Advantages and Disadvantages of a Brush DC Motor (2012-10-11 16:10:07)
The DC Brush Motor is one of the earliest of all electrical motor designs. It is usually the motor of choice for the majority of torque control and variable speed applications. The following discusses the advantages and disadvantages of using a Brush DC Motor in machinery and processes.
1. Advantages - Brush DC Motor
The DC Brush Motor is one of the earliest of all electrical motor designs. It is usually the motor of choice for the majority of torque control and variable speed applications. The following discusses the advantages and disadvantages of using a Brush DC Motor in machinery and automated processes.
• The Brush DC Motor has a simple construction, therefore may not require a controller. When a controller is chosen, it is typically a simple and inexpensive drive design.
• Understandable design technology facilitates in the quick application of the DC Brush Motor
• The design of the Brush DC Motor is quite simple, in that a permanent magnetic field is created in the by either of two means; permanent magnets or electro-magnetic windings
• If the field is created by permanent magnets, a Brush DC Motor is said to be a "permanent magnet DC motor" (PMDC). If created by electromagnetic windings, the DC Brush Motor is often said to be a "shunt wound Brush DC Motor" (SWDC). Today, because of cost-effectiveness and reliability, the PMDC motor is the motor of choice for applications involving a fractional horsepower Brush DC Motor, as well as most applications up to about 2.0 horsepower.
• Opposing the stator field is the armature field, which is generated by a changing electromagnetic flux coming from windings located on the rotor of the DC Brush Motor. The magnetic poles of the armature field will attempt to line up with the opposite magnetic poles generated by the stator field. Next, the section of the rotor where the electricity enters the rotor windings is called the commutator. The electricity is carried between the brush motor rotor and the stator by conductive graphite-copper brushes (mounted on the rotor) which contact rings on stator.
Imagine power is supplied:
A Brush DC Motor typically rotates toward the pole alignment point. Just as the DC Brush Motor would get to this point, the brushes jump across a gap in the stator rings. Momentum carries the brush 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 Brush DC Motor begins accelerating again, to the opposite set of poles. (The momentum has carried the brush motor past the original pole alignment point.) This continues as the brush motor rotates. Typically a Brush DC Motor has several sets of windings or permanent magnets present to smooth out the motion.
Speed Control for a Brush DC Motor
• Controlling the speed of a Brush DC Motor is simple. The higher the armature voltage, the faster the rotation. This relationship is linear to the Brush DC Motor's maximum speed.
• The maximum armature voltage which corresponds to the rated speed of a Brush DC Motor (a Brush DC Motor is usually given a rated speed and a maximum speed, such as 1750/2000 rpm), are available in certain standard voltages, which roughly increase in conjunction with horsepower.
• The smallest industrial-type Brush DC Motor is rated 90 VDC and 180 VDC. Larger units are rated at 250 VDC and even higher (dependent upon the individual manufacturer).
• Most industrial Brush DC Motors 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 comparable AC motor. This fact is in part due to the fact of the mere simplicity of control. However, it is also partly due to the fact that most industrial Brush DC Motors were designed with variable speed operations in mind. The addition of heat dissipation features / devices provided for lower operating speeds of Brush DC Motors.
• NOTE: Specialty Brush DC Motors for use in mobile applications are typically rated 12, 24, or 48 VDC. Other tiny brush motors can be rated as low as 5 VDC. This type of Brush DC Motor is very popular among hobbyists.
• In a Brush DC Motor, torque control is also easy to accomplish. Output torque is proportional to current. Therefore, if the current is limited, you have also limited the torque which the brush motor can achieve.
• This fact makes the Brush DC Motor ideal for delicate applications such as textile manufacturing.
• Simple and inexpensive control 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 Brush DC Motor requires little more than an appropriate potentiometer. In practice, these have been replaced for all but sub-fractional horsepower applications by the SCR and PWM drives (sometimes referred to as controls), which offer relatively precisely control voltage and current. Common controls for Brush DC Motors are available at the low-end of the product offering (up to 2 horsepower). The cost will depend on the accuracy requirement, but many brush motors can be accompanied with controllers covering a wide range of prices.
2. Disadvantages - Brush DC Motor
• A Brush DC Motor is less reliable in control at lowest speeds
• A Brush DC Motor is physically larger than other motors producing equivalent torque
• A Brush DC Motor is considered high-maintenance, which is not true of rushless DC motors
• A Brush DC Motor is vulnerable to dust which decreases performance
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