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Text File | 1995-05-01 | 17KB | 299 lines

Archive-name: woodworking/motors
Last-modified: 3/17/94

Copyright (c) 1994 by James J. Roche. All rights reserved.

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.




--
Jim Roche
[e-mail]
University of Rochester Computer Science Department Rochester, NY 14627