Basic Motor Theory (11)

Field Windings

The field windings provide the excitation necessary to set up the magnetic fields in the machine. There are various types of field windings that can be used in the generator or motor circuit. In addition to the following field winding types, permanent magnet fields are used on some smaller DC products. See Figure 19 for winding types.


Shunt Wound – DC Operation
Typical Speed – Torque Curve
Shunt wound motors, with the armature shunted across the field, offer relatively flat speed-torque characteristics. Combined with inherently controlled no-load speed, this provides good speed regulation over wide load ranges. While the starting torque is comparatively lower than the other DC winding types, shunt wound motors offer simplified control for reversing service.

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Compound Wound – DC Operation
Typical Speed – Torque Curve
Compound wound (stabilized shunt) motors utilize a field winding in series with the armature in addition to the shunt field to obtain a compromise in performance between a series and shunt type motor. This type offers a combination of good starting torque and speed stability. Standard compounding is about 12%. Heavier compounding of up to 40 to 50% can be supplied for special high starting torque applications, such as hoists and cranes.

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Series Wound – DC Operation
Typical Speed – Torque Curve
Series wound motors have the armature connected in series with the field. While it offers very high starting torque and good torque output per ampere, the series motor has poor speed regulation. Speed of DC series motors is generally limited to 5000 rpm and below. Series motors should be avoided in applications where they are likely to lose there load because of their tendency to “run away” under no-load conditions. These are generally used on crane and hoist applications.

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Permanent Magnet – DC Operation
Typical Speed – Torque Curve
Permanent magnet motors have no wound field and a conventional wound armature with commutator and brushes. This motor has excellent starting torques, with speed regulation not as good as compound motors. However, the speed regulation can be improved with various designs, with corresponding lower rated torques for a given frame. Because of permanent field, motor losses are less with better operating efficiencies. These motors can be dynamically braked and reversed at some low armature voltage (10%) but should not be plug reversed with full armature voltage. Reversing current can be no higher than the locked armature current.

Figure 19. Field Windings

Separately Excited Winding
When the field is connected to an external power source, it is a separately excited field.

Straight Shunt Winding
This winding is connected in parallel with the armature. Shunt windings usually consists of a large number of turns of small size wire. This is a good winding for reversing applications since it provides the same amount of torque in both directions. The torque/ current curve is non-linear above full load. Shunt wound motors often have a rising speed characteristic with increased load.

Series Winding
This winding is connected in series with the armature. A series winding usually consists of a small number of turns of large size wire. With this winding, the motor can produce high starting and overload torque. This design is not used for applications with light loads or no load conditions.

Compound Winding
This winding consists of a shunt winding and a series winding. This is also known as compound excitation. The series winding can be designed as a starting series only or as a start and run series.
Stabilized Shunt Winding
Like the compound winding, this winding consists of a shunt winding and a series winding. The series or stabilizing winding has a fewer number of turns than the series winding in a compound wound machine. A stabilizing winding is used to assures a speed droop with overload. It also adds to the torque in one direction of operation and subtracts from torque in the reverse direction of operation and in regeneration.

Shunt Compensated Winding
Shunt compensated motors have a shunt winding and a pole face series winding made up of large conductors placed in slots in the face of the main field poles. The direction of current in the compensating windings is the opposite of the current in the armature conductors passing under the poles. The flux produced by the compensating windings neutralizes the flux of the armature conductors passing under the poles so that distortion of air gap flux is minimized. Shunt compensated motors maintain constant or set speed well at all loads, no load through overload. Unlike the stabilized shunt winding, the pole face winding adds to torque in both the forward and reverse direction of rotation. Shunt compensated windings, due to cost and difficulty of construction, are provided on large motors only, usually 840 frames and larger.

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