Can an Altivar Process Variable speed drive be bypassed like an Altistart soft starter?
Yes, a variable-speed drive can be bypassed, but the bypass circuit must be designed carefully.
When the variable speed drive (Altivar) is bypassed, the motor is connected directly to the AC supply, so the drive no longer controls the motor speed. Altivar drives are designed to provide variable motor speed by controlling the voltage and frequency supplied to the motor
Yet variable speed drives me be used only for the High-torque applications start, like mills, up-hill conveyors, crushers, mixers, etc. For low-starting-torque application the Altistart is better option.
Altivar is not so easy to bypass because there is energy conversion AC-->DC-->AC. The most sensitive part is the Altivar output. Therefore the bypass is possible only if the Altivar output gets disconnected from the motor before connecting motor to mains using a bypass element (contactor for instance). Take this sequence seriously because if Altivar output gets connected to mains, short circuit in Altivar occurs. Therefore the best idea is to use Altivar output contactor with additional mechanical and electrical interlock between output contactor and bypass contactor. After bypass activation, Altivar input side can remain connected to AC mains or can be disconnected, it is fully optional.
1. Connect Altivar to mains and to motor
2. Start the motor.
3. When motor achieves full speed, activate freewheel stop (to avoid output phase loss on Altivar if output contactor gets open)
4. Switch off Altivar output contactor
5. Switch on bypass contactor
6. Optionally you can switch off Altivar from mains or completely remove it from installation as it is not needed until new start is required.