AC drive can brake without resistors

Yaskawa has unveiled a new generation of variable speed drives that are smaller and about 10% cheaper than its previous generation. Technical innovations include a built-in emergency braking system that does not need electromechanical brakes or braking resistors. The Varispeed F7 inverter is a sensorless flux vector system spanning ratings from 0.4-300kW at 400V and 0.4-110kW at 200V. In the open-loop vector mode, the drive can deliver more than 200% torque at 0.5Hz. For basic fan and pump applications, there is a cutdown version known as the E7.

Technical innovations in the new drives include:

• reduced noise at low carrier frequencies;

• an automatic energy-saving function;

• improved PID control including a "sleep" function;

• a choice of three auto-tuning modes, including non-rotating and dynamic modes;

• a speed search function for catching motors spinning in either direction;

• DC reactors built in as standard on all drives larger than 22kW;

• the use of fourth-generation IGBT technology for faster switching and improved short-circuit protection; and

• removable control terminals to simplify installation and servicing.

The emergency braking technique manipulates the frequency to make the motor operate under poor efficiency conditions. This causes most of the regenerative energy to be dissipated in the motor, bringing it to a standstill. Although the heat-dissipating capacity of the motor limits the number of times this technique can be used, it can save space and money.

Yaskawa has managed to shrink the new drive so that an 11kW version occupies the same space as a 7kW model from the previous generation.