Mosfet control of AC motor

Thread Starter

dporay

Joined Oct 10, 2015
6
Can someone provide a schematic for turning an AC motor ( 110 v single phase ) on and off using Mosfets. Also, ideas for suppressing spikes caused by the motor switching on and off.
 

MaxHeadRoom

Joined Jul 18, 2013
21,415
What is the nature of the motor, HP etc.
VFD's (Variable Frequency Drives) do not fare well with 1ph motors as they tend to drop out of run when loaded or low RPM.
What is the application?
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
21,415
This probably would take care of the voltage spikes since the SSR switches at the zero crossing point of the AC wave.
I agree with @shortbus also, but in theory when switching an inductive load, the current and voltage waveforms do not co-incide so both cannot be at zero when crossing.
But generally an improvement over mechanical switching.
Max.
 

cmartinez

Joined Jan 17, 2007
7,083
I agree with @shortbus also, but in theory when switching an inductive load, the current and voltage waveforms do not co-incide so both cannot be at zero when crossing.
But generally an improvement over mechanical switching.
Max.
I believe that dporay meant to say that he considers it an advantage to use an SSR with integrated zero volts crossing detection during fire up. That is, it's best to switch on any kind of AC load when the waveform is at zero volts. As to when it's best to switch it off, wouldn't it be best when current is at its lowest? Is there such thing as a zero current detection switch off?
 

Danko

Joined Nov 22, 2017
989
As to when it's best to switch it off, wouldn't it be best when current is at its lowest? Is there such thing as a zero current detection switch off?
Most of SSRs turns OFF close to zero current:
https://www.jelsystem.co.jp/en/product/ssr.html
"When the switch is turned ON, the current flows into the light emitting diode and the optically coupled photocoupler turns ON. The zero-crossing circuit operates and turns ON the triac of the output circuit close to the zero voltage of the AC power voltage. Accordingly, the current flows to the load from the power supply via the triac. When the switch is turned OFF, the solid state relay (SSR) turns OFF close to the zero of the load current due to the operating characteristics of the triac. The current waveform flowing into this load changes by the type of load"
 

cmartinez

Joined Jan 17, 2007
7,083
Most of SSRs turns OFF close to zero current:
https://www.jelsystem.co.jp/en/product/ssr.html
"When the switch is turned ON, the current flows into the light emitting diode and the optically coupled photocoupler turns ON. The zero-crossing circuit operates and turns ON the triac of the output circuit close to the zero voltage of the AC power voltage. Accordingly, the current flows to the load from the power supply via the triac. When the switch is turned OFF, the solid state relay (SSR) turns OFF close to the zero of the load current due to the operating characteristics of the triac. The current waveform flowing into this load changes by the type of load"
Interesting... if I understand correctly, then an internal zero volts crossing detector fires up the SSR, and its TRIAC (or back-to-back SCR's, as I've seen other SSR's use) shuts down automatically at zero current... no need for additional circuitry. :)
 

Danko

Joined Nov 22, 2017
989
Interesting... if I understand correctly, then an internal zero volts crossing detector fires up the SSR, and its TRIAC (or back-to-back SCR's, as I've seen other SSR's use) shuts down automatically at zero current... no need for additional circuitry. :)
That is the operative word 'Close to zero'!
Max.
Yes, because of not zero exactly, small amount of energy, which still stored in inductance, effectively absorbs and dissipates by snubber:
1572040169737.png
 
Last edited:

shortbus

Joined Sep 30, 2009
7,938
Thanks to everyone for all the information. I have learned a lot from this discussion.
The SSR mentioned earlier will work for a small HP like the 1/4HP your working with but for one closer to 1HP and over, I'd probably go with what is called a motor starter contactor. https://www.springercontrols.com/news/magnetic-motor-starters-basics/ Using one of these will allow using a low voltage, by choosing the right coil voltage, to start a higher hp and voltage than the SSR.
 
Top