Bridgeport Series 2 VFD conversion.

[David Leahy 1Participant @davidleahy1]

I have a series 2 mill and have used it for many years with a rotary converter (RC). Today the converter died and prior to attempting to diagnose the problem with the RC I was thinking I may upgrade to a VFD. My question is can I use a cheap Chinese VFD, that can convert 240v to 380V?

I am familiar with posts on this forum about how the VFD needs to be connected direct to the motor but do I need to change the motor set up? ie DELTA or STAR?

The Chinese VFD’s look exceptional value if they work……??!!

Can anyone recommend a unit that will work my mill from a 240c 32amp fused outlet?

Thankyou in advance.

[Andrew SkinnerParticipant @andrewskinner94774]

Assuming the motor is currently in star, you have a few options:

1. Leave it in star, and get an AT4 VFD (voltage step-up type). The cheaper ones seem to have awful manuals, and there is some debate over whether they are electrically approved (EMC levels). They are often normal VFDs that have been ‘hacked’ to produce the higher voltage.

 

2. Convert the motor to delta* and get a VFD kit from Newton Tesla, if you don’t fancy a self-build.

 

I would go for 2. My TS light vertical runs on a normal VFD with a home-made control box, and it’s transformed the mill. The Newton Tesla kit will likely do the same for yours, being plug-and-play as well. Soft start, speed control, reverse, jog – you’ll wonder how you ever got on without one.

 

*some older motors need a bit of surgery to convert to delta. Either DIY or get a motor rewinder to do it, or even buy a modern 3ph replacement.

[FulmenParticipant @fulmen]

I would rewire the motor to delta. I run my A&S 2J on a 240 VFD off a 16A breaker, never had an issue with that setup.

[old martParticipant @oldmart]

If you can change the motor to delta it will be much easier and cheaper.

I have bought from The Inverter Drive Supermarket and did all the wiring with priceless help from their “quick start guides” which are downloadable and printable pdf’s. This one is 230v single phase input and 1.5Kw which is for a 2hp motor. That will only need a standard 13A plug.

If your motor is 4hp, then you will need a bigger inverter and direct wiring from your 32A supply. Having proper remote controls is recommended.

 

inverterdrive.com/…/Invertek-E3-Easy-Start-Guide-IP20-3ph

[Tony RayParticipant @tonyray65007]

Another vote for rewiring the motor to delta. I too am a repeat customer of Inverter Drive Supermarket they are very helpful on the phone should you need support. You can download the quick start and full user manuals and decide which one is for you before you buy, some VFD manuals are far easier to understand than others. Do not over specify the VFD you do not need a higher capacity than the motor rating, in your case you probably have a 3HP 2.2kW motor.

[FulmenParticipant @fulmen]

Actually you don’t need a larger VFD than the supply allows either. You can easily run a large motor on an undersized supply using a VFD, since they can limit the current by dropping the frequency.

[Robert Atkinson 2Participant @robertatkinson2]

Fulmen said:
“Actually you don’t need a larger VFD than the supply allows either. You can easily run a large motor on an undersized supply using a VFD, since they can limit the current by dropping the frequency.”

Pedant alert: The VFD does not limit the current by changing the frequency (dropping the frequency increases the current b.t.w) It controls the power into the motor by adjusting the ratio of on/off time of the high frequency switches. This is caused pulse width modulation (PWM).

I agree you should not need a VFD rated for more than the motor rating. Any VFD seller suggesting otherwise is peddling sub-standard items. Effectively making you pay more to reduce the chances of them having to replace the VFD.

Conversely, if you had a machine with a 4HP (~3kW) or larger motor, which would be marginal to run from a UK domestic outlet, you could fit a 2 to 2.75kW VFD and accept the lower rated power. A hobbyist is unlikely to making the demands of commercial production the motor was sized for.

