# How to calculate the braking resistor & wattage value for a 3.7 KW 3 phase inverter ?

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#### danishdeshmuk

How to calculate the braking resistor & wattage value for a 3.7 KW 3 phase inverter ?

How to calculate the braking resistor & wattage value for a 3.7 KW 3 phase inverter AC Drive ?

thanxx

#### kak111

Re: How to calculate the braking resistor & wattage value for a 3.7 KW 3 phase invert

There is not any simple formulas ...
The change in kinetic energy of the load during deceleration is dissipated in the brake resistor.
If you decelerate the load from full speed to zero speed, the energy dissipated in the resistor is equal
to the kinetic energy of the driven load and motor minus the load and rotational losses.
High inertia give high kinetic energy.
The deceleration time determines the rate of energy flow (KW).

The lower the ohmic value, the higher the power, shorter the brake time.
The minimum ohms are set by the manufacturer, and will produce braking power at the peak rating of the drive (or its chopper module)
Higher ohmic values can be used; they will reduce the braking power proportionately, and hence increase the stop times of any given load.
The ohmic value is less than the value recommended by the drive manufacturer – result-in VFD failure (or its Chopper module). The correct ohmic value of the brake resistor should be equal or slightly higher by 10% of the value set by the drive manufacturer.

Start with the drive and select the brake chopper that is rated to the drive.
This "chopper" is the voltage sensing system which detects high DC bus voltage and diverts some of the bus energy to the braking resistor.
This chopper will have a maximum current capacity which may be specified directly or indirectly by specifying a minimum acceptable resistance value.

Even if you don't need all of the drive hp as braking, it is suggested that you buy a resistor with that minimum resistance value.

Finally, you need to determine what wattage the resistor must be. A resistor will have a continuous wattage rating but,
for a few seconds on a cold resistor, you can push it to 10x the continuous value.
So, duty cycle is an important element in the wattage selection.
Certainly, the maximum possible wattage would be the drive hp converted to watts.
This would only apply to continuous braking as in an unwind tensioner drive.
If the required braking hp is less than the drive rating, the resistor wattage comes down accordingly.
Also, as the duty cycle becomes smaller, the wattage comes down further.

you find some tables here....
6.1 BRAKING UNIT AND BRAKING RESISTOR UNIT APPLICATION LIST

http://www.grelectronics.co.in/braking-resistors.pdf

GR Electronics

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Points: 2

### munzir

Points: 2

#### munzir

##### Full Member level 6
Re: How to calculate the braking resistor & wattage value for a 3.7 KW 3 phase invert

why don't we need braking resistor & hence brake in every motor controller ?

#### kak111

Re: How to calculate the braking resistor & wattage value for a 3.7 KW 3 phase invert

DC:
Unlike most AC motors, when removed from its power supply a DC motor can act like an electrical generator due to its permanent magnet.
The idea here is to disconnect the motor from its power supply and connect it to the braking resistors instead. The DC motor will then use its
rotational inertia to produce electricity, which does work by heating the braking resistors. This uses up the energy stored in the rotational
inertia of the motor, bringing it to a stop.

A logical extension of dynamic braking is the concept of regenerative braking, which uses the electricity to charge a battery instead of wasting
it as heat energy. The battery power can then be used for other things, such as restarting the motor.

AC:
Most AC motors do not act as generators when disconnected from the power supply. To electrically brake an AC motor requires either a DC injection
brake or a variable frequency drive to provide dynamic braking. DC injection braking, applying DC voltage to the stator windings, is the simpler of the
two options but is harder on the motor. Braking resistors are not involved in DC injection braking.

Dynamic braking of an AC motor is achieved by providing a slower frequency of electric current to the motor than that which would be necessary
to maintain its current speed. For example, the synchronous speed of a 2 pole motor fed by a 60 Hertz power supply would be 3600 RPM.
While the motor is at this speed, feeding it with a power supply operating at less than 60 Hertz creates a magnetic field in the stator which rotates
slower than the rotor is rotating, and the drag produced will begin to slow the rotor down.

Although the AC motor does not have a permanent magnet in the rotor, it does have an induced magnetic field in the rotor, created by the rotating
magnetic field in the stator. The energy lost in the difference between the stator and rotor speeds backfeeds into the VFD, which raises the voltage
on the DC bus in the VFD. The greater the difference between the output of the VFD and the rotor's actual speed, the more energy will be backfed
into the VFD. This means that if the VFD tries to dynamically brake the motor too quickly, the voltage on the DC bus will raise too high and damage
the VFD. Most VFDs will shut down as a safety feature before this happens, and the motor will coast to a stop by friction alone.

This is where the braking resistors come in. The braking resistors act as an additional load on the DC bus, which helps to drain the excess voltage
and keep it within safe tolerances. With appropriately sized braking resistors, the motor can be brought to a stop much more quickly without raising
the voltage on the DC bus to unsafe levels.

In both the AC and DC cases, the smaller the resistance of the braking resistors, the larger the load it creates and the faster the motor can be stopped.
However, less resistance means more current can pass through the resistor, and more current means more heat is produced. The extra heat must be
dissipated by heat sinks, or reduced by using multiple resistors in parallel which share the load. Either option drives up the cost of the braking resistor
system, so it is important to size them correctly for a given application.

text: Everything2.com by rootbeer277 Mon Feb 02 2004

Regenerative variable-frequency drives

Regenerative AC drives have the capacity to recover the braking energy of a load moving faster than the designated motor speed (an overhauling load)
and return it to the power system.
Line regenerative variable frequency drives, showing capacitors (top cylinders) and inductors attached, which filter the regenerated power.

Cycloconverters and current-source inverters inherently allow return of energy from the load to the line, while voltage-source inverters require an
additional converter to return energy to the supply.

Regeneration is only useful in variable-frequency drives where the value of the recovered energy is large compared to the extra cost of a regenerative
system, and if the system requires frequent braking and starting. An example would be conveyor belt drives for manufacturing, which stop every few
minutes.
While stopped, parts are assembled correctly; once that is done, the belt moves on. Another example is a crane, where the hoist motor stops and
reverses frequently, and braking is required to slow the load during lowering.
Regenerative variable-frequency drives are widely used where speed control of overhauling loads is required.

Text: en.wikipedia.org/wiki/Variable-frequency_drive

The motor slows the load down, the VFD controls the motor to make this happen, and the brake resistor is a place that can use the electricity
generated by the slowing operation.
If the slowing is gradual enough, then the generated electricity is less than the losses in the VFD and motor; so the resistor never sees any of the
energy or regenerative circuit can handle generated energy so that DC-bus voltage dont rise too high , in these cases brake-resistor is not needed.

Some VDF`s has an Internal Dynamic Brake Resistor and can handle braking torque for short periods.
Usually in smaller VDF´s ( < 20kW).

Points: 2

### danishdeshmuk

Points: 2

#### Mr.Cool

Re: How to calculate the braking resistor & wattage value for a 3.7 KW 3 phase invert

it is possible to have control function that decelerates your AC motor at a rate determined by the capability of the breaking resistor. monitor the DC link voltage to determine when to turn ON the breaking resistor (through an IGBT) and with a duty cycle. as the duty cycle increases due to aggressive deceleration, it will reach a max around 90% (last 10% is safety margin) and so your controller decides to lessen the deceleration rate until the breaking resistor duty cycle back to normal operating range.

danishdeshmuk

### danishdeshmuk

Points: 2
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