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84 Frequently Asked Questions about Electric Motors




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This article is from the Woodworking FAQ Collection 3, by multiple authors.

84 Frequently Asked Questions about Electric Motors

From: roche@cs.rochester.edu (James Roche)
Newsgroups: rec.woodworking
Expires:
Frequency: monthly


This article answers many of the frequently asked questions about electric
motors.


Motors:

There are many kinds of motors, but this article considers only two
kinds used frequently in woodworking tool applications: universal AC/DC
motors and single-phase induction motors. Universal motors have brushes
and commutators and are used for portable tools like routers, skilsaws,
and electric drills. Single-phase induction motors have no brushes,
run only on AC electrical power, and are usually found on stationary
tools such as table saws, drill presses, planers, and jointers.
There are exceptions to this: some stationary tools use universal
motors.

Horsepower: Motor horsepower is the most misunderstood (and misused)
electric motor rating. Neither motor, universal or induction, produces
usable horsepower unless it is slowed down (by applied mechanical load)
from no-load speed. For induction motors, this slowdown is called
"slip", and the horsepower "developed" by a motor increases with slip
(to a simple approximation). This is why induction motors are
typically rated at 3450 rpm (two pole motor) or 1750 rpm (four pole
motor). The rating speed allows for slip from the "synchronous"
speeds of 3600 and 1800 rpm, respectively. Universal motors do not
have a synchronous speed, but have a maximum no-load speed that depends
upon the voltage applied to the motor.

Most motors can put out a lot more maximum horsepower than they can
sustain continuously. By forcing more mechanical load on the motor,
slowdown is increased and so therefore is the output horsepower.
Mechanically, horsepower is torque times rpm, and increasing the
mechanical load means that the rpm is slowed slightly and the drag
torque is increased to obtain more torque times rpm. Electrically,
horsepower is volts times amps, and by conservation of energy, the
mechanical output horsepower must be balanced by electrical input
horsepower. Since the voltage is relatively constant, this means that
as a motor is loaded, the input current increases. But the electrical
winding impedance has a resistive component, so that higher current
means more power dissipated in the windings. In fact, the motor
windings heat up proportional to the square of the motor current.
Except for specially designed motors, the current that a motor can
sustain continuously without burning out its windings is a fraction
of the current at maximum load.

Unscrupulous vendors sometimes publish maximum "developed" horsepower
to make their products seem more capable than they really are.
Developed horsepower may be two to five times the continuous duty
rating of a motor. Such products should be examined to discover the
continuous duty rating to compare with other, more conservatively
rated products.

When the talk is of developed horsepower, the meaning is "peak" which
for an induction motor is typically the local peak of the torque curve
near synchronous speed. A typical induction motor torque curve is:

          |
          |
          .                                            
          |.
          |  .                                        
   Dev.   _    .                                           .
          |      .                                    .      .
          |         .                               .         .
          |              .                        .            .
          |                       .             .
   Rated  _                              .                      .
          |
          |                                                      .
          |                                       
          |                                                       .
          |                                  
  Torque  |                                                        .
          |
          |
          |                                                         .
          |__________________________________________________________._
                                                                     |
          0                   RPM                              1800 or 3600


As you can see, the curve is very steep in the operating region and in
fact, the observed operation is typically that once you load the motor
past the local maximum torque, the speed jumps to the corresponding
point on the initial portion of the curve or simply stops. The actual
operation depends upon the shape of the curve near 0 RPM.

The Rated HP is typically the torque level at which the motor can be run
continuously without exceeding the temperature at which the winding
insulation beaks down. Since there is thermal mass involved, you can
operate the motor at higher than rated torque for less than 100% of
the time and not exceed this temperature if the motor is cool preceding
the run etc. etc. etc.

Typically, two motors with different rated HP develop different HP in a
ration close to the same as the difference in rating.


The story is somewhat different for a universal motor such as is used on
most hand held tools. In these motors, for a given input voltage, the
torque goes up as the speed goes down. The more you load them, the slower
they run until they stall, at which point their torque is a maximum.
In this case, the developed horsepower is a the point along the torque
curve where the speed X torque is a maximum. As with the induction motor,
the rated horsepower means you can run the motor there at 100% duty cycle.
Again, you can load the motor more and it will produce more torque but you
may only do this on a limited basis.

