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Electric motor question - Dave Diaman - 02-27-2017
I have a question for you electric motor guru's. I have quite a few 3 and 5 HP motors in my shop. Up until now almost all my 3 HP motors pull about 17 amps. What I have noticed is a lot of the new 3 HP import motors are rated at 22 amps. I just bought a new Grizzly dust collector and checked it and it is indeed pulling close to 22 amps. Is this an effort by the import motor manufactures to make their motors produce a true 3 HP at a lower efficiency than American made motors or something else? I can say the new motor on my dust collector is massive when I compare it to my other motors with equal HP ratings.
RE: Electric motor question - Roly - 02-27-2017
I'm going to take a guess and say the import motors are probably designed for 50 cycle power rerated for 60 cycle power. Because of the higher iron mass needed for 50 cycle results in a loss when ran on 60 cycle. Now tied into this the motor will run at a higher speed thus increasing HP. The increased speed also increases the cooling.
I have not found ratings for identical motors when used for the different frequencies. A lot of European motors are ratted in KW instead of HP to add to confusion. Roly Hopefully TDKPE ? will see this and comment.
RE: Electric motor question - Rick F - 02-27-2017
I checked the Grizz website.. It looks like a misprint to me ..
The 5hp Grizz dc is 22.4 amps.. 3hp motors are 15-17 amps typically ...
Why would the 5hp and 3hp be the same ..
RE: Electric motor question - ez-duzit - 02-27-2017
My Grizzly dust collector is rated at 3 hp and 18 amps.
RE: Electric motor question - daddo - 02-27-2017
Are they all the same speed rating?
You would have to connect each motor to an identical load and read their actual amps while running. One motor may have more or less torque and a different speed than another.
At 22 vs 17 amps, I would say it is working harder- not so much having more HP. Perhaps it is capable of higher running temps.?
RE: Electric motor question - Dave Diaman - 02-28-2017
(02-27-2017, 10:09 PM)Rick F Wrote: I checked the Grizz website.. It looks like a misprint to me ..
The 5hp Grizz dc is 22.4 amps.. 3hp motors are 15-17 amps typically ...
Why would the 5hp and 3hp be the same ..
Rick, it isn't a misprint. I assumed the same but after assembling my dust collector and installing the duct work I started the DC up on the 20 amp circuit I had installed for my last DC and it almost immediately popped the breaker. When checked, the machine is pulling between 21 and 22 amps. I looked at a few other import dust collectors and I am seeing the same ratings. Not all are rated a 22 amps but the ones that are seem to have higher CFM ratings. If the CFM ratings are accurate who knows though. Laguna who just redesigned their cyclones is the first one that comes to mind. The only machines I am seeing this on is dust collectors though. I looked at some other grizzly machines and the ratings are all over the place. A grizzly 5 HP planer (G0454) is 5 hp and rated at 5 HP and 20 amps. The Grizzly G1030X which is the same 5 HP 4 post 20" planer design is rated 5 HP and 30 amps. Personally I don't understand the variation but the dust collector has amazing suction and I installed new 10 gauge wire and a 30 amp breaker and it is working great now so I guess in the end it doesn't really matter.
RE: Electric motor question - TDKPE - 02-28-2017
Does the motor have two capacitors? It does in the web sales page, but does your actual motor?
RE: Electric motor question - Dave Diaman - 02-28-2017
Yes, it does have two.
RE: Electric motor question - Robert Adams - 02-28-2017
(02-27-2017, 10:09 PM)Rick F Wrote: I checked the Grizz website.. It looks like a misprint to me ..
The 5hp Grizz dc is 22.4 amps.. 3hp motors are 15-17 amps typically ...
Why would the 5hp and 3hp be the same ..
Makes me wonder if it's the same motor.. Kind of like what briggs and Stratton used to do with their riding mower engines. They would make one engine and put different size carbs on them to give the different hp ratings.
RE: Electric motor question - TDKPE - 02-28-2017
The presence of the run (second) capacitor would put the power factor very close to 1.0, typically 0.98-0.99, at full load and rated voltage and frequency. At less than full load, the power factor will drop, as will the efficiency. My 3 hp Unisaw motor, built by Marathon, has a power factor of 0.98 and an efficiency of 0.80 (full load values), for a nameplate 12.4A at 230V and 60Hz. I actually have the data sheet on that motor, which I got when it was still possible to talk to someone at Delta who knew what they were doing and was willing to give a peek behind the curtain. But I digress.
For that Grizzly motor to have such a high current rating, it would have to have low power factor (no run capacitor) and low efficiency, which is typical of less expensive motors. With the run cap, the efficiency would have to be extremely low. The two multiplied together would have to be around 0.46, which sounds like a combination of low efficiency motor and possibly actually being a larger motor than the nameplate states, which would put its operation at well below rated output, which in turn means it's running at a point on the curve where the efficiency and power factor are lower than rated (they both drop as the load is reduced, so using a big motor at light load is inefficient compared to a smaller motor running at rated load).
Electrically, that motor nameplate shows just under 6.5 hp (electrical input power), assuming a power factor of nearly 1.0. Even though Grizzly and most other Asian motors of this caliber tend to live at the high end of input volt-amps for a given shaft power, that motor is waaay beyond even typical Chinese motors. The NEC (Table 430-148) shows 17A as the design FLA for a 3 hp single-phase motor at 230V, which is supposed to give plenty of headroom for wiring and disconnect sizing for motor installations so that future motor replacement will not require changes to the branch circuit and control (other than possibly a heater change, the value of which is actually based on the nameplate FLA, whereas the wiring is based on the tabulated values). In other words, no standard NEMA-type motor should even reach 17A. The Grizzly is way beyond that. Which is suspicious.
Operating at a higher voltage than nameplate will hurt the eff and pf values, and the Griz motor may be 220V instead of 230V, but it would only account for a small difference, especially going a little higher.
I suppose it's possible that the two capacitors are only working to start the motor, and it's running with a low power factor as well as a low efficiency. But that's really stretching it. Inexpensive construction accounts for some of the high current typical of these motors, but not all of it. Unless, as someone suggested, it's a 5 hp nameplated 3 hp, which is operating well below optimal values. Delta did that with their dual-hp CS motor, but that was for UL listing purposes. But there's also no rule against it. Or it could have a 5 hp stator and windings with a short rotor, though that's a waste of copper, so kind of unlikely.
I would be interesting to stick it on a dyno and measure power factor (and calculate efficiency once pf is known) for various points on the output curve.