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  1. #1
    rjibosh is offline Hot Rolled
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    Default How much over speed can a 3 phase motor tolerate?

    I have a 3hp, 50Hz spindle drive that has been running for 20 years on 60Hz. The name plate on the motor says 2850 rpm. I put a tach on it and it actually runs at about 3400 rpm. Obviously, the motor can and has run at 20% over the rated speed with no ill effects. Is there a rule of thumb limit to how fast you can run a three phase motor? If a 2 pole motor runs at about 3500RPM, and a four pole runs at 1750 here in the US on 60hz, are the 50 Hz motors standard speed options something like 2850 and 1425? Or is there some way to make a 3Ph motor run at 3500 rpm on 50Hz?

    Rick

  2. #2
    Marcibb's Avatar
    Marcibb is offline Hot Rolled
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    The seed of a an induction electrical motor is dependent on the frequency of the electrical current applied to it which, with the number of poles it has, determines the speed of rotation of the magnetic field with in it minus a slip/load factor (the motor will slow as the load increases). Mechanically the maximum rotation speed of the rotor depends on it's construction and the quality of it's components bearings how well it is balanced and so fort.

    I have a 3phase 3HP Weg motor powering my lathe. I use a VFD and routinely run the 60Hz rated motor at 75Hz I am not sure if there is a rule of thumb but most bearings more than meet there specifications and if keep well lubricated can handle the extra speed. Rotors are electronically balanced and sealed in some type high strength compound ( I want to say epoxy but I am not 100% sure) that will keep them from flying apart. I would think that most good quality motors would handle 20% over rating. I would not be so sure if I was using a Chinese knock off.

    Marci

    The best person to ask such a question would be the motion guru.

  3. #3
    JST's Avatar
    JST
    JST is online now Diamond
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    Default

    It's a question with no solid answer, unless the motor in question has a rating.

    I would feel no fear of 20 or 25% overspeed, and probably none from 1.5x to 2x speed on a good motor. The 2x might be shaving it on some.

    Faster than that is probably asking for trouble unless you KNOW the rating of the motor.

  4. #4
    Jraef's Avatar
    Jraef is offline Stainless
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    Default

    The "rule of thumb" is, there is no rule of thumb, but there are a couple of relatively safe bets.

    1) There are really only two main issues with operating speed: bearings and cooling. Many motor mfrs don't really bother stocking and using different bearings for the different speeds (6 pole, 4 pole, 2 pole) so most of the time you can run a motor at the 2 pole speed, i.e. 3600 / 3500 RPM nominal. Some cooling fan designs are fine with the higher speed, some will not move more air however, and if the motor is being over loaded, it may become a problem (see below).

    2) Motors are not really made for 50 or 60Hz, but they ARE made for a specific V/Hz ratio. As long as you maintain that same ratio, the torque remains the same. But if you change it, you change the torque profile and can get in trouble, especially if the motor is marginally sized. So for example in your case, your motor was rated for 50Hz operation, but you didn't say at what voltage. Let's assume for now that it is 230V 50Hz as designed. That gives it a V/Hz ratio of 4.6:1. But you have been feeding it 60Hz, which lowers the V/Hz ratio to 3.833:1, so 17% lower, the equivalent of lowering the voltage by that much. Torque relates to the square of the applied voltage, so you have lowered the torque to roughly 69% of what the motor was originally designed for. At the same time, you have increased the speed by 20%, so now you have 69% torque at 120% speed and because HP is a function of speed and torque, you have about 84% of the original HP available from that motor. If the motor was over sized to begin with, might be no problem, as it appears to be your case. But it is by no means a given.

  5. #5
    rotarySMP is offline Aluminum
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    I have a cheap 2 pole 550W motor on my lathe and routinely run it at 75Hz. Since it has survived about four years of this, it is time I upped the top speed to 85Hz.

  6. #6
    rjibosh is offline Hot Rolled
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    Ok so there is no theoretical max speed. I suppose the torque could go to zero as the speed increases. At some point the torque would be inadaquate to turn the rotor and an equilibrium would be struck. So not grinding the numbers, but my 3hp,380v, 50Hz motor is likely putting out maybe 2Hp on 380V, 60Hz. How about the European basic speeds? Anyone know what they are?

    Thanks everyone.

    Rick

  7. #7
    Waterlogged's Avatar
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    I'm not an expert on the subject but I've run 3 phase motors at 120 Hz for 20 hours per day, 5 days per week, for years on end without motor failures. As mentioned earlier, cooling and bearing quality are probably the biggest concerns. Bearings shouldn't be a factor if it's a decent quality 17XX rpm motor.

  8. #8
    mike_kilroy's Avatar
    mike_kilroy is offline Stainless
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    European motors are typically 50hz and also 208/380(400 today). this is roughly equiv to our 60hz 230/460v so that is why european motors run here don't loose any power or torque - they have the same v/hz curve!

    european motors are same 2 or 4 pole motors mostly as we use,. our base synchromous speeds are 3600 & 1800rpm. european at their 50 hz are 50/60 times slower so 1500 & 3000rpm.

    take off 25-100rpm (typical) for a given motor design, called slip, from these numbers. so all go synchronous speed at no load and this slower speed at nameplate HP load.

    I have never had any cheap 75hp or smaller motor mfgr say I cannot run their motor to at least 1.5x base speed (90hz). Most if asked will not allow 2x (120hz) on their std motors unless around 2hp or smaller. reason for this is rotor balance. there are specs for rotor balance and on cheap motors they don't get the good balance necessarily and so the shaking will tear up even good bearings if you go too fast.

    as for cooling, you can look in the back of your motor and see if it has a shaft mounted fan blade; if so, it will obviously cool enough above base speed. You are not pulling much if any more power or heat from the motor going above base speed since above base speed the voltage is already at max so the output HP is same nameplate rating.

    That leads to how low can you go if you crank the VFD down... rule of thumb here is 1/6 base speed/freq assuming a shaft mounted fan blade inside. below that it doesnt move enough air to cool the motor. if separately excited fan that goes a constant speed instead, most motors will go all the way to 0rpm and still delivery full nameplate rated torque.

    If you have a VFD and can crank it up, you can put your hand on the motor as you speed it up a bit at a time and feel the balance; Being mechanical types here, most can feel it and will be able to tell if the balance is off to much at say 75 or 85, or 120hz. If you have never made that motor go fast before, in addition to making sure the balance is still smooth, you should shoot the front bearing with a cheap IR temp gun as it runs to make sure the bearing doesnt get too hot too.

  9. #9
    DaveKamp is offline Titanium
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    Default Hmmm...

    Well, my Johannsen radial drill is equipped with an ordinary 2hp 1800rpm motor.

    I yanked the fan, mounted a muffin-fan on the shroud... and overspeed it to about oh... 6,000rpm... driving a Bridgeport J-head through a 3:1 reduction, and it's plenty powerful at the spindle...

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