Direct-Current Motors: Shunt-Connected DC Motor Performance

Shunt-connected motors are constructed similarly to series-connected motors, as shown in FGR. 99. The shunt motor has performance characteristics much like those of the PMDC motor. It has a straight-line speed-torque curve over a limited range of performance. In this motor the permanent magnets are replaced by wound field coils, much as they are in the series dc motor. A series motor generally has a relatively few turns of large wire in the field. The shunt motor has a large number of turns of fine wire in comparison. Further, the field winding is connected across the armature-that is to say, in parallel with it, as shown in FGR. 100.The armature then needs to have a larger number of turns, as it sees full line voltage.

TABLE 2: Performance Calculations for Sample Universal Motor

To calculate the performance of this motor, one first needs to calculate the field flux. This is done by calculating the shunt winding resistance and using it to calculate the shunt current. Then, from the known ampere-turns one can calculate the flux and determine the mmf in the magnetic circuit. This procedure is the same as for the PMDC motor except that the magnet is replaced by a coil which produces the field mmf.

The motor geometry and magnetic paths for the field and armature are calculated in the same manner as for the series-wound dc motor.

Windings are selected such that the ampere-turns in the armature are about 75 percent of the ampere-turns in the field at full load.

The required no-load flux fnl is determined by the desired no-load speed Snl and is calculated as follows:

FGR. 99 Shunt-wound dc motor field.

FGR. 100 Shunt-connected dc motor schematic diagram.

where Eg = armature generated voltage or c_emf

P = number of poles Pa = number of parallel armature paths Za = active armature conductors ...

... where: VL = line voltage

Ianl = estimated armature no-load current

Rat = armature terminal resistance

It’s obvious that this type of motor limits the maximum no-load speed Snl. The constant flux causes the speed to stop increasing when Eg = VL.

When designing the field winding one must add in enough full-load ampere-turns to account for the ampere-turns lost as a result of field distortion caused by armature reaction and brush shift.

According to Puchstein (1961), field distortion Fdl, A _ turn, is calculated as follows:

Where: ?p = pole arc ÷ pole pitch Za = active armature conductors Ia = current per armature path P = number of poles

Field distortion Fbl,A _ turn, caused by brush shift is calculated as follows:

... where ...

?b = brush shift angle, degrees Shunt windings produce a nearly constant flux over a limited range of loads. This allows for a fairly straight-line speed-torque curve ( FGR. 101).

As the load increases, the armature reaction causes Fdl to increase to a point where the soft-iron pole tips are demagnetized. This reduces the main pole flux, which results in a reduction of torque.

This field distortion can be seen in the finite element shown in FGR. 102.

To avoid commutation problems, shunt motors should be operated well above the point on the speed-torque curve where field distortion becomes severe.

FGR. 101 Shunt-connected dc motor field performance characteristics.

FGR. 102 Magnetic field of shunt-connected dc motor with field and armature energized.