VFD Current Limit To Protect Upstream Circuit

hkinkade · Nov 30, 2016

I just purchased a large table saw powered by a 7.5 hp motor. I plan on driving the motor by a 7.5 hp VFD (used as phase converter ). The circuit is 240V 30 amp. I won't likely need 7.5 hp for my applications so I plan on setting the rated current of the motor in the VFD below motor nameplate and below circuit rating (say 25-28 amps). This would limit the motor to just below or near 5 hp, but that should be fine for my apps. Does anyone see a prblem with my approach. I don't want to have to rewire the circuit.

Bill D · Nov 30, 2016

My lathe came with a 3hp motor running from a 1hp vfd. I not used it for heavy cuts so no problems.
For you saw you may want to apply braking when you turn it off. I would have it stop in not less then 3 seconds. If you ramp up the speed over say the same 3 seconds peak amps will be fine on your existing circuit.
Bill D.

traditional-tools · Dec 1, 2016

Bill D said:
My lathe came with a 3hp motor running from a 1hp vfd.

That doesn't make any sense at all.

JST · Dec 1, 2016

traditional-tools said:
That doesn't make any sense at all.

It's possible to do that, but that is about the limit, and a table saw is not the tool I would try it with. Many motors have idle current nearly at the current limit of a VFD rated 1/3 the HP of the motor, so the amount of allowable current past that (and saw power available) is low.

A lathe may be able to get away with that, because a lathe is not overloaded commonly the way a table saw is. Table saws get a lot of abuse as far as overloads. Even with a lathe, a heavy chuck, or work may lead to a trip on starting if the machine does not use a clutch. And there isnt much power available without going over the current limit.

5HP on a 7.5 HP saw may be OK if you don't need full power. You may need to get the acceleration set so it does not trip out when starting. Too fast and too slow can both have problems, there will probably be a range where it works best. A table saw typically has a good deal of inertia.

Forrest Addy · Dec 1, 2016

There's no reason you cn't use your VFD in the way you suggest however your saw may not accpeerat as briskly as you might wish. You should realize the VFD acts much like a variable transformer in that its demand VA is proportionate to the load and the output setting. ln a 60 Hz system, if the VFD is set for 30 Hz the motor volts delivered by the VFD to the motor will be half the line voltage. Demand by the VFD for a 10 amp load from the line will be 5 Amps.

If you set the VFD for 60 Hz forever and control the connected saw motor from the VFD fine. You can also use it to run other motors one at a time. If you turn down the VFD to run , say - a drill press, and forget to rest the VFD speed, your table saw will run at the existing set point provoking a face palm. These are no-brainer points but VFD's can be tricky to use if all the little details aren't taken into account:

Bill D · Dec 1, 2016

traditional-tools said:
That doesn't make any sense at all.

Well it has a 3 horsepower motor that they hooked up to a 120 volt input VFD that has a nameplate saying it is for one horsepower output maximum. I did not say it was something I would have done but it worked.
Bil lD.

motion guru · Dec 1, 2016

You say that you want to set the drive to limit current at the motor to something on the order of 25-28 Amps - this is significantly higher than a typical 7.5HP rated motor (normally 22A at 230VAC).

You also need to consider that you are feeding the drive with single phase. Equivalent current for a single phase line at 230V when sourcing a given amount of power is substantially higher than the 3 phase current being delivered to the motor at that same power level.

So if you want to limit your circuit draw to 30Amps (single phase) . . . 230VAC @ 30A = (6.9kW at unity power factor) . . . the 3-phase equivalent current at unity power factor is 17.33A. If you consider that your drive is roughly 95% efficient, you would need to set your nameplate data in the drive to limit current at the motor to roughly 16.5A.

And as others have suggested, the drive should be set up to limit acceleration to something reasonable.

Setting motor current limit to 16.5A in this way will ensure that the single phase line draw will not exceed 30Amps. Actual mechanical power delivered to the shaft at 230V and 16.5A will nominally be 5.4HP +/- depending on motor quality and efficiency.

Lakeside53 · Dec 1, 2016

I suspect you will trip the vfd often as it's very easy to overload a saw even briefly. Considering the cost of the single phase input vfd, I'd just replace the motor with say 3hp single phase.

You should also limit you single phase input to 80% of 30 amps to stop nuisance tripping of your breakers, particularly if you breakers are typcial resident types.

hkinkade · Dec 22, 2016

I'm a little farther into my conversion of the Powermatic 72. Motion Guru, I forgot the single to three phase conversion issue (probably explains why I became a mech. engr. and not an EE). Ok, so I'll set the VFD to a current limit of 16 ish amps. The next question / issue is I have a 5.5 kw VFD rated for single or three phase input. I plan on wiring it for the full 5.5 kw rating, so with single phase input I'll need 6 AWG wire size! Dang those terminal screws on my VFD seem really small (landing is around .3" wide with a screw diameter of around .15"). Does this seem right, where could I find a lug that would take 6 awg wire on connect to this terminal strip?

Lakeside53 · Dec 22, 2016

No, doesn't seem right. 6awg would be a 60 amp circuit. Does your vfd say you need that? A real single phase 5hp motor would have an FLA of about 25-28 amps at 240v - roughtly what your vfd will pull at that output. VFD often have input breaker/fuses higher than just FLA, but...

What is the exact model of your vfd?

dazz · Dec 22, 2016

Hello
There are two issue here.
Protection of the VFD/motor and protection of the supply circuit.

