# motor controller for excitation of a synchronous generator

Hello, does anyone see a problem in controlling the excitation of a synchronous brushless generator using a dc motor controller ?
I am working on a project involving the control of a variable speed wind turbine where the control variable is a number called Tip Speed Ratio (TSR) which is =angular speed of rotor * rotor radius / wind speed.
As you can imagine keeping TSR around any fixed number (8.5 actually) requires varying the torque of the generator, which depends on the excitation voltage.
Exitation power is max 160W, 70V DC, which can be supplied from a 24V battery, then a boost converter 24 to 70V and from there PWM from a motor controller...

chispas:
Hello, does anyone see a problem in controlling the excitation of a synchronous brushless generator using a dc motor controller ?
I am working on a project involving the control of a variable speed wind turbine where the control variable is a number called Tip Speed Ratio (TSR) which is =angular speed of rotor * rotor radius / wind speed.
As you can imagine keeping TSR around any fixed number (8.5 actually) requires varying the torque of the generator, which depends on the excitation voltage.
Exitation power is max 160W, 70V DC, which can be supplied from a 24V battery, then a boost converter 24 to 70V and from there PWM from a motor controller...

I have no thoughts because what you wrote does not make sense to this old man.

A generator generated DC electricity and uses brushes to collect the DC out to wires. An alternator generates AC electricity and uses brushes on slip rings to bring the power out to wires.

The use of the word "synchronous" tells me the device is somehow locked to an AC excitation signal. But you then describe control based on a DC signal.

Perhaps a link to the devices specifications would help me understand the device.

Paul

Knowing a bit about electric machines is required to understand this.

The rotor is excited via slip rings with DC to produce a magnetic field fixed relative to the rotor. Its a
synchronous generator because the output frequency is locked to the angular speed.

The stator windings are the AC output.

The output frequency depends on the speed, the output voltage on the speed times the excitation field strength.

When driving a grid output the current will depend on the difference between no-load voltage and grid voltage, so by tuning the output voltage you can control the current, and thus the torque.

Some AC generators have 3-phase AC rotor windings that allow the magnetic field to rotate w.r.t. rotor,
and these are not synchronous, allowing the output frequency to be locked to the grid even if the rotor speed
varies.

Thanks, Mark. Hope the OP has some input for his device.

Paul

chispas:

You need sensors to measure output current, shaft speed and wind speed, and nested control loops I think.

The outer loop will be maintaining TSR by monitoring wind speed and shaft speed, and also implements speed limiting / overload prevention... It controls the set point current for the inner loop, which is all about controlling
output current via excitation drive.

You also need a BMS for the battery to keep it charged properly.

BTW "Electric Motors and Drives: Fundamentals, Types and Applications" is a great book for learning about all types of electric machine.

Many thanks to all for your coments and book recommendation (indeed a very good introduction I just got it from the library!).

The control method is exactly what MarkT posts above.The machine is meant to operate in island mode (no grid).

I leave here a short description of the brushless synchronous generator of the type I want to use: BRUSHLESS ALTERNATOR - YouTube .

First part up to 3' 23" is the relevant part.

The control of the generator torque (and electrical output) oposing the wind turbine's torque is proportional to the DC current applied to the exciter satator windings. As generators of this kind are usually meant to keep voltage and frequency constant,the Automatic Voltage Regulator has to adjust excitation current constantly as the load varies.

But in order to be used in a Wind Turbine one needs to forget about constant voltage and frequency in order to achieve aerodynamic efficiency (you can either find a use for this fluctuating power or you can use power electronics in order to re-form it). So the Automatic Voltage Regulator must be by-passed.

Now the problem that arises is how to feed the exciter stator with DC power. One can think for example in using a continously adjustable DC-DC boost converter (such as https://www.aliexpress.com/i/32879168358.html ) and operate the voltage potentiometer with a microcontroller (some minor surgery to the circuit would be needed) or one can set this DC-DC boost converter to the maximum expected voltage and current and then use a motor controller to adjust voltage through PWM.

Alternatively, I could use a motorized rheostat after the DC-DC boost converter but this would complicate things mecanically, software-wise and would make the cost go up significantly (may be unfeasible).

So I am hessitant as I have seen a lot of PWM used in the control of brushed DC machines but not to control a synchronous alternator. Besides, I think that the pulses at low voltages could create mechanical punches leading to vibrations etc.

Thanks again. I am sure I have a lot to learn from you all.

Variable boost converter potentiometers are usually operating in the feedback loop at the output voltage
and are not suitable for replacement with a digital potentiometer. Perhaps the motor controller option is
a better bet, although you only need 1 quadrant control I think, perhaps just one MOSFET can serve?

I understand the potentiometer may be operating in a feedback loop from the output voltage. I haven't noticed that issue thanks for bringing it up. I will examine the variable boost converter and post a coment if I can find the operating principle. I will also re-visit the motor controller option.