The classifications of DC motors commonly used to power miniature diaphragm pumps include Brush, Coreless, and Brushless.
Fundamentals of DC Motors
A DC motor is an electromechanical device that converts direct current (DC) into rotational mechanical energy. A basic understanding of motor design parameters is useful in understanding the overall pump assembly. The selection of the motor requires knowledge of the diaphragm pump performance requirements and constraints imposed by the intended application. Considerations include torque, RPMs, energy efficiency, life expectancy, reliability, noise, size, weight, and cost when selecting the best motor for the specific application.
In selecting a proper motor design, the pump designer considers the necessary speed and torque (torque constant x current) of the motor at the customer’s specified operating voltage to meet the specified performance conditions (flow at given pressure or vacuum). A factor of safety is applied with Hargraves designs to allow the customer to dead head a pump indefinitely and not damage the motor.
Speed Control
The speed of the electric DC motor will increase or decrease with varying voltage. One method is to use an analog control technique that can be accomplished using a variable voltage power supply or a power rheostat (variable resistor). This method is effective but relatively costly to implement and dissipates considerable energy in the power supply or rheostat.
A more efficient and less costly method to manage motor speed is to use digital controls through Pulse Width Modulation (PWM). PWM changes the effective voltage applied to a motor by interrupting the supply for brief periods of time.
Brush Motors
Hargraves offers a wide range of cost effective Brush DC Motor technologies that can be recommended for specific miniature diaphragm pump applications. For many fluidic systems, the brush DC motor provides a cost effective solution to power the diaphragm pump.
Brush Motor Fundamentals
The rotor of the motor, also called the armature, is made up of copper windings that will produce a magnetic field when energized with current. The current must pass through carbon brushes that slide over a set of copper surfaces called a commutator, which is mounted on the rotor. The commutator copper surfaces are soldered to the armature coils. Spring loaded carbon brushes slide over the commutator as the motor rotates, coming in contact with different segments of the commutator. The brush and commutator connection makes a sliding switch that energizes particular portions of the armature. This mechanical switching process, called commutation, creates north and south magnetic poles on the rotor that are attracted to or repelled by north and south poles by the permanent magnet on the stator. As the motor turns, the windings are constantly being energized in a different sequence so that the magnetic poles generated by the rotor do not overrun the poles generated in the stator. It's this magnetic attraction and repulsion that causes the rotor to rotate.
Carbon Brushes
As mentioned above, iron core brush motors typically use carbon brushes to conduct the electrical input from the lead wires to the motor’s commutator. The constant rubbing of the brushes on the commutator causes the brushes to wear down like the lead in a pencil. Brush motors are designed to last from 500 hours to 6,000 hours, depending on the quality of the motor and how it is used. Some applications only require 500 hours of operation life, so a reliable 1,000 hour motor maybe the appropriate choice.
Brush motors that experience frequent on/off cycles per day wear out more quickly, as the brushes experience an electrical arcing upon each start up. The frequent arcing heats up the carbon brushes, causing them to wear more rapidly. A top quality brush motor can be expected to last 3,000 hours with frequent on/off cycles.
Brush motors used in high duty applications with more continuous operation can last longer. A top quality brush motor can run continuously for up to 6,000 hours. It must be stated that few applications allow a pump to run continuously. Frequent starts and stops are the norm. Occasional cycling may lead to motor stall due to carbon dust build up between the brush base and commutator. Tapping the outer housing to clear these deposit from the brush tips can usually restart the motor.
Coreless Brush Motors
Coreless motor technology differs from the standard brush motor in that the winding is wound onto itself on the rotor. The brushes are made from a highly conductive and efficient precious metal. No iron is on the rotor, making the lighter, coreless (or ironless core) rotor spin at a given performance level with less required input energy. This results in lower current draw required to power the respective diaphragm pump. As a result, coreless motors are commonly used in portable, battery-operated systems.
Advantages of Brush Motors:
Cost – Brush motors offer the lowest initial cost DC motor option. Motor cost is a major factor in the selling price of the pump.
Energy efficiency – the coreless class of brush motor is the most efficient motor section. Brush motors in general are more energy efficient than brushless motors.
