Can you explain why an Edwards vacuum pump won't run on 60 Hz? 1

[gornea]

Hi,

I'm really curious to understand the following problem. I brought a vacuum pump from Switzerland to Canada. Originally the pump was obviously not designed to run in Canada. Also, the pump is pretty old but used to work flawlessly in Switzerland. I attached a picture with the few labels still present on the pump. It seems it was designed to run on single phase 220 V at 50 Hz. In Switzerland it was actually running fine on 230 V 50 Hz. I thought in Canada I would be able to make it work using a step-up converter. This is the first thing I tried. It blew the fuse on the power line where the converter was plugged in. I thought I needed more current and so I changed the plug to be able to connect the pump directly to a single phase "208 V" power outlet. In the building where I'm now it seems the mains is a little bit old. The neutral to phase is actually 115 V and the phase to phase is 200 V only. Anyway, in this configuration, that's with a single phase 200 V and 60 Hz line, the pump works for a while, about 15 seconds and then a thermal protection on the pump kicks in. When it runs, the pump doesn't seem to run correctly. The sound it makes is different from what I'm used to, it vibrates more and there is a periodic loud click. Anyway, I'm surprised that increasing the frequency from 50 Hz to 60 Hz makes such a huge effect. Because the voltage is lower and the frequency higher, I would expect the motor would run cooler not hotter. So I don't understand why the thermal switch kicks in! Before I invest 2000$ in a frequency converter and discover that the pump was actually mechanically damaged during transport, could you comment about what can possibly go wrong when trying to run a Edwards pump at 60 Hz instead of 50 Hz, in case anybody knows more about these vacuum pumps? I guess fundamentally the problem is that the motor pulls too much current but why? Is it possible that the mechanical load increases so much at 60 Hz that the motor can not drive it properly?

Thanks a lot!
Razvan

 

[davidbeach]

The answer to your final question is yes. Even if you weren't grossly overloading it, it would need about 264V at 60Hz to match the V/Hz ratio of 220/50. Whether it ever runs again or not could be problematic.

 

[Compositepro]

For the motor to work correctly you must keep the volt/hertz constant. You have increased the frequency, which will make the motor run faster and thus do more mechanical work. You have also reduced the voltage (the voltage needs to be increased going from 50 Hz to 60 Hz), which would make the motor draw more current just to do the the originally designed work. But it is doing more actual work than designed due to the higher speed, so it draws even more current, and trips the breaker.
By the way, you cannot have 115 volts L-N and 200 volts L-L

 

[davidbeach]

115V L-N on a wye system would be 199V L-L, so 200V is a decent approximation.

 

[Compositepro]

Thanks, David. I missed that it was a 3-phase wye that he was connecting to.

 

[itsmoked]

Can you just replace it with a correctly powered pump? That might be less $$ than the converter kludge.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

The strange sound and actions you observed were likely because with the lack of torque due to the low voltage, the motor is unable to get to full speed regardless of the faster frequency. So the centrifugal switch in your single phase motor is not activating to take the starting winding and capacitor out of the circuit. Don't continue trying this, you will damage the motor.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[berkshire]

You may be better off taking the motor to a re-winder and getting it wound for the voltage and frequency to want to run it on.
B.E.

You are judged not by what you know, but by what you can do.

 

[waross]

Rewinding for the voltage is not the problem. At the higher frequency the motor may need more torque. That means more magnetic flux density. Motors are typically designed to run at a flux density close to saturation. For more torque you need more iron. If the motor does not need more torque, then an autotransformer will do the job the cheapest, quickest and easiest.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[OperaHouse]

Ebay is loaded with Edwards E2M2 pumps that are 120/60. That may be your best solution. The model number doesn't change so it is likely the pump can run at a higher speed id the motor can take it. Adding a transformer makes it less portable and that could be costly to get the size needed. When I was out of town a friend had a motor rewound about this size for $900. Machine still didn't work. Took less than 2 minutes to find the wire that fell off in control box. Just another bad story about a rewinder.

