3–18

“A” Group: Standard Functions

DC Braking Settings

The DC braking feature can provide additional stopping torque when compared to a normal deceleration to a stop. It can also ensure the motor and load are stopped before acceleration.

When decelerating – DC braking is particularly useful at low speeds when normal deceleration torque is minimal. During deceleration, the inverter injects a DC voltage into the motor windings during deceleration below a frequency you can specify (A052). The braking power (A054) and duration (A055) can both be set. You can optionally specify a wait time before DC braking (A053), during which the motor will free run (coast).

Output voltage

+

Running

 

Free run

 

DC braking

 

 

 

 

 

 

 

 

 

 

 

A054

0

t

–

A053

 

A055

 

 

 

Configuring Drive

Parameters

 

 

When starting – You can also apply

 

Output

 

 

 

DC braking upon the application of a

 

voltage

 

 

 

Run command, specifying both the DC

+

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

braking force level (A057) and the

 

DC braking

 

Running

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

duration (A058). This will serve to stop

 

 

 

A057

 

 

 

 

 

 

 

 

the rotation of the motor and the load,

0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

when the load is capable of driving the

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

t

 

motor. This effect, sometimes called

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

“windmilling,” is common in fan appli-

 

 

 

 

 

 

 

 

 

 

–

 

 

A058

 

 

 

 

 

cations. Often, air moving in duct work

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

will drive the fan in a backward direc-

 

 

 

 

 

 

 

 

 

 

tion. If an inverter is started into such a backward-rotating load, over-current trips can occur. Use DC braking as an “anti-windmilling” technique to stop the motor and load, and allow a normal acceleration from a stop. See also the “Acceleration Pause Function” on page 3–21.

You can configure the inverter to apply DC braking at stopping only, at starting only, or both. DC braking power (0–100%) can be set separately for stopping and starting cases.

You can configure DC braking to initiate in one of two ways:

1.Internal DC braking – Set A051=01 to enable internal braking. The inverter automatically applies DC braking as configured (during stopping, starting, or both).

2.External DC braking – Configure an input terminal with option code 7 [DB] (see “Exter- nal Signal for DC Injection Braking” on page 4–17 for more details). Leave A051=00, although this setting is ignored when a [DB] input is configured. The DC braking force settings (A054 and A057) still apply. However, the braking time settings (A055 and A058) do not apply (see level and edge triggered descriptions below). Use A056 to select level or edge detection for the external input.

a.Level triggered – When the [DB] input signal is ON, the inverter immediately applies DC injection braking, whether the inverter is in Run Mode or Stop Mode. You control DC braking time by the duration of the [DB] pulse.

b.Edge triggered – When the [DB] input transitions OFF-to-ON and the inverter is in Run Mode, it will apply DC braking only until the motor stops... then DC braking is OFF. During Stop Mode, the inverter ignores OFF-to-ON transitions. Therefore, do not use edge triggered operation when you need DC braking before acceleration.

CAUTION: Be careful to avoid specifying a braking time that is long enough to cause motor overheating. If you use DC braking, we recommend using a motor with a built-in thermistor and wiring it to the inverter’s thermistor input (see “Thermistor Thermal Protection” on page 4– 28). Also refer to the motor manufacturer’s specifications for duty-cycle recommendations during DC braking.