Is Back EMF Safe for Victors?

[Tom Bottiglieri]

When a motor is backdriven, it becomes a generator and creates a voltage across its input terminals. This is called Back EMF, and the voltage produced is proportional to the angular velocity of the output shaft.

Does anyone know if creating this Back EMF will damage a powered Victor speed controller? And if so, what is the max amount of current that can be "backdriven" into a Victor without damaging the electical components?

In laymens terms... Can I set a Victor to coast mode, hook up a CIM motor to it, turn the bot on, backdrive it at 5000 RPM and live to tell about it?

[Rick TYler]

Cool question, Tom. My first thought is that you should bring along a bucket to collect the shredded drivetrain components spread around the field...

[Justin_302]

i really do not think it will effect anything. the electricity created is not high voltage and is defently not high amperage.

[patTeam241]

Typically it seems that this is pretty safe, you can role a robot around, without disengaging the wheels, and not blow victors out. The board will light up, so there is definately some back feed. However, that said, I did have a victor die on me for an unexplained reason one time, and I'd always wondered if it did take too much back EMF, although it may have been something else. You're almost certainly safe though.

[KenWittlief]

logically, back emf is there when the motor is running. It is what limits the current at the motors top speed (otherwise the motor would always draw its stall current).

So.... if you power the motor up to full speed, it cannot generate more back emf than its already seeing when its running

therefore, unless you tow the robot behind your car, so that its wheels are spinning faster than it would when self powered (way faster), there should not be a problem.

[pathew100]

It is kinda freaky when pushing the robot around and first the Victors power up, then the RC, etc...

[Al Skierkiewicz]

Interesting discussion so far.
The Victors have internal surge protection (MOVs) so that spikes generated in the motors will not come back and eat the FETs in the output bridge. With the Victors powered and in the coast mode with no output (PWM 127), there is effectively no connection to the Victor and as such the open circuit voltage at the motor can have spikes of up to 100 volts. (These are very short duration and are sqaushed by the protection internal to the Victor.) Unlike the backdriving when the robot is turned off, no current flows through the Victor in Coast (all FETs are off). When no power is applied to the robot, the "bridge" circuitry in the Victor can become biased on when pushing the robot around on the floor and this is the LED illumination you see during this condition. If you wire your Victor fans directly to the power input of the Victor you may also get the fans moving during this time.
Remember that when the Victor is sending a PWM signal to the motor, it is like turning a switch on and off. During the "off" period there is considerable noise and voltage generated by the motor.

[Gdeaver]

With power off, the intrinsic diodes in the FET take the current. They do have a voltage drop and will generate heat. Short term they sould be fine but if you pushed the robot fast for a long time?

[sciguy125]

Originally Posted by Gdeaver

With power off, the intrinsic diodes in the FET take the current. They do have a voltage drop and will generate heat. Short term they sould be fine but if you pushed the robot fast for a long time?

We pushed our robot across a parking lot once. It got to the point that it smelled like burning electronics. On powerup, however, it still worked fine. Not that I advise doing it...

On a side note, the back EMF turns the fans on, but I doubt they're going fast enough to do very much.

[KenWittlief]

so the general consenses is, if you have to drag your bot across a parking lot or another long distance, you are better off pushing it on its side and dragging it 'by its legs'

that will do less damage :^)

do they still put castors it the kit of parts? its very easy to make a bot-dolly with 4 castors and a hunk of plywood.

[Al Skierkiewicz]

So that we don't confuse the casual visitor, the voltage generated when the robot is off and is being pushed is generated voltage. Back EMF (or counter EMF) is the condition where a motor connected to a power source creates an induced voltage that opposes the supply voltage. This induced voltage is opposite in polarity to the supply voltage and tends to reduce current flow in the windings of the motor. Back EMF increase with motor speed.

[Richard Wallace]

Originally Posted by Tom Bottiglieri

When a motor is backdriven, it becomes a generator and creates a voltage across its input terminals. This is called Back EMF, and the voltage produced is proportional to the angular velocity of the output shaft.

Does anyone know if creating this Back EMF will damage a powered Victor speed controller? And if so, what is the max amount of current that can be "backdriven" into a Victor without damaging the electical components?

In laymens terms... Can I set a Victor to coast mode, hook up a CIM motor to it, turn the bot on, backdrive it at 5000 RPM and live to tell about it?

The back-EMF that a CIM develops at 5000 RPM is just under 12 Volts. That voltage will not damage a Victor.

Backdriving at much faster speeds could create a problem, but you'd need to analyze the particular loading conditions. In general, backdriving above the 12 Volt free-speed of any motor will create the potential for it to regenerate back into the 12 Volt supply (battery), and extreme regeneration can overload circuit elements just as extreme motor loading can.

Someone at IFI may be able to comment on regenerative capability of Victors.