Motor Power vs Watts

[CPeter James]

I was in a large tool store today and had a lively discussion with one of the "smart" sales people about electric motors on power tools. He said that the Delta motor that drew 12 amps was a more efficient motor that the Jet that drew 18 amps. He said the because both data plates said that they were 1 1/2 hp that the Delta was obviously the better motor. I thought that the hp ratings on most of those data plates were just "fluff" and that the more accurate indication of the power was the amp (watt) rating.

CPeter

[Mike Cutler]

Not sure about this one. 1 1/2 Hp is equal to 1119 watts. On a 115 circuit there would be a theoretical amperage of 9.73 amps, discounting motor efficiencies and power factors. On a 220 circuit it would be 5 amps per leg.
Curious.

[Rob Wilson]

This says it pretty well. Just happened across it about an hour ago as I was researching a subpanel hookup.


It is a universal physical law that 1 HP is equal to 746
watts. Given heating loss, power factor and other inefficiencies,
it is usually best to consider 1 HP is going to need 1000-1200
watts. A 110V 15A circuit can only deliver 1850 watts to a motor,
so it cannot possibly be more than approximately 2 HP. Given rational
efficiency factors, 1.5HP is more like it.

Some equipment manufacturers (Sears in particular, most router
manufacturers in general ;-) advertise a HP rating that is far
in excess of what is possible. They are giving you a "stall
horsepower" or similar. That means the power is measured when
the motor is just about to stop turning because of the load.
What they don't mention is that if you kept it in that
condition for more than a few seconds your motor will melt - the
motor is drawing far more current than its continuous rating.

When comparing motors, compare the continuous horsepower. This
should be on the motor nameplate. If you can't find that figure,
check the amperage rating, which is always present.

http://www.faqs.org/faqs/electrical-wiring/part2/section-12.html

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[CPeter James]

I guess what I am trying to find out is if the Delta motor is really more efficient that the jet and the extra amperage in the Jet is just wasted as heat. both are data plate rated at 1 1/2 hp but the Jet uses 18 amps and the Delta uses12 amps.

CPeter

[Charlie Plesums]

I bet if you put those saws side by side, with identical cuts, you would find the power drawn to be nearly identical. I don't believe either company builds their own motors, but buys them, possibly from the same factory.

The starting current, current under load, and current idling are not constant, but varies, so I bet they are measured differently.

Incidentally, if the Jet drew 18 amps continuously, it would not be legal in residential use... that is more than 80% of the normal 20 amp circuit. Also, they would not be able to get UL rating if it couldn't run on a 15 amp circuit common in older homes (which is why so many "hobby" machines only have 1 1/2 hp motors.)

Of course, if the Delta only draws 12 amps to produce 1 1/2 hp, the motor better never get warm, because it is running nearly 100% efficiently, not generating any noise or heat. I don't believe that number either.

The way HP is measured in universal motors (lighter weight, short duty cycles, often used in hand tools) is entirely different than the way it is measured in induction motors (normally used in stationary tools). The HP in universal motors is somewhere between black magic and marketing BS.

[Gary Bordeaux]

If you look at the motor name plate there may be a SF number. If you multiply the rated HP by the SF number you get max HP the motor is able to produce. So you can have a high service factor low HP motor and a low service factor with higher HP and have the exact same motor.

[Stephen Dixon]

I checked the 2005 NEC Book. It shows the full load current for a 1.5 hp motor on 115v is 20 amps, I am willing to say that the newer more efficient motors will beat this by some amount, but the Delta is clearly closer to a 1 hp motor than it is to a 1.5. The Jet is about right for a true 1.5 hp motor.


Incidentally, if the Jet drew 18 amps continuously, it would not be legal in residential use... that is more than 80% of the normal 20 amp circuit. Also, they would not be able to get UL rating if it couldn't run on a 15 amp circuit common in older homes (which is why so many "hobby" machines only have 1 1/2 hp motors.)

Charlie that is simply not correct, the code and UL both allow motors larger than 12 amps ( 80% of 15amps) in residential service. You simply have to have the correct circuit for it.

