CNC Spindle & Stepper Motor Load Meter – power meter
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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. I hope you enjoyed this video. It has taken
the longest of any of the videos I’ve made to date. If you liked the video
please hit the like button and consider subscribing. More videos are coming. Until
the next video, thank you for watching.

5 thoughts on “CNC Spindle & Stepper Motor Load Meter – power meter

  1. Good video. Im trying to work through these calculations too, I like the meters idea, think I'll get some of them too. Thanks

  2. Awesome video and great explanation! I've been looking for a power meter like that to estimate forces on a CNC router. Those meters are surprisingly accurate – thanks! Too bad they don't have any sort of external interface. This other approach looks too hard and expensive http://www.ti.com/lit/an/slyt545/slyt545.pdf

    It appears that your high speed drilling example (2600 RPM? and IPR?) stalled due to inadequate spindle power. Is that right?
    Are speed changes on that spindle accomplished via drive belt pulley changes or the motor controller?

  3. If Main Spindle Power of CNC1 is 5.5/7.5 kW and CNC2 is 9/11 kW, a same machining job is performed on both the machines, will the power(energy) consumed by both the machines will be same ? Or if different, then approximately how much difference ?

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