Toshiba W7 - Manual (Page 18)

12 W7 ASD Installation and Operation Manual
resistor. The braking resistor must be suitably matched to the load. Dynamic braking is also effective in
reducing the DC bus voltage during a momentary over-voltage condition.
If under extreme conditions the dynamic braking system or a component of this system were to fail, the
dynamic braking resistor may experience an extended over-current condition. The DBR circuit was
designed to dissipate excessive amounts of heat and if the extended over-current condition were allowed
to exceed the circuit parameters, this condition could result in a fire hazard.
To combat this condition, the 3-phase input may be connected using contactors that are configured to
open in the event of an extended DBR over-current condition or an internal circuit failure. Using a
thermal sensor and/or overload protection as the 3-phase input contactor drive signal, the contactors will
open and remove the 3-phase input power in the event of an extended DBR over-current or system over-
voltage condition.
Motor Braking
The motor may continue to rotate and coast to a stop after being shut off due to the inertia of the load. If
an immediate stop is required, a braking system should be used. The two most common types of motor
braking systems used with the W7 ASD are DC Injection Braking and Dynamic Braking.
For further information on braking systems, see DC Injection Braking Current on pg. 65 and
Dynamic Braking Enable on pg. 68 .
ASD Characteristics
Over-current Protection
Each W7 ASD model was designed for a specified operating power range. The ASD will incur a trip if
the design specifications are exceeded.
However, the ASD may be operated at 100% of the specified output-current range continuously or at
120% for a limited time as indicated in the section titled
Current/Voltage Specifications on pg. 155 .
Also, the Overcurrent Stall Level setting may be adjusted to help with nuisance over-current trips.
When using the ASD for an application that controls a motor which is rated significantly less than the
maximum current rating of the ASD, the over-current limit (Thermal Overload Protection) setting will
have to be changed to match the application. For further information on this parameter, see
Electronic
Thermal Protection #1 on pg. 70 .
ASD Capacity
The W7 ASD must not be used with a motor that has a significantly larger capacity, even if the motor is
operated under a small load. An ASD being used in this way will be susceptible to a high-output peak
current which may result in nuisance tripping.
Do not apply a level of input voltage to an ASD that is beyond that which the ASD is rated. The input
voltage may be stepped down if required with the use of a step-down transformer or some other type of
voltage-reduction system.
CAUTION