Series-wound motors, also known as series-excited motors, are DC motors where the stator and rotor windings are connected in series. DC motors can be classified by excitation type into series-excited, shunt-excited, compound-excited, and separately-excited. Series excitation is merely one excitation principle among DC motors, with no fundamental difference. These motors are primarily used in power tools and belong to the category of AC/DC dual-purpose motors or DC motors.
What is a Series-Wound Motor?
Series-wound motors derive their name from the series connection between the field winding and armature winding. Single-phase series-wound motors are dual-purpose AC/DC motors capable of operating on either alternating or direct current. Characterized by high speed, compact size, light weight, and easy speed regulation, they serve as the power source in most power tools. However, single-phase series-wound motors in power tools experience frequent starts, high rotational speeds, significant vibration, uneven loads, and susceptibility to stalling, which can cause rapid temperature rise in the armature. These characteristics impose stringent manufacturing quality requirements on single-phase series-wound motors. The motor primarily consists of three major components: the stator, the armature, and the housing, with the armature demanding particularly high standards.
I. Structure of Series-Wound Motors
The motor primarily consists of three parts: stator, rotor, and frame. The stator comprises a salient-pole core and field winding, while the rotor consists of a non-salient-pole core, armature winding, commutator, and shaft. The field winding and armature winding form a series circuit via brushes and the commutator.
II. Principle of Series-Wound Motors
When powered by an AC source, the principle generating rotational torque can still be explained using the operating principle of DC motors.
When current flows through a conductor, a magnetic field is generated around it, with the direction of the magnetic lines of force determined by the current direction. When a current-carrying conductor is placed within this magnetic field, the interaction between the two magnetic fields induces a force F on the conductor, causing it to move. The conductor will migrate from areas of high magnetic flux density toward regions of low density. Similarly, when a coil composed of two opposing conductors is placed in a magnetic field, both sides of the coil experience opposing forces, generating a torque.
When the coil rotates within the magnetic field, the two corresponding sides pass from beneath one magnetic pole to another. As the magnetic field polarity changes, the direction of the force acting on the conductor reverses, altering the direction of the torque. This causes the coil to rotate in the opposite direction, resulting in the coil oscillating back and forth around its central axis.
III. What is the typical speed of a series-wound motor?
Also known as universal motors, series-wound motors are widely used in power tools and household appliances. They feature high speed, high efficiency, compact size, and high starting torque. Typically equipped with a single pair of poles, their speed can only be adjusted by varying the voltage, current, or through mechanical means—it is completely unrelated to frequency.
The rotational speed of a series-wound motor is primarily determined by factors such as supply voltage, armature current, and mechanical load. Consequently, different series-wound motors may exhibit varying speeds under different operating conditions. Generally, the rotational speed of a series-wound motor ranges from several hundred to several thousand revolutions per minute (RPM), with the exact value depending on the motor's specifications, design, and application requirements.
For example, a 12V, 3000rpm series-wound motor will achieve approximately 3000rpm under a 12V supply voltage and rated current. However, in practical applications, actual speed may vary due to differences in mechanical load and circuit conditions.
It is important to note that series-wound motors have a relatively narrow speed range and are typically suited for low-speed, high-torque applications such as small machinery, toy models, and bicycle generators. For applications requiring higher speeds and power output, alternative motor types like brushless DC motors or AC induction motors may be necessary.
IV. How to Adjust the Speed of a Series-Wound Motor
The speed of a series-wound motor can be adjusted by regulating the armature current, following these steps:
Measure motor speed: Use a tachometer or similar device to determine the current rotational speed.
Adjust armature current: Modify the armature current by altering the power supply voltage or using a rheostat to achieve the desired speed. Generally, increasing the armature current raises the motor speed, while decreasing it lowers the speed.
Verify speed: After adjusting the armature current, re-measure the motor speed to confirm it reaches the desired value.
Note: When adjusting armature current, strictly follow operating procedures to prevent circuit failures or accidents. Additionally, due to the relatively stable output characteristics of series-wound motors, their speed adjustment range is limited. Therefore, in practical applications, select an appropriate speed control method based on specific requirements.
