HomeInstallation7.5 hp VFD for 5 hp motor in elevator

7.5 hp VFD for 5 hp motor in elevator

Case: Putting 7.5 hp ac drive in 5 hp motor in elevator,May be due to regeneration (DBR put as per recommanded) drive dc bus ckt failed recent out of 10 nos 8 nos burnt.

The variable frequency drive sizing of 7.5hp with a 5hp motor is due to debating requirements when running the drive at high switching frequencies along with the starting torque requirements of the application. DBR need to be sized to match the braking IGBT within the VFD as far as a minimum resistance is concerned. And the resistor needs to of sufficient kW's to take into to account the duty cycle of the braking.

We had a similar situation with a larger VFD at 200 hp. This type of variable frequency drive is not meant to control the amount of power being returned to the system by slowing down the elevator. This was back in the early days of VFD's and the solution for us was to install a much larger dynamic brake resistor and we also had to increase the size of the IGBT to match. If you find that you are blowing the reverse IGBT's in the regenative drive then you will have to upsize the VFD to a point where it can handle power requirements in the regen mode of operation.

You can choose a Dynamic or regenerative braking solutions depending of the regenerative peak power at the starting of the deceleration, cabin speed, the regenerative rated power, required deceleration time, lowering time and duty cycle.

Usually we use dynamic brake for elevators with 5,5HP asynchronous motors with speed ranges between 0.6 to 0.8 m/s. We select the minimum resistance corresponding to our VFD brake unit and a rated power of the resistor between 25% to 80% of the rated motor in function of the lowering cycles. It is important the overload capacity of the resistor power from the manufacturer. Please, take care with the minimum resistance considering even the ambient temperature because it could damage the DC link of the VFD due to overcurrent if there would be lower resistance than the required resistance of VFD model.

For higher performance elevators we prefer to use regenerative units. We can offer 2 types of solutions based on sinusoidal (THD<2.5%) or quasi-sinusoidal current. We need to estimate the regenerative current, the maximum overload current and duty cycle.

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