Author: Radoje Jankovic

All calculations and drawing by author.

Here are few practical calculation of drives with three-phase motors from practice.

These step-by-step calculations are useful for almost all motor drives with three-phase electric motors.

PROBLEM # 1.

PROBLEM #2

PROBLEM #3.

b) For the conveyor  No. 1 the motor M1, with power of 3 kW, are used high rupturing capacity fuses of 16 A, and for the motor of conveyor No.2 of nominal power of  11 kW, a 35 A blade fuse is used. The fuses should be fitted with micro contacts that will automatically shut off the motor if any of the three fuses are blown.

c) The single-line of both motors scheme is shown in Figs. 1 with contactors for remote control , for mains current of 22 A and bimetallic relays (overload relays) with 5-8 A and 16-25 A adjustments respectively.

Fig. 1.  Single-line diagram for M1, 3 kW and M2, 11 kW.

d) Three-line motor circuit diagrams shown in Fig. 2.a. and Fig. 2b. The power factor correction capacitors connect via a separate cable to the motor’s regular terminals in the distribution box.
When looking at the schemes in FIG. 2.a. and Sl. 2.b., in principle, they are exactly the same, but in the functional sense of the drive are the same as interdependent and different in their electrical characteristics, as we have already seen in the previously mentioned and discussed calculation problems.

Fig. 2.a. Three line power circuit diagram for motor M1 of 3 kW with power factor compensation capacitor C1 of 1 kVAr.

Fig. 2.b. Three line power circuit diagram for motor M1 of 3 kW with power factor compensation capacitor C1 of 3 kVAr.

PROBLEM #4.

PROBLEM #4.

PROBLEM #6.