Brush-commutated dc motors

July 1, 2002
Things you need to know about sizing and applying.

How they work

Permanent magnet direct current (dc) motors convert electrical energy into mechanical energy through the interaction of two magnetic fields. One field is produced by a permanent magnet assembly; the other by an electrical current flowing in the motor windings. Pushing against each other, the fields produce a torque that tends to rotate the rotor. As the rotor turns, the current in the windings is synchronously switched (commutated) to maintain continuous torque output.

Brush dc motors can be operated over a wide range of voltages, speeds, and loads. Output power is a product of speed and torque; input power is a product of applied voltage and motor current.

General sizing procedures

The first step in motor selection is deciding whether or not you need a gearbox. This will typically depend on the maximum required load speed. If speeds are below 1,000 rpm, it’s a good idea to use a gearmotor. If speeds exceed 1,000 rpm, a motor by itself should work fine.

To select a gearbox, start by considering the torque requirements of your application. Gearboxes are usually rated by their maximum allowable output (load) torque. Once you’ve chosen a specific type, you need to select the appropriate ratio. The easiest way to do this is to divide the maximum acceptable (gearbox) input speed by the maximum desired output (load) speed, then select the closest available ratio. Acceptable gearbox input speeds vary, but are typically on the order of 6,000 rpm.

The next step is to calculate motor speed and torque requirements using the following equations:

WM = WL × N and TM = TL/(N × n)

where WM = motor output speed
WL = load speed
N = gear ratio
TM = motor output torque
TL = load torque
n = gearbox efficiency

Once the motor requirements have been determined, choose a motor type and frame size capable of producing the required motor torque. For continuous operation, select a motor with a continuous torque rating greater than that of the required torque. For intermittent operation with a sufficiently short on-time, select a motor with a continuous torque greater than that of the rms value of the required torque.

Motor makers specify continuous torque based on certain operating conditions, including ambient temperature (often 25 or 40°C) and thermal resistance (heat sink, etc.) Be sure to read the fine print when comparing continuous torque ratings as they may need to be adjusted if these assumptions do not match your actual operating conditions.

After choosing a frame size, you need to specify the proper windings. Voltage and torque are generally known; speed and current need to be determined. The best winding choice will be that which comes closest to providing the desired speed and current draw given the supply voltage and load torque. The governing motor equations to determine speed and current follow:

W = (Vs - I × Rmt)/KE and I = TL/KT + INL

where W = speed
Vs = supply voltage
I = current
Rmt = motor terminal resistance
KE = back emf constant
T = load torque
KT = torque constant
INL = no-load current

While these equations are suitable for most applications, it is important to realize that they are only the basic formula and do not account for thermal considerations. Motor heating will alter some of the parameters in these calculations, including resistance, torque constant, and back-emf constant. Accounting for these effects adds more complexity to the process.

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Related components

“Off-the-shelf” brush-commutated dc motors are the exception, rather than the rule, and they are frequently customized to meet specific design and performance criteria for an application. Among those components typically specified:

Optical encoders
Because closed-loop servo applications require velocity and/or position feedback, common motor options include incremental optical encoders, which supply accurate position, velocity, acceleration, and direction feedback for precision motion control. Encoders can be added to any motor or gearmotor with wires or side-exiting power terminals and can be metalhoused or open air. They can be factory-mounted or prepared for mounting in the final stages of endproduct assembly. Encoders are usually specified with either two or three-channel, TTL-compatible quadrature outputs. The maximum frequency, which limits the maximum operational speed, is typically 100 kHz. In a three-channel unit, the third channel provides an index signal or pulse once per revolution of the codewheel.

Motor shafts can be customized as needed, incorporating flats, journals, cross holes, keyways, slots, grooves, gears, and/or pulleys. In many cases, several options are combined to meet application requirements; for example, a cross hole can allow a pulley to be pinned to the shaft or a journal can include a groove. Shaft material, typically 416 stainless steel, can be made from other grades of steel, such as 303 and 316 stainless with different hardness ratings. Standard and common optional shaft diameters include a variety of sizes from 4 to 8 mm and from 5/32 to 3/8 - in.

Gearheads are typically used to increase output torque and decrease speed. Though there are several types — varying in efficiency and output range — spur gearheads meet the needs of most applications in terms of torque, noise, and cost. Standard spur gearheads feature sintered nickel-steel gears, which provide moderate power handling with average audible noise. The sintering process allows for close tolerances (AGMA Q7-8) at a low cost. The sintered gear functions as a lubrication holder and helps dampen sound. For more strength, a hybrid cluster (an assembly of a cut-steel pinion and a sintered gear) or precisioncut steel gears may be chosen.