[Robert Atkinson 2Participant @robertatkinson2]

The Inverter Drive Supermarket quick start guide Old Mart linked has significant limitations.
They cover themselves by saying it is only a supplment to the OEM instructions. It make virtually no mention of electrical safety, how to wire a E-Stop, fuses, isolator etc.
Most importantly they make no mention of the need for an enclosure. The OEM manual for the drive says that unless th drive is in a pollution degree 1 environment it must be in an enclosure. Pollution degree 1 is no pollution or completly dry and any particulates not capable of causing any harm.
This is clearly not a model engineering workshop and certainly not on the wall behind the lathe. Also required is adequate cable support.

VFD’s MUST BE INSTALLED IN A SUITABLE ENCLOSURE!

Robert.

[FulmenParticipant @fulmen]

I’m not one to argue against sound theory, but in reality the current draw drops with frequency. Now for the same power output I’m sure you’re right, but if you try to pull a higher load than the VFD can supply it will simply reduce the frequency. And it works, as I have seen for myself countless times.

We had an old mill with a rather large gear train for the feeds. In the cold the VFD would bog down to a low frequency as the start up current got too high. The fix turned out to be a 5 second soft start.

[Fulmen]

On Fulmen Said:

…argue against sound theory, but in reality the current draw drops with frequency.

There seems to be some confusion about how a motor works as opposed to a VFD. Robert is correct in that for an induction motor, below base speed, the current will rise as the frequency decreases. Basic VFD control counters this by reducing the voltage and keeping the current the same.

When a current limit is reached a simple VFD solution is to shut down with an overload. But more complex VFDs can be programmed to reduce frequency, and hence speed, while maintaining current. This is a very short term solution as the power is also reduced. So if the mill is struggling with a cut then reducing power is not a long term solution. But it might help, especially for a manual mill, where the operator can also reduce feedrate.

Andrew

[Robert Atkinson 2Participant @robertatkinson2]

During a soft start the Voltage (by PWM) is reduced to reduce the current.  The frequency is reduced to lower the mechanical loading not the current. The power required is proportional to the accelleration. Zero to full speed instantly requires infinte power. Having a low inital speed “target” (frequency) and slowly increasing it reduces the power required. Note the energy required is the same (OK some losses are present for a longer time so technically a bit more energy but pretty insignificant).
If you reduce the frequency without reducing the voltage the current rises in inverse proportion. For an inductance, i.e. motor at rest, current is voltage divided by impedance and impedance is proportional to frequency (2Pi fL). As the motor sppeds up the back EMF reduces the current. In theory a perfect motor with no load runnning at syncronous speed would consume no power.

Trying to “soft start” a motor at say 5Hz without reducing the voltage would result in 10 times the current not a reduction.

This is also why current, and thus torque, drops off as you run a motor at higher than rated speed (frequency) unless the VFD has voltage boosting capability.

 

Edit: Crossed with Andrew’s post. Andrew knows more about the internal operation than I do.

 

[Robert Atkinson 2]

On Robert Atkinson 2 Said:

If you reduce the frequency without reducing the voltage

I’ll take your word for it, it’s obvious that you guys know more about motors than me.

But I never made that claim. All I said was ” they can limit the current by dropping the frequency”, and they do. At least mine Lenze does this, and it’s a pretty average unit.

Please bear in mind that I’m talking about supply current here, not motor current. We were running a larger 2-3HP, 3 phase mill on a shared single phase 10A circuit at the time, so we had to keep the current draw down.

[Robert Atkinson 2Participant @robertatkinson2]

Saying input current clarifies things.

It would still be more correct to say that “The VFD reduces the start-up input current by starting at a low speed and slowly accelerating. The speed is reduced by lowering the frequency of the VFD output”.
Spreading the acceleration from zero to running speed over a longer period reduces the mechanical power required by the start-up. Energy required stays the same.

Robert.

[Fulmen]

On Fulmen Said:

On Robert Atkinson 2 Said:

If you reduce the frequency without reducing the voltage

…I’m talking about supply current here, not motor current.

Ah, we only had half the story, and the wrong half at that!

It makes more sense now. If the VFD reduces the motor speed, and reduces motor voltage to keep the phase currents constant, then the power going to the motor will be reduced. That power reduction will be reflected in a decrease in supply current.

Out of idle curiosity how is the supply current being measured?

Andrew

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