# Can you use reverse as regenerative braking in a 2 quadrant motor?

I'm having trouble understanding 4 quadrant versus 2 quadrant drive for a 3 phase permanent magnet brush-less motor.

Let's say you are riding a scooter with such a motor at a certain velocity forward and slam it into reverse. The scooter will slow to a stop and then start accelerating backwards.

What is the difference between doing this and using regenerative braking (besides the obvious fact that you continue backwards after stopping)?

Does using reverse to brake the scooter as described above put energy back into the battery the same as regenerative braking would?

For drive manufacturers that sell drives with optional regenerative braking capability, what is really the difference between a 2 quadrant version of the drive? Can't the 2 quadrant version be used for regenerative braking with reverse?

• I'm not sure I know enough about brushless drives to answer well, but maybe see How can I implement regenerative braking of a DC motor? The explanation still applies, even if both side are switching. Jun 28, 2013 at 2:12
• The basic difference between 2Q and 4Q is that the electronics has to support negative current; current flowing into the battery instead of out of it. Jun 28, 2013 at 6:33

Putting a DC voltage on a brush motor with polarity opposite what the motor would generate itself is called "plugging". It will cause the motor to consume current in excess of its stall current (up to 2x), but it will stop the motor faster than would dynamic braking. Indeed, the torque trying to stop the motor may be twice the motor's starting torque. All of the electricity fed into a motor under such circumstances will be turned into heat (which could cause overheating if one isn't careful); further, the extreme torque may damage whatever the motor is connected to. Nonetheless, if one really needs to stop a system instantly, plugging is a way to do it.

Things are a bit different with AC, however. If one drives a motor with an AC signal which is connected to a battery in "forward" polarity some fraction of the time and in "reverse" polarity the remainder of the time, and if the frequency is high enough the motor current doesn't have time to change much during each cycle (due to the motor's inductance), one may by varying the "forward" duty cycle control the motor speed to be anywhere from forward full to reverse full. Three really nice things about this control approach:

-1- Its behavior is relatively linear; for example, driving the motor 75% forward 25% reverse will make its no-load speed be about 50% of its forward no-load speed.

-2- If one is willing to drive the motor with less than its maximum stall current for a given supply voltage, the supply current will be reduced proportional to the square of the current one does use. For example, if one is willing to settle for half the stall current, supply current while starting will be reduced by 75%. If one only needs a third of maximum stall current, supply current can be reduced by almost 90%.

-3- Provided that one tries to drive the motor at some speed in its direction of motion, it will automatically perform regenerative braking (maximum regeneration power can be achieved by driving the motor at a speed half its current speed; maximum efficiency can be achieved by slowing down the motor as gradually as is tolerable).

The amount of power a motor will waste as resistive heat is proportional to the square of the torque it's generating, which is in turn proportional to the difference between the motor's present speed and its "requested" speed. Although trying to switch motor polarity many thousands of times per second may incur some switching losses, trying to keep the requested speed close to the actual speed can help achieve some very good efficiency.

What is the difference between doing this and using regenerative braking (besides the obvious fact that you continue backwards after stopping)?

Regenerative breaking actually turns the motor into a generator instead of running backwards it puts a variable load (i.e. recharging the battery) on and "re-generates" power.

Does using reverse to brake the scooter as described above put energy back into the battery the same as regenerative braking would?

If you are actively driving current against the back EMF then no. You are actually doing work by putting more power into the motor to increase the force in the opposite direction.

For drive manufacturers that sell drives with optional regenerative braking capability, what is really the difference between a 2 quadrant version of the drive? Can't the 2 quadrant version be used for regenerative braking with reverse?

You need a 4 quadrant motor to do regenerative breaking and be able to run in both directions.

# 4 = Regen CCW

So there are either 1, 2 or 4 quadrant drives.

A drive that is 1Q is #1 only. A drive that is 2Q can be #1 + #3, or it can be #1 + #2. Saying just "2 quadrant" is technically insufficient to differentiate. You have a #1 + #3 from your description. A 4Q drive is of course #1, #2, #3 and #4. Which offers an active free quadrant for pulling power to regenerate and still have the ability to run both directions.

• The moment I switch my scooter to reverse, the wheel rotation will continue in the same direction. The battery voltage across the motor will swap. But given inductors 'don't like current change' there will be a large voltage across the motor that... will push current back into the battery until that voltage collapses and current flow starts flowing in the other direction. So... I don't see how you can instantly go from 1 to 3 (even if it is a 2 quadrant 1 & 3 drive).
– bt2
Jun 28, 2013 at 11:12
• @bt2 you will indeed get regen braking for a short time if you do that, but if you stay there long ("long" being relative to the inductance of the motor, maybe a few ms at most) then the battery and back-EMF have reversed the current, and now the battery is sourcing current again, and you aren't getting regen braking. Jun 28, 2013 at 11:33
• @bt2 also, as I understand it, a quadrant 1&3 drive won't be able to detect the help its getting from the back-EMF in this condition, its feedback mechanism will be confused, and it may not operate as intended. openservo.com/forums/viewtopic.php?t=142 Jun 28, 2013 at 11:37
• @bt2 As phil says you'll get some momentary power. However your breaking comes from driving power into the fields to oppose the current direction of rotation. What you want is to pull power from the fields loading down the magnetic field which slows the rotation because it is generating power. Jul 1, 2013 at 3:22
• @bt2 you should also read the first paragraph in supercat's post. Jul 1, 2013 at 3:25