9. Define: Pull out torque of stepper motor?

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1 UNIT II STEPPING MOTORS PART - A 1. Define: Stepper motor? (June 14) Stepper motor is a motor which rotates step by step and not continuous rotation. When the stator is excited using a DC supply the rotor poles align with the stator poles in opposition such that reluctance is less. 2. What are the advantages of Stepper motor? No feedback is normally required for either position control or speed control, Positional control is non cumulative, Stepping motor are compatible with modern digital equipment 3. Mention the different types of stepper motor? Variable Reluctance stepper motor (Single stack, Multi stack), Permanent magnet stepper motor, Hybrid stepper motor, Outer rotor stepper motor. 4. Mention the features of stepper motor?(dec 13) Small step angle, High positioning accuracy, High torque inertia ratio, Stepping rate, Pulse frequency 5. Define: Step Angle of stepper motor?(june 2013, Dec 13, June 2016) A stepping motor rotates through a fixed angle for every pulse. The rated value of this angle is called the step angle and expressed in degrees. 6. Define: Holding torque and Detent torque of stepper motor? (Dec 15) Holding torque is defined as the maximum static torque that can be applied to the shaft of an excited motor without causing continuous rotation. It is defined as the maximum static torque that can be applied to the shaft of an unexcited motor without causing continuous rotation. 7. Define: Resolution of stepper motor? It is defined as the accuracy of positioning of the rotor pole at a particular step angle with respect to stator pole. 8. Define: Pull in torque of stepper motor? These are alternatively called the starting characteristics and refer to the range of frictional load torque at which the motor can start and stop without loosing steps for various frequencies in a pulse strain. 9. Define: Pull out torque of stepper motor?

2 These are alternatively called the slewing characteristics. After the test, motor is started by a specified driver in the specified excitation mode in the self starting range; the pulse frequency is gradually increased; the motor will eventually run out of synchronism. The relation between the frictional load torque and the minimum pulse frequency with which the motor can synchronize is called pull out characteristics. 10. Define: Slewing frequency of stepper motor? This is defined as the maximum frequency (stepping rate) at which the loaded motor can run without losing steps is alternatively called the maximum slewing frequency. 11. Define: Stepping frequency of stepper motor? The speed of rotation of a stepping motor is given in terms of the number of steps per second and the term stepping rate is often used to indicate speed. 12. Define: Maximum starting torque of stepper motor? This is alternatively called as maximum pull in torque and is defined as the maximum frictional load with which the motor can start and synchronize with the pulse train of frequency as low as 10 Hz. 13. Why interleaving is done in a stepper motor? Interleaving is done in the stepper motor to decrease the step angle and thus increasing the resolution. 14. Explain: VR type stepper motor? It is a basic type of stepping motor in which the motor step by step rotation is achieved when the rotor teeth and stator teeth are in alignment such that the magnetic reluctance is minimized and this state provides a rest or equilibrium position. 15. Explain: PM type stepper motor? A stepping motor using permanent magnet in the rotor for step movement is called a permanent magnet motor. 16. What are the different modes of excitation?(may 12) (Dec 15) Single phase excitation, two phase excitation, Half step mode, Mini-step drive. 17. Compare closed loop control and open loop control in stepper motor.(may 12) Closed loop control is more accurate, oscillatory motions are avoided for certain speed ranges, Speed remains constant for high inertial load, follows the input pulses at stepping frequency are some of the advantages over open loop control. But it is costly and complex.

3 18. Calculate the stepping angle for a 3 phase, 24 pole permanent magnet stepper motor.(dec 12) Step angle β = 360/(no. of stator phases * no. of rotor teeth)=5º. 19. Define torque constant of a stepper motor.(dec 12) The torque constant of the stepper motor is defined as the initial slope of the torque current curve of the stepper motor. 20. What is the function of driver circuit in stepper motor. (June 13) The stepper motor is a digital device that needs binary signals for its operation. The power driver is essentially a current amplifier, since the sequence generator can supply only logic but not any power. 21. Give the difference between single and multi stack stepping motors?(june 14) Sl.No Single Stack Stepper Motor Multi Stack Stepper Motor 1 The number of stator poles should be different that of the rotor poles in order to have self starting capability and bidirectional rotation. 2 In single stack each and every stator pole carries a field coil. The stator and rotor have same number of poles and same pole pitch. It is used to obtain small step sizes. It consist of m identical single stack variable reluctance motor with the rotor mounts on the single shaft. 22. Distinguish the half step and full step operation of a stepper motor.(dec 14) SL.NO HALF STEP OPERATION FULL STEP OPERATION 1 It is defined as the alternate one phase on and two phase on mode operation. 2 Rotor rotates on each step angle is half of the full step angle. It is the one phase on mode operation.it means at that time only one winding is energized. By energizing one stator winding the rotor rotates at some angle.it s full step operating. 23. Define the micro stepping mode of stepper motor. (May 15)

