Motor-Drive Systems: FINAL EXAM

This is the final exam for this guide. Review the "QUIZ" questions in each section before taking this final EXAM set of questions.

1. How is line notching corrected in a DC drive system?

2. What is RFI and how is it controlled in a DC drive system?

3. What are harmonics and what are the corrective actions to reduce them?

4. What are the effective shielding and grounding methods used with AC drives?

5. What is PID and how is it used?

6. Name three factors that cause the efficiency of an AC motor to improve.

7. What are the two separate electrical circuits in a DC drive sys tem?

8. What is the purpose of the commutator?

9. What is the V/Hz ratio?

10. Why are the laminations in the armature skewed?

11. Where would an operator look in a drive to obtain specific information on status or fault conditions?

12. For current to flow, what must be added to overcome resistance?

13. What is a pulse tach and how does it work?

14. Describe the difference between static and dynamic performance of a closed-loop system.

15. What is the difference between a series wound and shunt wound DC motor?

16. Why is proper tension control important in a web-fed system?

17. Describe what the following circuits are used for in a DC drive: summing circuit current controller current measuring/scaling

18. What NEMA design class would provide the highest amount of starting torque, when connected across line power?

19. What are the two main parts of a DC motor and what is the purpose of each?

20. What is the definition of base speed?

21. What is the difference between a DPFG and a TEFC motor?

22. How does a synchronous motor differ from a standard AC induction motor?

23. How is speed determined in an AC induction motor?

24. What is the purpose of compensation windings?

25. What is a dancer control?

26. What is the common DC bus configuration and why is it used?

27. What is meant by serial communications? How is it used with drives?

28. How is a VFD used to save energy in an outlet damper application?

29. What are jumpers or DIP switches used for, connected to analog input signals?

30. Name three or more devices that are used for set point or feed back conditions in closed loop systems.

31. If a speed reducer has a 10:1 ratio, with 3 lb-ft and 1200 rpm input shaft, what speed and torque would the output shaft be (assume 85% efficiency)?

32. How does an SCR operate?

33. What is a Darlington bipolar transistor?

34. What is electromagnetism?

35. What is power factor?

36. Give an example as to where inductance may be helpful.

37. What are the three main procedures in maintaining a drive?

38. Why is accumulated dust on circuit boards a potential problem?

39. What is the easiest method of determining if the drive has a problem?

40. Coordination of variable-speed drive systems in industry are typically controlled by what type of device?

41. What is dynamic braking, and how is it accomplished?

42. What is the difference between scalar and vector drives?

43. What is voltage reflection and how is it corrected?

44. Explain the difference between single-, two-, and four-quadrant systems.

45. What is the operating principle of temperature and humidity sensors?

46. What is meant by an encoder PPR?

47. Describe the EMF, current, and IR compensation feedback loops in a DC drive system.

48. Inductance has the ability to block - and let - pass.

49. What is the definition of torque?

50. What is the procedure for removing accumulated dust on boards?

-- ANSWERS --

1. Line notching is corrected through use of a line reactor connected ahead of the drive.

2. RFI is radio frequency interference. This interference is caused by the control circuits, contactors, and oscillators used to control the drive. The best corrective action is to use shielded control cable, as well as shielded input and output power cable in extreme cases. The use of a shielded transformer will also reduce the conducted noise that could be transmitted back to the power line.

3. Harmonics are described as distortion-they provide no usable work, yet are fed back to the AC line. This distortion is superimposed on the fundamental waveform of 60 Hz. For a standard six-pulse drive, the 5th, 7th, 11th, and 13th harmonic will be generated, and will be the most destructive in value. Line reactors and isolation transformers help in the mitigation of voltage harmonics. Trap filters and 12-pulse drive units will reduce the harmonic content to a significant level.

4. Control wiring needs to be shielded, with the shield cut back and taped at the signal source end. In addition, control wiring must be kept away from power wiring or at a minimum of 12 inches away. Input and output power wiring also need to be separated. A continuous ground wire throughout the entire system is a requirement and will reduce the possibility of conducted noise.

5. PID stands for proportional integral derivative. PID is the ability to automatically control temperature, pressure level, humidity, or any other medium that can be supplied as a feedback signal. The drive has the ability to make corrections in speed because of the error given by the summing junction. This is considered a closed-loop system. It operates at a very-high-performance rate, with small feedback errors translating to thousandths of an inch rotation of the shaft.

