Braking chopper
Encyclopedia
Brake choppers are used in the DC voltage
intermediate circuits of frequency converters to control voltage
when the load feeds energy back to the intermediate circuit. This arises, for example, when a magnetized motor
is being rotated by an overhauling load and so functions as a generator
feeding power
to the DC voltage intermediate circuit.
braking is a method based on motor
losses. When braking in the drive system is needed, the motor flux and thus also the magnetizing current
component used in the motor are increased. The control of flux can be easily achieved through the direct torque control principle. With DTC the inverter
is directly controlled to achieve the desired torque and flux for the motor. During flux braking the motor is under DTC control which guarantees that braking can be made according to the specified speed ramp.This is very different to the DC injection braking typically used in drives. In the DC injection method DC current is injected to the motor so that control of the motor flux is lost during braking. The flux braking method based on DTC enables the motor to shift quickly from braking to motoring power when requested.
In flux braking the increased current means increased losses inside the motor. The braking power is therefore also increased although the braking power delivered to the frequency converter is not increased. The increased current generates increased losses in motor resistances. The higher the resistance value the higher the braking energy dissipation inside the motor. Typically, in low power motors(below 5 kW) the resistance value of the motor is relatively large in respect to the nominal current of the motor. The higher the power or the voltage of the motor the less the resistance value of the motor in respect to motor current.
In other words, flux braking is most effective in a low power motor.
The other possibility to limit DC bus voltage is to lead the braking energy to a resistor through a braking chopper.
The braking chopper is an electrical switch that connects DC bus voltage to a resistor where the braking energy is
converted to heat. The braking choppers are automatically activated when the actual DC bus voltage exceeds a specified level depending on the nominal voltage of the Variable-frequency drive
Drawbacks of the Braking chopper and resistor
When to apply a braking chopper
When not to use Braking Chopper
Resistor
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
intermediate circuits of frequency converters to control voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
when the load feeds energy back to the intermediate circuit. This arises, for example, when a magnetized motor
Motor
Motor is a device that creates motion. It usually refers to an engine of some kind. It may also specifically refer to:*Electric motor, a machine that converts electricity into a mechanical motion...
is being rotated by an overhauling load and so functions as a generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...
feeding power
to the DC voltage intermediate circuit.
The concept of flux braking
FluxFlux
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...
braking is a method based on motor
Motor
Motor is a device that creates motion. It usually refers to an engine of some kind. It may also specifically refer to:*Electric motor, a machine that converts electricity into a mechanical motion...
losses. When braking in the drive system is needed, the motor flux and thus also the magnetizing current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
component used in the motor are increased. The control of flux can be easily achieved through the direct torque control principle. With DTC the inverter
Inverter (electrical)
An inverter is an electrical device that converts direct current to alternating current ; the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits....
is directly controlled to achieve the desired torque and flux for the motor. During flux braking the motor is under DTC control which guarantees that braking can be made according to the specified speed ramp.This is very different to the DC injection braking typically used in drives. In the DC injection method DC current is injected to the motor so that control of the motor flux is lost during braking. The flux braking method based on DTC enables the motor to shift quickly from braking to motoring power when requested.
In flux braking the increased current means increased losses inside the motor. The braking power is therefore also increased although the braking power delivered to the frequency converter is not increased. The increased current generates increased losses in motor resistances. The higher the resistance value the higher the braking energy dissipation inside the motor. Typically, in low power motors(below 5 kW) the resistance value of the motor is relatively large in respect to the nominal current of the motor. The higher the power or the voltage of the motor the less the resistance value of the motor in respect to motor current.
In other words, flux braking is most effective in a low power motor.
Functioning of braking chopper
]The other possibility to limit DC bus voltage is to lead the braking energy to a resistor through a braking chopper.
The braking chopper is an electrical switch that connects DC bus voltage to a resistor where the braking energy is
converted to heat. The braking choppers are automatically activated when the actual DC bus voltage exceeds a specified level depending on the nominal voltage of the Variable-frequency drive
Benefits of the braking chopper and resistor solutions
- Simple electrical construction and well-known technology.
- Low fundamental investment for chopper and resistorResistorA linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
. - The chopper works even if AC supply is lost. Braking during main power loss may be required. E.g. in elevatorElevatorAn elevator is a type of vertical transport equipment that efficiently moves people or goods between floors of a building, vessel or other structures...
or other safety related applications.
Drawbacks of the Braking chopper and resistor
- The braking energy is wasted if the heated air can not be utilised.
- The braking chopper and resistors require additional space.
- May require extra investments in the cooling and heat recovery system.
- Braking choppers are typically dimensioned for a certain cycle, e.g. 100 % powerElectric powerElectric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
1/10 minutes, long braking times require more accurate dimensioning of the braking chopper. - Increased risk of fire due to hot resistor and possible dust and chemical components in the ambient air space.
- The increased DC bus voltageVoltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
level during braking causes additional voltage stress on motor insulation.
When to apply a braking chopper
- The braking cycle is needed occasionally.
- The amount of braking energy with respect to motoring energy is extremely small.
- Braking operation is needed during main power loss.
When not to use Braking Chopper
Resistor
- The braking is continuous or regularly repeated.
- The total amount of braking energy is high in respect to the motoring energy needed.
- The instantaneous braking power is high, e.g. several hundred kW for several minutes.
- The ambient air includes substantial amounts of dust or other potentially combustible or explosive or metallic components.