed speed curve. This may be needed in centrifugal pump and blower drives to avoid surges due to excessive acceleration.

VFDs from standard manufacturers offer intelligent voltage boost programming to provide not only maximum starting electromagnetic torque , but also a maintained electromagnetic torque that can reduce motor overheating in applications requiring high electromagnetic torque at low speeds.

3.4 Stopping a Motor on VFD

Controlled decceleration of a motor is possible in two ways in a VFD. The applied frequency is reduced to a value such that the slip of the machine becomes negative and it becomes an induction generator. The machine then delivers its kinetic energy to the dc bus through the inverter. This energy goes to the a.c mains if the AC-DC converter is bilateral. Otherwise this energy is dissipated in a resistor switched on across the D.C bus capacitor. In the first case it is regenerative braking and in the second case it is dynamic braking. VFD in this braking mode is controlled by stator current and the D.C. bus voltage (in the case of dynamic braking) and the stator frequency is controlled in such a manner that the stator current is limited to its maximum safe value and the D.C bus voltage is limited to its maximum safe value till the motor achieves zero speed. It is also possible to program this mode in such a way that the motor deccelerates as per a pre-programmed speed curve. This pre-programming may be needed in certain cases to avoid material slippage or shifting due to jerk during coasting down.

3.5 Voltage Boost in VFDs

Stator resistance and leakage inductance have been neglected in all the explanations till now. Now their effect on the performance will be examined.

Assume that a 15HP, 1440rpm, 400V motor has 90% full load efficiency and 90% full load power factor. Its full load current will be 20Amp and reactive component will be about 9Amps.Assuming that the maximum efficiency point is at full load and that the stator and rotor resistive losses are equal, the stator resistance will account for about 2.5% loss i.e. when full load current is flowing through it the stator resistance will take about 2.5% of 400V i.e. 10V across it. Similarly the leakage may also cause a voltage drop of about 15V.Thus the net drop will be about 25V across the stator impedance. If the applied voltage is as per the V/f rule only a portion of this V will reach the magnetising inductance and hence the flux in the machine will be lower than the expected value. Obviously this effect will be more at low frequency values since the applied voltage will be small then. Hence it is necessary to apply a voltage even more than the value permitted by V/f rule so that the stator impedance drop will get compensated. This extra voltage is called voltage boost and it is a function of load on the machine. A well designed VFD will take care of this voltage boost needed when it decides the voltage to be applied as per the V/f rule-V/f rule is not the issue; maintaining the air gap flux at constant level is the issue.

3.6 Features of a Commercially Available Adjustable Speed Drive

Allen-Bradley make 1336 PLUS ASD is available in the 0.5HP-600HP range at voltages of 230V,460V and 575V rating. This drive is based on sensor-less field vector control technique. However, the drive allows a user selectable constant V/f mode of control too. Some features of this drive are listed below for illustration.
 

1336 PLUS uses third generation IGBTs in its inverter and uses Sinusoidal Pulse Width Modulation with variable carrier frequency to ensure quiet operation. M/s Sandra Power Engineers, Ernakulam, sell this drive in Kerala.
List of Addresses of Some Firms Dealing with VFDs. (The data below may be outdated ..)