DC Motors and its functioning

What are dc motors and how do definitions vary? Technical summary for engineers

DC motors are motion components that take electrical power in the form of direct current (or some manipulated form of direct current) and convert it into mechanical rotation. The motors do this through the use of magnetic fields that arise from the electric currents to spur rotation of a rotor fixed with an output shaft. Output torque and speed depends on the electrical input and motor design.

A direct current (DC) motor is an electric machine that converts electrical energy to mechanical energy.

According to the most common industry naming conventions of today, there are three DC motor sub types: DC brush motors, DC permanent-magnet (PM) motors, and DC universal motors. As we’ll see, there are some caveats and sub-classifications.

Many larger DC motors still employ brushes and wound fields, but PM motors dominate fractional and integral-horsepower applications below 18 hp. That said, PM motors are increasingly common for myriad designs.

What are DC brush motors and what kinds are there?

ome engineers call DC brush motors wound-field motors, because it’s a wound and lacquered coil of copper wire that makes the electromagnetic field. Some engineers also argue that all DC motors are brush DC motors, and that the term “brush less DC motor” is a misnomer.

No matter the term, there are permanent magnet, shunt, series, and compound-wound brush DC motors.

All except the former use two currents:

1. Current through armature (rotor) windings to interact with a stator magnetic field (for output of mechanical rotation) and

2. Current through stator windings to make the magnetic field in question.

In contrast, permanent-magnet brush DC motors use:

1. Current through armature (rotor) windings to interact with a stator magnetic field (for output of mechanical rotation) and

2. Permanent magnets on the stator to make the magnetic field in question.

The armature and field coils in a shunt-wound motor connect in parallel so the field current is proportional to the load on the motor.

The armature and field coils in series-wound motor connect in series so current passes only through the field coils.

The armature and field coils in compound-wound motors include both series and shunt windings.

No matter the setup, brush DC motors have commutations and brush contacts to pass current to the rotating rotor’s copper-wire windings. Designers can control speed by changing rotor voltage (and current with it) or by changing the magnetic flux between rotor and stator through adjustments of the field-winding current. Brush orientation to the rotor’s commentator bar segments mechanically controls the phase commutation.

In fact, the way DC brush motors let designers control field and rotor winding means they’re suitable for applications that need simple and cost-effective torque and speed control.

That said, increased functionality from electronics for PM motors means that this advantage in less pronounced than it once was. What’s worse, current on both rotor and stator generate heat that limits the motors’ continuous-current ratings. The motors also present a spark hazard, so can’t go in explosive settings. At certain periods during the dc motor rotation, the commentator must reverse the current, reducing motor life with arcing and friction. So, brushed dc motors require more maintenance in the form of replacement of springs and brushes that carry the electrical current, and replacement or cleaning of the commentator. These components are important for transferring electrical power from outside the motor to the spinning coil winding of the rotor inside the motor.

Note: The brushes in DC brush motors wear and need replacing, and brush-wear particles mean that designers shouldn’t use DC brush motors in clean rooms. Same goes for applications that need high precision, as friction from brush-commentator engagement make for long position-settling times.

Series-wound DC motors

As mentioned, the armature (rotor) and field coils in series-wound motors connect in series. That means the entire armature (rotor) current passes to the field winding. So, these motors only need one input voltage supply. Torque equals current squared. Increasing armature (rotor) current induces a field-current increase. Regenerative braking isn’t possible; field current collapses when rotor current passes through zero and reverses.

Torque is highest when the motor stops because the armature (rotor) generates no back electromotive force (bEMF) when at rest. When the armature (rotor) accelerates, bEMF increases. That in turn reduces effective current, voltage and torque. Without loading, the motor accelerates to dangerous speeds. In contrast, increased load slows the motor but lowers bEMF … and increases torque to turn the load.

Series-wound motors can’t regulate speed well, as speed control depends on adjustments to the supply voltage. Even so, they’re inexpensive and can drive designs that need high starting torque. For example, designers use series-wound motors in low and high-power automotive mechanisms, consumer products such as power tools, toys, and sewing machines, and industrial traction drives with fixed and variable speed.  Designers can reverse series-wound motors by reversing field or armature (rotor) winding connections.

Shunt-wound DC motors

As mentioned, armature and field coils in shunt-wounds motor connect in parallel … so field current is proportional to the load on the motor. Variable-voltage input allows for speed adjustment. Supply fixed voltage to a shunt-wound motor to make it run at constant speed. Then supply increasing motor current to a shunt-wound motor to increase torque without significant slowing.

In shunt-wound motors, the field (stator) winding connects in parallel with the armature (rotor) winding.

With these motors, a technique called field weakening can control speed without forcing the controls to change input voltage. A field-winding rheostat reduces field (stator) current and with it the magnetic flux between armature and field (across the air gap that separates them). Speed is inversely proportional to flux, so this accelerates the motor. One caveat: Torque is directly proportional to flux, so the acceleration comes with diminished torque output.

Stabilizing winding prevent acceleration as load increases at weak field settings. The only catch is that reversing applications need reversal of this winding to go with armature (rotor) voltage reversal. That necessitates reversing contractors. So for reversing motion, sometimes manufactures just design shunt-would motors with higher stability and omit stabilizing winding.

Reversing a shunt-wound motor’s connections on either rotor winding or field reverses the motor’s direction of rotation; self-excitation maintains the field when the rotor current reverses, which means the motors can alternatively brake.

Shunt-wound motors drive machine tools and automotive fan and wiper applications.

Compound-wound motors

Separately excited motors (sometimes called compound-wound motors) are DC brush motors with independent voltage supplies to the field (stator) and armature (rotor) … for better control over motor output. Input voltage on either winding can control motor output speed and torque. Most manufacturers build compound-wound motors with series and shunt-wound field (rotor) winding. The direction and strength and direction of two winding dictates the motor’s speed-torque curves.

Compound-wound motors work well for traction in automotive or rail-train applications