Other gearhead options include planetary gearheads for lower backlash and much higher torque or Delrin (moldable polymer) gears that produce less noise than sintered gears.

Wire and cable assemblies
Custom wire and cable assembly options are designed to speed motor installation and boost component reliability. Almost any connector style and wire type can be specified for motors, gearmotors, and encoders.

EMI/RFI suppression components
A number of cast and stamped component solutions have been developed to reduce the amount of electrical noise generated by a motor. For low-frequency RFI (typically below 30 MHz) capacitors are generally effective, with an inverse relationship between the value of the capacitor and the attenuated noise frequency. Capacitors installed by the motor manufacturer enable strategic placement inside the motor frame for optimum filtering as close to the noise source as possible.

For high-frequency noise (generally above 30 MHz) ferrite beads can help reduce RFI. A combination of ferrite beads and capacitors provides the most effective suppression by creating a low-pass LC filter that is inductive-capacitive at low frequencies and dissipative at high frequencies. Mounting for each component may vary from slipping ferrite beads over wires to soldering chokes near the motor terminals.

Developed as a safety and energy- saving feature, rear-mounted power-off and power-on electromagnetic brakes prevent a motor or gearmotor from rotating freely. Brakes typically are offered for 16 and 48 oz-in. static torques and 12, 24, 28, 48, and 90-Vdc operation, although other voltages, including 120 Vac, are available.

A power-off brake stops a motor when power is removed and releases the motor when power is re-applied. In low-duty applications, the brake saves energy by maintaining a known motor position without power. An added safety feature is that if power is lost while the motor is lifting an object by pulley or lead screw, the brake will lock the motor and prevent the object from falling. A power-on brake holds the motor in place upon application of power and releases the motor when power is removed.

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Dealing with extremes

When brush-commutated dc motors are used to drive gears or pulleys, it is imperative to avoid excessive side loads. These can push a motor to extremes and lead to failure. If side loads will be present, ball bearings are usually recommended.

Environmental conditions also influence operation and performance. For example, moisture in the air acts as a lubricant and, where humidity is low, the resulting lower lubrication will accelerate brush wear and shorten motor life. (Special brushes are designed to solve this problem.)

Installation and troubleshooting tips

• Know the proper rating of the motor for an application and recognize and understand the importance of continuous operation vs. duty cycle.

• Do not press fit components on a motor’s shaft (in any direction) without proper support at the other end of the shaft. This action could lead to motor failure.

• Do not apply adhesives — or other foreign material that could contaminate bearings — directly to shafts. This could negatively influence performance. If such materials must be used, it is generally advised to apply them to the component to be secured to the shaft. This will reduce the risk of contamination.

• Consult with your motor manufacturer before, during, and after a motor is specified for an application.

Cost-saving strategies

Users can save money (and headaches) at the outset by partnering with a motor manufacturer from the beginning of the design stage. This will minimize (and likely eliminate) costly mistakes and ensure that a motor performs as intended and required in an application. Early involvement also can open a window to available motor features and options, which could help initially to reduce labor and handling costs and provide for easier installation.

Failure modes

The primary cause for failure of brush-commutated dc motors over time is ongoing brush wear.

The traditional method for mounting copper or silver graphite brushes in motor assemblies has been to solder the brushes onto standard cantilever springs to enable the required constant contact with the commutator. This conventional spring design, however, carries inherent drawbacks as force levels diminish over time, and motor failure can result.

One solution is to house brushes within a specially designed cartridge and use torsion springs to ensure desired even force over the life of the motor.

The cartridge, which fits into the motor base, consists of a twopiece, high-temperature plastic, snap-together assembly in which each of two brushes is seated securely within its own specially constructed slot. This design effectively restricts the brushes to traveling in a track in a desired linear motion.

The cartridge design further provides for an ideal region of pressure (6 to 8 lb psi) for the brushes to withstand the detrimental effects of mechanical wear.

Other common causes of motor failure include motor overloading, contamination of the armature, and electrical or mechanical malfunctions. There are many others, depending on motor design, operating parameters, and in-use service and safeguards.

Standards and regulations

NEMA publications are the most relevant sources for standards relating to traditional motor products and devices. Other standards include ANSI and IEC for rotating machinery, as well as IEEE standards for motor-related test procedures.

In addition to product standards, a set of quality-oriented standards also applies to motor suppliers. Manufacturers that have achieved ISO certification demonstrate documented adherence to procedures and operations consistent with international quality standards.

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