Static Kramer Drive

UKZN Electrical engineeringStatic Kramer DriveThuven Moodley 211 511 466

8/4/2014

This report on the static Kramer drive and its modified version incorporates both theoretical and simulated information. The static Kramer drive is a more efficient means of controlling induction motor speed and its characteristics are discussed in this report.

Table of Contents1.)Introduction to Slip recovery drives11.1.) Applications of slip recovery drives12.)Literature Review23.)Working Principle23.1.) Static Kramer Drive23.1.1.) Principle of operation33.2.) Modified Kramer System43.2.1.) Principle of operation43.3.) Slip control using Kramer drive53.1.1.) Principle of operation53.1.2.) Starting Methods64.)Simulation64.1.) Simulink model64.2.) Analysis of simulation74.3.) Harmonics85.)Conclusion86.)References97.)Tutorial107.1) Example 1107.2) MCQs11

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1.) Introduction to Slip recovery drivesInduction motor drives are widely used in industry. A cage type or a wound rotor machine can be utilized in the drive. Wound-rotor machines are more expensive, heavier, have higher rotor inertia and speed limitation, along with reliability issues due to the brushes or slip rings. One of the simplest methods of AC motor speed control is the wound-rotor machine with a mechanically varying rotor rheostat. Slip power is obtained easily from the slip rings and can be electronically controlled in order to control the speed of the motor. Taking into account the extra expense if utilizing a wound rotor machine, it is observed that the cost reduction can be substantial.

Figure 1 - Basic slip control

Using the method of rotor resistance control the slip power is usually wasted as heat in the external rotor resistance (figure 1). The Static Kramer drive is a popular and more energy efficient method of slip power control. The efficiency of the drive is increased substantially by using this method.1.1.) Applications of slip recovery drivesThe applications of these slip-recovery drives are as follows: Fan drives Pumps Variable speed wind energy machines Variable speed hydro pumps/gens Utility system flywheel energy storage

2.) Literature ReviewWound rotor induction motor drives are common in industrial applications due to their key characteristic of readily available slip power (Kumar et al, 2011). Slip power recovery drives overcome the flaw of controlling the speed of these motors by using the simple method of adjusting the external rotor resistance is the wastage of the slip power. Many authors (Bose, Drury, Biswas, Leonard) all state the static Kramer drives more efficient method of slip power control and recovery. There was seen to be a grey area with respect to the actual definition of a static Kramer drive from the more primitive Kramer drive that is discussed further in this report. According to S. Siyanagaraju, the static Kramer drive is mechanically coupled with a DC motor. This is the approach the researcher has taken toward this report however the other types of Kramer drives are touched.3.) Working Principle3.1.) Static Kramer DriveThe Static Kramer drive is used mainly for large power pump and fan-type loads. The speed control is limited near, but below synchronous speed. This is suitable for most fan or pumping applications. The static Kramer drive is also very efficient with a low power rating.

Figure 2 - Static Kramer drive\

3.1.1.) Principle of operationThe slip power is obtained from the slip rings of the induction motor.This voltage is then converted to voltage Vd by the diode bridge rectifier. The current Id is then fed through an inductor (choke) which reduces the ripple voltage. This inductor is connected to a mechanically coupled DC motor. Vd is proportional to slip Idis proportional to torqueThe voltage Vd1 is given by:

Where S = slip, V = line to line source voltage, n = stator turns ratioThe voltage Vd2:

Where and a , = firing angleThe current Id:

The torque of the DC motor is given by:

Where

3.2.) Modified Kramer SystemA slight change from the static Kramer system so that the speed control is extended so that a speed of zero can be obtained. DC motor is replaced by a synchronous motor fed by a load commutated inverter. By using this arrangement the speed can be controlled by adjusting the thyristors firing angle.

Figure 3 - Modified Kramer system

3.2.1.) Principle of operationThe modified Kramer drive operates similarly to the static Kramer drive. These drives consist of a commutatorless DC motor which essentially is a synchronous motor fed by a load commutated inverter. This motor is also mechanically coupled to the wound rotor induction machine. Again the slip power is obtained from the slip rings and is converted to DC by the diode bridge rectifier which is then fed through an inductor and then converted back to AC by the load commutated inverter.

3.3.) Slip control using Kramer drive

Figure 4 - Static Kramer drive used as SPRD3.1.1.) Principle of operation The static Kramer drive avoids this unnecessary wastage by converting the slip power back into 60Hz AC and feeds it back into the line via a transformer. The static Kramer drive only allows the control of speeds below the synchronous speed.The voltage at slip rings is the slip frequency, however the power that is to be fed back to the supply must be at the stator frequency.The rotor voltage is converted into DC (i.e. Vd) by the diode bridge rectifier. The current Id is then fed through a smoothing inductor. This inductor acts as a choke and reduced the ripple voltage. The DC voltage is then converted into AC by the use of the commutated thyristor inverter and fed back to the 3 phase supply therefore increasing efficiency.

