measuring voltage on dc motor?

Brandon,

Can you provide a link to the spec sheet you are looking at? I don't recall seeing a specification called "measuring voltage" anywhere. Could be something specific to that particular manufacturer.

In the meantime, here is a link to some good info regarding dc motor specs:

Richard

"Brandon"

** Almost certainly means the DC test voltage that was used to find the other figures that are quoted.

Like the rpm v. torque & power output curve.

....... Phil

specs for a motor and one of the things it lists is

specs:

Here the spec sheet I'm referring to:

specs:

--
There are three different motors; one rated for 6 volts, one rated
for 12, and one for 24.

The columns under the voltages specify how that particular motor
will perform with that voltage connected across it.

Hmmm, well I'm still a little confused, but maybe I just don't understand enough about motors. Take a look at this spec sheet from the same company.

Consider the 36V motor and the torque vs. speed chart in the lower right hand corner. If I get a no-load speed of 7200 rpm at the rated voltage of 36V, then how is it possible to get up to 12000 rpm (which is the maximum continuous speed according to the the chart) without pushing the voltage higher than the rated voltage???

--
What I meant by "rated voltage" was that at that voltage the
characteristics of the motor were described in the column under the
stated voltage.

Perhaps stating that with the voltage measured at what is called the
"measuring voltage" (the voltage at which the measurements were
made) on the data sheet would have been clearer.

As I see it, the chart only shows the torque and speed limits for
any of the motors with 3.8 watts out of the shaft.

The figure of 36V is the voltage at which the measurements in the table were obtained, not the maximum voltage (for which they don't give a figure).

In general, motors are limited by current rather than by voltage. The limiting factor is usually the amount of current the armature windings can take (the amount of flux required to saturate the core and/or demagnetise the permanent magnets can sometimes be a factor, but these are also dependent upon the current).

When spinning, a motor generates back-EMF which reduces the effective voltage across the windings. The back-EMF is proportional to the speed.

[The datasheet quotes a Back-EMF constant for each motor, in V/1000RPM. For the -105, the figure is 4.95 V/1000RPM, which is where the figure of 7200RPM no-load speed at 36V comes from. At 7200RPM, the back-EMF will be 4.95*7.2 ~= 36V, i.e. the back-EMF cancels out the applied voltage, resulting in zero torque and hence zero acceleration.

To get 12000RPM, you would need to apply 4.95*12 ~= 60V.]

A consequence of the increase of back-EMF with speed is that the RMS current will decrease as the speed increases. Or, from another perspective, as the motor speed increases, you can apply a higher voltage without exceeding the maximum current.

If the motor stalls, it stops generating back-EMF and the voltage ceases to alternate. In this situation, the windings' resistance becomes the limiting factor.

For small motors, this is usually high enough to adequately limit the current at typical operating voltages. E.g the datasheet quotes 200Ohms for the -150. With a maximum continuous current of 0.14A, you can apply up to 28V without having to worry about stalling the motor for an extended period.

For relatively simple uses, it's common to just apply a fixed voltage which is low enough that the maximum current is never exceeded. If you want to push the motor harder, you need to explicitly limit the current. E.g. if you want to apply 60V to push it to 12000RPM, you will need to start it off at a lower voltage and only increase the voltage as the speed increases.