The final word is heat. If you exceed the winding insulation temperature
rating, you will fail the insulation and ruin the motor ( or pop the
thermal cutout if so equipped).



Application areas: Universal motors are compact, have high starting
torque, can run at high rpm, and deal well with rapidly varying
loads. They are often used with triac or thyristor speed controls.
This makes them ideal for portable power tools. Single-phase
induction motors are efficient, have a limited rpm selection,
are relatively heavy and bulky, and are almost maintenance-free.
They work well in stationary tools that run at one rpm or that have
a variable-speed transmission.

Voltage: Both kinds of motors are supplied in popular mains voltages
(115 or 230) but only induction motors are supplied with winding
taps that allow either voltage to be selected. As far as the motor
is concerned, there is no difference in efficiency when selecting
either 115 or 230 volts. This is because such motors have two
identical sets of windings that are connected in parallel for the lower
voltage and in series for the higher. Neither connection results
in the individual windings seeing a different voltage. However,
inadequate wiring can make a difference to motor operation, because
higher current at 115 volts may give unacceptable wiring voltage drops
in some shops or garages. Some wiring voltage drop is expected and
built into the motor rating. Nominal pole transformer output (to
your house) is about 120/240 volts. Motors are rated for 115/230
volt operation, which allows for 5/10 volts wiring voltage drop.
More voltage drop than this can cause low starting torque and
overheating at rated load.

115 or 230 volt operation makes no difference to your power company
either. The watt-hour meter at your electrical entry measures watts
regardless of the voltage used. Your power company does not give
you a single watt for free, and your PUC (Public Utility Commission)
won't let the power company charge more than the legal rates.
Watt-hour meter accuracy is a matter of law in most States.

Current: Motors have a nominal current rating which is supposed to be
the current at rated horsepower and rated voltage. A motor will not
draw exactly rated current except in the unlikely circumstance that
the voltage applied is exactly the rated voltage and the load applied
is exactly the rated horsepower. As a matter of fact, most woodworking
tools spend much of their life spinning without applied load and drawing
only a small fraction of nameplate rated current. When the tool begins
to cut, motor current varies widely depending upon cutting load. In
some tools which have relatively small motors, motor current may approach
several times rated current as the tool is momentarily loaded close
to stall or breakdown torque. An exception to this wide variation
would be something like the motor driving the fan on a dust
collection system; such motors operate at about rated horsepower all
the time because the fan presents a constant load.

For both universal and single-phase induction motors, the full-load
current is given by

I = (746 * hp) / (eff * pf * voltage)

where eff is efficiency, pf is power factor, and the others are
obvious. In AC systems, the voltage and current waveforms are
(nominally) sine waves and may differ in phase from each other
by an angle called the phase angle. There are 360 phase angle
degrees in one sinusoidal cycle. Power factor is the cosine of
the phase angle, and for motors this angle is normally between
zero and 90 degrees, current lagging voltage. In DC systems,
there is no phase angle, and power factor is defined as 1.0.

Typical values for single-phase induction motors running at 115
volts AC are pf = 0.8 and eff = 0.9. This gives a rule-of-thumb
value for amps/horsepower at 115 volts of

9 amps / horsepower

This figure is probably OK for rule-of-thumb comparison of induction
and universal motors or reasonability checks as long as you
remember that it is based on typical values.

If you are contemplating operating a 115 volt universal motor
on DC, performance should be slightly better at 115 volts DC
than it was on AC. The proper voltage to use is 115 volts DC.
This is because AC voltages are given as RMS values, which
are their power-equivalent DC values. The tool will actually
endure less voltage stress under DC operation because the
peak voltage experienced under DC is 0.707 times the AC peak
voltage. Switches and contacts, however, may not last as long.

Starting current can be as much as ten times rated motor current.
This is usually not a problem for the circuit breaker feeding the
motor, because modern circuit breakers are typically rated to trip
instantaneously at about ten times breaker nameplate rating. For
currents less than the instantaneous value, the breaker trips due
to internal heater elements which mimic the heatup characteristics
of the wiring the breaker is supposed to protect. Since starting
currents last only a second or two (unless the motor is jammed),
motors usually will not trip circuit breakers on starting current if
the breaker is rated at higher current than the motor nameplate
current. This may not be true if you start the motor on a circuit
which is already loaded close to rating.

