Alternative Hybrid-Electric Vehicle

An Alternative Hybrid-Electric Vehicle

Sunday, October 12, 2014

3-Phase Sinusoidal Variable Frequency & Voltage Inverter

Now the next question is: "What drives the 3-phase, 60-Hz, squirrel-cage induction motor?"

The prototype uses transformers to step up the 3-phase power to the operating voltage of the induction motor. One would say that this is an inefficient configuration. And that comment is absolutely true. Beyond the prototype, the motor should be driven directly from the battery, through a control switching system, of course.

We have not yet answered the question. Ok, now we will.
  • We start with a battery and have a DC power supply
  • You could have any voltage you want
  • In our case, we decided upon 84 volts for the prototype, so that we do not unnecessarily expose ourselves to electrocution, i.e., death!
  • Now generate a square wave that will go from 0 to 84 volts, stay there for 1 millisecond, then go from 84 to 0 and stay there also for 1 millisecond
  • Connect a 10k resistor to the square wave that is generated and connect the other terminal of the resistor to a 1 micro-farad capacitor and measure the voltage on the capacitor
  • You will see the voltage to be half of 84, or 42 volts
  • Make the square wave stay at 84 and when the voltage on the capacitor hits 62, bring the voltage down to 0
  • When the voltage on the capacitor hits 58, switch the resistor back to 84 volts
  • When the voltage on the capacitor hits 64, bring the voltage back down to 0
  • When the voltage reaches 60 volts, switch it back up
  • The first set point is 60 +/- 2 volts, the second is 62 +/- 2 volts 
  • Make this set point follow a reference sine wave in time and the average voltage at the capacitor will approach a sine wave
  • Call this phase A
  • Build another set-up like this and make it lag 120 degrees from phase A, and you have phase B
  • Build a third set-up and make it lag 240 degrees from phase A, and you have phase C
  • NOW, you have built a 3-phase inverter, i.e., converting Direct Current to Alternating current
  • Example of a clean sine wave is given below

Sine Waves spaced 120 degrees apart characterizes a 3-phase electrical source.

Our Inverter builds these sine waves with variable frequencies, but maintaining an electrical distance of 120 degrees.

It is able to supply a peak current of 90 amperes at 84 peak volts. This is more than enough power to drive our 3-phase squirrel-cage induction motor.

We have partially answered the question of "what drives our electric motor?" The answer is an INVERTER that takes its input from a battery and produces a 3-phase power source for the motor.

Ok, we have an inverter. But how does the inverter produce AC from DC? 

The process is as follows:

  • First, we generate a half sine wave for each phase, whose frequency we can vary - "A" above
  • This is the reference we said earlier
  • Second, we build a square wave, whose period coincides with the reference sine wave - "C" above
  • Third, switch the 0 and 84-volt span, each time reversing when the reference is reached +/- the desired band - "B" above
  • And the resulting wave is "D" above that can supply 90 peak amperes at 60 volts AC
  • Feed this to each of the phases of the WYE-connected transformer primaries and you have 240 volts on the -WYE-connected secondaries

This description sounds very easy and straight-forward. And it is. The issues are normally found in the components and their corresponding ratings. In this work, the most critical are their voltage and speed ratings. Current ratings have a significant tolerance, voltage hardly any, and inadequate speed blows up your IGBT's (a term that stands for "insulated gate bipolar transistor").

Very Important Landmarks

At this point, let me give three videos, which I recommend that you watch.

  • The first one shows the beginnings of the creation of the sine wave at the laboratory
  • The second shows the inverter driving a 3-phase electric drill
  • The third is my YouTube video of the HEV

Video 1 - The beginnings of the Inverter

This is a video of the early stages of the variable frequency, variable voltage inverter, with current controller. Its output could be a SINUSOIDAL VOLTAGE or it could be a SINUSOIDAL CURRENT. These combined features make it technically ready to synchronize to the Power Grid, WITHOUT sophisticated and expensive synchronizing equipment.

Video 2 - The Inverter drives a 3-phase industrial drill

This video demonstrates that this HEV's 3-phase inverter drives a 3-phase industrial drill and can also drive irrigation pumps, supply electricity to homes, and to other applications. 

This is one very important feature that makes it UNIQUELY attractive to developing countries abroad, where only single-phase power is available, especially in farms and remote areas. You will use the hybrid to transport produce from the fields, such as rice or corn, and bring them to one area where you have a 3-phase electric motor that drives a rice or corn mill. Or you could drive irrigation pumps with your HEV.

Many developing countries have capabilities to build their own indigenous vehicles. 







(Photo courtesy of NiƱo Uy, 2005)
This photo shows an irrigation pump that is driven by a single-cylinder diesel engine. Our HEV is able to fulfill the diesel engine's function through a 3-phase induction motor of the same rating. While the HEV is able to fulfill the function, a study will need to be undertaken on each and every condition, in order to confirm that the economics is advantageous.

Video 3 - My YouTube Video

This 4.5-minute video walks you through the whole concept behind the technology of this HEV. You will note that this alternative technology is very doable, and produce very affordable products. 

How do I control the speed of the motor? There are several ways that are acceptable engineering practices.

  • First Method. 
    • Run the motor up to synchronous speed (less induction motor "slip")
    • Adjust the frequency up or down and the motor will change speed to follow the frequency
  • Second Method.
    • Impress a reduced voltage on the motor
    • Increase the voltage to reduce the "slip" of the motor and it will run faster
    • The speed and torque of the motor will change as the voltage is varied
  •  Third Method.
    • This is could be used on single-phase systems
    • As the system is running, skip one complete wave, or Period
    • Then skip 2, or 3, or 4, and so forth
    • When I do this method, I perform ALL the switching at "Zero Crossing"
  •  Fourth Method. (I perform this on my 3-phase inverter)
    • Since I can control the voltage output of my 3-phase inverter (variable-frequency, variable-voltage), I reduce this voltage to ZERO on all three phases
    • When I do this, I cut off the magnetism to the rotor, thus removing the prime-moving torque
    • At no load, it will coast, and with load, its speed will be reduced faster
    • I could maintain the speed of the motor as I bring the voltage output all the way up, and all the way down, depending upon the load requirements
    • Now, some care will be exercised, because we do not want to introduce too many transients into the motor that produce harmonics that can generate iron-core heating 
My HEV performs all these speed control techniques, except the Third Method, which I use only for single-phase operation.

In summary, my controller has three set points, with which I control the speed and power of the 3-phase AC motor. They may be manipulated individually, or in sequence, or concurrently/parallel, to achieve the desired effect. These are:

  • The amplitude of the reference sine wave - Controls the magnetism and the coupling between the stator and the rotor, resulting in "slip". The reference signal maintains the sinusoidal form of the inverter's output.
  • The frequency of the reference sine wave - Controls the speed of the rotating magnetic field at the stator
  • The AC current controller - Also controls the magnetism, except that it clips the top portions of the 3-phase sinusoid.

I repeat that one must avoid or minimize the production of harmonics at the iron pieces.