Introduction to DC Motors

[Magneto]

The brushed DC motor is one of the earliest motor designs. Today, it is the motor of choice in the majority of variable speed and torque control applications.

Advantages

Easy to understand design
Easy to control speed
Easy to control torque
Simple, cheap drive design
Easy to understand design
The design of the brushed DC motor is quite simple. A permanent magnetic field is created in the stator by either of two means:
Permanent magnets
Electro-magnetic windings
If the field is created by permanent magnets, the motor is said to be a "permanent magnet DC motor" (PMDC). If created by electromagnetic windings, the motor is often said to be a "shunt wound DC motor" (SWDC). Today, because of cost-effectiveness and reliability, the PMDC motor is the motor of choice for applications involving fractional horsepower DC motors, as well as most applications up to about three horsepower.

At five horsepower and greater, various forms of the shunt wound DC motor are most commonly used. This is because the electromagnetic windings are more cost effective than permanent magnets in this power range.

Caution: If a DC motor suffers a loss of field (if for example, the field power connections are broken), the DC motor will immediately begin to accelerate to the top speed which the loading will allow. This can result in the motor flying apart if the motor is lightly loaded. The possible loss of field must be accounted for, particularly with shunt wound DC motors.

Opposing the stator field is the armature field, which is generated by a changing electromagnetic flux coming from windings located on the rotor. The magnetic poles of the armature field will attempt to line up with the opposite magnetic poles generated by the stator field. If we stopped the design at this point, the motor would spin until the poles were opposite one another, settle into place, and then stop -- which would make a pretty useless motor!

However, we are smarter than that. The section of the rotor where the electricity enters the rotor windings is called the commutator. The electricity is carried between the rotor and the stator by conductive graphite-copper brushes (mounted on the rotor) which contact rings on stator. Imagine power is supplied:

The motor rotates toward the pole alignment point. Just as the motor would get to this point, the brushes jump across a gap in the stator rings. Momentum carries the motor forward over this gap. When the brushes get to the other side of the gap, they contact the stator rings again and -- the polarity of the voltage is reversed in this set of rings! The motor begins accelerating again, this time trying to get to the opposite set of poles. (The momentum has carried the motor past the original pole alignment point.) This continues as the motor rotates.

In most DC motors, several sets of windings or permanent magnets are present to smooth out the motion.
Easy to control speed
Controlling the speed of a brushed DC motor is simple. The higher the armature voltage, the faster the rotation. This relationship is linear to the motor's maximum speed.

The maximum armature voltage which corresponds to a motor's rated speed (these motors are usually given a rated speed and a maximum speed, such as 1750/2000 rpm) are available in certain standard voltages, which roughly increase in conjuntion with horsepower. Thus, the smallest industrial motors are rated 90 VDC and 180 VDC. Larger units are rated at 250 VDC and sometimes higher.

Specialty motors for use in mobile applications are rated 12, 24, or 48 VDC. Other tiny motors may be rated 5 VDC.

Most industrial DC motors will operate reliably over a speed range of about 20:1 -- down to about 5-7% of base speed. This is much better performance than the comparible AC motor. This is partly due to the simplicity of control, but is also partly due to the fact that most industrial DC motors are designed with variable speed operation in mind, and have added heat dissipation features which allow lower operating speeds.
Easy to control torque
In a brushed DC motor, torque control is also simple, since output torque is proportional to current. If you limit the current, you have just limited the torque which the motor can achieve. This makes this motor ideal for delicate applications such as textile manufacturing.
Simple, cheap drive design
The result of this design is that variable speed or variable torque electronics are easy to design and manufacture. Varying the speed of a brushed DC motor requires little more than a large enough potentiometer. In practice, these have been replaced for all but sub-fractional horsepower applications by the SCR and PWM drives, which offer relatively precisely control voltage and current. Common DC drives are available at the low end (up to 2 horsepower) for under US$100 -- and sometimes under US$50 if precision is not important.

Large DC drives are available up to hundreds of horsepower. However, over about 10 horsepower careful consideration should be given to the price/performance tradeoffs with AC inverter systems, since the AC systems show a price advantage in the larger systems. (But they may not be capable of the application's performance requirments).

Disadvantages

Expensive to produce
Can't reliably control at lowest speeds
Physically larger
High maintenance
Dust