DC Electrics - DC Motors — Page 110, Lesson 109

I want to walk you through the two main types of DC motors we use in aviation: the series wound motor and the shunt wound motor. Let's start with the series wound motor. In a series wound motor, the field coil is connected in series with the armature. That means the entire current flowing through the armature also flows through the field coil. The field coil itself is made of a few turns of heavy wire. Because all the armature current passes through it, the strength of the magnetic field — what we call the field strength — varies directly with the armature current. More armature current means a stronger field; less current means a weaker field. Now, here's the key behavior: if the load on the motor increases — say the motor has to turn something heavier — the motor slows down. When it slows down, the back EMF decreases. Back EMF is the voltage the motor generates as it spins, which opposes the supply voltage. With less back EMF, more current flows into the armature and through the field coil. That increased current produces a stronger magnetic field, which gives the motor the heavier torque it needs to drive the increased load. So series motors run slowly with heavy loads and very rapidly with light loads. Here's the critical safety point: if the load is completely removed — if the motor is running with nothing attached — it can dangerously overspeed and possibly disintegrate. Why? Because with only a light load, the current required to rotate the motor is very small. That small current produces only a weak magnetic field from the series wound field coils. The motor cannot spin fast enough to generate enough back EMF to restore the balance between supply voltage and counter-voltage. So it keeps accelerating until it may tear itself apart. Because of this, series wound motors are variable speed motors — their speed changes with the applied load. For that reason, they are not used where a constant speed is needed, nor where the load is intermittent. They do have a very high starting torque, and because of that high starting torque, they must never be started off load — you always need some load connected. In aviation, you'll find series wound motors mainly in electric actuators, starter motors, and landing gear actuation. Now let's move to the shunt wound motor. In a shunt wound motor, the field coil is connected directly across the voltage source — in parallel, or "shunt," with the armature. That means the field current is independent of variations in load and armature current. The field coil consists of many turns of fine wire, which gives it a relatively high resistance, so it draws a small, steady current. The torque developed by a shunt motor varies directly with the armature current. Here's how it responds to an increased load: if the load increases, the motor slows down. That reduces the back EMF — and remember, back EMF depends on both speed and field strength. In a shunt motor, the field strength is constant because the field is connected directly across the supply. So the reduced back EMF allows the armature current to increase, and that increased armature current furnishes the heavier torque needed to drive the increased load. So the key difference between the two: in a series motor, field strength varies with load; in a shunt motor, field strength is constant, and torque varies directly with armature current alone.