The advance of new technology and electric cars has been taking place recently. As a result, many companies want to ensure their electric engines are the best in the market regarding reliability, autonomy for the client, and durability.
Therefore, these companies perform tests digitally to prevent any damage to the actual product and find the best temperature interval in which the engines work the best.
Performance optimization can be applied to understand the behavior of the electric motors.
We have gathered an extensive data set of several sensor data collected from a permanent magnet synchronous motor deployed on a test bench for this study.
Having robust rotor and stator temperature estimators helps the automotive industry improve their motors, reducing power losses and, eventually, heat build-up.
1. Application type
This is an approximation project since the variable to be predicted is continuous (engine temperature).
The fundamental goal is understanding how voltage and current affect numerous car features and the temperature of different parts of an electric motor.
2. Data set
The first step is to prepare the data set, which is the source of information for the approximation problem. It is composed of:
- Data source.
The file permanent_magnet_synchronous_motor.csv contains the data for this example. Here, the number of variables (columns) is 14, and the number of instances (rows) is 107.
In that way, this problem has the following variables:
- temperature_ambient, ambient temperature measured by a thermal sensor located close to the stator.
- temperature_coolant, coolant temperature. The motor is water-cooled. Measurement is taken at the outflow.
- voltage_direct, voltage d-component.
- voltage_quadrature, voltage q-component.
- current_direct, current d-component
- current_quadrature, current q-component.
- voltage_module, vector module from the d-q voltage components.
- current_module, vector module from the d-q current components.
- speed_motor, vehicle motor speed.
- torque, torque induced by current.
- temperature_stator_yoke, stator yoke temperature measured with a thermal sensor.
- temperature_stator_tooth, stator tooth temperature measured with a thermal sensor.
- temperature_stator_winding, stator winding temperature measured with a thermal sensor.
We have different variables in the study: ‘temperature_ambient’, ‘temperature_coolant’, ‘voltage_direct’, ‘voltage_quadrature’, ‘current_direct’, ‘current_quadrature’, ‘u_module’, and ‘i_module’ are inputs. On the contrary, ‘motor_speed’, ‘torque’, ‘stator_yoke’, stator_tooth’, and ‘stator_winding’ are the targets of this study. Indeed, our main goal is to describe the behavior of the electric motor to prevent overheating. That is why these output variables show the temperature of the engine’s internal parts.
They are divided randomly into training, selection, and testing subsets, containing 60%, 20%, and 20% of the instances, respectively. More specifically, 65 samples are used here for training, 21 for validation, and 21 for testing.
Once all the data set information has been established, we will perform some analytics to check the quality of the data.
For instance, we can calculate the data distribution. The next figure depicts the histogram for one of the target variables.
In this diagram, we can see a normal distribution of the stator tooth temperature representing one of the parts of the stator temperature. We could affirm it is normal because this output depends on many input variables simultaneously. During this experiment, the inputs constantly varied, causing the targets to have this type of distribution.
The next figure depicts inputs-targets correlations. This might help us see the different inputs’ influence on the motor temperature.
As this machine learning study has various target variables, we will show the correlations diagram of one.
The above chart shows that a few instances have a critical dependency on the variable ‘torque’. As we can see, an instance is highly correlated to this target, which is the case of the input ‘current_quadrature’.
At first sight, we could have predicted this behavior simply by looking at the data set and realizing the torque is induced by the current, in this case, by the quadrature coordinate of the current.
We can also plot a scatter chart with the stator winding temperature versus the ambient temperature.
In general, logically, the more the ambient temperature, the more stator winding temperature.
3. Neural network
The neural network will output the different motor temperatures as a function of the current, voltage, coolant temperature, and ambient temperature.
For this approximation example, the neural network is composed of:
- Scaling layer.
- Perceptron layers.
- Unscaling layer.
The scaling layer transforms the original inputs to normalized values. Here, the mean and standard deviation scaling method is set so that the input values have a mean of 0 and a standard deviation of 1.
Here, two perceptron layers are added to the neural network. This number of layers is enough for most applications. The first layer has eight inputs and three neurons. The second layer has three inputs and five neurons.
The next figure shows the resulting network architecture.
4. Training strategy
The next step is to select an appropriate training strategy, which defines what the neural network will learn. A general training strategy is composed of two concepts:
- A loss index.
- An optimization algorithm.
Once the strategy has been set, we can train the neural network. The following chart shows how the training (blue) and selection (orange) errors decrease with the training epoch during the training process.
The most critical training result is the final selection error. Indeed, this is a measure of the generalization capabilities of the neural network. Here, the final selection error is selection error = 0.083 NSE.
5. Model selection
The objective of model selection is to find the network architecture with the best generalization properties. We want to improve the final selection error obtained before (0.083 NSE).
The best selection error is achieved by using a model whose complexity is the most appropriate to produce a good data fit. Order selection algorithms are responsible for finding the optimal number of perceptrons in the neural network.
The final training error continuously decreases with the number of neurons. However, the final selection error takes a minimum value at some point. Here, the optimal number of neurons is 9, corresponding to a selection error of 0.0432.
The following figure shows the optimal network architecture for this application.
6. Testing analysis
The objective of the testing analysis is to validate the generalization performance of the trained neural network. The testing compares the values provided by this technique to the observed values.
A standard testing technique in approximation problems is to perform a linear regression analysis between the predicted and the real values, using an independent testing set. The following figure illustrates a graphical output provided by this testing analysis.
From the above chart, we can see that the neural network is predicting the entire range of temperature data well. The correlation value is R2 = 0.990, indicating the model has a reliable prediction capability.
7. Model deployment
We can plot a directional output of the neural network to see how the targets vary with a given input for all other fixed inputs.
The next plot shows the stator tooth temperature as a function of the quadrature coordinate of the voltage, through the following point:
- temperature_ambient: -0.603. (scaled: Mean=0, Deviation=1)
- temperature_coolant: -0.393. (scaled: Mean=0, Deviation=1)
- voltage_direct: -0.359. (scaled: Mean=0, Deviation=1)
- voltage_quadrature: -0.235. (scaled: Mean=0, Deviation=1)
- current_direct: 0.0834. (scaled: Mean=0, Deviation=1)
- current_quadrature: 0.231. (scaled: Mean=0, Deviation=1)
- voltage_module: 1.26. (scaled: Mean=0, Deviation=1)
- current_module: 1.19. (scaled: Mean=0, Deviation=1)
The electric_motor.py contains the Python code for the electric motor temperature Neural Network.
- Kaggle Machine Learning Repository. Electric Motor Temperature Data Set.