 

[waross]

One thing to try before spending big money is to see if the motor may be re-connected for 120 Volts. If it doesn't start and run right away stop before doing damage. If it does run on 120 Volts (115 Volts) then check the current to see if it is below the rated current for the low voltage connection. Going from the high voltage connection to the low voltage connection will give you a 15% gain in voltage to the actual windings. That may be enough to do the job.
I normally would not suggest this, but from looking at vacuum pumps on Ebay I suspect that some Edwards pumps have dual rated 50/60 Hz motors. I also suspect that the motor may be slightly overpowered for the load. With the high voltage connection, each of the windings will see 200 Volts /2 or 100 Volts. Going to the low voltage connection will supply 115 Volts and that may make the difference.
Be sure to check the current at both the start and the end of the pump-down cycle.
Good luck

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Operahouse is likely on the right track. That pump is at least 25 years old just based on the motor manufacturer, and the oil doesn't look as fresh as it should be in a well-maintained pump. When I was in the electronics industry the oil we used in our Edwards pumps started as a pale yellow and by the time it was a deep yellow it was time for a change.

 

[gornea]

Thanks a lot for so many useful answers! Yes the transformers are three-phase "600V to 208V" but the voltages are only 200V/115V (measured with a multimeter). Maybe the 600 V that's coming in is not really 600 V, that I couldn't measure. I moved a rather large setup from Switzerland to Canada, I have 7 turbo pumping stations and gas recirculation pumps, all normally working on 50 Hz. So I really want to understand the physics involved before making changes to the equipment (anyway I'll have to schedule upgrades because I don't have the money for all this). I started to play with this pump which is the oldest model I have and eventually I could survive even if I burn it although I would like to still use it for a while. Yes probably the oil should have been changed long time ago. Equipment and instrumentation maintenance is a nightmare in my field because we lack long term or permanent technician support and do a lot in the lab with students. I'll replace the oil soon, thanks for pointing to that.

I had a look at how AC motors work. I'm still interested to find a solution if possible without using a frequency converter since indeed they are rather expensive. So I wonder if it is even possible to run reliably a motor made for 50 Hz on 60 Hz. My basic idea is that the mechanical torque required would increase proportional with the frequency, roughly 20% only. I looked where the V/freq comes in and, if I'm talking about the correct type of motor, it seems like the maximum torque generated by the motor varies with the square of this ratio. On the other hand, the position where the maximum occurs, in terms of the "slip" variable, that's s=(w-w_m)/w [that's AC freq minus shaft rotation freq over AC freq], s_max also changes and actually becomes smaller (it's inverse proportional to the AC freq). So I'm thinking that to match the torque curve in terms of s is enough to rise the voltage at ~ 240 V which would match the torque curve to that when using the motor at 50 Hz. The motor will have to work a bit harder because the mechanical torque is 20% greater so it will actually work at a larger s which will lower the efficiency and make it dissipate more heat but maybe without tripping the thermal fuse.

Right now with the voltage at 200 V and 60 Hz I get that the maximum torque is only 57% of that when operated at 220 V and 50 Hz. So clearly this is a huge problem and explains why it doesn't work properly. The s is probably very large which explains why it heats up. But at 240 V and 60 Hz I get the maximum torque at 83% of what it would be at 220 V and 50 Hz. Considering that the s_max occurs closer to s = 0 I think the torque curve as a function of s is matched to that when the motor is used at 220 V and 50 Hz. The advantage of 240 V is obviously that is more realistic to obtain it than 264 V. I'm hoping that a 240 V transformer would be a cost effective solution to power all the electric motors on my setup.

What do you think about this? Any considerations that would actually kill this idea? I haven't understood the comment about the magnetic flux density. Clearly I lack the understanding how it affects the operation of the motor but I was thinking that for a 20% mechanical torque increase there will be no saturation. Lets say somehow the motor can not deliver the torque required and the shaft rotates much slower than the AC freq. then the s is larger than at full speed and the frequency of the induced current in the rotor is larger by the same amount the mechanical frequency is smaller. Isn't this a similar configuration to when the motor starts up? It will heat more but otherwise work?

