In burners with ventilation motor capacity of 22kW and above, the power circuit and control circuit need to be installed separately due to destructive effects of electrical noise that power circuit or high voltage has on the control devices. With regard to this reason, raadman motor starter (RMS) in which the power circuit is embedded, is introduced. The Most common start ventilation motor is star-Delta and the frequency converter is also called VSD (Variable Speed Drive).
RMS-…-SD-…
A star-delta starter is the most commonly used method for starting a three-phase induction motor. In star delta starting, an induction motor is connected via a star connection throughout the starting period. In order to use this starting method, the motor must be delta connected during a normal run. This starting system has some disadvantages that are mentioned below:
Overload current drawn from electric grid at the time of starting
Current and mechanical shocks at the load torque starting moment -Current and mechanical shocks to the motor at the moment of changing status from Star to Delta which reduces the motor useful lifespan
by increasing the current range, the contactors and bimetals sizes are increased
RMS-…-VSD-…
The frequency converter is also called VSD (Variable Speed Drive), VFD (Variable Frequency Drive) or simply drive. The drive consists primarily of two parts; one part converts AC (50 or 60 Hz) to DC and the other part converts DC back to AC, but with a variable frequency of 0-250 Hz. The drive can control the motor speed by controlling the frequency. During start, the drive increases the frequency from 0 Hz up to the network frequency (50 or 60 Hz). By increasing the frequency gradually, the motor can run at its rated speed for that frequency. Since the motor can run at its rated speed, the rated torque is available already from start and the current will be around the nominal current. Normally, the drive trips if the current reaches 1.5 times the rated current. The followings are limitations of starting with a drive:
causing noise and harmonics to the network (needs filter) and high cost