# SCR (forklift) and PWM controller differences



## Stunt Driver (May 14, 2009)

Come on, Guru of EV drive, share a bit! Is the key differences in type of transistors used? Or there is a different idea?


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## TheSGC (Nov 15, 2007)

A PWM controller uses MOSFETs or IGBTs, usually around 95-98% efficient. From what I can gather, SCRs are a thyristor based system that is around 50% efficient, at least according to a few EV sites I have found, like this: http://www.seattleeva.org/wiki/Talk:Motor_controller

I could go into specific technical things about the why PWM is more efficient, but it would take a while. So long story short, PWM is more efficient than SCR due to the technology of the components. Do some google searches on the theory of PWM and SCR and you will see why SCR has been discontinued.


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## Tesseract (Sep 27, 2008)

TheSGC said:


> A PWM controller uses MOSFETs or IGBTs, usually around 95-98% efficient. From what I can gather, SCRs are a thyristor based system that is around 50% efficient, at least according to a few EV sites I have found, like this: http://www.seattleeva.org/wiki/Talk:Motor_controller


SCRs are less efficient than IGBTs or MOSFETs - mainly because of the higher on drop and much slow turn-on time - but they do have the highest I^2t rating of any semiconductor (i.e. - they can carry more amps for a given size semiconductor die).

That said, they are SLOW (think switching frequencies in the 300Hz range) and once they turn on they can't be turned off unless the current drops to zero or via an external commutation circuit (usually another SCR in series with a capacitor shunting the main SCR). Successful commutation depends heavily on the nature of the load, which means an SCR controller is much less flexible about the type of motor it drives.

The low switching frequency also requires HUGE input capacitors. For example, a 1000A SCR controller running at 300Hz would need somewhere between 21,000 and 42,000uF to reduce ripple to the recommended range of 20% to 10% peak to peak.

On the plus side, SCRs are downright cheap for a given amp/voltage rating. IGBTs are nipping hard at their heels, though.


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## JustinUther (Oct 18, 2009)

SCR's have their place. I wouldn't use them as switching devices unless your power supply is a 3 phase AC generator. 

I could explain the how PWM and SCR controllers work but if since our power supply is DC there really isn't any question.


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