4 Micro stepping means, the step angle of the VR stepper motor is very small. It is also called mini stepping. It can be achieved by two phases simultaneously as in 2 phases on mode but with two currents deliberately made unequal. 24. Name the various driver circuits used in stepper motor. (May 15, June 2016) Driver circuit for stepper motor are broadly classified in to Unipolar and Bipolar driver circuits. Based on the supply voltage given to stator windings they are classified as L/R driver circuit, Chopper drive circuit, H bridge drive circuit. 25. Define: Maxwell s stress It is defined as curving of magnetic lines of force at the end of the poles of the stator when rotor rotates. 26. Define Lead angle(dec 2016) The angle difference between the phase to be de-energized to bring the stepper motor to the position of equilibrium (stopping the motor) and energization of next phase winding to start the motor during closed loop operation is known as lead angle. The relation between the rotor s present position and the phase to be excited specified in terms of lead angle. 27. What is the need for suppressor circuits in stepper motor? (Dec 2016) The suppressor circuits are needed to ensure the fast decay of current through the winding whn it is turned off Part B 1. Explain the construction and principle of operation of Variable Reluctance Stepping motor? (May 12, Dec 12, Dec 13,June 14 )( Working of Single stack type and multi stack type( June 13, June 2016))(Micro stepping Dec 13) Variable Reluctance Stepping motor: 1. Single stack type, 2.multi stack type(refer 10) Construction:

5 The variable reluctance motor does not use a permanent magnet. As a result, the motor rotor can move without constraint or "detent" torque. This type of construction is good in non industrial applications that do not require a high degree of motor torque, such as the positioning of a micro slide. The variable reluctance motor in the above illustration has four "stator pole sets" (A, B, C) set 15 degrees apart. Current applied to pole A through the motor winding causes a magnetic attraction that aligns the rotor (tooth) to pole A. Energizing stator pole B causes the rotor to rotate 15 degrees in alignment with pole B. This process will continue with pole C and back to A in a clockwise direction. Reversing the procedure (C to A) would result in a counterclockwise rotation. If your motor has three windings, typically connected as shown in the schematic diagram in Figure, with one terminal common to all windings, it is most likely a variable reluctance stepping motor. In use, the common wire typically goes to the positive supply and the windings are energized in sequence. The cross section shown in Figure is of 30 degree per step variable reluctance motor. The rotor in this motor has 4 teeth and the stator has 6 poles, with each winding wrapped around two opposite poles. With winding number 1 energized, the rotor teeth marked X are attracted to this winding's poles. If the current through winding 1 is turned off and winding 2 is turned on, the rotor will rotate 30 degrees clockwise so that the poles marked Y line up with the poles marked 2. To rotate this motor continuously, we just apply power to the 3 windings in sequence. Assuming positive logic, where a 1 means turning on the current through a motor winding, the following control sequence will spin the motor illustrated in Figure 2.6 clockwise 24 steps or 2 revolutions: Winding Winding Winding time ---> There are also variable reluctance stepping motors with 4 and 5 windings, requiring 5 or 6 wires. The principle for driving these motors is same as that for the three winding motors, but it becomes important to work out the correct order to energize the windings to make the motor step nicely. Principle of operation: One phase on or full step operation, 2 phase on mode, half step operation, micro stepping. 2. Explain the construction and operation of hybrid Stepping motor? (Dec 14) Hybrid motors combine the best characteristics of the variable reluctance and permanent magnet motors. They are constructed with multi-toothed stator poles and a permanent magnet rotor. Standard hybrid motors have 200 rotor teeth and rotate at 1.80

