# A question about braking and deceleration of an AC motor

Here is an info about regeneration due to braking:

Most of the time, in most applications, a variable frequency drive controls the motor by supplying it with energy which then powers the load. However, occasionally the energy flow will be in reverse, that is, from the load, through the motor, back to the drive. This will occur if the load is giving up energy, such as when a crane or elevator is lowering a load, or maybe when a conveyor is transporting material downhill. Regeneration, as it is called, will also take place if a high inertia load is decelerated; in this case, the energy stored in the rotating mass flows back through the motor to the drive. Fans often regenerate when slowed quickly.

I can understand first examples where a crane or an elevator is lowering a load it acts as a generator. But the info says this regeneration will take place if a high inertia load is decelerated. How can this happen in practice?

If I decelerate a motor does that mean I have to mechanically stall its shaft and it will cause regeneration? What can be an example to deceleration in this context? How does such braking happen so that it causes regeneration?

• – JRE
May 3, 2018 at 11:20
• Think for example of a big circular saw, if you want to reduce the speed at which it is rotating you need to remove some energy from the rotating metal disc. May 3, 2018 at 11:36
• If you mechanically stall its shaft you are taking energy out using friction, thus you will get less electric energy. May 3, 2018 at 11:49
• Basically, the VFD ramps down the frequency, and this causes the motor to decelerate (while also converting mechanical energy into electrical energy). May 4, 2018 at 5:15
• Perhaps a refresher on the DC motor, which is simpler, may help. Have a look at Why does a DC motor spin when there is no load applied to it? and my answer and see if that helps. May 4, 2018 at 5:57

Induction motors 101.

In an induction motor, the supplied voltage creates a rotating magnetic field around the rotor. If you consider a two-pole motor running on a 60 Hz system, the rotating field moves at 60 Hz * 60 sec/min = 3600 RPM.

In this case, 3600 RPM would be called the "synchronous speed." Meaning that if the rotor actually was spinning at 3600 RPM, then the rotor and rotating field are in-sync.

But in normal motoring operation, the rotor spins a bit slower than 3600 RPM. This speed difference, called "slip" gives rise to a current in the rotor which then generates torque. The larger the slip, the more torque the motor will produce, to a point. So, basically, as you apply a load to an induction motor, it slows down a bit, and the torque increases, until the motor and load balance each other out (or maybe if the load is too much, the motor will stall).

The way you put an induction motor into regeneration is to simply increase the mechanical speed of rotation higher than synchronous speed. In our example, this means you need to make the rotor spin faster than 3600 RPM. You could do this, for example, by connecting another motor (maybe a gas motor) to your electric motor, then over-drive the electric motor with the gas motor. Now you have an induction generator rather than an induction motor. You will be supplying power to the electrical grid instead of using the power from the grid. You don't need to do anything else. It just happens.

Now lets consider a motor running from a variable frequency drive (VFD). The VFD can control the frequency of the voltage applied to the motor, which means it can control the motor speed. If the motor is turning a giant heavy turntable, for example, and the VFD needs to slow it down quickly, then the VFD will apply a frequency lower than the actual rotation of the motor. Because the electrical frequency is lower than the actual rotor frequency, the motor will be in regen. This will cause mechanical energy in the turntable to be converted into electrical energy. The VFD may even have an over-voltage failure when this happens, unless it has some way to dump the extra energy (for example into a load resistor). Because of the way they are designed, VFD's usually cannot put energy back into the grid.

There is a lot of other stuff that could be written about this topic, but these are the basics. If the rotor is spinning slower than the electrical frequency, then the motor is operating in the usual fashion, as a motor. But if an external force speeds up the rotor faster than the electrical frequency, then the motor will naturally transition into regen as it converts mechanical energy into electrical energy.

• This is a really great concise explanation! May 4, 2018 at 14:04

Take an electric motor connected to a big fly-wheel. When you are driving it, it has a lot of energy. The moment you stop the power, it still has that energy. The fly-wheel will start driving the motor, in exactly the same way as a crane load drives the motor.

I have seen this used in the past in back-up generators. e.g. Airport, hospitals. There is a big electric motor/generator which is driven by the power net and has a huge fly-wheel. The moment the power net stops, the motor becomes a generator using the fly-wheel energy. This lasts only a short time but just enough to start a diesel generator which then takes over: constant energy without the need to run the diesel generator all the time.

(p.s. The diesel generator was kept warm all the time by using pre-heated cooling oil. )

But how is electrical braking done and how does it cause declaration and regeneration? Does the input power to the motor have to be stopped for these to happen?

Regeneration means the motor starts generating energy instead of using. This by definition means the power to it is stopped.
Compare this to the standard 'generating' which means making energy with a generator by applying mechanical energy.

It also means the energy from the motor has to be used. If you just stop the power, leave the electric connections open there is no re-generation.

I have failed to find a you-tube demo but I have seen one myself. Take a DC motor with fly-wheel.

• Spin it up, stop the power, leaving the motor wires open: the motor slows down in X seconds.
• Spin it up, stop the power, short circuit the motor wires: the motor slows down in less then X seconds.

In the first case the energy is used by the friction of the system.

In the second case there is also friction but additionally it is generating electric energy which, through the short circuit, is converted into heat. Thus it slows down faster

• But how is electrical braking done and how does it cause declaration and regeneration? Does the input power to the motor have to be stopped for these to happen? May 3, 2018 at 12:49
• @panicattack for ac induction motors, there are several ways to implement electrical braking. One of them is to use DC source to stop the motor. With this method two of three inputs of ac input terminals have to be used. May 4, 2018 at 8:02

A variable frequency drive (VFD) works by changing the synchronous speed of the motor. When the frequency is changed, the torque vs. speed curve moves to a different torque vs. speed curve with a different synchronous speed defined by the intersection of the curve with the speed axis. The figure below shows a motor first operating at a stable operating point, 1, on curve A. The operating point is stable because it is the point where the load curve intersects the motor curve.

If the operating frequency is reduced slightly, the motor curve becomes curve B. The speed does not change instantaneously, so the new operating point becomes point 2. Since that point, is below zero on the torque axis, the motor has become negative, braking the load and operating as a generator. The characteristic curve of an induction motor naturally extends into the negative torque region. The motor supply energy to the VFD increasing the DC bus voltage as the energy is stored in the DC bus capacitor bank. If the capacitors are charged to an excessive voltage, the capacitors or something else will fail. To prevent that, VFDs have a means of limiting the speed at which the frequency will decreased when the speed command is reduced. In addition, braking resistors can be connected in parallel with the capacitor bank to dissipate the braking energy. A a backup, the drive will shut down if the voltage gets too high.

At the application of negative torque continues, the speed decreases until the motor is at point 3, a new stable operating point at a lower speed.

If the motor is simply shut off, some energy will be returned to the source, but the motor's magnetic field will generally decay quickly and the motor will coast to a stop.

The overall performance in the scenario described above will be influenced by the friction and inertia of the load and the amount of energy lost in the motor and VFD due to inherent inefficiency.