# AC vs DC regen braking



## kirwoodd (Jul 15, 2008)

In the news forum I saw this post:


jamzky said:


> Based from avtron.com, AC is cheaper than DC motors. DC can easily pump braking energy back to the AC mains while the AC drives more complex. Take a look on here, www.avtron.com/ac_vs_dc.htm


Which sounds like the opposite of what I have read in these forums, so I followed the link, and it says


> DC drives can easily pump braking energy back to the AC mains. In fact the cost for full regeneration is so reasonable that Avtron offers regenerative braking bridges for free up to about 300HP. Above that non-regenerative DC bridges are available and cost less than their AC drive counterparts.


Did I misunderstand? Which does regen better, AC or DC? Or is that too simple a question?


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## etischer (Jun 16, 2008)

An EV does not have AC mains. 

A normal AC powered DC drive uses SCRs which turn on at a certain threshold voltage on the sine wave. Since EV's are powered by DC batteries, EV drives are using a PWM signal to switch the high power. A DC motor typically designed to spin 5000rpm at 200 volts, will only put out 2 volts at 50 rpm, how do you put 2 volts back into a 200 volt battery?

The way I've read controllers regen to zero speed is by shorting out the armature, and opening it really fast. Upon opening the short, the collapsing field would normally create a high voltage arc, but they have the battery connected there to take the voltage spike. I can't imagine this is very efficient.

Putting 2 volts back into an AC sine wave is easy, the sine wave crosses 0 volts every 1/60th of a second, three times if powered by 3 phase.

AC is generally considered better than a brushed DC motor. Much better control over speed and position, and regen ability to zero speed.


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## ElectriCar (Jun 15, 2008)

AC drives, industrial type, rectify the supply AC into DC and put it on the DC Bus with capacitor smoothing. Any regenerated energy will be placed in this bus I believe. AC is easier to regen I suspect because tht voltage is constant. The AC motor derives it's speed by varying the frequency while the DC motor via voltage.


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## kirwoodd (Jul 15, 2008)

Thank you!
So that sounds to me like "Regen works better with AC motors"


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