3
\$\begingroup\$

As the title says, what should I expect a DC brushed motor's current to look like over time?

I'm using a standard 12V DC brushed motor in a long-life application, and I've got a current sensor constantly monitoring it. I was wondering if I would be able to predict the wear on the motor from the current.

Does anyone know of any papers that have this information? I searched extensively online, but I couldn't find any data on this.

\$\endgroup\$
  • 1
    \$\begingroup\$ Changing the brush is part of the DC brushed motor's maintenance, if you cannot do that you should reconsider the choice of the motor. \$\endgroup\$ – MathieuL Jul 4 '16 at 15:47
  • \$\begingroup\$ @MathieuL Unfortunately I don't have control over the type of motor. This motor is pre-built into a pump housing. \$\endgroup\$ – 0xDBFB7 Jul 4 '16 at 16:06
  • \$\begingroup\$ It'll depend on brush wear to some extent, but it'll also depend on all sources of friction including bearing wear, lubrication and corrosion issues.Prediction? probably not. But a steady increase in current could well indicate friction issues to recommend preventative maintenance. \$\endgroup\$ – Brian Drummond Jul 4 '16 at 16:30
  • \$\begingroup\$ One company i worked at used DC brushed pumps. Then they switched to stepper pumps, and all problems around them were gone instantly. \$\endgroup\$ – Gregory Kornblum Jan 6 '17 at 20:28
  • \$\begingroup\$ @GregoryKornblum Very interesting...do you mean brushless pumps or literal stepper motors?. \$\endgroup\$ – 0xDBFB7 Jan 7 '17 at 20:26
5
\$\begingroup\$

The current taken by the motor will be the (torque required to turn the load at the speed the motor is running at) divided by (the field strength of the motor).

Temperature will affect the motor in several ways. Assuming ferrite magnets, their output drops slightly with temperature, 0.2%/degreeC for one grade I've looked up. This will increase current slightly with temperature. However, the windings will increase in resistance (0.4%/degreeC assuming copper), which will reduce the speed, which may reduce the load torque required, reducing the current. Either may win for your particular motor and load. However, the effect with temperature is expected to be small.

The motor bearings will be expected to wear, and as a result, friction should rise, increasing the current drawn. The effect may be small compared to the load, and may be difficult to detect, right up to bearing seizure.

While wear on the commutator and brushes will affect speed and therefore load torque slightly, the largest variable in the conductivity of that system is the surface condition of the commutator segments, which can be very different from each other without indicating any problem. The wear here is unlikely to be detectable reliably compared to the load and the normal variability, until it is catastrophic.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.