The VFD requires short circuit and overload protection. These are usually provided by a fuse and circuit breaker mounted in close proximity to the VFD (ie. in the same enclosure). The manual for the VFD should provide recommended protection options.

The cabling between the switchboard and machine (fitted with the VFD) also needs protection. In your case, it should be a HRC fuse or circuit breaker. 30Amps is not a lot for a 240VAC single phase supply even for a domestic house. Ovens, spa pools and electric water heating will use more than that. I would look at the options around increasing the supply current capacity. If you can't do that, then telling the VFD is is driving a small motor work.

What ever you do needs to comply with local wiring regulations to ensure that your insurance remains in place.

hkinkade · Dec 22, 2016

The supply circuit is protected by a 30A / 220 breaker in the main box. The motor is protected by the VFD (Set to 5HP max. current of about 16 amps on inverter output). VFD is rated to 7.5HP. Motor is 7.5 HP rated, I'm limiting its power electrically at the VFD. I don't expect to ever tap more than 5HP from the motor; hence, shouldn't ever trip breaker or VFD. I'll do a soft start of motor, about 3 seconds, and slow stop 7-10 seconds (the powermatic naturaly takes 10 seconds to shut down when power cut). It does look like there are narrow lug 6 AWG connections available ($6 a pop though, ouch).

Steve in SoCal · Dec 22, 2016

I don't get the need for 6 gauge wire? Your limiting the current to the motor so you can wire the inverter on a 30 amp circuit, 10gauge THHN is fine for this aplication. If you run the saw with a 50 amp service at full power then 6 gauge would be called for.

Steve

dazz · Dec 22, 2016

hkinkade said:
... The motor is protected by the VFD (Set to 5HP max. current of about 16 amps on inverter output). ...

A circuit breaker / fuse combination will protect the VFD A VFD of that size is worth protecting.

JST · Dec 22, 2016

The manual for the VFD will give you specific requirements for both wiring and circuit protection needed to run it. Regardless of the expected output needs, it is best to wire the input and protect the circuit for the full VFD power. That will be good for any later uses, and avoid questions.

You may not be able to do that.

As for limiting the output, yes it is perfectly possible to do that in such a way as to limit input power.

With 30 A, and a motor FLA of 22A(3 phase), you can't really supply the current for that motor at full load. Probably you will be limited to 3 HP or so on 30A.

First, the 22A is going to be more at single phase. It would be 38A at full load in the best case. Then also the VFD draws power at a crummy power factor of perhaps 0.6 or 0.7. So the 38A is really more like 55A, possibly a bit over 60A. That will limit you to about half power, or 3.75 HP.

Allowing 20% over for "headroom", you should not likely plan to draw more than about 3HP. That works with my experience, where a 3 HP VFD draws between 24 and 27A.

So, bottom line is that you should not expect more than 3HP output to the saw, and ought to set the VFD limits for about that, or roughly 9 or 10A to the motor..

Lakeside53 · Dec 22, 2016

dazz said:
Hello
There are two issue here.
Protection of the VFD/motor and protection of the supply circuit.

The VFD requires short circuit and overload protection. These are usually provided by a fuse and circuit breaker mounted in close proximity to the VFD (ie. in the same enclosure). The manual for the VFD should provide recommended protection options.

.

This is not quite true. The vfd is NOT protected by the input power protection. The reason for the specific fusing is 1) to allow the vfd to operate at choosen full power level, and 2) to disconnect the input power on VFD FAILURE. i.e. prevent fires.

The vfd output protection is provided by the vfd, and very well.

Input wire size is determined by the source breaker/fuse protection - by code.

Lakeside53 · Dec 22, 2016

hkinkade said:
The supply circuit is protected by a 30A / 220 breaker in the main box. The motor is protected by the VFD (Set to 5HP max. current of about 16 amps on inverter output). VFD is rated to 7.5HP. Motor is 7.5 HP rated, I'm limiting its power electrically at the VFD. I don't expect to ever tap more than 5HP from the motor; hence, shouldn't ever trip breaker or VFD. I'll do a soft start of motor, about 3 seconds, and slow stop 7-10 seconds (the powermatic naturaly takes 10 seconds to shut down when power cut). It does look like there are narrow lug 6 AWG connections available ($6 a pop though, ouch).

On the derated motor 3 seconds may be somewhat aggressive and result in over-current on acceleration, but a easy fix.

You should not try to do programmed stop less than the natural deceleration time to stop unless you have a braking resistor installed AND/OR program the VFD to take a specific action on DC buss over-voltage (like extend deceleration time). For a table saw.. much easier to simply set it for "freewheel" stop.

As for your wire size, there is no point in putting 6awg wires onto a 30 amp breaker. 10awg is all that is required.

Lakeside53 · Dec 22, 2016

dazz said:
A circuit breaker / fuse combination will protect the VFD A VFD of that size is worth protecting.

The input fusing does not protect the vfd. Assuming it's sized for the output, it's there to protect the input wiring on a vfd failure.

Lakeside53 · Dec 22, 2016

To help the conversation, what is the make/model of your vfd?

Toms Wheels · Dec 22, 2016

I have an RPC of 7,5HP, going into that is a box fused for 60A, wired to match. The feed from the RPC to each of the 4 machines it runs, is #6, each machine has a disconnect box, 2 are fused, the others are just disconnects. At each of those points the output is wired with 10, 12 14 awg depending on HP. The two wired with VFD's one is 10 the other is 12.

No need to run 6 into the VFD, just run it to the disconnect, then small stuff to the vfd.

VFD Current Limit To Protect Upstream Circuit

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