Thanks a lot!

 

[davidbeach]

New motors would be your best bet. Lacking that you need to get close to 264V while on 60Hz. To the extent that the loads are speed dependent you're probably still hosed. A typical fan or pump will require more than 70% more power at 60Hz than at 50Hz. Equipment like that doesn't move well between 50 and 60Hz.

 

[itsmoked]

I think you should get a proper replacement and quit screwing around and ultimately failing.

You will ever get that wrong motor to run that far out of its design region for the periods demanded in vacuum service. Sometime you can get away with this type of misapplication because the load isn't continuous. For instance a hydraulic punch or press which has very cyclic loading with periods of virtually no load interspersed, with perhaps gross overloads during the end of a press cycle. In this example the motor has time to cool off regularly with no extended overheating period. But, compressors, pumps, and vacuum pumps can sit there demanding full load out of the driving motor for minutes, hours, and even days in some cases. Those type applications don't pan out in your, "I'll just tweak things and see if it works" case. It leads quickly to burnouts that sometimes cause thousands in damage, big delays, and ruined work/product.

Look what effort you're spending hoping to get this scheme to work! It isn't worth it and the results are not going to fly. You can't transport cattle on a bicycle. Replace the unit with a used one (your budget) and move on to doing things you can save on.

faq237-1224

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

1 You won't get more torque without overloading the motor.
2 To get maximum torque at 60 Hz you will have to provide 264 Volts.
You need to gain 64 Volts.
If you use an auto-transformer with a primary of 240 Volts, you will need a secondary of 53 Volts to get a 64 Volt boost with 200 Volts on the primary. Not a standard transformer.
If you can find a transformer rated at 240:48 Volts and rated for boost, it will give you a 20% boost. Those are standard ratings even though 48 Volt secondaries may not be plentiful. That will take your 200 Volts up to 240 Volts. Now a second auto-transformer rated 240:24 Volts will take you up to 264 Volts.
After you locate and price out these transformers, check the price of a VFD to run the pump on 50 Hz. If the pump won't run on 200 Volts/50 Hz, then drop the frequency to 45 Hz. That should give you full torque at 200 Volts/50 Hz.
Bear in mind, I don't know the speed/torque characteristics of a vacuum pump. I don't know how heavily loaded the motor is.
If the motor is loaded up to the design horsepower and if the torque demand increases with speed, the pump probably won't run on 264 Volts 60 Hz without overloading anyway.
A small VFD for each pump may be more economical than one large inverter to power all the equipment.
Re torque and saturation:
The maximum torque is determined by the maximum magnetic field strength. Motors typically have a field strength close to saturation.
Slip and heat. You are correct. starting is a high slip event and the heat builds up rapidly. That is why some motors are limited to 3 or 2 or even 1 start per hour. A start only lasts for a few seconds. A few minutes at high slip conditions may destroy the motor from excess heat.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[gornea]

Hi,

Thanks a lot for all the tips! I experimented a little bit and finally managed to make it work with 277 V at 60 Hz. I used the neutral to phase of a 480 V line. The pump is running very smoothly and reaches an ultimate vacuum of about 15 mtorr with the motor pulling 2.4 A. The temperature on the body of the motor reaches a maximum in the middle of the distance between the fan and the pump itself. The temperature increases with time but gets flat at ~ 48.5 deg. C after a few minutes. I have never measured the temperature when it run on 50 Hz but feeling by hand it was similar. Possibly in the long run the lifetime of the motor is going to be shorter. The major remaining issue is electrical certification and maybe here I'll run out of luck!

Cheers,
Razvan

 

[waross]

In Switzerland it was actually running fine on 230 V 50 Hz.

230 V x 60 hz/50Hz = 276 Volts.
Nice solution.
Thanks for the update.

Bill
--------------------
"Why not the best?"
Jimmy Carter