Philippe MISSIRLIU

Lycee Newton Enrea

1 Pl. Jules Verne

92110 Clichy

France

The use of a simulation software is an essential requirement to
teach electrical engineering :

- Simulation technique gives results, when lacking sophisticated hardware for tasks such as: current measurement in a transistor, calculation of Fourier transform of a signal, measurement of magnetic fluxes.
- Students being what they are (and not what they should), many phenomena are too complex for a theoretical presentation. Simulation allows the presentation of findings, when time is short for explanation.
- Simulation is increasingly used in the industry, familiarising students with its use is therefore mandatory.

Amongst the many software available on the market, Pspice has many
advantages:

- It is based on the industrial standard Spice and therefore gives access to many libraries of models, developed by manufacturers.
- Pspice is widely used in industry.
- It allows the mixing of digital and analog parts without any problems.
- Learning the software is quite easy.

The professional version of Pspice is not within the range of
budgets allotted to our high school. Fortunately an evaluation
version is available for absolutely free, either to be downloaded
from Internet or as a CDROM ordered from the distributor. An
evaluation version has limitations of course. But here we are limited
only in the number of nodes of the scheme to be simulated. All the
many other features are available.

We have used Pspice in our department, in student projects, for
three years. To that end, we developed a number of easy steps, along
with a library of optimised parts. We have thereby managed to cover
all the aspects of an electromechanical system: power electronics ,
mechanics, digital and analog electronics, regulation, all with one
software, moreover one free software!

The aim of this paper is to illustrate the above stated, by way of
a frequently used topic in EE: the speed control of a DC motor fed by
a 3-phase controlled rectifier.

The evaluation version of Pspice offers a series of traditional switches such as : bipolar transistors, field-effect transistors, IGBT (starting from version 8), SCRs, triacs. However, an attempt at simulating a simple 3phase controlled rectifier, will produce two problems:

- The SCR available in the evaluation version has a VRRM of 50V and a max current of 1Amp.
- Using more than three SCRs results in the message : " EVALUATION VERSION analog Node Limit (64 Nodes) Exceeded! "

The solution consists in designing a "home made " SCR. This
part does not completely simulate a real SCR, as there is no
provision for a triggering circuit. Phenomena such as the hold
current, or dV/dt triggering, are not simulated. Nonetheless the
model proved satisfactory for our use.

Figure 1 shows a 3phase controlled rectifier using this part. It
will be noticed that there is no apparent triggering circuit for the
SCRs. The simulation results are presented in figure 2. Pspice has
powerful means of visual presentation of results. One can visualise
dimensions as instantaneous, average, rms values, and calculate an
harmonic analysis.

There are also more sophisticated functions available. In this
case, one could contemplate visualising the variations of the average
value of output voltage, as a function of the triggering delay.

A software usable in electrical engineering must be able to simulate mechanical dimensions. This does not necessarily constitute an initial feature. We did however accomplish this by using a classical analogy between mechanical and electric dimensions:

Speed |
Voltage |

Torque |
Current |

Rotational inertia |
Capacitance of a capacitor |

Viscous friction |
Reciprocal of a resistor |

Elastic connection |
Reciprocal of an inductance |

By using this analogy, we developed a model of a DC MOTOR along with a model of a mechanical load. The parameters of these models may be modified. Figure 3 shows the modeled DC MOTOR fed from the 3phase rectifiers above mentioned. The simulation results are seen in figure 4.

Pspice allows the use of blocks defined by a mathematical
function, such as Laplace transforms. We used this possibility to
model the speed control of a DC motor. In order to avoid prolonged
calculating time, the 3phase rectifier of the preceding paragraph,
had to be replaced by a simpler model. For a start a simple gain.
Eventually,the simulation of the influence of the rectifier may be
added, by introducing a delay. The classical structure with two
intertwining loops with PI correctors is presented in figure 5.

Figure 6 illustrates the response of the system. Pspice obviously
allows the study of harmonics and the tracing of classical diagrams
ie: Bode, Nyquist, and Black. One may easily wiew the effect of a
modification of a parameter on the system’s behavior. This makes
Pspice a great teaching tool.

Simulation of a control circuit does not call for specific
adjustments. The software is designed for this type of application.
Digital and analog parts may be combined. The example presented is a
project developed at the ENREA: a singlephase 50Hz inverter with PWM
control, see schematics in figure 7. The aim is to suppress the first
two harmonics.

The control sequence of the switches is introduced by way of an
EPROM in which the first 1024 addresses are scanned every 20ms.
Simulation results are presented in figure 8. Harmonics analysis of
the data is presented in figure 9. The first harmonic of the tension
measured is of the 7-th order. Note that the file used to program the
virtual EPROM is identical with the one used for the real EPROM.

** **

Spice is the de facto standard in the industry. One resulting advantage being the frequent offer of Spice compatible models by manufacturers. As an example: we use an AD633 analog multiplier manufactured by Analog Devices at the ENREA. We downloaded a Spice model of the part from their Internet site. We just created a corresponding symbol and added it to our own "home made" library. Another means of creating a part model is the ABM behavioural modelisation, using blocks defined by mathematical functions. The technique was used to create a model of the AD736 RMS converter of Analog Devices, of which no model was available. We used the synopsis of the part and the specifications mentioned in the manufacturer literature.

We have been using the demo version of Pspice at Lycee Newton ENREA for three years to our complete satisfaction. People interested may download the library we have created, from the attached Internet site, together whith the simulations contained in this paper.