Richard Green
Senior Engineer
Pittman
Harleysville, Pa.
Selecting dc motors usually requires consulting the manufacturers’ catalogs and matching published parameters to the application. Though this is generally the right way to go, catalog parameters are usually nominal values and can lead to a motor that’s only marginally suited for a job. Sometimes these values can compromise motor reliability or life. But there is usually some leeway in picking motor parameters. So take advantage of the possibilities and match performance characteristics that are most critical to a specific machine, such as speed for one or torque for another.
Regardless of application, operating voltage is one parameter most often overlooked. It’s the first one that must be examined to get a motor into the right ball park, but is usually ignored after that. But applying a voltage less than the rated value makes the motor run slower — which can extend brush life by more than 20%. Less voltage and lower motor speed also increase gear life, reduce audible noise, and decrease EMI while keeping efficiency up.
Analyzing factors that comprise the voltage equation can lead to some other considerations that increase motor life. For example, applied voltage is:
where Tf = friction torque, oz-in.; Sm = motor speed, rpm; Df = viscous damping factor; Tl = load torque, oz-in.; r = gearhead reduction ratio; E = gearhead efficiency, %; Kt = torque constant, Rt = terminal resistance, ohms; and Ke = back-EMF constant.
Consider Va as the applied voltage in an openloop system, but the minimum voltage in a closedloop system. The open-loop system may require a nonstandard power supply, usually containing an adjustable voltage regulator, to maintain the applied voltage. Changes in loads affect motor speed in open-loop systems, so give this type of system consideration when tight speed control is not required.
In a closed-loop design, however, use a standard power supply because the control system adjusts voltage to regulate speed. The system’s feedback device, typically an encoder, sends position data to the controller, which derives velocity and acceleration. During operation, the controller continuously monitors motor speed, calculates the difference between actual and desired parameters, and sends modified error signals to the voltage amplifier, which adjusts the motor speed.
Although low applied voltage should be considered for new designs, it may not benefit all motors. For a particular application and selected group of motors, compare the motor speed as well as the current and gearhead-reduction ratio at identical operating points. Usually, the lowest speed indicates the motor that offers the longest gear and brush life. This guideline applies to motors operating at speeds above 1,000 rpm. At lower speeds, however, dust from motor brushes can quickly accumulate in commutator slots and electrically short across two terminals. Ideally, motor speeds should be as low as possible to increase gear and brush life while high enough for centrifugal force to remove the dust from the commutator slots.
Next to motor speed, low current translates into longer brush life. When considering tradeoffs, higher currents often can reduce EMI. For the quietest motor in terms of audible noise, look for the lowest gearhead reduction ratio.
These benefits may be found in one motor, or several motors offering one or two advantages. In the latter case, decide which benefit is most important to the application and choose the motor accordingly.
To compare motors at a single operating point, use a spreadsheet program such as that available from Pittman simulating motor performance when supplied with nominal values and an applied voltage. For example, in a programmed simulation using 100 datapoints, motor speed is:
where Sn = new speed, rpm; Sn-1 = speed at the previous data point, rpm; S0 = no load speed, rpm; Va = applied voltage, Vdc; and V = rated voltage, Vdc. Gearmotor current In is:
where Tf = gearmotor torque at the current data point, oz-in.
Consider an application that requires a motor supplying 10 oz-in. of torque at 300-rpm output speed. Three gearmotors are found to meet the requirements. The first gearmotor has a gearhead reduction ratio of 30.9:1, rated at 24 V, and runs at an applied voltage of 22.6 V to obtain the proper load point, as shown in the table. The second and third gearmotors both have a gearhead reduction ratio of 6.3:1. The rated and applied voltages for the second motor are 24 and 6.9 V, respectively. The third motor is rated at 48 V and runs at 13.8 V.
Of the three motors, the first one appears to be an obvious choice because the application comes closest to rated specifications — running a 24-V motor at 22.6 V to meet the operating requirement. But what is needed and not evident from specifications and catalog sheets comes from simulations that reveal the current in this motor is the lowest of the three, which helps extend brush life. On the other hand, the simulations reveal that the motor runs at 9,270 rpm, which is much too fast for extending life.
With voltages reduced from rated levels, the second and third motors offer similar speeds and gear life. At 1,890 rpm, these motors also afford about 25% longer brush life than the first motor. Additionally, the second and third motors operate with nearly identical efficiency. However, because the third motor operates at half the current, its brush life will be significantly longer. By contrast, the higher current in the second motor reduces EMI. Thus, select one of the two motors for either long brush life or low EMI.