Synchronous Motors

SPEED34(a)6-Pulse Converter With Delta/WYE TransformerFourier Series:i = 2 3 I d (cos θ + 1 ⁄5 cos 5θ - 1 ⁄7 cos 7θ - 1 ⁄11 cos 11θπ+ 1 ⁄13 cos 13θ + 1 ⁄17 cos 17θ - 1 ⁄19 cos 19θ —)(b)6-Pulse Converter With Delta/Delta TransformerFourier Series:i = 2 3 I d (cos θ - 1 ⁄5 cos 5θ + 1 ⁄7 cos 7θ - 1 ⁄11 cos 11θπ+ 1 ⁄13 cos 13θ - 1 ⁄17 cos 17θ + 1 ⁄19 cos 19θ —)(c)Current in Delta/Delta and Delta/WYE TransformersFourier Series:I total = 2 3 I d (cos θ - 1 ⁄11 cos 11θ + 1 ⁄13 cos 13θπ- 1 ⁄23 cos 23θ + 1 ⁄25 cos 25θ —)Figure 63 Sine wave simulation of VFD output.I dI dI dsynchronized with the very low frequency source.Greater starting torques may be available at thesame time that lower starting currents aredemanded when compared with across-the-linestarting. Reduced speed operation is controlled at aconstant volts per hertz basis so the magneticstructure is not saturated.Occasionally, a by-pass contactor or circuitbreaker will be supplied to connect the linedirectly to the machine. This will affect the cost ofthe installation, but it may also be desired by theuser as a backup system. It is also possible to startor operate multiple machines while using only oneLCI. For example, there may be two pumps; onemay be operating via the LCI, but its capacity hasbeen exceeded by the demand. This machine canbe transferred to across-the-line operation via theby-pass contactor and the LCI transferred to thesecond machine, which it can bring up tooperating speed, increasing the total capacity in avery smooth and bumpless manner. Refer toFigure 64.Occasionally, it will be desired that voltagewill be increased to rated level at speeds over 80%(of line frequency rating). For this condition, themotor is built with enough iron so that magneticsaturation does not become problematic. Theresult is that the machine can run without a powerfactor penalty. Low voltage means delayed firingangles in the front (input) end of the converter,and that translates into a low power factor. Sinceit is common for large machines (forced draft andinduced draft fans) to run at speeds between 80%and 100%, it is desirable to operate at 100% voltageand realize the benefit of the improved powerfactor (as seen by the line) available from 80 to100% speed.While unity or leading power factors aregenerally associated with synchronous motors,this feature does not hold true when the motor isoperated through an LCI. The motor will beoperating at a leading power factor, but the linedoes not see the motor; it sees the input section ofthe LCI, and that is always a lagging power factorload. The lower the speed (voltage) the morelagging the power factor, and the best powerfactor is still only about 90% when output voltageto the motor is at maximum.Cooling the machine may be by use of anintegral fan if the load is variable torque and is notexpected to operate at speeds below about 50% forextended periods. If a constant torque load isbeing driven, however, and particularly if thespeed will be less than 50% rated, external coolingfans must be employed. Often, the motor will beoversized to lessen the effects of reduced coolingat lower speeds.Sometimes present in the output supplied tothe motor from the LCI are common mode