Motors and drives Module B- Introduction to input measurements
Introduction to input measurements There are a number of locations where input measurements may be made Which is the most important location to make the measurements? At the input of the drive The purpose of the input measurement is to establish that there is enough capacity and suitable quality
Objectives After completing this module, you will be able to: List the diagnostic measurements useful for troubleshooting motor drive inputs. Describe the sequence for using input side diagnostic measurements. Describe the purpose of each input side diagnostic measurement.
What is nominal supply voltage, current, and frequency? Nominal supply voltage, current, and frequency are the characteristics of the voltage supplied to the motor drive under normal operating conditions. Because supply voltage, current, and frequency can affect the health of the motor drive in a variable speed motor system Start your diagnostic efforts at the input side of the motor drive.
Measuring nominal supply voltage, current, and frequency Use a power quality analyzer connected to the drive input. First measure at the input side of the drive then, if needed, at the service entrance on the load side of the protection devices, NOT the utility unprotected side.
Interpreting nominal supply voltage, current, and frequency A measurement of more than 10% out of range means there is potentially a supply voltage problem during the measurement period. Attach a power quality analyzer for long-term troubleshooting. Next, check for unbalance.
Diagnostic benefits Arrive at problem resolution faster. Avoid nuisance inverter low voltage fault circuit tripping. Avoid damage to motor drive.
Advanced topic What are voltage and current unbalance? Occurs when three phase voltages or current differ in magnitude. 2 to 3 % voltage unbalance has the potential to cause drive problems
Advanced topic Measuring unbalance Use a 3 phase PQ analyzer properly connected to the motor drive input supply. Measure three phases simultaneously to assess any impact of load changes The source of unbalance voltage could be poor installation practices or loads which need to be optimized correctly
Advanced topic Manually measuring unbalance Use a DMM or Clampmeter to measure each phase of the input to the motor drive. Voltage Current
Manually calculating unbalance % unbalance = (Max deviation from average V or I/average V or I) x 100. Advanced topic Example: 30 + 35 + 30 = 95 95/3 = 31.7 average current 35-31.7 = 3.3 maximum deviation 3.3/31.7 =.104.104 x 100 = 10.4% current unbalance
Advanced topic Interpreting unbalance Voltage unbalance greater than 2% is potentially problematic. Current unbalance greater than 6% is potentially problematic. Next, check for transients.
Diagnostic benefits Advanced topic Avoid tripping current overload fault protection. Avoid elevated motor temperatures.
What are transients? A transient is a temporary unwanted voltage in an electrical circuit. Transients have a wide range of waveforms, amplitudes and durations.
Measuring transients Use a three phase power quality analyzer Transients function selected Set to greater than 50V above nominal voltage.
Interpreting transients Transients > 50v above nominal voltage are potentially problematic. If you measure no transients, measure for a longer period of time Sources Outside of the plant: lightning or utility infrastructure Inside the plant: switching electronic loads
Diagnostic benefits Prolong motor drive and motor life. Avoid tripping overvoltage circuits. Identifying transients helps isolate building wiring faults.
Advanced topic What are harmonics? Harmonics are multiple frequency components of the fundamental waveform. E.g., a third harmonic of 60Hz is 180Hz. The fifth harmonic of 60Hz is 300Hz.
Advanced topic Measuring harmonics Use a power quality analyzer to measure harmonics. Measure both the voltage and current harmonics
Advanced topic Interpreting harmonics Voltage THD (Total Harmonic Distortion) greater than 6% is potentially problematic. If THD is greater than 6%, then identify which harmonics are most prominent. Some current harmonic distortion is normal Next, check power factor.
Diagnostic benefits Advanced topic Avoid repair costs stemming from excessive harmonic distortion. Avoid transformer overheating. Avoid motor overheating.
What is power factor? Energy is the capacity to do work. Power is energy / time. Power factor is the ratio of the active power(w) used in an AC circuit to the apparent power (VA) delivered to the motor.
Measuring power factor Use a three phase power quality analyzer set to Power and Energy with both voltage and current probes connected using correct polarity and with the arrow pointing toward the load being measured.
Interpreting power factor A power factor of less than 1 indicates that the circuit is not operating at full efficiency. A measurement of less than 0.9 indicates a potential problem. A power factor less than 0.9 may be acceptable in some conditions.
Diagnostic benefits Identifying and correcting low power factor saves money on utility bills. Prevent voltage drops and overheating by identifying capacity restrictions.
Quiz on input side troubleshooting Q: What level of voltage unbalance is potentially a problem? A: Greater than 2%. Q: What level of nominal voltage over or undersupply could be a problem? A: Greater than 10%. Q: What type of measurement tool is best for measuring transients? A: A three-phase power quality analyzer, like the Fluke 434 or 435.