Following the Forrest Addy protacol, I am a journeyman electrician with more than 30 years experience in industrial envoirnments, read motors and motor controls.

[Charlie Plesums]

Incidentally, if the Jet drew 18 amps continuously, it would not be legal in residential use... that is more than 80% of the normal 20 amp circuit. Also, they would not be able to get UL rating if it couldn't run on a 15 amp circuit common in older homes (which is why so many "hobby" machines only have 1 1/2 hp motors.)

Charlie that is simply not correct, the code and UL both allow motors larger than 12 amps ( 80% of 15amps) in residential service. You simply have to have the correct circuit for it.


I am not up on current codes, but I am an EE who years ago contributed to the NEC, so I understand the electrons even if I am not up on the latest rules.

One HP plus all the inefficiency you can put into a cheap motor requires roughly 10-11 amps; 1 1/2 hp roughly 15-16 amps. Therefore I agree that the proper circuit for practically any stationary machine in residential use is 20 amps. The max design load of a 20 amp circuit (80% of 20 amps, or 16 amps) will cover up to 1 1/2 hp motors. (My conclusions agree with the NEC, as you noted.)

UL specializes in all the things that can go wrong, so I believed the people who told me that UL would only certify machines that could be plugged into residential outlets, with motors up to 1 1/2 hp, since that is not likely to cause a fire if it were inadvertently plugged into a 15 amp circuit - even if the CB failed. I assumed they were probably also biased by the fact that such machines have a low duty cycle.

Certainly there are larger motors in a home, like the AC compressor, but they don't have a dangling power cord that can be plugged into the wrong outlet.

[Dev Emch]

I bet if you put those saws side by side, with identical cuts, you would find the power drawn to be nearly identical. I don't believe either company builds their own motors, but buys them, possibly from the same factory.

The starting current, current under load, and current idling are not constant, but varies, so I bet they are measured differently.

Incidentally, if the Jet drew 18 amps continuously, it would not be legal in residential use... that is more than 80% of the normal 20 amp circuit. Also, they would not be able to get UL rating if it couldn't run on a 15 amp circuit common in older homes (which is why so many "hobby" machines only have 1 1/2 hp motors.)

Of course, if the Delta only draws 12 amps to produce 1 1/2 hp, the motor better never get warm, because it is running nearly 100% efficiently, not generating any noise or heat. I don't believe that number either.

The way HP is measured in universal motors (lighter weight, short duty cycles, often used in hand tools) is entirely different than the way it is measured in induction motors (normally used in stationary tools). The HP in universal motors is somewhere between black magic and marketing BS.

Hey Charlie...
Based on what you just said, I think we should all be weary weary quiet about all this.:D Your definition and my definition of ummm, residential use may be a bit in conflict.

I had this thing in storage and had to move it to my ummm residence. Oh yah, I plan on running this toy in my residential garage.... all 21,000 pounds of it.:D

[Barry O'Mahony]

UL specializes in all the things that can go wrong, so I believed the people who told me that UL would only certify machines that could be plugged into residential outlets, with motors up to 1 1/2 hp, since that is not likely to cause a fire if it were inadvertently plugged into a 15 amp circuit - even if the CB failed. I assumed they were probably also biased by the fact that such machines have a low duty cycle.

Not quite. UL will certify a device, with a standard 15 amp plug, only if it is safe to attach to a 15 amp or 20 amp branch circuit (since a 15 amp plug will plug into either a 15A or 20A receptacle). You seen that reflected in the NEC; e.g, 240-5(B)(1) in the 2005 NEC, that allows thin flexible supply cords to be used, as long as they are used in products listed by UL or other similar labs as safe for such use.

That's not the same as saying they will only list devices that can be attached to a 15A or 20A 120V circuit. There's no prohibition in the NEC to having higher-ampacity branch circuits in residential applicaitons. I have a UL-listed room air conditioner with a 30A, 240V 'dangling power cord'. It will only attach to a 30A branch circuit, and is designed for residential applications, although I use it in the shop..