6. Motor load, the horsepower rating, and the speed of operation.

7. The armature supply and the field supply.

8. The commutator acts as an electric switch to cause the armature flux polarity to be in opposition to the field winding flux.

9. The V/Hz ratio is the proportion of rated voltage to frequency. Every motor requires this proportion in order to develop rated torque (ex. 7.67 V/Hz for a 460-V line operated motor).

10. To allow for smooth rotational action when operating at low speeds.

11. The operator would look at the I/O status location for the conditions of the digital inputs, analog inputs, etc. A section called "operating data" would also be helpful in determining drive healthiness as well the last several faults that occurred.

12. Voltage 13. A pulse tach is basically a digital encoder. It uses a light source that shines a light beam through a slotted disk. A photo-detector receives the light beam in the form of pulses. The longer the light beam creates pulses, the slower the disk is rotating (the disk is connected directly to the motor shaft).

14. Static performance (static speed regulation) is a change in steady-state speed, which is caused by a change in load. Dynamic performance describes the operation during the transition from one operating point to another.

15. A series wound DC motor includes a series field winding within the armature circuit. One supply voltage is required. A shunt wound DC motor has two separate circuits-one for the armature and one for the field winding.

16. Tension control is required or the product quality will suffer. Too little tension on the web will cause bunching and poor quality of the product.

Too much tension could stretch the material, causing a thinner product, or in the worst case, cause the web to break.

17. Summing circuit-takes the speed feedback signal and matches it against the speed reference to obtain an error signal, used for drive speed control.

Current controller-controls the firing angle of the SCRs and signals the firing unit how long and when to gate the SCRs on.

Current measuring/scaling-takes a sample of the current feedback signal and sends it to the summing junction error processing and correction of current output.

18. NEMA design D, per the classification curves.

19. Armature-rotating part of the motor, it contains windings and magnetic flux that repels the flux developed in the field winding.

Field winding-stationary part of the motor that develops magnetic flux that interacts and repels the armature flux. The interaction of both magnetic fields produces rotation.

20. Base speed is the nameplate speed in RPM, when the motor is operating at rated voltage, rated frequency (Hz), and rated load applied.

21. A DPFG motor has no means of external cooling. It does have vents that allow outside air into the motor housing but reduce the input of airborne materials. A TEFC motor contains an integral fan that circulates the air around the motor housing. It does not allow the entry of air or particles into the unit.

22. A standard AC induction motor uses an induced magnetic field in the rotor, caused by the stator, to produce rotation. A synchronous motor has two separate circuits-rotor and stator. A rotor power supply is used to create magnetic flux. The stator field is energized another power supply (three-phase) that creates a rotating magnetic field. The rotor exactly matches the rotating magnetic field of the stator and the speed synchronizes.

23. Speed is determined by either the frequency applied (Hz) or by the number of pole pairs constructed in the motor. As applied frequency increases, so does the motor speed. The number of pole pairs has an inverse affect on speed.

24. Compensation windings are additional poles installed to the magnetic poles of the field winding (stator). They tend to smooth the flux developed across the pole.

25. A dancer is a device that changes resistance per the amount of force being applied. This device is typically used to monitor and regulate the amount of tension placed on web-fed material.

26. Common DC bus arrangements are used when the likelihood exists for an overhauling load. The extra energy generated by the motor is fed back to the common bus and then transferred to another inverter unit connected to the bus. This increases the overall efficiency of the system.

27. Serial communications is the transmission, in sequence of bits of data to or from a controller device. Communications speeds range from 4800 to 19,200 baud. Drives are wired in parallel (daisy chained) to the control device. The controller could be a computer or any device that talks the same language as the drive. The drive may need a language "interpreter" if the language is not pre-loaded in the drive.

28. The VFD is used to vary the speed per the feedback from the pressure transducer in the duct (if PID control is used). The outlet dampers are fixed in the open position, with the drive varying the speed of the fan per the feedback received from the transducer. Lower fan speed and pressure in the duct is the result. The fan only supplies the airflow required to meet the application needs.

29. Jumpers or DIP switches match the analog input signal with the drive input setting. Typical input signals are 4(0) to 20 mA and 0 to 10 VDC.

