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Subject: rec.woodworking Electric Motors Frequently asked Questions

This article was archived around: Mon, 5 Jan 1998 19:44:28 GMT

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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 roche@cs.rochester.edu University of Rochester Computer Science Department Rochester, NY 14627