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Abstract
This paper describes how Event Capture can be used to analyze motor startups and covers the typical initial settings for capturing and analyzing a motor startup. All PMI recorders, old or new, have the capability of analyzing motor startups using the event capture function. Event Capture was supported by WinScan and is still supported by ProVision even with PMI legacy box-type recorder hardware.
The Event Capture function was developed before the Waveform Capture functions and is basically a substitute for the waveform capture in older recorders that cannot record raw waveforms. The event capture function has the advantage of using much less memory than the waveform capture function. The timestamp of a significant change event can be used to find the same disturbance in the Event Change Report if further analysis is required.Event capture report works well in analyzing motor startup due to its relatively high resolution when configured to record power disturbances on a cycle basis. The Event Change Report gives much more detailed information but is somewhat more complicated to examine that some of PMI’s other reports.
To capture a motor startup, a recorder must have the ability to trigger at the correct time and have enough resolution to capture the extremely fast inrush current of the motor. To properly analyze the whole motor startup, it is also necessary to make sure the complete startup sequence has been recorded and observe what happens right before the motor is energized. PMI recorders have very sophisticated triggering functions that allow them to buffer pre-trigger information that can be included so no startup information is missed.
The Induction Motor and What to Expect
All induction motors are not created equal. The inrush current varies with the design of individual motors. Some of the newer motors offer improved efficiency because of the reduced wire size and reduced wire length of their coil windings, which results in a reduction of the I2 x R losses. These motors tend to have a much higher current when the motor is first starting up.
The voltage induced in the rotor depends upon the rotor frequency which further depends on relative speed between the rotor and the synchronous speed of rotating magnetic field. The relative speed at the time of starting or standstill is maximum and hence large voltage is induced in rotor conductors or winding. This causes very high current flow in the rotor– generally 5 to 8 times the full load or running current of a typical induction motor. The inrush current or starting current will generally be 8-10 times higher than the motor’s rated current because the motor is at rest.
The motor appears to be a transformer with a shorted secondary before it starts moving. This results in a very low impedance to the system voltage and the motor has a “locked rotor” current of typically 5 times full load current, but can be up to 8 and sometimes as high as 10 times the current.
At motor startup during the first couple of AC current cycles, transient currents cause some of the phases to have asymmetrical values high enough to sometimes cause circuit breakers feeding the motor to trip, when protection settings are too low. PMI recorders can help the operator analyze the power during motor startup and pick a protection level that would protect the system without causing the breakers to trip. The basic equation for the current is: I = P / cos Ф / (1.732 x V), and the cos Ф can be 0.3 during starting. Since the voltage for the most part stays constant, the current must rise, but is limited to the locked rotor value for the specific motor as power is required to start the motor and to keep it running. Some voltage drop will occur for most power systems during a motor start. This voltage drop with constant locked rotor impedance will cause the starting current to be reduced proportionally.
The startup current is equal to the voltage divided by 1.732 divided by the rotor’s impedance.
When nearly full running speed is reached, the current drops rapidly to full load current or less, depending on the actual load attached.
When analyzing a motor start situation, ideally the stripchart is set to a small interval, such as 1 second, or even 1 cycle. This gives the highest resolution for looking at starting current details. Older recorders may not be able to record at 1 cycle, or it may not have been known in advance that a motor was involved. In this whitepaper, the default 1-minute stripchart interval is used, to illustrate the power of the Event Capture report.
