DC Motor Types

Shunt DC Motor

In a Shunt DC Motor, the magnetic field is generated by an electromagnet whose coil is connected in parallel with the armature. The voltage equation can be written as:

$$V=i_f{R}_f=i_a{R}_a+E$$

• When $V$ is constant, the is effectively the same as a permanent-magnet motor
• If $V$ gets smaller, $i_a$ and $i_f$ get smaller (called field weakening)

The torque equation changes due to field weakening:

$$T=-\frac{k_t{k}_e}{R}\omega +\frac{k_t}{R}V$$

If $V$ is halved, then the $i_f$ also halves, reducing the torque constant $k_t$ by half as well.

The motor’s speed is less affected by changes in load parameter compared to a PMDC motor, making it suitable for maintaining a relatively constant speed. The torque-speed curve shows a smaller drop in speed with an increase in load.

Series DC Motor

In a series DC motor, the field coil is connected in series with the armature. This gives a voltage equation of:

$$V=i_a(R_f+R_a)+E,i_f=i_a$$

From the primitive motor model, the force is proportional to the product of magnetic flux density $B$ and armature current $i_a$

$$F=Bl{i}_a⟹T\propto B\cdot i_a$$

Applying magnetic Ohm’s law, the armature current is proportional to the magnetic field strength $B$:

$$E=Blv⟹B\propto \frac{1}{\omega }$$

Combining the above equations gives the torque relation:

$$T\propto B^2\propto \frac{1}{{\omega }^2}$$

The speed of a series DC motor varies significantly with changes in load. Under load, the motor slows down, while it can reach very high speeds with reduced load. The torque-speed curve is highly nonlinear, showing a steep drop in speed as torque increases.