In certain applications, for instance centrifuges, it is desirable to bring the motor to a stop more rapidly than can be achieved through controlling via

ramp down or by free-wheeling. In such applications, dynamic braking with a braking resistor may be utilized. Using a braking resistor ensures that the

energy is absorbed in the resistor and not in the frequency converter.

If the amount of kinetic energy transferred to the resistor in each braking period is not known, the average power can be calculated on the basis of the

cycle time and braking time also called intermitted duty cycle. The resistor intermittent duty cycle is an indication of the duty cycle at which the resistor

is active. The below figure shows a typical braking cycle.

The intermittent duty cycle for the resistor is calculated as follows:

Duty Cycle = t

T = cycle time in seconds

is the braking time in seconds (as part of the total cycle time)

Danfoss offers brake resistors with duty cycle of 5%, 10% and 40% suitable for use with the FC202 AQUA drive series. If a 10% duty cycle resistor is

applied, this is able of absorbing braking power upto 10% of the cycle time with the remaining 90% being used to dissipate heat from the resistor.

For further selection advice, please contact Danfoss.

NB!

If a short circuit in the brake transistor occurs, power dissipation in the brake resistor is only prevented by using a mains switch or

contactor to disconnect the mains for the frequency converter. (The contactor can be controlled by the frequency converter).

2.14.2. Control with Brake Function

The brake is to limit the voltage in the intermediate circuit when the motor acts as a generator. This occurs, for example, when the load drives the motor

and the power accumulates on the DC link. The brake is built up as a chopper circuit with the connection of an external brake resistor.

Placing the brake resistor externally offers the following advantages:

The brake resistor can be selected on the basis of the application in question.

The brake energy can be dissipated outside the control panel, i.e. where the energy can be utilized.

The electronics of the frequency converter will not be overheated if the brake resistor is overloaded.

MG.20.N4.02 - VLT

is a registered Danfoss trademark

2. Introduction to VLT AQUA Drive

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