# Three phase induction motor control

I have been developing control software for below given power electronics circuit.

This circuit consists of two three phase inverters with common DC link. One inverter supplies a three phase squirrel cage induction machine working as a motor and the second one supplies also three phase squirrel cage induction machine but working as a generator. Both induction machines are coupled together via sheaves and belt. The common DC link is connected to utility grid via three phase diode rectifier.

Both inverters are controlled by one microcontroller. The sequencing should be following. At first the precharging step is needed. After that step the average DC link voltage value is 3*sqrt(6)*230/pi V. Then the motor is started with scalar control algorithm in open loop until the prescribed frequency is achieved. After that the second machine should be started.

The second machine should work as an induction generator. So it is necessary to somehow identify the desired stator frequency so that the machine will actually work as a generator (the synchronous speed must be lower than the actual mechanical speed which is prescribed by the motor).

My problem is in the generator inverter control. I am not sure whether I understand the "physics" correctly. The generator rotor is rotated by the motor. As soon as I set some value of synchronous speed (i.e. stator frequency) so that the generator mode is achieved the generator will start deliver electrical energy into the DC link. This results in current flowing from the generator into the DC link capacitor via freewheeling diodes in the generator inverter. This results in rise of the DC link voltage. As soon as the DC link voltage achieve the voltage produced by the generator and the current cannot flow and the generator stops deliver the energy. So I need DC link voltage controller which will control the DC link voltage by changing the generator synchronous speed (i.e. delivered power by the generator). The problem is that the DC link voltage is also influenced by diode rectifier wich isn't controllable by the DC link voltage controller. Please can anybody tell me whether my thinking is correct and whether it is possible to control this circuit with DC link voltage controller and what conditions must be fullfilled? Thanks for any suggestions.

• The DC link voltage should be controlled by the rectifier that is connected to three phase utility power (mains). The DC link capacitor needs to be sized to supply the reactive volt-amperes for both machines. It seems to me that the system as drawn is operable with the right control scheme. If the IG would be connected to utility power, it would be quite easy to control, but limited to the speed dictated by the utility frequency. This is essentially a "four square test stand." You might search to see how it has been done by others. The IG control transistors must carry current also.
– user80875
Oct 10, 2017 at 16:08
• What is the purpose behind this? A thought experiment? Are you needing the final result, or is the process of design the ultimate goal? Because you can buy commercial-off-the-shelf VFDs that can be used in this manner, you don't need to invent anything. Oct 10, 2017 at 20:35

## 2 Answers

You should really add a diagram. But I think I get the setup. Two induction motors. Each one controlled by an inverter. The two inverters are connected to the same DC link voltage. The motors are mechanically coupled so that they spin at the same shaft speed. So, if you want one of them to act as a generator, the easiest way would probably be to command negative torque in the generator. The other motor would be commanded to operate at a specific speed. I don't think there is any danger of over-driving the DC link, because there is no source of power other than the grid.

I hope you are not thinking of this as some kind of free energy or perpetual motion idea. You probably should explain why you want this arrangement. Maybe there is an easier way.

• Thanks for your answer. I don't think about that as a type of perpetual motion. I have two problems. I don't know whether I understand the behavior of the circuit during generator operation (I think that the stator current of the generator reverse its flow direction and will flow through freewheeling diodes in the inverter which will act as three phase diode bridge rectifier and this current will charge the DC link capacitor). The second problem is that I don't know how to control this system. Is it sufficient the scalar control (V/f) of both machines? Oct 11, 2017 at 7:17
• In the setup you describe, there is no energy source. So the DC link voltage will not rise any higher due to generator action. All you are doing is converting electrical energy to mechanical energy and back to electrical energy. Efficiency less than 100% guarantees that the returned energy is less than the energy used. Oct 11, 2017 at 7:29
• In a more general case, where there is some other source of mechanical energy, I suspect v/f control might be sufficient. Or, at worst, you could implement a PID control loop that observes the link voltage and adjusts the generator frequency to maintain the link voltage in regulation. But this is a bit outside my experience, so take it with a grain of salt. Oct 11, 2017 at 7:32
• Do you mean that it is possible to identify that the induction machine works as a generator based on negative stator current? Oct 11, 2017 at 7:45
• Sure. Doesn't that make sense? If current flows from induction machine to inverter, then the machine must be operating in regenerative mode. (Acting as a generator). Oct 11, 2017 at 16:08

Both machines can act as well as a motor or a generator. This all depends on their electrical frequency in respect to the common speed. Think of the large inertia of the arrangement to understand you may also have two motors or two generators in some situations.

Given the instant mechanical speed is fixed by the inertia, a machine whose synchronous speed (given by the electrical frequency) is higher acts as a motor. A machine whose synchronous speed is lower acts as a generator.

So the solution is to supply two different frequencies to the two machines. You don't have to care about the DC stuff and the supply rectifier. It's your rail, it has plenty of power and there's no way to shove power back to the grid – the rectifier has no controls. You don't have to care about the DC voltage either, it settles to the voltage delivered by either the generator or the rectifier, whatever is higher.

EDIT: To sum it up: The frequencies of the two inverters in regard to the mechanical speed control the direction and value of the power consumed/delivered by the rotating machines, and the phase angles of the two inverters between their respective current and voltage control the direction and value of the power transferred by the inverters. These should all match, but care about losses.

• Thank you for your reaction. So you mean I should only set the frequency of the motor inverter and generator inverter in appropriate fixed manner so that the generator delivers the exact amount of energy which is consumed by the motor? Oct 11, 2017 at 6:03
• To the OP, if possible, configure the generator by commanding negative torque. If the inverter does not support torque commands, then a fixed frequency difference between the two motors should produce a similar result. Please note that it is NOT POSSIBLE for the generator to supply the full amount of energy consumed by the motor. That would be equivalent to a perpetual motion machine. Oct 11, 2017 at 6:34
• It's not possible in steady state, but sure possible while consuming the energy stored in the rotating mass. Of course, speed drops quickly then. Oct 11, 2017 at 12:58
• @Steve: you have to understand the power consumed/delivered by an asynchronous machine depends on the slip between mechanical speed and synchronous speed. You cannot set the mechanical speed in your arrangement (no turbine connected), so you have to measure it and set the synchronous speeds (electrical frequencies) accordingly. Then, the motor can consume and the generator can deliver power. Second, you have to set the phase angle of your transistor arrays that way they are able to shove the DC power to AC for the motor and vice versa for the generator. Oct 11, 2017 at 13:05
• @Janka, yes. Energy is stored in the rotating mass. My comment only applies to the steady state. Oct 11, 2017 at 16:14