Event Capture Settings Window
Under the Event Capture Settings, Event Recording Parameters, the first column is the Nominal Voltage. The default setting under Nominal Voltage is 120 volts. This is a good setting in most cases for a 120 volt system, however in some cases depending where in the system the recorder is installed, this default setting maybe a little low or a little high. A good rule of thumb for the initial setup is to adjust this setting to the current line voltage during installation. If the voltage tap is close to a substation, this Nominal Voltage may be set to 125 volts. The threshold is added and subtracted to the nominal to form the voltage trip point. These trip points can be entered down to zero volts and up to the maximum recorder voltage using multiples of the threshold voltage. For example if the threshold setting in the second column is set to the default of 6 volts, and the nominal voltage is set to 120, then the first set of trip points would be at 114 volts and 126 volts. The normal voltage band would be between 115 volts and 125 volts. This is where the voltage could wander without triggering the recorder. Since the threshold is set at 6 volts, if the voltage wanders beyond this normal voltage band, a trigger will occur every 6 volts from 120 volts down to zero and from 120 volts up the maximum voltage of the recorder. For example going down the trip points would be on a 6 volt grid, starting at 114, 108, 102, 96, 90, 84, 78… down to 0 volts and a 6 volt grid starting at 126, 132, 138….up to maximum voltage of the recorder. After the recorder has been triggered, this event change will continue until the voltage returns back into the normal band or moves past another trip point. Each time the voltage passes another trip point, the current event is terminated and a new event change is started. This data is compiled in the Event Change Table Report. This report can be accessed in ProVision by moving the mouse cursor to Reports, right clicking Header Reports and then right clicking Event Change.
Figure 1 shows a partial screen capture of the Event Change Report Table.
Figure 1. Event Change Report
A test recording was made on the same motor used in the white paper Analyzing Motor Starts available by clicking HERE, but with a 1-minute stripchart interval. Here we will use the Event Capture report to analyze the motor startup time instead of the raw waveform capture data, which may not always be available.
Figure 2 shows an example of an RMS Voltage Interval Graph of the motor starting, running and stopping.
Figure 2. RMS Voltage Interval Graph
The circles in the stripchart mark the triggered Event Capture records. These are clickable annotations; clicking on a circle opens up the Event Change Report for that event.
The graph in Figure 3 shows a zoomed in view of a stripchart of the motor start up at 21:26:00.
Figure 3. Zoomed in view of a stripchart of the motor start up at 21:26:00
As the motor starts, the line voltage drops slightly as the currents spikes to over 30 amps and then returns to around 10 amps running current. This is exactly what is expected in a motor startup. The 30A starting current pulls the voltage down to 112V. What isn’t known is how long the motor start lasted. The running current is around 11A, but we can’t tell from this graph how long the motor drew 30A, and how long the (112-112)/122 = 8.2% sag lasted.
Clicking on a circle in the RMS Voltage Graph Interval launches the Event Change Report for the motor start as shown in Figure 4.
Figure 4.Clicking on a circle in the Interval Graph shows the Event Change Report
This event is event number 1 of a group call Super Event 1. It was triggered on channel 1 on June 6, 2013 at 21:26:00:09, (timestamp), with duration of 6 cycles with a negative slope, (-S). The Maximum voltage was 116.1 volts with a minimum voltage of 113.9 volts. The Maximum current during the event was 33.10 amps and went down to a minimum current of 23.20 amps towards the end of the event. The Prev. 0.10 is the current in amps 1 cycle before the event started and Post 14.00 is the current in amps of one cycle after the event.
We see that the motor starting current lasted around 6 cycles. The fact that the prior current was low (basically zero), and the post current is lower than the max is indicative of a motor start causing voltage sag. The current before the sag was low, the current during the sag was high, and the current after the sag is lower than during the event. If a sag was NOT caused by the monitored load, typically the current during the event is lower, or around the same as before and after the event.
As shown above, the event change table report show a lot of very important information needed to analyze a motor startup, and this can all be done with any PMI recorder using the Event Capture Function.
Conclusion
All PMI recorders have the capability of analyzing motor startup using the event capture function.
Event capture is supported by all PMI’s software, WinScan and Provision
Another big advantage of using event capture to analyze motor startups is it requires a lot less memory than using the waveform capture function.
When initializing the recorder use a setting that will give enough resolution for motor startup analysis. A good place to start is with the 1 cycle setting.
Cowles Andrus III
Communications Specialist
candrus@powermonitors.com
https://www.powermonitors.com
(800) 296-4120
