In this video I'd like to describe the use of power meters for measuring load
on the spindle and stepper motors of a CNC mill.
This is an update of a previous video where I used a current and voltmeter to
estimate the power. The previous approach also measured apparent power instead of
actual power. To understand the difference will have to examine some
definitions. Mechanical power is measured in horsepower while electrical power is
measured in watts. One horsepower is defined to 746 watts. To measure the
power delivered by an electrical motor we'll measure its consumption of watts.
Power going out can't be greater than power going in and we'll estimate the
load on the motor as a ratio of its delivered to its rated horsepower.
Unfortunately since there'relosses in the motor, belts, pulleys, bearings, and so on,
the system is not 100% efficient and not all supplied power
will be delivered at the spindle. Electrical power can be divided into
actual, apparent, and reactive power. Actual power is what the electrical
company charges for and, over time, measures the work done.
Actual power is measured in watts.
Reactive power is power that does no work. It is power borrowed from the
electric company to charge electric motors coils and it's later returned.
Apparent power is the total power delivered
including actual and reactive power. Apparent power is measured in KVA or
kilovolt-amp and is used to size wires and circuit breakers.
Unlike watts, basic multimeters can be used to calculate kilovolt amps by
measuring and multiplying RMS voltage and current. To measure watts a
specialized meter is required.
The ratio between actual and apparent power, or the ratio between useful and
total power, is called the power factor.
Ideally, a load would have a power factor of one. Where all delivered power is used.
This is ideal because of benefits for production and distribution electrical
power. As the motor's load increases its power factor increases. The motor becomes
more efficient to a point. For a milling machine we can use load as an estimate
of reserve capacity, and to guide us to take heavier or lighter cuts.
While our previous meters behaved like a multimeter, where we measure current and
voltage separate, and multiply them together to produce apparent power,
our new meter is able to measure these plus actual power, in terms of watts, and
power factor.
These meters are produced by Murata and consists of a panel interface with a
selection button. On the back, there is a current transformer and screw terminals
to attach to the mains. Current is measured by induction from a wire that passes
through the current transformer, while voltage is measured at the screw
terminals. A microprocessor inside of the power meter is taking into consideration
the phase difference between the voltage and current waveforms. From this it is able
to measure watts and power factor. The top meter will measure the spindle motor
while the bottom meter will measure the stepper motors. Both meters are fused with
an external half amp fuse.
There is no internal fusing. Here the spindle motor is a DC motor and the
current is measured as its passed to the driver. Power delivered to the stepper
motors is measured as power delivered to their switch mode power supply. A small
sub panel for these meters was cut from wood on the CNC mill. This graph measures
watts and the volt-amps delivered to the spindle motor as various RPMs.
It also shows the power factor as the RPMs are changed. The spindle was turning
a half inch drill bit at the time.
A repeat of these measurements without any tool inserted into the spindle showed no
difference.
This graph might be useful for describing the power necessary to simply
turn the spindle. In that case, at 2,600 RPM, approximately one-tenth of a
horse power is necessary simply to turn the spindle. For most of this graph the
power factor is between 0.5 and 0.6.
However, under significant load, the power factor will increase to
0.7 to 0.8. If the power factor remained within this tight band between 0.5 and 0.6,
it may be reasonable to use a current meter as an indication of load.
However using a power meter removes all doubt by removing the reactive component
of apparent power.
While a power meter may be a better approach for measuring power, and thus
load, over a current meter, in practice, as we'll see,
they each have their advantages.
The milling machine that the power meter is attached to is a modified Precision
Matthews 25-MV (PM25MV). The spindle motor is a one horsepower DC motor. The maximum
speed of the spindle is 2600 RPM. The advertised drilling capacity is one inch,
but to drill 1 inch without step drilling is overly optimistic. In some
CNC mills the drilling capacity is not limited by the spindle, but instead
by the thrust capacity of the axis motor. With this CNC mill, horsepower delivered
to the spindle is the limiting factor. An advantage of the current meter over the
power meter is faster response time. The power meter is only update every second
or so. In this case, the spindle motor was not able to deliver the necessary
horsepower to drill the hole and ultimately stalled. The current meter
spiked to 15 Amp or 1,800 volt-amps of apparent power at 120 volts. The power meter
spiked to approximately a thousand watts.
I had hoped to use a power meter as an indication that the spindle is about to
stall, but the slow response time of the power meter may not allow this.
The combined power consumed by the three stepper motors is rarely above 70 watts.
I found this to be surprisingly low and if it turns out to be representative there
may have been no need to install power meter for the stepper motors. The second
drilling test uses a slower spindle speed and feed rate.
While the spindle is under considerably less load this is not an efficient
drilling operation. The stringy chips in addition to being dangerous are
representative of the feed rate that is too low.
We can estimate the horsepower necessary to drill a half-inch hole in
the aluminum at 1000 RPM. These equations and constants can be found in
the Machinery's Handbook.
While this method has been derived from the experiences of drill operators over
decades,
it is important to recognize this is still an estimate. We'll first estimate
torque necessary, then power necessary of the cutter, and then power necessary at
the motor. The constant of 63,025 is the torque necessary to produce one
horsepower in one revolution. And one horsepower was originally defined as the
energy required to lift 33,000 pounds one foot in one minute.
It was an estimate derived from experience with horses.
Needless to say the SI units for power are a lot cleaner. Inserting the constants into
the equations, we arrive at 11.3 inch-pounds of torque required.
This is 0.179 horsepower at the cutter and assuming an efficiency of
eighty-five percent 0.211 horsepower at the motor. Given a horsepower is 746 watts,
157 watts are necessary to drill a half-inch hole in aluminum at
1000 RPM.
On this mill, 35 watts are necessary to spin a spindle without a tool.
This is the spindle's overhead. Adding 35 watts to the 157 estimated watts,
we arrive at 192 watts of estimated power necessary to
drill a half-inch hole in aluminum at 1,000 RPM. A rough average of the
measured power is 200 watts.
These two values are surprisingly close and this experiment will be repeated to
verify consistency between estimated and actual horse power requirements.
To make measurements of power truly useful they need to be captured by the
control computer. And unfortunately these meters have no means to feedback
information to the computer. Because of the slow update rate of these meters,
they may be more useful for steady state operations than as an early indication of
spindle stall.
I have a few other ideas of how these meters might be useful and if those ideas
work out I'll make videos on those topics as well. If you have ideas for
experiments or noticed a mistake that I made
please make a comment below.
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