Dynamic braking question
Dynamic braking question
(OP)
Is dynamic braking really "dynamic braking" with a vfd? I know that with a dc motor, the method of dynamic braking using a db contactor and a db resistor across the armature gives true dymic braking. However, with a vfd for instance when a three phase motor regenerates and causes the dc bus to rise let's say. When using a "braking" resistor shunted across the dc bus controlled by a chopper for instance. Does this do the same thing as dynamic braking on a dc motor when it comes to actually stopping the motor? Or does it just prevent the drive from tripping on dc bus overvoltage?





RE: Dynamic braking question
If a VFD is using a resistor on the DC bus, it is dynamic breaking. If the VFD has the circuitry to invert the DC bus energy and feed it back into the grid that is regenerative breaking.
Bill
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"Why not the best?"
Jimmy Carter
RE: Dynamic braking question
RE: Dynamic braking question
Hang in there until some of the fellows get back to work tomorrow. We have some pretty good drive people here who can better explain the details. But, neither the AC nor the DC resistor cause a current to flow.
Both DC motors and recently disconnected AC motors generate a back EMF. The resistor supplies a path for the current that results from the back EMF.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Dynamic braking question
Once your decel value reaches the DC bus max, you either need to decrease the accel rate setting or add a braking resistor to allow maintenance of the DC bus below the fault tripping point. The decel rate needs to still not exceed the resistor's ability nor the VFD's ability to PWM the regen'd power off safely.
The VFD keeps the AC motor properly magnetized so it will generate, unlike just a resistor dropped across a DOL AC motor.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Dynamic braking question
When the DC bus voltage exceeds a threshold, the switch is closed, allowing bus power to be dissipated through the resistor. Yes, this protects the power circuitry from being damaged due to overvoltage conditions, but having this protection permits aggressive deceleration that would otherwise destroy the power circuitry.
Operating the drive and motor to engage this shunt circuitry is often part of normal operation of the motor/drive system. It is used in control of DC motors, synchronous AC motors (including "brushless DC" motors), and AC induction motors (especially under vector control), functioning in fundamentally the same way for all types of motors.
The other location for dissipating resistors is between the leads of the motor armature. (For a 3-phase motor, you will have 3 resistors connected in Y or Delta.) In this configuration, a relay for each motor lead can switch the lead from being connected to a drive output to an end of the resistor.
This is really an "emergency stop" circuit. In a stop condition, the motor is automatically disconnected from the drive and connected to the load resistors, which help decelerate the motor more quickly due to their loading of the motor acting as a generator. This is quite commonly used for DC motors and synchronous AC motors.
It is a bit more problematic for AC induction motors, because it works only when the field is present. In an AC induction motor, once the motor is disconnected from the drive, the rotor field will decay quite quickly, and with it, the braking effect. For this reason, I am unfamiliar with its use on AC induction motors, but perhaps others here have different experience.
Curt Wilson
Delta Tau Data Systems
RE: Dynamic braking question
A note on braking using dump resistors. It is strongly advised that a thermal cut-out measuring the heat generated on the resistor is interlocked with a coast stop in the VFD or other method of immediately shutting the VFD down in case the resistor gets too hot and potentially short-circuits your DC link. There are a lot of installations I've seen that do away with this added protection and it can leave you vulnerable to severe damage on the VFD if something were to go wrong with the dump resistor, short the brake transistor and then things tend to go pop quite loudly.
RE: Dynamic braking question
The end result is the same either way, the motor shaft resists rotation and the spinning load is more rapidly brought down in speed to a stop or near-stop.
RE: Dynamic braking question
To brake an AC induction motor you connect it to a resistor via a 3-phase inverter section.
In the end both take the energy from the motor and dump it into a resistor allowing you to stop the motor quicker. So, I will continue to call them both dynamic braking.
RE: Dynamic braking question
When DC and synchronous motors are run from inverter stages (as in servo control), braking in normal operation is done with a shunt resistor in the inverter -- any time the capacitor bank cannot absorb the generated energy, and regeneration to line is not worth it.
In these applications, resistors connected to the motor leads through contactors are reserved for those (relatively rare) applications where (1) a controlled stop is not trusted for some cases; (2) a coasting emergency stop is not permitted; and (3) a mechanical brake is not needed for these emergency cases.
Curt Wilson
Delta Tau Data Systems