Dynamic braking question 1

[2217116]

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?

 

[waross]

Generically, dynamic breaking is wasting the returned energy through a resistance. Regenerative breaking is introducing the returned energy back into the supply. In some cases DC motors may be regeneratively braked by reducing the supply voltage and/or over exciting the motor.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[2217116]

Thanks Waross for the reply. Let me ask you this...Whereas when with a dc method of dynamic braking, when the db contactor drops out, disabling the drives output from powering the drive and the combination of the now closed db contact in series with the db resistor also flows through the armature allowing the motor cemf to bring the motor to an abrupt stop. Does the db resistor in a vfd cause a similar current to flow through the ac motor that may help to bring the motor to an abrupt stop?

 

[waross]

2217116;
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

 

[itsmoked]

Yes the VFD can control an AC motor to stop abruptly when teamed with a resistor. You can set a VFD to slow the motor as quickly as you desire. But as it slows the DC buss voltage will skyrocket. If your deceleration setting causes the DC buss to exceed its high limit then you will get a fault and the VFD will go to coast mode until either the drive is reset or the DC bus drops below some threshold.

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

 

[cswilson]

We have to distinguish here between two different locations of the dissipating resistor. In one case, the resistor is connects between the plus and minus DC bus of a drive in series with some kind of control switch, usually a power transistor. In my experience, this is usually called a shunt resistor and circuit.

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

 

[ozmosis]

There is a function (my previous company called this 'compound braking') in some VFD's that can basically recirculate the regenerated energy back into the motor in pulses of DC current that is superimposed over the AC waveform going out to the motor. It provides some level of improved braking but not as absolute as dumping it into a resistor. It was sometimes called "ABS Braking" for VFD's internally as it dynamically measures the DC link voltage and tries to stop the motor as quickly as possible without tripping on overvoltage.
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.

 

[DickDV]

In my experience, the term "dynamic" braking is reserved for DC motors. When an AC drive has a voltage sensing circuit usually called a "brake chopper" and a resistor for dissipating energy, this arrangement is called "snubber" braking since it snubs the DC bus voltage at a level just below the High DC Bus Fault level.

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.

 

[LionelHutz]

To brake a DC motor or synchronous motor you connect it to a resistor to brake it via a contactor or possibly some type of electronic switch.

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.

 

[cswilson]

Lionel:

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