3.1.2.) Starting Methods

1- The motor is started with switch 1 closed and switches 2 and 3 open.2- As the motor builds up speed, switches 2 and 3 are sequentially closed until desired smax value is reached3- After which switch 1 is opened and the drive controller takes over. 4.) Simulation4.1.) Simulink model

Figure 5 - SimPowerSystems Model of Static Kramer Drive System (Ajay Kumar-Performance analysis of a microcontroller based slip power recovery drive)4.2.) Analysis of simulationSimulation results show that by varying firing angle (above 90 degree) in small intervals, motor speed can be controlled from zero to nominal speed.

Figure 6 - Motor speed for (a) 90 degree (b) 100 degree firing angle

Figure 7 Voltage and current waveforms at the sourceThe motor speed vs. time characteristics at two different firing angles have been shown in Figure 6. It is seen that steady state speed for higher firing angle is less as compared to lower firing angle. It can be seen in all of these waveforms that there is some distortion caused by the system that the source is feeding. This can be seen in the most detail in the voltage waveforms.

4.3.) HarmonicsThe rectification of slip-power causes harmonic currents in the rotor which are reflected back into the stator. This results in increased machine losses.

Figure 8 DC current waveformThe presence of power electronic converter and inverter circuit in this system, also cause low frequency odd harmonics injection to the network. This may lead to an increased torque ripple which results in the heating of the motor. Figure 8 shows the DC current waveform with a lot of harmonics present.The harmonic torque is small compared to average torque and can generally be neglected in practice.

5.) ConclusionThe static Kramer drive utilizes the usually wasted slip power that is obtained from the slip rings on the wound rotor machine. This power can be fed to a DC motor that can be used as an auxiliary fan or to run a separate process altogether. Key differences between the modified Kramer system and the primitive Kramer system do exist however the characteristics of the static Kramer drive hold true in both these models.The torque of the drive varies linearly with the dc link current. Hence, the drive has similar characteristics as that of separately excited dc motor. The increase in efficiency has been observed as compared to rotor resistance method of speed control.In the slip control area the presence of converter and line commutated inverter results in harmonic injection on the source side which can be ignored. The presence of power electronic converter and inverter circuit in this system, also cause low frequency odd harmonics (3rd, 5th,7th ) injection to the network.This harmonic distortion can cause the motor to heat up. These effects however are negligible when the use of a static Kramer drive is concerned.

6.) References1.) Ajay Kumar, S. K. Aggarwal, L.M. Saini, and Ashwani Kumar, 2011, Performance analysis of a microcontroller based slip power recovery drive.2.) Sivanagaraju, S., Power Semiconductor Drives (Text book)3.) Bose, B. K., 1988, .Power Electronics and AC Drive. Englewood Cliffs, NJ: Prentice-Hall.4.) Bose, B. K., 2002, .Modern Power Electronics and AC Drive. Englewood Cliffs, NJ: Prentice-Hall.5.) Drury, Electrical Machine Drives6.) Dubey, G.K., 2008.Power semiconductor controlled drives., PHI. Edition.7.) Robert John Kerestes, ESTIMATION OF HARMONICS, INTERHARMONICS AND SUB-HARMONICS IN MOTOR DRIVE SYSTEMS8.) Paul Blaiklock and William Horvath, Saving energy, TMEIC GE, USA

7.) Tutorial

7.1) Example 1A 3-phase, 440V, 50Hz star connected 1000rpm, 6-pole induction motor has the following parameters referred to the stator: R1 = 0.2, R2 = 0.15, X1 = X2 = 0.4. The stator to rotor turns ratio is 3.5. The motor is controlled by the static Kramer drive. The drive is designed for speed range of 30% below the synchronous speed. The maximum firing angle is 170o.Calculatea) The turns ratio of the transformer.b) Torque for a speed of 750 rpm and = 140o.SolutionGiven:Frequency, f = 50 HzRated speed = 970 rpmNumber of poles, p = 6 m = 170oR1 = 0.2, R2 = 0.15 X1 = X2 = 0.4n = 3.5 for 30% speed rangeSm = 0.3a)

b) At N = 750

Here Rd = 0

Therefore:

7.2) MCQs1) A static Kramer drive uses what type of rotor design?(a) Wound rotor(b) Cage type rotor

2) Which of these applications is not suitable for the use of a static Kramer drive?(a) Fan drive(b) flywheel energy storage(c) bi-directional conveyor(d) pumps

3) Slip power is wasted in ______________.(a) Static Kramer drive(b) Static Scherbius drive(c) Rotor resistance control(d) None of these

4) Write any one disadvantage of rotor resistance control.(a) High Cu loss(b) Low Cu loss(c) Both of these (d) NoneSolution to MCQs1) A2) C3) C4) A