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ScopeMeter
®
Test Tool
MDA-550-III Test Tool
67
Motor Drive Output
The Motor Drive Output functions check the drive output conditions. The modulated output
voltage varies with the motor speed and load. The output current depends on load and correct
function of the motor. Unbalance between the phases can cause or indicate problems. The stress
on the motor insulation can be determined by measurement of the risetime of a fast modulation
pulse.
Voltage and Current (Filtered)
Voltage and Current (Filtered) are measurements of the voltage, current, and frequency at one of
the phases of the motor drive output. The measurement is done with a 10 kHz bandwidth filter so
that a sinewave shaped voltage waveform shows instead of the pulse width modulated signal.
The voltage measurement is done between two phases (Phase-Phase). The current
measurement is done on a single phase. Repeat the measurement for the other phases.
The display shows the voltage waveform in red and the current waveform in blue. The PWM
voltage, rms current, frequency, and Volt/Hz factor (ratio between voltage and frequency) show
as readings at the top of the display. The PWM voltage shows and not the rms voltage, as PWM
voltage represents the effective voltage of the switching output based on the average value of
samples over a whole number of periods of the fundamental frequency.
2 changes the readings on the display to Volt Peak readings or Current Peak readings:
Peak-Peak, Maximum Peak, Minimum Peak, and Crest Factor (ratio between peak and RMS
value).
Tips:
• Use the V/Hz ratio to check if the ratio is within specified limits for the motor.
• If V/Hz is too high the motor will overheat and if V/Hz is too low the motor will lose torque.
Note
The peak voltage readings here are the peak of the effective voltage and not the peaks of
the actual PWM voltage. Use the Voltage Modulation function to measure PWM voltage.
• The Voltage and Current (filtered) function detects motor overloading. Stable Hz with unstable
V readings points to dc bus problems. Unstable Hz with stable V readings points to IGBT
problems. Unstable Hz and unstable V readings points to speed control circuits.
• Check the output voltage on the motor drive with the nameplate rating. The current must be
within the specified Full Load Amps for the motor. Consider the motor service factor that
specifies the percentage of overloading the motor can handle for short periods of time.
• If the output current is too high, the motor can run hot. A 10-degree temperature rise can
equal 50 % decrease in stator insulation life.
1.888.610.7664 sales@GlobalTestSupply.com
Fluke-Direct.com
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