240v supply to 400v motor (to our electrical gurus)

mitsue

mitsue

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I have an Excello milling machine that the motor was rewound to 440v only. I found this article at the link below.


http://www.inverterdrive.com/HowTo/240V-Supply-to-a-400V-AC-Motor/

(quote)

400_240_Graph.gif

It is not uncommon for home workshops to include machinery requiring a 3 phase 400V supply (often marked 380 – 420V). This can appear to be a problem where a 3 phase supply isn’t immediately available. However, it is possible to run such equipment from a domestic 230V supply using nothing more than an off the shelf Inverter Drive.
If you are unable to change the connections or windings to 240V 3 phase, then read on for the next best solution... The speed of an Electric Motor is determined by Voltage and Frequency. So, a 400V 50Hz Motor will run at rated speed at 400V/50Hz and half rated speed at 200V/25Hz. Provided this Voltage/Frequency ratio is maintained, the Motor will operate at full Torque – ideal in a Lathe application where speed must remain constant even when a load (the tool) is applied (to a work piece).
An Inverter Drive is not only able to convert a 230V single phase supply to 230V 3 phase but it also controls both the output Frequency and Voltage to maintain the correct ratio. It therefore follows that a 400V/50Hz Motor will operate normally at 230V and 29Hz, just at two thirds the speed (eg. 1000rpm instead of 1500rpm).
The parameter to set in the Inverter Drive is "Base Frequency" or "Nominal Frequency" (depending on the manufacturer) in Motor Settings.
Above 29Hz, the Motor will become progressively "Under-Fluxed" (Torque will reduce) as a 230V Inverter does not normally exceed its supply voltage. However, this will only be evident if the Motor is fully loaded. If it is, the Motor will look for more current to meet the load. A correctly set Inverter will provide protection against this by reducing the speed automatically to bring the load current under the maximum "over-current" set.
It is important to note that when an Inverters output voltage does not match the Motors rated voltage, specifying the correct Inverter is not a simple case of matchingkW and it should instead be matched to the Motor full load current.
Full Load Current of 400V x 50Hz 4 Pole motors are as follows:-

  • 5.5kW (7.5HP) - 11.4A
  • 4.0kW (5.4HP) - 8.6A
  • 3.0kW (4HP) - 6.4A
  • 2.2kW (3HP) - 4.9A
  • 1.5kW (2HP) - 3.6A
  • 1.1kW (1.5HP) - 2.7A
  • 0.75kW (1HP) - 2.0A
  • 0.55kW (0.75HP) - 1.5A
  • 0.37kW (0.5HP) - 1.2A
  • 0.25kW (0.33HP) - 0.8A
  • 0.18kW (0.25HP) - 0.63A
  • 0.12kW (0.16HP) - 0.44A
Three Phase Output Current of 230V Single Phase Input Inverters are as follows:-

  • 3.0kW (4HP) - 12.6A
  • 2.2kW (3HP) - 9.8A
  • 1.5kW (2HP) - 7.5A
  • 1.1kW (1.5HP) - 6.7A
  • 0.75kW (1HP) - 4.7A
  • 0.55kW (0.75HP) - 3A
  • 0.37kW (0.5HP) - 2.4A
  • 0.25kW (0.33HP) - 1.7A
  • 0.18kW (0.25HP) - 0.63A
  • 0.12kW (0.16HP) - 0.44A
If the sacrifice of some top speed performance is acceptable for the application, this method provides an excellent low cost solution in allowing domestic workshop use of small Industrial Machine tools.
(endquote)



If I get this right, I should be able to set the drive at 40hz, my 11/2hp 1200rpm 440v motor should run 1hp, 800rpm.(machine 66- 2500 rpm spindle, running 44-1630 rpm with gearbox and variable belt drive) I loose power only if I want to run the motor at higher rpm/hz ,probably won't be a problem with smaller cutters.
This works on a lathe, should be no problem on a mill.
What do you think?

Darcy
 
Well, that's what they are saying. That's the first time I've ever seen anything in print that said it, but if they are willing to stand behind it with their product, I don't see what the gamble would be unless that's way off and the motor is hurt by running under-voltage. But the main effect of under voltage is a higher current draw, and they claim to be able to compensate for that and protect the motor.
 
I never heard of this before either. It made me remember that I have used 240v component fans at half voltage to make them run quieter on amplifiers and on home theater units. 10 years later they are still running.

