3ph 240VAC motor control and switching

[Joy Biswas]

I have searched this forum for an answer but did not find anyone that helped me.
If this question is already answered then please point me to the thread.

Question 1: We want to control a 5HP 3 phase motor (220VAC) electronically (switch off and switch on). Assuming current needed approx. 4000W/340 (rms of 240VAC), can we use BTA41 in 3 lines to simultaneously control?

Question 2: To regulate the speed, pumping PWM (simultaneously) will help?

Hope the question is clear.
Dear moderator, request you to move this thread to appropriate section (if needed).

Thanks,
- JB -

 

[FvM]

1. You can use triacs for electronic on-off switching. They should have suffcient overcurrent capability to stand the motor inrush currents which can easily reach tenfold the rated current. 40 A triacs are probally O.K. The tricas and it's gate control circuit, e.g. opto triacs should be rated for the phase-to-phase voltage, at least 600 Vpk.

2. Induction motors can't be effectively speed controlled other than using variable frequency drives. I'm not sure what you mean with "pumping pwm"? Do you mean multicycle control? It would cause large current peaks and probably overload the motor, triac and supply net.

Phase angle speed control of induction motors is possible in special cases, e.g. for fans that have a strongly speed dependant torque. But you won't use it at a multi kW power level.

 

[zsolt1]

Hi,
I suggest to look after an appropriate inverter . They are cheap now. You can find a lot of inverters using scalar control like V/F, and (a little more expensive) ones that use some of the vectorial control methods like FOC,SWM, DTC . Some are also senzorless, so you don't have to use encoder .

 

[Joy Biswas]

Thanks for the answers. We are clear on (1)
Yes we are using 40A Traics and OC with p-p around 800V (though we think it is an overkill of design), but better be on the safer side.
We have done a lot of phase angle cut to control speeds of fans (by making and selling this product). The way we control of single phase fan motors, but chopping of the % sine wave, can we synchronously do for 3 phase motor?
Obviously yes. Question is what is that synchronous here mean?
Because zero crossing of the phases are apart by 120 deg. Suppose we want 50% on time.
2 ways to check, 1st the zero crossing of 1 phase and all 3 phases cutoff 50% from there
Or 3 independent zero crossing and then cutout of 50% from each of the ZC's.

Probably, we should give some more time to read the technologies available.
I'll get back with questions, once we understand completely the operation and complete the experimentations.
We have to build this whole system quickly :|

 

[FvM]

What's the application, how is the load torque versus speed characteristic?

I'll presume you'll try symmetrical phase angle control, same phase angle (relative to respective zero crossing) applied to all phases. That's what you e.g. do in controlled three-phase rectifiers. There aren't much alternatives, in so far I don't understand which problem about "synchronous" operation you see.

Phase angle control of motors is also restricted by power quality regulations, may be nobody cares in your country?

 

[Joy Biswas]

We have supplied this sort of a wireless remote mechanism to a few customers.
Check the link: https://www.forbixindia.com/electronics/products/industrial-automation-control/remote-motor-control/ this is simple ON/OFF of a single phase 0.5HP motor.
Now there are other people demanding the same with higher motor capacity, and speed modulation.
Speed modulation requirement is something we have to understand from customer.
Let me talk to the people and ask what is the exact application.
I'll get back.

- JB -

 

[Joy Biswas]

I discussed with the customer.
The application is for feeding grains the cattles (running on a conveyer).
I got them convinced that speed controlling may not be necessary at this point of time.
The demand quantity of this system is huge.
Just controlling ON/OFF will do. They seem to be satisfied.
And we will supply only on/off control.
Thanks for all the valuable inputs.
You can close the thread for now

- JB -