# Can an electric motor create torque in a direction opposite to the current axle's spin?

Let's imagine a wheel spinning clockwise. There's an electric motor on the axle, and we want to spin the wheel counterclockwise.

Can we just switch on the motor and it will work? I want it to slow down, stop, then start spinning counterclockwise. In other words, create torque that's initially in the opposite direction of the axle's current spin. Will this "retrograde" action damage the motor? Or maybe be much less efficient than "prograde" motor operation?

I have no idea if it will be a DC or AC electric motor because I don't really understand the difference between them that well. Does the answer depend on which one? Does it maybe depend on the RPMs involved too?

EDIT: Apparently a motor can be easily used as a generator. I know that will cause countertorque, but this is not exactly what I want. What I'm imagining is an electric power source powering a motor to brake an axle. So the power source will be drained, not charged.

• I know that the Boosted Board electric skateboards allow you to apply retrograde torque. Also, apparently generating retrograde torque unavoidably runs the motor as a generator, generating electric power. Apparently if the Boosted Board's battery is fully charged, it doesn't let you apply retrograde torque, because it wouldn't have anywhere to dissipate the generated power. – AlcubierreDrive May 24 '16 at 5:25
• um, DrZ, where does that power go? power comes outa the source, you say. and since the torque is opposite of the spin, then I say that power is coming out of the "motor". in the "generator" or "regenerative braking" case, that power flows back into the battery and charges it. if it's some kinda power grid, the power goes back into the grid and relieves the power requirement from the generators feeding the grid. where does the energy that is removed from the rotating axle go?? – robert bristow-johnson May 24 '16 at 6:02
• Please explain what you want to happen. Don't mention motors and powering them - stick to the high level requirements. – Andy aka May 24 '16 at 8:12
• You can't draw power from both the mechanical system and the electrical system. The energy from one will be transfered to the other. But if you just want to provide break a spinning motor/generator, a simple resistor break would do the job and the energy would be transfered from mehanical to heat in the resistor. – winny May 24 '16 at 8:15
• Anecdote: If you spin a ceiling fan (or a table fan) the wrong direction and then turn it on, it will slow down and start turning the other way. It's a low torque motor, so it takes time, but it does slow and reverse. – JPhi1618 May 24 '16 at 15:08

If this is a brushed DC permanent magnet (PM) motor, then yes it will work, but could work too well. If you switch a high capacity battery in reverse across it, it will draw a huge current which would have the capability to damage the motor, by demagnetisation, the vehicle, and the battery.

If you are using a motor controller, that has a programmable current limit, then it depends on the software of the controller. As the motor will be generating power, and delivering to the battery, the power flow is not what the motor controller was designed for. Some will continue to work as an old skool current source, others will detect a fault condition and shut down in some way.

If you are using a 3 phase BLDC motor, then it all depends on the software in your ESC. Unless the ESC has been designed to handle this power flow, it is almost certain that it will shut down at the error condition.

When you brake a load, it generates power. This is physics 101. That power has to go somewhere. If you don't store it in a battery, then you will have to allow something to get hot. It actually simplifies the control if you can force the motor controller to idle, and then switch a resistive load across the motor terminals to absorb the power. Note that if you put a short circuit across the motor, the braking torque will be high and uncontrolled, and the motor windings will get hot to absorb the energy.

There are two main options for what you do with the energy. One is to store it, and then allow it to dissipate it slowly once the braking event is finished. The second is to dissipate it fast as it's generated.

In the first option, you might bolt power resistors to a large metal heatsink, say the alli chassis of your vehicle. In the second, ordinary filament bulbs, especially auto bulbs that are cheap and readily available, can dissipate significant power, mainly because they are designed to get very hot. The temperature coefficient of resistance of a filament works very well for braking, as their resistance increases an order of magnitude as they get hot, which means they tend (tend ==> more so than a resistor, less than a controlled current sink) to a constant current, that is constant torque load, over a wide range of voltage (speed). They could double as a brake light as well ;-)

• Note that if you put a short circuit across the motor, the braking torque will be high and uncontrolled, and the motor windings will get hot to absorb the energy. What does "short circuit across the motor" mean exactly? Do you mean there's no load? But the countertorque will be the load itself, won't it? (otherwise how would it be getting hot?) – DrZ214 May 25 '16 at 2:50
• @DrZ214 If you simply close a switch across the motor terminals, that is short circuit the motor terminals, the voltage generated by the motor will cause a very large braking current to flow. As the only significiant resistance in series with this current is the motor itself, it is what gets hot. If you control the current by connecting a resistor to the motor terminals, the braking current and hence torque will be lower, controllable by selecting the resistor value, and dump most of the kinetic energy of the vehicle in heating the resistor, not heating the motor. – Neil_UK May 25 '16 at 5:45
• @Neil_UK: The "short-circuit" braking current at a given speed will be roughly equal to what the stall current would be if the motor were driven with the open-circuit voltage for the aforementioned speed. The amount of energy resistively dissipated in a motor that is shorted out to stop it would generally be less than the energy dissipated while it gets up to speed if full run voltage is applied without current limiting, so if the latter isn't a problem, the former likely wouldn't be either. – supercat Mar 17 '20 at 20:02

sure. but then we call the motor a "generator". i know with DC motors, that if you increase the current of the field, so that at the present speed of the motor the "back e.m.f." (i don't remember the current term for that, it's what we used in my college days) of the DC motor exceeds the applied voltage, then power will flow out of the motor (it's acting as a generator) and the torque will oppose the spin rather than be in the same direction.

• Uh oh this isn't exactly what I wanted. Using a motor in generator mode requires somewhere to dump that power. The thing I'm imagining does not have anywhere to dump that power. Instead, I want an electric power source that brakes a spinning axle. I'll make an edit to my OP. – DrZ214 May 24 '16 at 5:40
• Trains, Electric cars, electric bikes, etc all do this but call it restrictive or regenerative breaking. The harder a load they put on the running motor the harder it breaks. This is because the induced EMF from the spinning motor drives a current through the load. This current causes a force on the motor slowing it. The more current, the more is slows. Once the motor is stopped you can turn it back the other way with your power supply. – Andrew Cragg May 24 '16 at 5:56
• @DrZ214 if you arrange a switch or relay to disconnect the power and THEN simply short circuit the terminals of the DC motor, the reverse current and reverse torque should not exceed the stall current and stall torque of the motor. This will give you some hefty braking with the energy absorbed by the motor itself. Once it stops, reverse it. If it is a small motor, it should be able to take that kind of treatment. Don't brake a large motor in this way (or stall it at full supply voltage) though! (If you try this ensure your changeover relay or switch is open before close.) – Level River St May 24 '16 at 13:32
• @LevelRiverSt Interesting, but why do you say don't try it with a large motor? Shouldn't a larger one be more stalwart and capable of sustaining more power and/or abuse? – DrZ214 May 24 '16 at 21:16
• might be that the explosion that melts down the entire laboratory is bigger with a large motor than a smaller one. – robert bristow-johnson May 24 '16 at 21:18

Induction motors try to run synchronously with the frequency of the AC supply so if they are mechanically driven to go faster they will create a counter torque.

An induction motor can also create counter-torque if you supply a DC current to the coils. This will create eddy currents in the squirrel cage of the rotor. However this counter torque is proportional to the rotation speed. So this will never reverse direction.

A multi phase induction motor can create zero-speed starting torque and reverse directions by swapping the leads on one of the coils.