6 step angles. Other hybrid motors are available in 0.9ºand 3.6º step angle configurations. Because they exhibit high static and dynamic torque and run at very high step rates, hybrid motors are used in a wide variety of industrial applications. 3.Discuss the static and dynamic characteristics of stepper motor with neat sketch? (June 14)(May 12)(June 14, Dec 2016, June 2016) There are two characteristics- torque displacement characteristics, torque current characteristics

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8 Dynamic characterictics: At low stepping rates, the rise and fall times of the current through the motor windings has little effect on the motor's performance, but at higher speeds, the effect of the inductance of the motor windings is to reduce the available torque, as shown in Figure

9 Torque Normal mode Slewing mode Speed An ideal torque-speed characteristic for a stepper motor 2 distinct modes of operation: Locked-step (normal) mode Slewing mode In the first, the rotor comes to rest between steps (mode commonly used to achieve a given rotor position); rotor can be started, stopped, reversed Slewing mode does not allow stopping or reversal of the rotor, although it advances in synchronism with the stepping sequence (e.g. rewinding a tape drive) Torque Curve A: Pull-out torque Curve B: Pull-in torque Curve B Curve A Normal mode Slewing mode Pull-in rate Max pull-in rate Pull-out rate Speed Max pull-out rate The motor's maximum speed is defined as the speed at which the available torque falls to zero. Measuring maximum speed can be difficult when there are resonance problems, because these cause the torque to drop to zero prematurely. The cutoff speed is the speed above which the torque begins to fall. When the motor is operating below its cutoff speed, the rise and fall times of the current through the motor windings occupy an

10 insignificant fraction of each step, while at the cutoff speed, the step duration is comparable to the sum of the rise and fall times. Note that a sharp cutoff is rare, and therefore, statements of a motor's cutoff speed are, of necessity, approximate. The details of the torque versus speed relationship depend on the details of the rise and fall times in the motor windings, and these depend on the motor control system as well as the motor. Therefore, the cutoff speed and maximum speed for any particular motor depend, in part, on the control system! The torque versus speed curves published in motor data sheets occasionally come with documentation of the motor controller used to obtain that curve, but this is far from universal practice! Similarly, the resonant speed depends on the moment of inertia of the entire rotating system, not just the motor rotor, and the extent to which the torque drops at resonance depends on the presence of mechanical damping and on the nature of the control system. Some published torque versus speed curves show very clear resonances without documenting the moment of inertia of the hardware that may have been attached to the motor shaft in order to make torque measurements. The torque versus speed curve shown in Figure is typical of the simplest of control systems. More complex control systems sometimes introduce electronic resonances that act to increase the available torque above the motor's low-speed torque. A common result of this is a peak in the available torque near the cutoff speed. Characteristic Parameters Holding torque: the maximum torque which can be applied to an energized stationary motor without causing spindle rotation Pull-out torque: the maximum torque which can be applied to a motor, running at a given stepping rate, without losing synchronism Pull-in torque: the maximum torque against which a motor will start, at a given pulse rate, and reach synchronism without losing a step. Pull-out rate: the maximum switching rate at which a motor will remain in synchronism while the switching rate is gradually increased. Pull-in rate: the maximum switching rate at which a loaded motor can start without losing steps Slew range: the range of switching rates between pull-in and pull-out in which a motor will run in synchronism but cannot start or reverse. There are following two modes of operation with regards to dynamic characteristics start stop mode, slewing mode 4. Explain in detail the drive system of a stepping motor? (May 12, Dec 12,Dec 14,June 14) There are two main drive circuits for stepper motors, namely; Uni-polar and Bi-polar drive circuits.

11 Uni-polar Drive Circuit Uni-polar drive circuit for three-phase variable reluctance stepper motor Bi-polar Drive Circuit One phase of a Bi-polar drive circuit for permanent magnet or hybrid stepper motors DRIVERS The stepper motor driver receives low-level signals from the indexer or control system and converts them into electrical (step) pulses to run the motor. One step pulse is required for every step of the motor shaft. In full step mode, with a standard 200 step motor, 200 step pulses are required to complete one revolution. Likewise, in microstepping mode the driver may be required to generate 50,000 or more step pulses per revolution. TYPES OF STEP MOTOR DRIVERS For industrial applications there are basically three types of driver technologies. They all utilize a "translator" to convert the step and direction signals from the indexer into electrical pulses to the motor. The essential difference is in the way they energize the motor winding. The circuit that performs this task is known as the "switch set."

