This paper covers the setup fundamentals and techniques required to obtain the high-quality waveform capture needed to analyze a motor startup. It also reviews the basic electrical characteristics of a typical AC motor.
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 the 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 the 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.
The initial startup current is not directly related to the load that is applied to the motor, but can add to the inertia that the motor has to overcome and increases the time for the motor to reach its top speed. During acceleration, part of the starting current used to overcome the system’s inertia is converted to kinetic energy. Lenz’s Law and Joule’s first law can be used to explain where the remaining energy goes in causing the rotor to heat up, especially when the inertia is large and results in a longer startup time. Lenz’s Law states that an induced voltage (EMF) always gives rise to a current whose magnetic field opposes the original change in magnetic flux . Joule’s first law explains that heat is generated due to this current flowing through a conductor.
At motor startup, during the first couple of AC current cycles, transient currents make some of the phases have asymmetrical values high enough to sometimes cause circuit breakers feeding the motor to trip if protection settings are too low. Our recorders can help the operator analyze the power during motor startup and pick a protection level that would protect the system yet not cause the breakers to trip. The basic equation for the current is: I = P / cos Ф / ( 1.732 x V), and cos Ф can be 0.3 during starting. Since the voltage usually stays constant, the current must rise, but is limited to the locked rotor value of the specific motor as power is required to start the motor and 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.
Initialization And Setup
In order to capture useful information, it is an imperative to have the recorder setup properly in the initialization phase. Figure 1 shows a typical stripchart Initialization setup for a motor start.
Figure 1. Typical stripchart initialization setup for a motor start
Since the motor startup happens very quickly, for the preliminary recording, the following guidelines are recommended:
1. Use a very small stripchart interval such as 1 second or ideally 1 cycle. With the 1 cycle interval, it is possible to get cycle RMS voltage, current, and other data for the maximum time resolution.
2. It is a good idea to turn off all unneeded channels and measurements, such as reactive power, phase angle and anything else that is not necessary. This allows for maximum recording time with a short interval.
3. For waveform capture, set the trigger on current and the post size to something long enough to capture the entire start such 60 or 120 cycles, or maybe a little more. Be sure to turn off all unneeded channels and consider reducing the sample rate to 128 or even 64 samples per cycle to maximize the number of waveforms. Higher rates are not needed for RMS measurements. Figure 2 shows a screenshot of a waveform capture initialization setup for a motor start.
Figure 2. Waveform capture settings for a motor start recording
It is also important to note that initial motor startup currents can sometimes be quite high and for the preliminary initialization, it might be wise to set a high current range, so as not to clip peak currents. For example, a motor that has a 90 amp rating may need the recorder set to the 1000 amp range to capture the peak startup current.
When doing motor startup studies, initialize and start the recorder and then start the motor as soon as possible. This is the best method for high-resolution captures. If this is not possible, and the recorder will be placed and left for some time before a motor start, a slower interval may need to be used or data overwrite may need to be enabled.
In order to get the complete picture of what is happening with a motor, it is good to not stop the recording just after startup but to continue to record until the motor is powered down. Also, if there is not a constant load on the motor, it is good to capture the motor during the load changes and especially during the peak load conditions.
Figure 3 shows a screen capture of a typical small 120V motor startup.
Figure 3. Voltage and current cycle stripchart of a small 120V motor startup
The voltage is initially at around 123 volts and drops by about 10 volts as the current inrush peaks at over 30 amps and then settles out at just over 9 amps. Channel 2 of the recorder shows how the voltage between neutral and ground reacts during the motor startup current inrush.
Figure 4 shows a waveform capture that clearly defines how the current peaks at startup return to a lower level after a few cycles, once the motor is up to speed.
Figure 4. Current peaks at startup return to a lower level after a few cycles, once the motor is up to speed.
Figure 5 shows the RMS Capture graph from the waveform capture data in Figure 4.
Figure 5. RMS capture graph of motor start waveform
Here, a sliding RMS window has been applied, giving a continuous RMS value for voltage and current. The 10V sag during the motor start is much easier to see than in Figure 4. This is the highest resolution available for RMS data, especially if cycle stripchart data is not present.
Figure 6 shows the RMS stripchart of the same motor, but with the default 1-minute interval setting.
Figure 6. Motor starts with the initialization set at the default values and not optimized
Here the motor start is reduced to a single stripchart min/max/ave value, with no indication of how long it lasted.
Conclusion
1. Motor startup currents will vary depending on the specific design of the motor; however, all motors always have a much higher peak startup current than what is required to keep them running under normal load conditions. It is important to make sure that the range is set high enough to record the peak startup currents. If you are not sure what current range to select, double-click the current range.
2. Use a very small stripchart interval such as 1 second or even 1 cycle to allow maximum time resolution.
3. Turn off all unneeded channels and measurements to allow for maximum recording time with a short interval.
4. When using waveform capture, set the trigger on current and the post size long enough to capture the entire start. Consider reducing the sample rates to 128 or even 64 samples per cycle to maximize the number of waveforms captured.
5. If possible, initialize and start the recording and then start the motor as soon as you can. This is the best method for high-resolution captures. If this is not possible be sure to have the data overwrite enabled.
6. Consider the peak starting current when selecting the current range. It may be necessary to use the next higher range compared to the running current.
Cowles Andrus, III
Communications Specialist
candrus@powermonitors.com
https://www.powermonitors.com
(800) 296-4120