Why is the Starting Torque of a DC Motor Greater Than That of an AC Motor?
DC (direct current) motors generally have higher starting torque compared to AC (alternating current) motors. The starting torque of a motor is crucial for applications where the load initially resists motion, such as in conveyors, crushers, and other heavy-duty machinery.
In a DC motor, the torque is produced by the interaction of magnetic fields generated by the stator (field winding) and the rotor (armature winding). The direction of the current in the armature winding is controlled by a commutator, which ensures that the torque-producing magnetic field is always in the same direction relative to the rotor. This design allows for a smooth and consistent torque production throughout the rotation of the motor.
The starting torque of DC shunt motor is given by:
The starting torque of a DC machine can be adjusted and controlled easily by varying the armature voltage or by using additional external resistors. This flexibility in controlling the starting torque makes DC motors suitable for applications that require a high torque at startup.
In applications where high starting torque and precise speed control are needed, DC motors are preferred, such as in vehicles, cranes, elevators, winches, crushers, CNC and conveyor systems, material handling/lifting equipment, traction control, etc.
On the other hand, AC motors, especially induction motors, rely on the induction of a magnetic field in the rotor by the stator’s rotating magnetic field. During the startup of an AC motor, there is a brief period when the rotor speed is not synchronized with the rotating magnetic field, resulting in lower torque at startup. This is known as “slip,” and it can affect the starting torque of AC motors.
The starting torque of AC induction motor:
For this reason, AC motors, especially induction motors, may have lower starting torque compared to DC motors. However, some AC motors, such as synchronous motors, can provide higher starting torque under specific conditions.
In applications where low starting torque and precise control are needed, AC motors are preferred to be used, such as in pumps and compressors, fans, washing machines, refrigerators, air conditioners, and vacuum cleaners; HVAC systems; cranes, elevators, and escalators; winches; printing and textile machinery; CNC and conveyor systems; material handling/lifting equipment; traction control, etc.
While advancements like variable frequency drives (VFDs) have improved the control and performance of AC motors, DC motors still offer certain advantages in terms of controllability and starting torque. The ability to control the direction and magnitude of the current in the armature winding of a DC motor allows for precise torque control, making DC motors suitable for applications where high starting torque is essential.
FAQ
Why does the DC series motor have high starting torque?
DC Series Motors
On starting, because the windings are low resistance, a large current can be drawn producing a high starting torque. This is an advantage for high starting loads such as traction, crane and other heavy applications
Do DC motors have more torque than AC motors?
Which Motor Is More Powerful: AC or DC? AC motors are generally considered to be more powerful than DC motors because they can generate higher torque by using a more powerful current. However, DC motors are typically more efficient and make better use of their input energy
Why starting current is high in DC motor?
Value of armature resistance is very small i.e., nearly equal to 0.5 ohm. Due to this small value of armature resistance and no induced emf at starting, the starting current is very high
Why do AC motors have lower starting torque?
The reason for low starting torque is the poor power factor of the rotor circuit. At the start, the rotor inductance is very high as compared to the rotor resistance. The starting torque of an induction motor is low because the rotor of an induction motor is not directly connected to the stator’s magnetic field
Which motor has high starting torque, AC or DC?
DC motor is used for high starting torque as well as for wide range of speed control is required. DC series motor gives the maximum torque at starting
Why the starting torque is low in DC motor?
The high number of turns allows a strong magnetic field to be generated, but the smaller gauge wires provide a high resistance and limit the current flowing through the shunt coil. Therefore, the starting torque of a DC shunt motor is low, meaning that the shaft load must be small at startup
Which DC motor has the highest starting torque?
DC Series Motor: In a DC series motor, torque is directly proportional to the flux and armature current. The torque-speed characteristics of different DC motors are shown below. Therefore, the starting torque of the DC series motor is highest among all the DC motors
Why DC motor is better than AC motor?
Due to induction current loss and motor slip, the efficiency of the AC motor is less. The efficiency of the DC motor is high as there is no slip and induction current loss. AC motors require less maintenance as brushes and commutators are absent
Why AC motor starting current is high?
What causes high start-up or inrush current? An induction motor with a stationary rotor behaves like a transformer with shorted secondary windings; there is no back-emf to counter the emf in the stator windings, so a very high current flows
Why do DC motors have inrush current?
DC motors do not have current inrush. And the DC inrush current should be the DC resistance divided into the applied voltage. Once the motor is spinning it generates a back emf which reduces the current draw
