60Hz power for a 50Hz Motor

Greetings All.

Once again I am thrown into new territory. I have been asked to assist troubleshooting a machine that has a 480V, 3-Ph 1.5Kw Hauptantrieb motor that causes the circuit breaker to pop (how frequently, I don't know). I have very little to go on at this point, but the limited documentation available makes reference to the motor being 50Hz.

Before I go pulling my resources together and trudging to another building, I thought an insightful bit of input from one of y'all might get me off on the right foot, or at least keep me off a rabbit trail or two.

What affect does operating a 50Hz motor with 60Hz power have on the motor itself?

-Grover

[QUOTE]What affect does operating a 50Hz motor with 60Hz power have on the motor itself? [/QUOTE]

The motor runs faster--20% faster than rated RPM.

We actually bought a machine, that had originally been bound for the old USSR, equipped with 50Hz/380VAC motors on it and ran them with 60Hz/480VAC. That was 6 years ago and they're still running fine. The extra Hz increases the inductive reactance, which actually offsets the current added by the 380/480 difference, and they don't self-destruct--they just go faster. It's not really a good idea, but we decided to try it and maybe buy a new motor every once in a while, rather than about 20 all at once--haven't lost one yet!! You gotta be careful about pumps, because of the added pressure that comes with the RPM.

The motor runs faster--20% faster than rated RPM.

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Yes, but isn't it like running a 60 HZ motor on a VFD @ 72 HZ?
You lose Torque???

Verify, Verify!!

Gentlemen, thanks again for the response and adding to my education. It turns out the breaker was tripping because the motor brake was still on during motor start. What a drag, you know?

The contactor controlling the brake had a bad set of contacts which intermittently would keep the brake applied.

I will keep the comments about speed and torque in my little mental tidbit file (heavy emphasis on little). I'm sure I will apply them elsewhere.

Kind regards, fellows and Happy Thanksgiving!

-Grover

Yes, but isn't it like running a 60 HZ motor on a VFD @ 72 HZ?

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I don't think so--because in a VFD, you reach 460 volts at 60 Hz then increase only Hz to 72, and the volts stay at 460.

With the 50/60 Hz motor thing, you have full VAC at 60Hz the motor is wound for 50 Hz (and probably rated 1500 RPM), and runs at 60 Hz (probably 1800 RPM)--full torque, full HP.

I think that's right, I could be mistaken.

Mmmmmmmmm now! Be careful boys! Just because you end up with a motor running 20% faster doesn't mean that you now have a motor with 20% more horsepower.

And that's fairly obvious, I suppose, but your machine DOESN'T BELIEVE IT! While the motor torque must be derated 20% when operated at 20% overspeed, the machine likely requires either constant or increasing torque at these higher speeds.

Those 50Hz motors that were closely sized for their rated KW or HP at 50Hz are going to roast at 60Hz even with their fans turning faster. Those motors that had plenty of spare capacity at 50Hz will operate very well at 60Hz generally. If you are unlucky and find your motors closely rated at 50Hz, you can dodge a bullet, so to speak, if the motors are belt or chain coupled to the machine. Simply increase the reduction ratio in the belt or chain system and your machine is back down to its design hp and speed and your motor is happily turning faster but being asked to produce less torque.

I often see these motors being operated on 60Hz with a 380V transformer feeding them. This is a mistake for sure. It makes the motor run faster and reduces the voltage needed to overcome the added impedance of the coils at 60Hz. The result is a voltage starved motor that slips way to much and draws too much current under load. Short service life is almost always the sad result.

In my experience another thing about 50 HZ motors is that a lot of them come from countries that use lighter windings than we would normally use here in North America, and this practice of saving on copper also caries on into some of their wiring practices.

I would have to agree with John on this one. Assuming you maintain the volts/hertz ratio the speed will go up by the ratio of the frequencies and the torque at rated slip will stay the same with no appreciable increase in motor heating. This means your shaft horsepower increases by the frquency ratio. The trick is the volts/hertz ratio, which maintains a constant airgap flux level. This is the basis of constant torque drives.
Two things to be careful of are that the motor is a 380VAC/50Hz motor. I don't think the original post specified the voltage of the motor. We just assumed 380/50 as the design.
The other was already mentioned. Be careful of variable torque applications like pumps and fans. With most pumps and fans the shaft torque required increases as the square of shaft speed. This means shaft horsepower increases as the 3rd power of shaft speed. This is where you can get into real trouble. My company cooked a couple of fan drive motors ding this. But if you have a standard conveyor or pull roll application, which is typically constant torque, you will be OK.

Keith

I cannot agree with kamenges concerning motor overspeed. There is simply no justification for taking a motor designed for, say 10kw at 380V 50Hz and running it up a constant V/Hz ramp to 460V 60Hz and expecting to get 12kw out of it.

This would be equivalent to taking a 10hp 460V 60hz Nema motor and running it up to 552V 72hz and getting 12hp out of it at the higher speed.

In both cases, it can't and won't happen! If it did, don't you think the motor mfgr's would advertise these amazing, laws-of-physics-defying motors as a new breakthrough and get a big price for them?

No, the reality is that, above the motor design basespeed, whether 50 or 60Hz, you must limit voltage to the design voltage and expect not more than constant hp (declining torque) as speed goes above the base speed. This is well tested territory and, except for minor extras gained from motors with large design service factors, the rules must be followed for reliable motor operation.

As mentioned before, some, maybe even many, 380V 50hz motors have the ability to reliably withstand 460V and therefore develop full rated torque at 60hz intermitently, but not for long at their rated ambient temperature. The motor nameplate data is mostly a safe operating point AT DESIGN AMBIENT TEMPERATURE and running these motors at full torgue and 60hz violates the thermal conditions of the nameplate considerably. Now, if you are running these motors in an ice house or at the North Pole, you can cheat on the thermal limits and get away with it. Many do! And many in warmer climates and working environments don't!

It's still cheating and needs to be called that! And I'm not opposed to doing it as long as you understand why!