Choosing a DC motor

Inherent problems of DC motors such as noise andEMI can be mitigated if the EE chooses wisely

BY TED SEVERN
MicroMo Electronics
Clearwater, FL
http://www.micromo.com

Manufacturers of brush-type DC motors typicallyoffer a wide variety of features that may either meet or clash withthe needs of your design. Here are some general notes on theapplication of DC motors and some general guidelines for care andhandling of precision motors.

Audible noise

Audible noise is a concern in some types ofmotor applications. Good design practice requires that the noise belimited as much as possible.

In large machines, the combination of hundredsof DC motors and gears operating simultaneously can be very loudand distracting to the employees who have to work in closeproximity to the machine. Probably the best method of ensuring lowaudible noise in motors is to specify quality components.

Motors using cheap or poorly fitted bearings aremore likely to be noisy. Poorly designed or loose fitting brushsets can contribute to audibly noisy commutation. The designer mustconsider whether low cost takes precedence over quiet operation inthe priorities of the customer.

Bearing choice

The use of ball bearings without preload is apotential source of audible noise. Where the specific applicationpermits, ball bearings should be preloaded.

This means that the balls will not be able tomove axially in the race and cause the minute intermittent rattlingthat can sometimes be associated with ball bearings that have notbeen preloaded. Smaller ball bearings can be sensitive to heavyshaft loads.

They are easily damaged when press-fitting addedcomponents and by short radial or axial overloads. Care should betaken not to exceed the shaft loading ranges specified in thedatasheets. A damaged ball bearing can be a significant source ofaudible noise and can affect motor life.

Sintered sleeve bearings are a very good choicewhen limiting audible noise if the application does not require themotor to endure significant continuous or intermittent changes inthe shaft loading characteristics. If the bearing is overloadedradially, however, this film breaks down and the shaft will grindthe bearing down causing audible noise and reducing the operationallifetime of the motor.

Vibration

Rotor vibration can be a significant source ofaudible noise. Vibration and noise increases with speed. Even aslight imbalance in the rotor can cause major vibrations at speedsof 10,000 rpm.

Brush options

Copper-graphite-type brushes tend to be bothaudibly and electrically noisier than precious metal systems.Graphite-based brushes are capable of withstanding considerablyhigher current densities, however, and they are often required inan application for that reason. Where a choice is available for aspecific application and audible noise is important, precious metalbrushes are the better choice.

Electromagnetic interference

DC motors can also be a source of electricalnoise. Electromagnetic interference (EMI) can be radiated by motorterminals and lead wires and may cause problems with othercomponents in the vicinity of the motor.

It is also possible for spikes to be coupledonto data lines or output lines from encoders. The result can befalse data or encoder information.

There are a number of methods that can be usedto minimize EMI in motor applications. Like many otherconsiderations in DC motor applications, each has its advantagesand disadvantages and must be evaluated within the context of theapplication.

Motor commutation

The most common source of EMI problems is thecommutation of motors. At each commutation point, when the brushbreaks contact with a commutator segment, the energy stored in themotor winding as a magnetic field causes an arc or voltage spikebetween the brush and the commutator segment. This occurs not onlyduring normal commutation, but also when the brushes“bounce” on the rotating commutator.

Lead wires and shielding

Motor lead wires should be placed as closetogether as possible so that EMI radiated from the two leads cancancel each other. This canceling effect can be improved by usingso-called “twisted pairs,” where the positiveand negative lead wires are twisted together.

Motor leads should be physically separated fromdata lines or encoder outputs to reduce the possibility of couplingmotor noise onto them. This means that when using shieldedpigtails, the feedback lines should be shielded separately from themotor leads. If noise is still causing problems with the encodersignals, it may be time to consider using a differential encoder toeliminate the effect of interference on the encoder.Pulse-width-modulated (PWM) switching noise is another source ofEMI problems. Most commonly, PWM switching results in radiatednoise from motor lead wires. Shielding and lead wire placement canalso help mitigate the effect of PWM generated EMI.

Passive components

EMI problems can often be significantly reducedby the simple act of installing a capacitor across the motorterminals. In some types of applications, this method may not besuitable since a resonant circuit is created, which can cause“ringing” problems near resonantfrequencies.

In these instances, an RC snubber network acrossthe motor terminals may be more effective. Component values are notcritical for motors driven with DC, but care must be taken inselecting components for PWM driven systems.

DC motor selection is made easier if the EEknows about audible noise, EMI, reliability, and a number of othercharacteristics.

Operating point

In most applications, the torque and speeddemands placed upon a DC motor determine its overall operationallifetime. As the torque requirements on the motor increase, thecurrent through the armature increases proportionally, thusincreasing the current density at the brush-commutatorinterface.

High current densities promote electro-erosionof brush and commutator materials, a limiting factor in motorservice life. In addition, high rotational speeds shorten motorservice life by accelerating mechanical wear.

Although each application has its own specificrequirements to be addressed, it is usually advisable to operate aDC motor with precious metal brushes and commutator continuously atno more than one-third of its rated stall torque. Motors withgraphite-on-copper commutation systems should be run continuouslyat no more than 1/2 of the motor’s rated stalltorque.

Drive profiles

Operating conditions other than torque and speedalso affect service life. The application may require frequentstarting and stopping or reversals of direction.

Both situations result in periods of highcurrent density and a resulting shortening of service life. Asimilar effect is seen in applications where PWM drives are used.If the PWM frequency is too low, the motor is constantlyaccelerating and decelerating with an accompanying increase incurrent density.

The motor’s environment

Environmental conditions can have a profoundeffect on motor service life. One good example is the rapid dryingand wear of graphite-based brushes in a vacuum or very dryenvironment.

Very warm and dry conditions also hasten thebreakdown of bearing and commutator lubricants. The ambienttemperature has a cumulative effect on the motor’soperational temperature and can lower performance by limiting theoperational temperature range of the motor.

External cooling by contact, air, or forced aircan produce significant gains in motor performance. At the oppositeextreme, very cold conditions increase the viscosity of lubricantsand cause the motor to run at a higher current.

Shaft loading

Excessive axial or radial shaft loads decreasethe life of bearings, sometimes significantly. In continuous dutyapplications with low radial shaft loads, sintered bearings are theinexpensive choice. Ball bearings are typically specified when theapplication calls for higher radial and axial shaft loads.

Quality and workmanship

Lifetime performance is enhanced considerably byhigh standards of workmanship and quality. ISO certification andprecise component design and assembly all help to maximize productlifetimes and value to the customer. ■