Although they are both forms of regulated switching circuits, dynamic braking and choppers each have their own specific advantages and disadvantages. A dynamic brake is a self-contained module containing a transistor, a controller and a resistor and it works by using the controller to monitor DC bus voltage levels. The controller then registers these levels and switches on and off with the switching circuit (or transistor) as and when is needed. Another regulated switching device (generally used for more heavy duty systems) is a 'chopper', and we will go into the differences between dynamic braking and choppers later in more detail.
To explain how dynamic braking works, we're going to be using a 460-volt drive as an example. In a drive of this voltage the DC bus voltage at which it will work most efficiently will be 650 volts, at 800 volts it will overheat and fail. The dynamic brake regulates the voltage so that when it hits 750 volts, the transistor switches on and the current moves into the resistor. The resistor burns off this energy as heat, causing the DC voltage to start dropping, when the voltage gets down to 735 volts, the transistor will switch off. The difference between the turn on and turn off voltage is known as the 'history' and this is why when a dynamic brake is active the voltage will be constantly bouncing between (in this example) 735 and 750 volts.
As we have already mentioned, a dynamic brake is constructed with a controller, resistor and switching device in one unit. A chopper however, only has a regulator circuit and switching device. They are also rated differently, a dynamic brake is rated in 'horsepower' and a chopper is rated in 'amps'. In theory the chopper is generally a more durable and 'heavy duty' choice as it can be mounted in an enclosure with the resistor mounted up to 100 feet away. This means that the large amount of heat generated by the resistor never needs to come into physical contact with the chopper module itself, a dynamic brake is far more likely to overheat.
There are still applications however, in which it would be preferable to use dynamic braking. Loads with a duty cycle greater than 20% are better suited to a chopper (if a dynamic brake exceeds a 20% duty cycle load it WILL fail), but a low regenerative duty cycle can be handled more efficiently with dynamic braking.
No matter what regulating circuit you use, precautions need to be taken. Both devices will need to be connected at the capacitor bank nodes of the DC bus. Some drives will have a DC bus connection at the input bridge rectifier nodes but these connections points should definitely not be used. If the connection is made here then the current in either the chopper or dynamic braking system will pass through the DC link choke that will produce a voltage spike when the power switch is turned off, this will cause an instant system failure.
To explain how dynamic braking works, we're going to be using a 460-volt drive as an example. In a drive of this voltage the DC bus voltage at which it will work most efficiently will be 650 volts, at 800 volts it will overheat and fail. The dynamic brake regulates the voltage so that when it hits 750 volts, the transistor switches on and the current moves into the resistor. The resistor burns off this energy as heat, causing the DC voltage to start dropping, when the voltage gets down to 735 volts, the transistor will switch off. The difference between the turn on and turn off voltage is known as the 'history' and this is why when a dynamic brake is active the voltage will be constantly bouncing between (in this example) 735 and 750 volts.
As we have already mentioned, a dynamic brake is constructed with a controller, resistor and switching device in one unit. A chopper however, only has a regulator circuit and switching device. They are also rated differently, a dynamic brake is rated in 'horsepower' and a chopper is rated in 'amps'. In theory the chopper is generally a more durable and 'heavy duty' choice as it can be mounted in an enclosure with the resistor mounted up to 100 feet away. This means that the large amount of heat generated by the resistor never needs to come into physical contact with the chopper module itself, a dynamic brake is far more likely to overheat.
There are still applications however, in which it would be preferable to use dynamic braking. Loads with a duty cycle greater than 20% are better suited to a chopper (if a dynamic brake exceeds a 20% duty cycle load it WILL fail), but a low regenerative duty cycle can be handled more efficiently with dynamic braking.
No matter what regulating circuit you use, precautions need to be taken. Both devices will need to be connected at the capacitor bank nodes of the DC bus. Some drives will have a DC bus connection at the input bridge rectifier nodes but these connections points should definitely not be used. If the connection is made here then the current in either the chopper or dynamic braking system will pass through the DC link choke that will produce a voltage spike when the power switch is turned off, this will cause an instant system failure.
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