VFD for 165 Volt motor? have some questions...

Hi Guys i have an old mazak quick turn 10 with some spindle drive issues. Im considering replacing the drive and have some questions. This machine has a motor rated at 7.5kw, 165v and 40a. Replacement drives are going to be rated at 240 of course so it looks like i would have to move up to a 11kw drive to get the 40a that this motor needs right?

Heres the big concern, Im running off a 30hp phase converter so dont have a ton of power available. If i buy a 11kw drive and set amps right for my motor, will the drive draw more current or would it be the same as before? I realize the higher voltage will have the potential to provide more torque at higher speed which would mean more power, but lets say a low speed of maybe 1000 rpm. Will the new drive draw the same amps as before or more?

You can set motor rated voltage to be the 165V, and the frequency there to be whatever is required, That will fix the issue of over-voltaging the motor. Current will be whatever you set, max, up to the drive capability..

if you set up the drive to work the same as the old one, odds are that the input current will be similar, unless the old one is in some other way special, as some can be in similar equipment. Will it be exactly the same? hard to say, likely not exact, since some design factors will likely be different. But also unlikely to be radically different.

For determining the drive current, it matters if the 40A was continuous, or shorter term. if continuous at 40A, then, yes, you want that on the replacement. Don't worry so much about the kW, look at current capability.

Typically what this is, comes from the fact that the intended use is at 300Hz. So they take a VFD set up for 200V 400hz maximum (establishing the V/Hz ratio) and run it at 300Hz, which comes out to be 165V. If the motor needs 40A at 165V, you must have 40A available from the VFD, which as you said makes it a 7.5kW (10HP) 240V drive. Amp draw equates to torque produced and it stays relatively flat if the machine torque requirement remains flat. So at 1000RPM if the load requires the full amount of torque that motor is capable of, the current will still be 40A. The input current will be slightly less, but not enough to get excited about.

Actually - I have helped a few guys on this board who have replaced older spindle drives on Japanese machine tools (and a few US made machines as well) with similar spindle V/Hz characteristics.

Our observations have been as follows . . .

Line current = motor current x 165/240/.95

As such, if your motor is pulling 40Amps - generally we see line amps at the drive at 29Amps. We did several tests and found that this scales all the way to zero speed . . . if you are running 40A at 10% rated speed (16.5V) you only pull about 3Amps at the line. These were all spindles with encoders running in closed loop FOC mode.

Ok great, so looks like id probably be fine to replace it with an 11kw drive. I actually got the old drive working today, but ill definitely keep this in mind for the future. This drive is 35 years old so not sure how much longer it will last. Thanks again for the help guys.

Which is more or less to say that you draw based on the power used, just like the original unit.

When you run below max voltage, the drive acts as a buck regulator, and you SHOULD expect to see a reduction of input amps in proportion to output vs input voltage. That's part of the "power transformation". Power must go to the load or the losses, and losses are small.

Power factor for 3phase inputs is generally good, much better than with single phase. So not much added current due to bad pf.

The original drive should have done the same, assuming the same input voltage, so the new and old drives should be pretty similar as to input current.

Yes, but remember that a drive is rated by its OUTPUT current, i.e. what the transistors can handle not what it draws from the line. If the motor is going to draw 40A, you need a drive rated for 40A on the output side.

Curious: base speed on nameplate is 1800rpm? What is max speed?

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Jraef said:

Yes, but remember that a drive is rated by its OUTPUT current, i.e. what the transistors can handle not what it draws from the line. If the motor is going to draw 40A, you need a drive rated for 40A on the output side.

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Naturally. That's a basic requirement.

A very significant one for any drive designer, that affects the choice of output devices and everything else about a drive.