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Scott35 Online Happy OP
Broom Pusher and
Discussion thread for: Dynamic braking of AC induction motor

Feel free to make comments here!!!

Scott SET

Scott " 35 " Thompson
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JBD Offline
Just a couple of thoughts.

You should always include fusing on the DC side of the rectifier.

Also be aware of the requirement for DC rated contacts on the TDO relay. UL listed devices rated for breaking DC voltages in excess of 48V are not very common.

The TDO delay must be set long enough to bring the motor to a stop, but not long enough to overheat the motor windings. The addition of a zero-speed switch can be helpful when braking time is variable.

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Scott35 Online Happy OP
Broom Pusher and
JBD has made very good points here.

Let me add a few more:

The object with this type of braking system is to inject DC power into the windings of an AC induction motor for a temporary time.
Before the Rotor has stopped rotating, the DC power should be removed. Leaving the DC power connected to the windings while the rotor is stalled [not turning] will fry the windings.
DC power should be opened at somewhere like 20% of rotor speed [long before the rotor stops].

The DC braking current can be from 2x FLA of the motor [for gradual stopping], upto 6x FLA of the motor [for rapid stopping]. So for a motor with a FLA of 10 amps, a DC value of 20 amps will slow the rotor down gradually - a DC value of 60 amps will slow it down real suddenly.

The reason for the Transformer in front of the Rectifier is to get some high DC amperes to inject into the almost zero resistance of the motor's windings, but at a low enough voltage as to not produce extreme heat in the windings, nor to require a large input from the Line power [AC power]. This being no less than twice the motor's rated full load amperes [FLA].
The braking is done with high levels of DC current, so keeping the voltage down low also lowers the input KVA required for braking.

Once again, this schematic was drawn to be a very basic and simple example, so many key items have been eliminated for ease of understanding the basic concepts and designs. Next series of schematics will have more details.

FYI: If you are into experimenting, you can see how DC effects an AC Induction motor at home!!!
Fun at parties!!! [Linked Image]

A bench grinder works the best, since there's a nice Dynamic load on the shaft to keep it spinning [the inertia of the grinding wheels].
Turn the grinder on, let it run to full speed, then unplug it.
While the rotor is slowing down [and the switch is still closed], quickly connect a low voltage DC power supply to the plug [something like a 10 watt DC power supply for charging a child's toy, or whatever...].
When the DC flows in the windings, it will slow the rotor down.
Use a low power DC supply, so there's no risk of frying the motor's windings.

Try it!!!

Extra Credit Q!!! [Linked Image]

Just what effects will DC biasing have on AC motors while they are in operation??? [how will an AC Squirrel Cage Induction motor react with DC imposed on the AC supply].

Will it effect all Induction motors equally???

Scott SET

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Aug 2001
Posts: 31

The vfd world defines the two types of motor braking as follows.

The type you speak of in this thread is called DC injection braking.

The other type called Dynamic braking consists of a switch (HS dc contactor or transistor) and a resistor. When the motor changes quadrants from motoring to generating, the switch closes and allows the resistor to become the generator load. The purpose is to prevent the dc generation from exceeding the DC bus voltage.


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