12 UNIPOLAR The name unipolar is derived from the fact that current flow is limited to one direction. As such, the switch set of a unipolar drive is fairly simple and inexpensive. The drawback to using a unipolar drive however, is its limited capability to energize all the windings at any one time. As a result, the number of amp turns (torque) is reduced by nearly 40% compared to other driver technologies. Unipolar drivers are good for applications that operate at relatively low step rates. R/L R/L (resistance/limited) drivers are, by today's standards, old technology but still exist in some (low power) applications because they are simple and inexpensive. The drawback to using R/L drivers is that they rely on a "dropping resistor" to get almost 10 times the amount of motor current rating necessary to maintain a useful increase in speed. This process also produces an excessive amount of heat and must rely on a DC power supply for its current source. BIPOLAR CHOPPER Bipolar chopper drivers are by far the most widely used drivers for industrial applications. Although they are typically more expensive to design, they offer high performance and high efficiency. Bipolar chopper drivers use an extra set of switching transistors to eliminate the need for two power sources. Additionally, these drivers use a four transistor bridge with recirculating diodes and a sense resistor that maintains a feedback voltage proportional to the motor current. Motor windings, using a bipolar chopper driver, are energized to the full supply level by turning on one set (top and bottom) of the switching transistors. The sense resistor monitors the linear rise in current until the required level is reached. At this point the top switch opens and the current in the motor coil is maintained via the bottom switch and the diode. Current "decay" (lose over time) occurs until a preset position is reached and the process starts over. This "chopping" effect of the supply is what maintains the correct current voltage to the motor at all times. In the below circuits, the details of the necessary switches have been deliberately ignored. Any switching technology, from toggle switches to power MOSFETS will work! Figure contains some suggestions for implementing each switch, with a motor winding and protection diode included for orientation purposes:

13 Each of the switches shown in Figure is compatible with a TTL input. The 5 volt supply used for the logic, including the 7407 open-collector driver used in the figure, should be well regulated. The motor power, typically between 5 and 24 volts, needs only minimal regulation. It is worth noting that these power switching circuits are appropriate for driving solenoids, DC motors and other inductive loads as well as for driving stepping motors. ` The SK3180 transistor shown in Figure is a power darlington with a current gain over 1000; thus, the 10 milliamps flowing through the 470 ohm bias resistor is more than enough to allow the transistor to switch a few amps current through the motor winding. The 7407 buffer used to drive the Darlington may be replaced with any high-voltage open collector chip that can sink at least 10 milliamps. In the event that the transistor fails, the high-voltage open collector driver serves to protect the rest of the logic circuitry from the motor power supply. The IRC IRL540 shown in Figure is a power field effect transistor. This can handle currents of up to about 20 amps, and it breaks down nondestructively at 100 volts; as a result, this chip can absorb inductive spikes without protection diodes if it is attached to a large enough heat sink. This transistor has a very fast switching time, so the protection diodes must be comparably fast or bypassed by small capacitors. This is particularly essential with the diodes used to protect the transistor against reverse bias! In the event that the transistor fails, the zener diode and 100 ohm resistor protect the TTL circuitry. The 100 ohm resistor also acts to somewhat slow the switching times on the transistor. For applications where each motor winding draws under 500 milliamps, the ULN200x family of darlington arrays from Allegro Microsystems, also available as the DS200x from National Semiconductor and as the Motorola MC1413 darlington array will drive multiple motor windings or other inductive loads directly from logic inputs. Figure 3.8 shows the pin out of the widely available ULN2003 chip, an array of 7 darlington transistors with TTL compatible inputs:

14 The base resistor on each darlington transistor is matched to standard bipolar TTL outputs. Each NPN darlington is wired with its emitter connected to pin 8, intended as a ground pin, Each transistor in this package is protected by two diodes, one shorting the emitter to the collector, protecting against reverse voltages across the transistor, and one connecting the collector to pin 9; if pin 9 is wired to the positive motor supply, this diode will protect the transistor against inductive spikes. The ULN2803 chip is essentially the same as the ULN2003 chip described above, except that it is in an 18-pin package, and contains 8 darlingtons, allowing one chip to be used to drive a pair of common unipolar permanent-magnet or variable-reluctance motors. For motors drawing under 600 milliamps per winding, the UDN2547B quad power driver made by Allegro Microsystems will handle all 4 windings of common unipolar stepping motors. For motors drawing under 300 milliamps per winding, Texas Instruments SN7541, 7542 and 7543 dual power drivers are a good choice; both of these alternatives include some logic with the power drivers. 5.Explain in detail the multi stack Variable Reluctance Stepping motor?(june 13, Dec 2016,June 2016) Multi-Stack Variable Reluctance Stepper Motors Cross section view of a typical three-stack variable reluctance stepper motor

15 Teeth position for a 3-phase, 4-pole, 12-teeth, three-stack, variable reluctance stepper motor when phase a is energized The VR stepper motors mentioned up to this point are all single-stack motors. That is, all the phases are arranged in a single stack, or plane. The disadvantage of this design for a stepper motor is that the steps are generally quite large (above 15 ). Multi-stack stepper motors can produce smaller step sizes because the motor is divided along its axial length into magnetically isolated sections, or stacks. Each of these sections is excited by a separate winding, or phase. In this type of motor, each stack corresponds to a phase, and the stator and rotor have the same tooth pitch. 6. What is the motor torque Tm required to accelerate an initial load of 3x10-4 kg m 2 from f1 = 1000Hz to f2 = 2000Hz during 100msec. The frictional torque Tf is 0.05 Nm and the step angle is T L2Ti ( t)sint 2 7.With a neat block diagram, explain the closed loop operation of stepping motor. (June 13) 8.a Explain the concept of torque production in variable reluctance stepping motor.(dec 13)

16 Let e(t) = voltage applied per stack R = winding resistance per stack L(θ) = winding inductance per stack (a function of rotor position only and independent of coil current because of linear magnetic circuit assumption) i(t) = current per stack θ(t) = angular position of rotor d Kirchoff s mesh equation for stator winding is e( t) Ri( t) dt where λ = flux linkages of stator winding = il(θ). Therefore, di dl( ) d Ri( t) L( ) i...(1) dt d dt Transformer emf speed emf Energy stored in air gap is W = ½ L(θ) i 2 (t)..(2) Mechanical torque developed is given by T W ( i, ) 1 2 dl( ) i () t...(3) 2 d Rotor dynamics is governed by 2 d d T J f.(4) 2 dt dt In a toothed structure, reluctance and therefore winding inductance varies continuously (even function) as function of θ over and above an average value, i.e., L( ) L L cost.(5) 1 2 Substituting in equation 3, 1 2 T L2Ti ( t)sint 2 8.b.A single stack, 3-phase variable reluctance motor has a step angle of 15º. Find the number of stator and rotor poles. (Dec12) Nr=8, when Ns>Nr, Ns=12, when Ns<Nr, Ns=6 9. What is the motor torque Tm required to accelerate an initial load of 2x10-4 kgm 2 from 500Hz to 1500Hz in 50 ms. the frictional torque is 0.03 Nm and step angle is 1.18º. (Dec 12)

17 1 2 2 T L2Ti ( t)sint 10. a.with a neat block diagram explain microprocessor control of stepper motor (June 2013) A positional signal is feedback to the block of hardware which monitors the rotor movement and exchanges information with the microprocessor. The software must be programmed so that the microprocessor determines better timings for changing lead angles. b. What are the advantages of closed loop control of stepper motor. Closed loop control is more accurate, oscillatory motions are avoided for certain speed ranges, Speed remains constant for high inertial load, follows the input pulses at stepping frequency are some of the advantages over open loop control.

18 11.Explain the modes of excitation of a stepper motor with neat diagram(dec 2016)

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26 12. A stepper motor has resolution of 180 steps/rev. Find the pulse rate required in order to obtain a speed of 2400rpm. (Dec 2016) Solution: Ressolution = 180 steps/rev Required motor speed = 2400 rpm Speed in rps (n) = Speed in rpm/60 = 2400/60 Speed in rps (n) = 40 rps

27 Step angle (β) = / (Number of steps/ rev) = 360/180 β = 2 0 From the expression, n = (β*f)/ f = (n *360)/ β f = ( 40*360)/2 f = 7200 pps