Disadvantages of Brush Motors:
Life expectancy - Low to moderate life expectancy based on brush wear
RFI noise – brush motors will introduce electrical noise into a system. Coreless motors generate less electrical noise than std. brush motors.
Contamination – The particulates generated by brush wear must go somewhere, usually inside of customers system. For applications requiring the highest quality levels, we select motors with fully enclosed brushes to minimize dust contaminants.
Brushless Motors
Hargraves Brushless DC Motors (BLDC) are designed to endure long life applications that under certain conditions exceed 20,000 hours of operation. The Hargraves engineering team took the initiative to improve standard motor designs and manufacturing processes to optimize brushless motor technology with regards to performance, reliability and endurance. We offer our BLDC motor in both a “Short Stack" and a “Long Stack" configuration. The Long Stack motor consists of magnets and windings that are twice the length, and is used for exceptionally high load applications.
Brushless Motor Fundamentals
Brushless motors, as the name implies, do not use brushes for commutation. Instead, they are electronically commutated. The stator consists of stacked steel laminations that are axially cut along the inner periphery. Numerous coils are interconnected to form each winding. An even number of magnetic poles are produced from each of these windings that are distributed over the stator periphery. The rotor is a permanent magnet with alternating magnetic poles built in.
To rotate a DC motor, the windings need to be energized in sequence. The position of the rotor will determine which winding will need to be energized. Unlike brush motors that commutate by mechanical switching, brushless motors position using an electronic sensorless method or by utilizing Hall effect sensors. These sensors are embedded on the stationary part of the motor and monitor the exact position on the rotor’s position. As the rotor’s magnetic poles rotate near the Hall effect sensors, they give a high or low signal that signifies a north or south pole is passing near the sensors. The exact sequence of commutation can be determined from the combination of the three signals the sensors are reading. Each commutation sequence has current entering one of the windings resulting in positive magnetic flow, current exiting a second winding resulting in a negative magnetic flow, and a third winding that would be non-energized. The direction of the magnetic flow would affect the winding’s polarity. The interaction of these changing poles of the magnetic field between the stator and the permanent motor produces the torque that turns the motor’s shaft.
As the requirements for miniature diaphragm pumps to fit in even smaller package envelopes, the motors powering these systems have had to become smaller. These smaller brushless motor sizes will most likely use sensorless commutation methods since Hall effect sensors take up more space. It should be noted that sensorless motors are limited to the initial startup torque they can produce. Brushless DC motor applications that require restarts under a high load will need to utilize Hall effect sensors with an instant start capability. Therefore, to achieve very small motor size, there may need to be a tradeoff on restart capabilities.
Advantages of Hargraves BLDC Motor:
Life expectancy – The innovative design and proprietary manufacturing processes contribute to the Hargraves Brushless DC Motor being able to operate under demanding conditions up to 20,000 hours.
Cost – In producing our own motor, Hargraves controls the cost and quality.
Low mass – The Hargraves BLDC motor is a unique compact, space and weight saving design.
Compact, integral commutation circuit - The Hargraves design has a broader voltage control range that facilitates greater operational flexibility. In addition to voltage surge suppression, reverse polarity protection is built in so that the motor cannot be damaged by improperly connecting the lead wires.
Reliable restarts under high loads -The Hargraves BLDC motor is a higher torque design than comparable motors, allowing it to reliably re-start under higher loads than expected.
Broader operating temperature range -The commutation circuit is designed to operate in a 110 ° C maximum ambient temperature, allowing the pump to operate in a broader temperature range. A special high temperature lubricant is used to maintain proper lubrication in bearings at elevated temperatures. Although the standard operating temperature range is 5 – 50 ° C, the BTC pumps with the Hargraves BLDC motor can operate in –30 ° C to 70 ° C range with a 50% pump duty.
Low RFI emission - The removal of the brushes eliminates the introduction of RFI noise into the system circuitry.
Hargraves Technology, www.hargravesfluidics.com, designs and produces high performance, long life and high efficiency DC brush and brushless motors for miniature diaphragm pumps, vacuum pumps and micro compressors. For more information on DC motor technologies and their application in powering miniature diaphragm pumps, please visit Hargraves Advanced Brushless DC Motors for Diaphragm Pumps. |
