VLT ® 2900 Series All types of three-phase asynchronous standard mo- tors can be connected to a frequency converter. Nor- mally, small motors are star-connected (230/400 V, Δ / Y). Large motors are delta-connected (400/690 V, Δ / Y). The correct connection mode and voltage can be read from the motor nameplate. NB! In motors without phase insulation paper, an LC filter should be fitted on the output of the frequency converter. The frequency converter is able to control several mo- tors connected in parallel. If the motors are to have different rpm values, use motors with different rated rpm values.
Motor rpm is changed simultaneously, which means that the ratio between the rated rpm val- ues is maintained across the range. The total current consumption of the motors is not to exceed the maxi- mum rated output current I INV for the frequency con- verter. Direction of motor rotation Problems may arise at the start and at low rpm values if the motor sizes are widely different. This is because the small motors' relatively high ohmic resistance in the stator calls for a higher voltage at the start and at low rpm values. In SYSTEMs with motors connected in parallel, the electronic thermal relay (ETR) of the frequency con- verter cannot be used as motor protection for the individual motor.
For this reason further motor protec- tion must be used, e.g. thermistors in each motor or an individual thermal relay. (Circuit breakers are not suit- able as protection). NB! Parameter 107 Automatic motor adaption, AMT cannot be used when motors are connected in parallel. Parameter 101 Tor- que characteristic must be set to Special motor characteristics [8] when motors are connected in parallel. The factory setting is for clockwise rotation with the frequency converter transformer output connected as follows: Terminal 96 connected to U-phase. Terminal 97 connected to V-phase. Terminal 98 connected to W-phase.
The direction of rotation can be changed by switching two phases on the motor terminals. As leak current to the earth is higher than 3.5 mA, the frequency converter must always be earthed accord- ing to prevailing national and local regulations. To guarantee that the earth cable has a good mechanical connection to terminal 95, the cable cross section must be at least 10 mm 2 or 2 rated earth wires that are ter- minated separately. To increase safety you can install an RCD (Residual Current Device), which guarantees that the frequency converter trips when leak current Earth connection Parallel connection of motors 64 MG.29.A3.02 - VLT ® is a registered Danfoss trademark