Motors and drives Module C- Drive and Output Measurements Module

Introduction to drive output measurements module

Objec tives

After completing this module, you will be able to: List the diagnostic measurements useful for troubleshooting

motor drive outputs Describe the sequence for using output side diagnostic

measurements. Describe the purpose of each output side diagnostic measurement.

What is DC Bus?

DC Bus functions as a voltage buffer Direct current (dc) voltage derived from AC rectifiers Constant energy supply to switching circuit

Measuring DC bus voltage

Measure the DC Bus voltage across the + and - terminal. Portable Oscilloscope AC or DC input coupling to measure the absolute or ripple voltages Make sure the Scope and probe are rated appropriately to measure the voltage level

Interpreting DC bus measurements

DC bus voltage is ~1.414 x the RMS line voltage (480 Vac drive, the DC bus should be ~ 678 Vdc) Too low can trip the drive Cause malfunction of drive output due to reduced energy available to the output

devices

What is voltage unbalance?

Occurs when three phase voltages differ in magnitude Unbalance can adversely affect motor operation

Voltage unbalance at motor terminals indicates an impedance difference between the phases Potential faults may be either in the cabling to the motor The terminations at the motor The windings themselves

Measuring voltage unbalance

Measure the voltage on each terminal at the drive output Next check the voltage at the motor terminals Any unbalance could be problematic for the motor Use a multimeter or ScopeMeter with a low-pass filter function (Vpwm) to measure the effective voltage to the motor

Calculating voltage imbalance

% Voltage (V) unbalance = (Max deviation from average V/average V) x 100

Example

Interpreting voltage unbalance measurements

Imbalance greater than 2% is problematic. This measurement rules out motor overheating due to voltage unbalance. Cant rule out other overheating causes. Next, check for current unbalance.

Diagnostic benefits

Avoid tripping current overload fault protection.

Avoid elevated motor temperatures.

Avoid causing current unbalance.

What is current unbalance?

Occurs when three phase currents differ in magnitude

Measuring current unbalance

Use an oscilloscope with a current clamp on all three drive output terminals separately to measure the current draw on each terminal.

Calculating current unbalance

% Current (I ) unbalance = (Max deviation from average I/average I) x 100

Example

Interpreting current unbalance measurements

A current unbalance of more than a few %, should be investigated further Should not exceed 10 percent

Diagnostic benefits

Avoid tripping current overload fault protection.

Avoid elevated motor temperatures. Current unbalance can lead to burnt

windings

What are sigma current and protective earth (PE) current?

Sigma: The sum of the current of the three phases. PE: Capacitive coupled current through Protective Earth (ground). Ideally the leakage should be low, increased leakage currents can cause arcing

across bearing races

Adv

ance

d to

pic

Measuring sigma current

Oscilloscope with current clamp. Attach clamp around all three conductors attached to the motor terminals.

Adv

ance

d to

pic

Measuring PE current

Oscilloscope with current clamp Measured at the ground wire.

Adv

ance

d to

pic

Interpreting sigma and PE current

Sigma current and/ or PE current flow through the bearings and shaft.

Adv

ance

d to

pic

Diagnostic benefits

Avoid damage to bearings from stray current.

Fluting of a bearing race caused by bearing currents. Photo courtesy of Electro Static Technology.

Adv

ance

d to

pic

What are output reflections?

Reflections: Occur as a result of impedance mismatch or change in the circuit. Have a wide range of waveforms, amplitudes, and durations. Show up as spikes on an oscilloscope display.

Measuring output reflections

Fast output transients and reflections on pulse width modulated waveform can only be measured with an oscilloscope.

Attach to motor terminals

Interpreting output reflection measurements

Reflections or transients > 50% of nominal voltage are problematic. Fast switching DC voltage from the IGBT (dV/dt). Reflections of the PWM signal as a result of a too long cable. Make sure the level does not exceed the motor insulation rating

Diagnostic benefits

Prolong motor drive and motor life.

Reduce potential of winding insulation breakdown

Avoid tripping overvoltage circuits.

What is a disturbance?

Signal disturbances are signal integrity deviations induced onto low voltage digital network signals.

Adv

ance

d to

pic

Measuring disturbances

Look for disturbances that correlate with other devices on the plant floor. Is a big load elsewhere turning on and causing the problem?

Adv

ance

d to

pic

Interpreting disturbance measurements

Adv

ance

d to

pic

Diagnostic benefits

Prevent irregular signaling problems.

Identify wiring and power problems elsewhere in the plant.

Adv

ance

d to

pic

What is volt per hertz ratio?

The ratio of voltage to frequency determines the amount of torque produced by an AC induction motor.

By keeping this ratio constant the magnetic field inside the motor is kept at a constant level which results in a constant torque.

Adv

ance

d to

pic

Measuring volt per hertz ratio

DMM with Vpwm option or oscilloscope with pwm function. Vpwm: Reflects the amplitude of the fundamental frequency Use current clamp to measure the frequency

Adv

ance

d to

pic

Interpreting volt per hertz ratio measurements

If V/Hz ratio is correct but the speed is off, check the drive program An incorrect volts-per-hertz ratio can create a variety of problems

High: motor will overheat Low: Loss of torque Unstable: rough running motor and loss of torque Unstable frequency: Varying speed of the motor. Fluctuating in tandem: potential loss of torque.

Adv

ance

d to

pic

Diagnostic benefits

Reduce excessive heating. Prevent a loss of torque.

Adv

ance

d to

pic

Diagnostic shutdown

Shutdown systems to solve the problem

Adv

ance

d to

pic

Steps in a diagnostic shutdown when the system stops running

Isolate motor from drive 1. Verify Drive proper operation 2. Check motor cabling,

connections 3. Perform insulation test on

motor/cabling

Adv

ance

d to

pic

Check motor cabling

Potential problem: Motor cables or connections between drive and motor

Check each cable connection for tightness

Perform insulation resistance test on cables and connections

Check winding resistance

Adv

ance

d to

pic

Motor winding insulation Current unbalance can be

caused by bad wiring or insulation problems within the motor.

To check the motor for insulation problems, disconnect the motor and do an insulation resistance test.

We can do a detailed motor insulation test by removing the delta/wye interconnection strips inside the motor terminal box

Adv

ance

d to

pic

Quiz on output side troubleshooting Q: What can be damaged by excessive sigma current? A: Bearings and shaft of the motor. Q: What can excessive PWM waveform reflections cause A: Eventual motor winding insulation breakdown Q: What are the three steps in checking a diagnostic shutdown? A:

Verify Drive proper operation Check motor cabling, connections Perform insulation test on motor/cabling

Motors and drivesIntroduction to drive output measurements module ObjectivesWhat is DC Bus? Measuring DC bus voltageInterpreting DC bus measurementsWhat is voltage unbalance? Measuring voltage unbalanceCalculating voltage imbalanceInterpreting voltage unbalance measurementsDiagnostic benefits What is current unbalance? Measuring current unbalance Calculating current unbalanceInterpreting current unbalance measurements Diagnostic benefits What are sigma current and protective earth (PE) current? Measuring sigma currentMeasuring PE current Interpreting sigma and PE current Diagnostic benefitsWhat are output reflections? Measuring output reflections Interpreting output reflection measurements Diagnostic benefitsWhat is a disturbance? Measuring disturbancesInterpreting disturbance measurementsDiagnostic benefits What is volt per hertz ratio? Measuring volt per hertz ratioInterpreting volt per hertz ratio measurementsDiagnostic benefits Diagnostic shutdown Steps in a diagnostic shutdown when the system stops runningCheck motor cabling Motor winding insulationQuiz on output side troubleshooting