# three phase induction motor controller

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#### dimper129

##### Member
hi
i need a circuit to control the speed of three phase induction motor used in my university project.
the motor is 2 HP three phase. i want to control it with micro controlor.

##### Banned
2HP? That's almost 1500 watts. That's a big project! Hopefully the vets here will be able to help you, what you're asking isn't particularly easy and three phase induction motors are like black voodoo to me still =) Good luck.

#### dknguyen

##### Well-Known Member
THere's more theory behind the speed control of an induction motor than any other motor I've come by in class...if you didn't already learn the circuit you didn't already take the right class which also has everything else you need to know in it too (and there's way way more theory than the circuit).

That said, you first need a 3-phase bridge rectifier with filtering to convert AC into DC (a very very large 3-phase AC to DC converter).

Then you need a 3-phase inverter that will take this DC power and use PWM to produce a sinusoid output that can vary in magnitude and frequency. PWM makes square waves only, so you must modulate the PWM duty cycle sinusoidally, and choose a frequency high enough that the square wave harmonics will be filtered out by the motor to it only sees the sinusoidal fundamental frequency waveform, or you must add filters of very high power on your output circuit to do the filtering if you cannot get PWM frequency is not high enough.

Just my opinion...but for a project like this, with such complexity and high power levels, if you are asking where to start, then you do not know enough to finish the project. At the very least, reduce your power level to 200W. Many, many problems arise in high power motor drives which can normally be ignored in lower power drives.

Attached are the two circuits you need to build and connect together (without the control circuitry or many of the inverter components needed in real life like flyback diodes and capacitors to make it work). If you weren't sure, the switches for the inverter are supposed to transistors. If you couldn't figure that out then you need to switch projects right away.

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##### Banned
If the incoming lines can handle the current (household outlets in the US can do close to 1500 watts) there's no need for incoming three phase, just the three phase inverter.

#### tcmtech

##### Banned
Pick up an actual VFD on eBay . I get 2 hp VFD's from time to time to build custom speed controlable drives for my customers. ($35 -$150) The little ones like you need are single phase and can be driven off a 120 volt circuit with a booster autotransformer or a diode and capacitor voltage doubler. (done both before with reliable results)

Strip it, rearange the components and make it look homemade and your set!

If your instructors questions it just say that "in the real world" you dont reinvent the wheel. You just go to the local tire center and get aready made ones!
then say " those who can, do! those who cant, um... well, they have jobs just like yours."

If your going down in flames you might as well make the crash a memerable one!

#### Ubergeek63

##### Well-Known Member
If the incoming lines can handle the current (household outlets in the US can do close to 1500 watts) there's no need for incoming three phase, just the three phase inverter.
that is only with PFC. Without PFC you might get 1200W safely.

#### tcmtech

##### Banned
All my stuff is 20 amp. I can run 2200 watts/VA's all day without tripping the breakers. You must have old house wiring? 15 * 120 = 1800 ???

#### smanches

##### New Member
2200 watts at PF 1.0

#### tcmtech

##### Banned
I am a power systems uber geek! You think I dont have a power factor meter?
P.S. (watts / VA's) I already acounted for less than ideal loads in my post. hehehe
Good catch though!! A+

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