30. Pressure, humidity, temperature transducers, potentiometers, a variety of electro-mechanical devices that change voltage output given the input signal.

31. 102 RPM, with 30 lb-ft of torque.

32. An SCR conducts current in one direction-and only when the trigger circuit (gate) is powered on. The device shuts off when it’s subjected to an opposite polarity between the anode and the cathode.

33. A device that basically has two or more power transistors, internally connected in one package. It can be turned on and off by the use of a command signal (base).

34. The ability to produce a magnetic field through the use of a voltage and a coil.

35. Power factor, given in %, is the measurement of the phase difference between the utility generated voltage and current waveform.

36. In the DC bus circuit of an AC drive (filters out unwanted AC ripple) so that the voltage delivered to the next stage will be as clean. Also, inductors placed ahead of the drive tent to reduce the amount of voltage harmonics back to the AC line.

37. Keep it clean, keep it dry, and keep the connections tight.

38. Dust attracts moisture. Moisture can cause the microelectronic circuits to misfire or develop a short. In addition, moisture assists in the development of corrosion. Corrosion on circuit board traces will eventually cause the electrical contacts to disintegrate.

39. Isolate the drive from the external control signals. If the drive satisfactorily operates the motor, when in keypad mode, then the drive is not the problem. Something external to the drive is causing the problem. If the drive does not operate the motor satisfactorily, then check all inputs to the drive. The drive must have two conditions to operate-start command and speed reference.

40. A programmable logic controller (PLC) or other computer device.

41. Inertia built up in the motor is transferred back to the drive by means of regenerative voltage. The voltage fed back to the drive is sent to a power resistor for dissipation as heat. A DB contactor is used to connect the motor voltage to the resistor. At the same time, the output contactor of the drive is opened so no voltage can be fed back into the drive output section.

42. Scalar is the term given to standard operation of a PWM voltage con trolled drive. With this drive mode, the motor must have several percent slip in order to rotate. The drive simply supplies the volts and hertz output, and the motor responds with rotation, per the designed slip characteristic. Vector is the term given to a specialized drive that causes full motor torque at zero speed. Flux vector is another term used to describe the control of flux, which is also the control of torque.

43. Voltage reflection is the phenomenon of drive output voltages combining with voltage bouncing back from the motor. These combined peak volt ages can cause damage to the stator windings, if they are not rated to handle the voltage stress. Precautions would include using an inverter duty motor, adding drive output reactors and/or dv/dt filters. Inductors will reduce the high-voltage spikes that can occur and keep the values within the range of the motor lacquer insulation.

44. Single-quadrant systems allow for motoring in the forward direction only.

Two-quadrant systems allow forward and reverse direction, with reverse torque available for braking, Four-quadrant systems allow for forward and reverse direction operation, plus the capability of forward and reverse torque available for braking in either direction.

45. Both devices use the principle of variable resistance. A temperature sensor uses a material that changes resistance per the ambient temperature.

A humidity sensor uses a material that changes length per the amount of humidity in the atmosphere. A hair hygrometer is a device that uses the changing length of human hair to operate a pivot connected to a variable resistor. In both cases, the output is a variable resistance, which if fed to a power supply, will generate a variable voltage, which can be fed to the drive (as a reference or feedback signal).

46. PPR means pulses per revolution. This number is a representation of the resolution of the unit. The resolution is the smallest amount of device rotation able to be detected by the encoder. For example, a 1024 tach indicates a shaft movement of 1/1024 of a revolution (i.e., 0.35º of shaft rotation).

47. The armature voltage feedback (EMF) is a representation of the output voltage, which is a direct indication of speed. The current feedback is a representation of current output from the drive. The IR compensation feedback is used to increase the speed reference circuit to increase speed due to loading of the motor. All three signals are brought back to the summing circuit. If the current feedback signal indicates the drive is exceeding the current capability, the drive will automatically reduce the speed reference. The speed reference will remain at a lower value until the current drops to an acceptable level.

48. AC, DC

49. Torque is a turning motion, twisting of a device. It’s also defined as force × distance.

50. Special non-ESD-generating air should be used. Cans of ionized air are available for this purpose. Static electricity (ESD-electrostatic discharge) is the silent killer of small microcircuits that cannot handle several hundred volts of static charge.