Join 60,000+ competitive exam aspirants
or with email
Your Cart
Subtotal₹0
Total₹0
Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
Join 60,000+ competitive exam aspirants
If a D.C. series motor is operated on A.C. supply, it will have
Poor power factor
Poor efficiency
Spark excessively
All of these
All of these
Quick Summary: A D.C. series motor is not designed for A.C. operation because the reversal of supply polarity leads to bidirectional torque and severe operational deficiencies. When connected to A.C., the motor experiences excessive sparking at the brushes, high iron losses, and poor power factor due to the high inductance of the series field winding.
A D.C. series motor is not designed for A.C. operation because the reversal of supply polarity leads to bidirectional torque and severe operational deficiencies. When connected to A.C., the motor experiences excessive sparking at the brushes, high iron losses, and poor power factor due to the high inductance of the series field winding.
Te∝ΦIa — Average torque production in series motor
XL=2πfL — Inductive reactance causing low power factor
Et=4.44fNΦm — Transformer EMF in short-circuited coil causing sparks
In a D.C. series motor, the torque is proportional to the product of flux (Φ) and armature current (Ia). When supplied with A.C., both the field flux and armature current reverse polarity simultaneously, maintaining unidirectional torque. However, the rapidly changing flux causes induced eddy currents in the yoke, high inductive reactance (XL=2πfL) reducing power factor, and commutation difficulties due to transformer EMF induced in the short-circuited coils under the brushes.
The magnetic circuit of a standard D.C. series motor is solid; using A.C. causes high eddy current losses.
The high reactance of the series field winding leads to a lagging power factor.
Commutation is hindered by the transformer EMF induced in the coils short-circuited by the brushes.
To operate on A.C., motors must be redesigned as 'Universal Motors' with laminated yokes and field cores.
Unidirectional torque is maintained
Series motors have high starting torque
Excessive sparking leads to rapid brush and commutator wear
High heat generation due to iron losses
Extremely low power factor
Universal motors (small household appliances)
Series-wound traction motors (specifically designed for A.C./D.C. compatibility)
Standard D.C. motors have solid yokes, while Universal motors require laminated cores to minimize eddy current losses.
Option A, B, and C are all valid consequences of operating a standard D.C. machine on A.C.
D is correct — operating a D.C. series motor on A.C. results in poor power factor, low efficiency, and severe sparking, making 'All of these' the correct choice.
Remember that a 'Universal Motor' is essentially a modified D.C. series motor; always look for the word 'laminated' when determining if a machine is suitable for both A.C. and D.C.