Reply by ondablade

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Posted on Old three phase motors and Variable Frequency Drives (VFD)

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105 posts in 3224 days

#1 posted 04-04-2010 07:53 PM

It may be a little different to the US, but over here in Ireland i’m using a 4kW Omron/Yaskawa V1000VFD to step up the rpm of the 3 phase 50Hz motor on the Bill Pentz dust sysytem i’m building from the standard 2 pole 2850 rpm to the 3,450 rpm you get on the US 60Hz motor the blower was designed for.

The variable speed is incidental so far as i’m concerned, it’ll be set and forgotten. The VFD does have some other advantages however from what i’ve read in recent weeks though:

1. The programmable soft start the VFD almost eliminates the additional current drawn by the motor on start up. (it would draw six times the full line current if started normally DOL or direct on line) This is important for me as i would otherwise risk popping the incoming 62A fuse on my 220V single phase supply.

2. A decent VFD gives very high levels of protection in the event of overloading and so on. It also avoids the need so far as i know for an NVR (no voltage relay – drops the contacts out for safety in the event of loss of supply) in the starting switch as it contains this function.

3. They give more or less full torque right down to the lower RPMs – but you need to be careful about the limited cooling that results from lowered fan RPMs.

The downside my supplier said is that it’s not reckoned to be a good idea to run a cheap eastern or old motor with one, presumably because of the insulation and other extra stresses it can result in if run over or under the stock RPM.

It also happens that for the majority of single phase to three phase VFRs that don’t step up the voltage (i for example am getting 3 phase power at our single phase 220V – instead of the usual 380V) you need a dual voltage motor connected in delta. This isn’t an issue for most modern motors, but older and cheaper motors may not have this facility.

The reason for this is that to get the same HP at 220V as 380V the windings have to handle a much higher current – which in the specific situation where old three phase motors are being run on single phase power via an inverter without a voltage step up transformer is presumably the reason they often quit.

Don’t ask any complicated questions as this is just what i’ve gleaned in the past week – i’m no expert!!! :-)


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