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Registered Member #989
Joined: Sat Sept 08 2007, 02:15AM
Location: São Paulo, Brazil
Posts: 476
I am finishing the theory and starting to make the prototype, but now I am thinking here:
My project is a Three Fase Brushless motor controller (like used on Eletric R/C Planes, or HDD Motors), and I see various types of inverters, some uses Square wave, others sine. What is the best method for these motors?
I attached the output A B C from the Three Fase Bridge, as you can see it has three voltage levels, 0V 6V 12V, with that, the motor will work (I see other circuits arround the internet that uses this concept) but I dont know if its the best, if its sine, I can integrate the signal with an Op-amp. At simulator (Proteus) the motor is consuming about of 100A! (With set 10mH and 6 Ohms DC Resistence for each coil, but actually the motors that run on rc planes have MUCH lower inductance for each coil). I am switch with P and N mosfets ( IRFZ44NS and IRF5303 ) If someone knows anything else that can help me, I will be grate =)
Soon as finishes that project, I will post it on 4hv. Thanks!
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
You must keep in mind that the fact that the motor is turning induces a voltage in the winding which works against you, when the motor is stopped (stalled) you only have the inductance of the winding, and when the motor is running at top speed (as determined by the motors kv rating) the emf from the turning motor will equal the drive voltage. A surprisingly good resource I found was from microchip at
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Square wave is easier to generate, but the harmonics make the motor run hotter and less efficiently.
Sine wave gives the ultimate motor efficiency, but needs high frequency PWM that make the MOSFETs in your controller run hotter and less efficiently.
You'll probably need to use PWM of some sort anyway, in order to meet the motor's Volts/Hz characteristic from a fixed voltage source like a battery. For an efficient motor, the winding resistance can be neglected, so the back EMF is the only thing that limits the current. You left back EMF out of your simulation and that's why the current is excessive. Back EMF is defined by the "Kv" coefficient that Peter mentioned, which has units of volts per RPM, volts per Hz or whatever.
Microchip have application notes for controlling all sorts of motors on their PICs.
Registered Member #989
Joined: Sat Sept 08 2007, 02:15AM
Location: São Paulo, Brazil
Posts: 476
Hmm, I was using PIC16F84A to make a controller, but I think I will need to jump to 16F877A because the built-in ADC's (Here 84 and 877 its about the same price, so its doesnt make any difference)
Also, reading that I see that I was making the speed control wrong, I though that the fases frequency that defines the motor speed, not the voltage. So I will need to use PWM to control the speed. So its more complicated than I imagined =P, but I will try it.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
you will quickly open up a large can of worms with different motor control techniques, trust me!
In a nutshell, a sine wave type drive is usually more efficient, assuming your EMF is sinusoidal (just spin it up and connect to a scope). Ideally, you build a lookup table which matches your EMF waveform.
Any voltage you apply to the motor will create current on 2 axis - direct and quadrature. Depending on how you define these terms, current on your direct axis (in sync with the EMF) creates torque, and current on the quadrature axis creates losses and heat. Warning these terms can be reversed when they are relative to flux! A fairly standard 'vector control' type control loop takes the quadrature current, and tries to minimise it to zero, by altering the applied voltage. Frequency is proportional to load (i.e. faster running indicates less load). Any fixed 'position sensing' will probably be either optimised for starting torque, or for maximum torque at a certain speed - because as the motor speeds up, the flux shifts (I forgot the term!) Good luck making it all work
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Three Fase Brushless Motor - Square Wave or Sine Wave?