A motor may trip your circuit breaker on time-overcurrent (the
heaters) even if the motor nameplate current rating appears to be
within the breaker rating. This can happen if you continuously
overload the motor; motor current will then be several times the
nameplate rating. There may be other signs of this. The motor may
become extremely hot (spit sizzles on the casing). This is General
Electric's way of telling you to slow down.

Breakdown torque: Single-phase induction motors, unless they are
designed for torquemotor operation, have a "breakdown" torque rating.
This refers to the motor torque-versus-rpm curve, which has a peak
torque somewhere between zero rpm and rated rpm. If the motor is
running and load is applied, the motor slows and torque increases
until breakdown torque is reached. At this point, further rpm
reduction causes a reduction of motor-supplied torque, and the motor
rpm reduces rapidly to zero (it "breaks down"). This is why a saw,
for instance, appears to suddenly stall as it is overloaded.

Ventilation: Most motors have one of two kinds of ventilation: fan-
cooled open housing, or totally enclosed, fan-cooled (TEFC) housing.
In the former type, a fan attached to the motor shaft draws air
through the internal parts of the motor and blows it out of
ventilation slots cut into the motor housing. Most universal motors
are of this type because of the need to cool the brushes and to
exhaust brush carbon dust and commutator copper fragments. In the
TEFC type, the motor housing is completely enclosed and no air
gets to the internal parts of the motor. Instead, internal heat
is conducted through the metal housing to fins, where air blown
by an external fan removes the heat. Some induction motors have
this kind of (more expensive) ventilation and they are often used
in applications where excessive dust or flammable conditions exist.

Drive gear: Surprisingly enough, even though many people will look
at motor horsepower rating, they often completely ignore the drive
gear attaching the motor to its load. The drive gear is often a clue
to the real power rating of the motor-drive combination. It's
difficult to determine the rating of enclosed gears, but v-belts
can give an immediate visual clue. While larger pulleys increase
a v-belt rating, a nominal rule of thumb is about one horsepower
per 1/2 inch v-belt. Two 5/8 v-belts on large pulleys may be good
for 4 or 5 horsepower. One small belt on a motor which "develops"
3 horsepower is cause for some suspicion. Actual belt drive ratings
can be found in manufacturers handbooks (see Gates, for example) or
in Machinery's Handbook.

Motor Starters: Motor starters are big relays mounted in expensive
metal boxes with heater overloads matched to the motor they start.
They serve two purposes: 1) The relay contacts are heavy duty and
are rated for the motor starting current. Delicate contacts, such
as those on a pressure switch, will fail if used directly to
start a large motor. Delicate contacts are therefore wired to
operate the motor starter relay rather than the motor. 2) Wall-
mounted circuit breakers are designed to protect building wiring,
not motors plugged into wall receptacles. If your electrical box
circuit breaker trips before your motor burns up, it is incidental,
not on purpose. However, motor starters are designed to trip on
heater overload before the motor they start burns up.

How much horsepower: This question is often asked and has no easy
answer. This is because the amount of horsepower you need depends
upon your patience, your preferences, and the way you use the
machine in question. Here are some pros and cons. A larger
horsepower motor (and associated drive gear) has a thicker shaft
and is typically more robust than a smaller horsepower motor. It
responds to overloads and hard cuts more strongly, and may not stall
in your application. It does not use very much more power, since
electric motors use only power demanded plus some motor losses (which
are somewhat larger for higher rated motors). On the down side, the
initial expense of the motor and drive gear is greater. Higher
horsepower often requires 230 volt wiring. The motor and associated
drive gear and mountings are heavier. A smaller horsepower motor
is cheaper, lighter, and may run on 115 volts. For a careful worker,
the torque supplied may be sufficient. On the down side, the tool
may stall more often and wet wood may be impossible to cut. The
drive gear may be less robust and may require more maintenance. If
the tool is operated in overload, the 115 volt circuit breaker may
trip.







 

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