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by Bob DeLucas
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* TEFC = Totally Enclosed Fan Cooled
There are some basic things to look for in a motor:
How long is the motor and tool warranty? Most motors if they are going to fail because of an internal wiring or part problem, will do so in the first 6 months of use. The warranty should be at least one year, and if possible, two years would be even better.
Capacitor start motors on stationary power tools fit into basically two categories:
Open or Guarded Dripproof and Enclosed.
Open or guarded dripproof motors (see photo A and B) have holes for ventilation, and they are designed for basically clean, dry applications. If you look through these ventilation holes in the housing you can see all the way into the critical motor windings.
Enclosed means the motor windings are sealed or closed off to prevent dust, dirt, or moist air from entering the motor windings (see photo C). If there are holes in the back end of the enclosed motor, it is only for a internal fan to help with cooling.
The enclosed motor, or an enclosed motor equipped with an internal fan (see photo D and E) for cooling is known as a totally enclosed fan cooled motor or TEFC. In my opinion, TEFC is the best design for dusty and dirty wood or metalworking applications.
Note: both of these types of motors are designed for use in non-hazardous environments.
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Photo A Open Guarded drip proof motor on Delta Contractor saw #43-444 (note open slots in motor case). |
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Photo B Open Guarded drip proof motor on Delta Contractor saw #43-444 (note open slots in motor case). |
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Photo C 1-1/2 HP TEFC motor on JET JWTS-10JF contractor style table saw |
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Photo D 1-1/2 HP TEFC motor on JET JWTS-10JF contractor style table saw (these holes on a TEFC motor are for the cooling fan only) |
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Photo E 1-1/2 HP TEFC motor on JET JWTS-10JF contractor style table saw (these holes on a TEFC motor are for the cooling fan only) |
Heat is a major cause of motor failure. The type of material a motor housing or casing is made of can effect a motors life . A motor housing or casing made of cast iron is the best dissipator of heat. It will allow the motor to run cooler and last longer. The cast iron housing/casing material pulls heat away from the motor windings faster and more efficient than steel, aluminum or plastic. So to sum up ranking motor housingcasing material for heat dissipation, cast iron is #1 steel is #2 aluminum is #3 plastic is #4.
NOTE: Cooling fins (see photo F) on a motor housing allow air to better pull the heat from the motor. Cooling fins on the motor housing are always better, than a smooth surface motor housing.
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Photo F 3/4 HP TEFC motor on JET JDP-17MF Drill Press (note cooling fins) |
This is an area where you have to pay attention. Some companies over estimate a motor's strength by rating motor horsepower under "no load" or use words in their advertising such as "develops". This sometimes could be misleading because it does not always reflect the motor's true power or horsepower in actual machine use under load. If a motor is weak or overrated, the tool could become bogged down in medium to heavy use.
Example: A weak motor on a table saw, planer, drill press, band saw, jointer, and bench grinder under a heavy cut or load on hard material or hard wood such as oak will lose speed and possibly stall. The same condition could even happen with tools such as dust collectors. Large volumes of saw dust from multiple ports or points and long distances could put extra stress on a weaker motor and cause the motor and impeller to slow down and decrease the dust collector's suction capability.
A general rule is check the motor amp rating on the data plate of the motor. Amps (or Amperes) is a general indication of the power or horsepower that the motor puts out.
If we look at stationary power tool standard efficiency capacitor start type motors, a general rule of thumb for amps / horsepower relationship for a 1700 rpm motor at 115 volt, 1 phase, 60 hertz operation is:
| 1/4 hp approx. 4 to 5.6 amps* |
| 1/3 hp approx. 5.7 to 7.3 amps* |
| 1/2 hp approx. 7.4 to 9.7 amps* |
| 3/4 hp approx. 9.8 to 11.6 amps* |
| 1 hp approx. 11 to 14.7 amps* |
| 1-1/2 hp approx. 14.8 to 18.4 amps* |
| 2 hp** approx. 18.9 to 30 amps @ 115v* |
| 3 hp** approx. 30 Amps and up @115v* |
*Note: 3400 rpm capacitor start motors may draw slightly less amps and a 1100 rpm motor may draw slightly more amps than shown on the chart above.
** Large motors 2 hp and up have such a high amp draw at 115 volts that we would only recommend them to be run on 230 volts. Running these motors at 230 volts will cut the amp draw in half (of the amount shown in the chart). All amp specs shown are based on 115 volts.
High or premium efficiency vs. Standard efficiency motors: We would all like to think the motors on our tools & machines are the most efficent available. Truth is, cost is a major factor and the machines motor is one of the highest cost items for all manufactures of stationary power tools. The type of motor they pick to be on their machines is almost always a standard efficiency motor.
Again, this general amp/hp information is for stationary power tool standard efficiency capacitor start motors at 115 volt, 1 phase, 60 hertz. These specifications may vary, but should help you to evaluate the power that a given motor puts out.
1. 1 to 2 year warranty is preferable.
2. Motor should be enclosed preferably TEFC (totally enclosed fan cooled). TEFC is the best for dusty, dirty applications.
3. Motor casing/housing should be cast iron or steel for better heat dissipation and longer motor life.
4. Don't believe horsepower specs alone. Check the motor data plate for amp rating. AMPS tell the TRUTH!
In future months, I will expand on motors and other wood and metalworking machinery topics.
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