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In the universal motor, The compensated winding is placed in the slots at _______________ with the main field axis.
A) 90╦Ъ Electrically
B) 90╦Ъ Mechanically
C) 180╦Ъ Electrically
D) 180╦Ъ Mechanically
90╦Ъ Electrically
In a universal motor (AC series motor), the compensating winding is placed in the stator slots at an angle of 90╦Ъ electrically with respect to the main field winding axis. This axis corresponds to the quadrature axis (q-axis) of the motor. Placing the winding in this position allows it to effectively oppose and neutralize the armature reaction flux generated along the q-axis.
In a universal motor (AC series motor), the compensating winding is placed in the stator slots at an angle of 90╦Ъ electrically with respect to the main field winding axis. This axis corresponds to the quadrature axis (q-axis) of the motor. Placing the winding in this position allows it to effectively oppose and neutralize the armature reaction flux generated along the q-axis.
ErтАЛ=4.44f╬жmтАЛNaтАЛ тАФ transformer M.F. induced in short-circuited armature coils during commutation
LqтАЛ=LaтАЛтИТLcтАЛ тАФ net quadrature-axis inductance after compensation
Armature current produces a cross-magnetizing armature reaction field along the quadrature axis, which creates severe commutation sparking and reduces the power factor. By connecting the compensating winding in series with the armature (or short-circuiting it magnetically), a counter-magnetic field is established along the same q-axis (90╦Ъ electrically from the main field axis). This counter-flux cancels the armature flux, improving commutation and motor power factor.
The main field winding produces flux along the direct axis (d-axis).
The armature flux is produced along the quadrature axis (q-axis), oriented 90╦Ъ electrically from the d-axis.
Compensating windings are positioned along the q-axis (90╦Ъ electrically to the main field) to neutralize the armature M.M.F.
Compensation can be conducted (conductively coupled in series) or induced (inductively coupled/short-circuited).
Significantly reduces sparking at the brushes during commutation
Improves overall motor power factor under AC operation
Increases the motor output power and torque capability
Increases motor weight, size, and manufacturing complexity
Increases overall motor cost
High-power AC series motors used in electric traction
Heavy-duty portable power tools and kitchen appliances requiring high AC torque
Option A is correct because electrical 90╦Ъ offset directly aligns the compensating winding with the q-axis armature reaction flux.
Option B (90╦Ъ Mechanically) is incorrect because mechanical angle equals electrical angle divided by pole pairs (P/2), so it only equals 90╦Ъ electrically in a 2-pole machine.
Options C and D (180╦Ъ) are incorrect because placing a winding at 180╦Ъ would align it directly with the main field axis (d-axis) instead of the cross-magnetizing q-axis.
A is correct тАФ The compensating winding must be placed along the quadrature axis, which is 90╦Ъ electrically perpendicular to the main field axis.
For multi-pole machines, remember the relation ╬╕electricalтАЛ=2PтАЛ╬╕mechanicalтАЛ. Exam questions often test whether an angle is specified in electrical degrees or mechanical degrees!