I will try the science experiment since the motor I have would have to be rewound to work on lower voltage anyway. I will monitor the motor temps and make sure the rpm's don't drop while cutting. It's that or I spend $500 for a 240 in, 415v out drive. I'd rather use the drive I have and get a new motor (or rewind) at 240v if it fails. I'll post the results next week. SWMBO has me busy this weekend.:))
 
It's not uncommon to use a dry transformer for stepping up/down voltages for motor use. I have a 480/240 transformer just in the case I run into a situation like yours. Might be worth looking at instead of the inverter they sell. Can't say how the prices would compare.
 
The transformer idea crossed my mind also. I have two 3hp and one 10hp drives sitting here so I thought I would set up the full load amps parameter to the old motor's 240v plate rating of 6.2 amps and give it a try. I've been looking for a 240/480 transformer to keep around for testing purposes. (I don't want to dedicate it to a single machine)
 
What is wrong with setting the xfmr up for the largest load and then wiring your other machines to it. I don't think you will be running more than one machine at a time will you? Xfmrs are rated for 100% duty cycle with a 125% overload capacity. It is just a chunk of iron with cu or al wrapped around it. If you supply the proper overload protection the worst you can do is trip something or blow a fuse. :lmao: If you are worried about it, set the machine connections up with plugs. Then unplug the machines not being used so someone can not overload the system.
 
rdhem2 said:
What is wrong with setting the xfmr up for the largest load and then wiring your other machines to it. I don't think you will be running more than one machine at a time will you? Xfmrs are rated for 100% duty cycle with a 125% overload capacity. It is just a chunk of iron with cu or al wrapped around it. If you supply the proper overload protection the worst you can do is trip something or blow a fuse. :lmao: If you are worried about it, set the machine connections up with plugs. Then unplug the machines not being used so someone can not overload the system.
Click to expand...

That is also a good way to go if I had three phase in the shop already, then I would reconnect my other machines to 440 volt and run less amps with the higher voltage. (Personal choice: I would have built a rotary converter years ago but I couldn't stand the noise or the wild leg balancing act with multiple machines. I fell in love with ac and dc drives.) If I had three phase in the shop I would add a circut with a disconnect into the transformer, then out to a 440v sub panel with plugs out from that.

Bear in mind I have 240 volt, 80 amp single phase service panel for the machines. All on twist lock plugs. If I go the transformer route then I have to buy a transformer and a 415v in, 415v out drive as well, or buy the 240v in/415v out drive from drives direct for $450+ shipping. Since I have three 240v in/out drives here not being used, either this will work or I will have the motor rewound to 240 volt. That still saves me the money for the drive and whatever the transformer costs.(probably $500. at least).

This mill is the only 440 volt machine in the shop that is likely to stay for a while. I like to have each machine power setup alone since they seem to come and go around here, and I find it far easier to sell 240 volt single phase input machines.

Lately I have been running the 8x24 surface grinder steady, 7x12 saw when I get to it, and either my old mill or the lathe at the same time. Makes it hard to drink my coffee before it gets cold. (I may have to switch to beer:drink:)

And I just want to see if this will work, feeding the 440v motor 240 volts at reduced hertz. This costs me nothing to try and only the price of a rewind if it doesn't.
thanks,
Darcy
 
I believe that your original idea to run the motor at reduced speed and voltage is valid. You will have to "hack" the VFD settings to get the proper V/Hz curve, but that should not be too difficult (just like the curves you present in your original post). You will have to probably limit the upper speed to keep from operating too far into the "reduced flux" region. The newest VFDs from Teco and others include a sensorless "torque boost" feature that recovers some of the torque lost by the older V/Hz design, but I doubt that they can get you back to the rated torque of the motor.

Just for reference, I have an 1800 rpm, 3 HP, 3PH lathe that I run on a VFD to get the 3 PH power. I have taken to running at 30 Hz just about all of the time because the lathe is much smoother and quieter at that speed. I have yet to come up short on torque with anything that I have done. The drive is a Teco JNEV series, and I use the torque boost feature with the original factory settings.

I was advised by a profession EE friend (who does a lot of work with industrial VFDs) to use lowest possible carrier frequency - usually around 4 kHz. Newer drives allow you to run the carrier freq well above 10 kHz where older ears cannot hear the whine produced in the motor. His advice is that the high frequency carrier is harder on motor insulation, especially on older motors. I run mine at 4 kHz and there is some whine in the motor, but the volume is small compared to all the belt noise and gearbox noise that comes along with running the lathe.

Terry S.
 
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