Motor Starting Analysis
Motors are a widely used industry to produce a mechanical power to run various applications such as compressors, pumps, fans, etc. Naturally, motors draw a huge amount of current when it powered-on/energize, under starting condition, high inrush current is flow which can reach up to six times the rated current of the motor, relatively with low impedance in attached bus-bar and loads. Serious problems may accrue in the power system like under-voltage tripping if the motors staring process is not well-managed.
This article is an introduction to a motor acceleration study using ETAP software. we will perform a motor acceleration study on a simple system content three induction motors.
In ETAP there are two kinds of motor acceleration study:
- Static motor starting
- Dynamic motor starting
This tutorial will cover both methods. Note: ETAP version 12.6 was used in this tutorial.
As always, the first step is to build your network, so I just start to establishing a simple network compose of three motors first and second motors are 260 kW and the third one is 150 km.
In the mode toolbar, select motor acceleration study mode. You can choose either static or dynamic study from the right-hand side of the ETAP’s window as shown below.
However, to run either static or dynamic study you will need to create a set of events to decide which motor will switched-on first, at what time and so on.
To do that, click on Edit Study Case > Event > add new event > add a new action and choose the motors you want to either start or stop it. You can set a series of events for other equipment as well like generators.
In the below example, I created three events as follow:
1. Event 1, Mtr1 will start at second 1.
2. Event 2, Mtr2 will start at second 6 and Mtr1 is still running.
3. Event 3, Mtr3 will start at second 11, Mtr1 and Mtr2 are still running.
Static Motor Starting
Let us start with a static study before running it, there are necessary information needed to perform static starting analysis. So, by double click on the motor in your single line diagram (SLD), you’ll see the popup window similar to the following:
go to the load page, and depending on your motor specifications, refer to catalog/datasheet, type the acceleration time both no-load & full-load.
Now, you can run static study and then simply drag the time-slider to the right (time in Sec.) as shown here.
ETAP entitle us to create different types of plots for any devise within our power system. But definitely, motor starting inrush current effects the bus which directly connected to the motors in addition to the loads that attached to that bus (in this case bus2). This is the most important step because it helps us to observe and manege the voltage drop levels.
As a designer, this process should be guardedly managed to avoid trip accident or exceeding the recommend voltage drop percentage which can effect the other loads performance.
As per International Electrotechnical Commission (IEC) “in general, a maximum allowable voltage drop of 10% is recommended during start-up of the motor”.
Source: Electrical installation guide, According to IEC international standards book.
With regard to generate plots just click on motor starting plots option.
From motor starting plots window select the devices that you may need to study/analyse their graphs.
ETAP will create graphs for you similar to the below. Notice that, the more power a motor draws, the more voltage dip can occur.
The above graph shows the voltage dip created by induction motors starting. During Mtr1 and Mtr2 starting the voltage percentage reach to 99.7% of 400V the system/normal voltage, and during Mtr3 starting it reach to approximately 99.9%. Under-voltage percentage will be less than the above example in case more than one motor start at the same time.
Dynamic Motor Starting
The main difference between static and dynamic is that dynamic starting current decrease gradually until it reach to rated-current unlike static. furthermore, dynamic analysis is more difficult than the static analysis and need more information but, it gives more accurate/reliable results.
Three data input are required to run dynamic motor starting:
- Motor model or it equivalent circuit
- Motor inertia
- load model
First of all, you need to adjust the motors information, just by double click on the motor symbol in the SLD, and open the Model page, ETAP provide two option, CKT which mean equivalent circuit and Characteristic. Here an example for equivalent circuit of the motor, you can select it from the library.
After selecting the motor model and click on “OK” you’ll see the popup window similar to the following:
Compare the data provided by ETAP to your actual motor specifications. Hit “Update” if it was approximately equal to your existing data.
Secondly, determine the load model. Load type (pump, fan, compressor, etc)
In the third place Motor inertia where,
inertia is the property of matter which resists change in its state of motion. Inertia is a measure of the force that keeps a stationary object stationary, or a moving object moving at its current speed.
Motor inertia is given by the manufacturer but, there are a typical values you may use in case motor inertia is unknown to you. However, ETAP will fetch/use only the total.
Under the same scenario (same events) as previously. Run dynamic study option and drag the time-slider to the right. As seen below current is decreasing gradually.
Finally, similar to what we have done in static study we can create graphs for study/investigate. For example, the below graphs is for bus voltage curve.
Here is the motor current curve:
Motor speed curve:
With this, we come to the end of this article on “Motor Starting Analysis”. Thank you for reading. As always, I hope you find this article helpful. If you have any questions, concerns, or you want to share more information about the topic discussed above, please feel free to ask through gmail or add me on LinkedIn.