Looking for 3 phase motor speed control.

[avitsur]

I would like to build a 3 phase (380V) motor speed control and will appreciate your recommendation for a circuit diagram for it. from what I've learnt about a single phase motor speed control, there is an torque issue. since I'm looking to use it to control the speed of a 5hp motor, it might be significant point. I'm aware that there are VFD's but I'm looking for a less sophisticated solution.

[ejeffrey]

A speed controller from an AC motor is a VFD, those are the same thing.  So I'm not sure what you are looking for that is less sophisticated.

[Siwastaja]

Of course you can do less sophisticated things but they colossally suck because anything else than VFD is based on the increase of motor slip, and the higher the motor efficiency under normal conditions, the worse it gets under low-speed conditions. So "less sophisticated" stuff is normally done on very small motors which are inefficient anyway, and usually something with mechanical load profile where the power needed reduces in second exponent when speed drops, e.g. fans, so that if your full speed condition is 100W(mechanical) with 60% efficiency, half-speed 25W(mechanical) with 30% efficiency is not a problem.

But once the motor speed falls below breakdown torque, not only efficiency plummets but the maximum torque that can be generated is significantly less than at rated speed, so with many types of mechanical loads, the motor just stalls to stop and starts to overheat. Small motors are designed to offer relatively better torque at low speeds, sacrificing efficiency.

For a 5hp, 3-phase motor you need an actual VFD, so don't waste your time any further thinking about anything else. Indeed AC induction motor control is non-trivial so unless you are going to spend years to learn everything needed to build one from scratch, just get an off the shelf unit, they are not expensive.

[MarkT]

The VFD _is_ the least sophisticated solution (unless you count cooking your motor and starting a fire a solution)...

For better control you have FOC - you can make an induction motor perform like a DC motor with FOC (but again many motors overheat with FOC)

[jpanhalt]

My understanding is the poster wants to control a 3-phase motor.  For that you have the option of VFD or PWM.  Here is a description of the PWM method: https://www.st.com/resource/en/application_note/cd00041736-pwm-management-for-3-phase-bldc-motor-drives-using-the-st7mc-stmicroelectronics.pdf

I don't believe torque problems are as severe with 3-phase as with single phase.  VFD control works quite well with 3-phase too.  Which is more complex?  That's in the eye of the beholder.

[Siwastaja]

The VFD _is_ the least sophisticated solution (unless you count cooking your motor and starting a fire a solution)...

For better control you have FOC

 

Maybe you should do a simple Google search to find out what these terms mean before posting this nonsense. Nearly every VFD on the market implements FOC, or some other (sophisticated) algorithm with similar outcome. e.g. see https://en.wikipedia.org/wiki/Variable-frequency_drive#Control_platforms