Soft Start/VFD for High Interia load?

We have a large 300 ton Southwark-Baldwin Traveling gantry straightening press, imagine a large gantry crane riding on crane rails on the floor, instead of a hoist on the I beams, it has a large hydraulic ram. The gantry itself is powered by a 5hp motor geared to jackshaft that runs to both sides of the machine, driving the wheels on the rails. I estimate that the gantry itself weighs in at at least 10,000 lbs. The problem is, it has no soft start, and at least 10 gears are being driven to make the gantry move along its path of 22 feet.As you can imagine, starting off, it dog tracks if there is anything on the wheels, or crane wheels causing slippage, not to mention the jerk from the instantaneous start.
I have looked at soft starters, but most are only rated for 10 starts per hour. We could do 10 starts in twenty minutes straightening parts, or moving the gantry out of the way to load/unload parts. I have also considered a VFD obviously, and have a AC Tech SMVector for a 5hp motor laying around from a previous project, however the motor on the gantry is an original GE motor, and don't want to burn it up putting a VFD on a non VFD compatible motor. The starts and stops are from momentary buttons, and would like to keep those, as there are three stations, 1 on each side of the press, and a pendant.The main objective to slow the wear and tear on the gears and drive mechanism, not to mention easier positioning of the gantry itself.
Thoughts and suggestions? I'm not an electrical nut, but should be able to accomplish a task such as this.

Get a GOOD VFD around 10 hp capacity with cooling fan (most GOOD units have them)

Wire the VFD for your application and set the ramp up and down parameters to best suit your needs.

Relax and enjoy!

A VFD is a good soft-starter, so it's a good enough plan.

As for the motor, if you put an inductor on the output of the VFD, that should protect the motor. You seem to just want to start it softly, and are not going to try to run it slow for long periods, so all you really need is some protection of the motor from spike voltages. The 3 phase inductor should do that. Talk to the VFD vendor about an appropriate one..

They have starters that can handle a start every 2 minutes.

Soft Starters Selector | AutomationDirect

Almost any soft starter can be used at a higher starts-per-hour, you just have to over size it. The example above is going to tell you to triple the size, indicating that their product is starting out with relatively low capacity. If you start off with a good quality heavy duty rated soft starter, you can likely get away with just doubling the size.

I would do it with a VFD though. A soft start is "soft" because it REDUCES the starting torque. If you NEED the starting torque, that actually works against you, meaning if the load is really high inertia like this, the soft starter will likely need to ramp up to 80% voltage before the load even starts to move. That will make your MOTOR starts-per-hour situation worse. A VFD on the other hand can make the motor develop FULL torque immediately, or ramp into it, but without causing the motor to over heat at all, so it will be BETTER on the motor from a starts-per-hour perspective.

THAT VFD might not be the best choice from the standpoint of longevity in this kind of application, but you already have it, so I would try it. And as JST said, put a load reactor between the VFD and motor or if you are more concerned about the motor than the money, use a dV/dt filter or even what's called a "Sine Wave" filter. Warning though, a Sine Wave filter may cost more than the VFD or motor...

Agree with Jraef.

This is not the correct application for a soft start. A soft start is really intended for starting centrifugal loads (like fans and pumps) where you have a soft power system and the voltage sags a lot when the motor starts. Using a soft start greatly reduces the voltage sag and the low torque is not a problem in an application like that. Sometimes folks misapply them and get away with it.

A good VFD is definitely the way to go on a crane.

Jraef said:

Almost any soft starter can be used at a higher starts-per-hour, you just have to over size it. The example above is going to tell you to triple the size, indicating that their product is starting out with relatively low capacity. If you start off with a good quality heavy duty rated soft starter, you can likely get away with just doubling the size.

I would do it with a VFD though. A soft start is "soft" because it REDUCES the starting torque. If you NEED the starting torque, that actually works against you, meaning if the load is really high inertia like this, the soft starter will likely need to ramp up to 80% voltage before the load even starts to move. That will make your MOTOR starts-per-hour situation worse. A VFD on the other hand can make the motor develop FULL torque immediately, or ramp into it, but without causing the motor to over heat at all, so it will be BETTER on the motor from a starts-per-hour perspective.

THAT VFD might not be the best choice from the standpoint of longevity in this kind of application, but you already have it, so I would try it. And as JST said, put a load reactor between the VFD and motor or if you are more concerned about the motor than the money, use a dV/dt filter or even what's called a "Sine Wave" filter. Warning though, a Sine Wave filter may cost more than the VFD or motor...

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Thank you sir!! I did call Wolf Automation which is whom I got the drive from years ago, and it should work, although the amp draw of the motor and amp rating on the drive are very close. The next step is wiring the momentary forward and reverse contacts since there are three locations on the machine. According to Wolf, this should be easy to do, we shall see.

The old fashion way is to put a heavy cast iron coolant fin on the motor
SEW used to sell those

Peter

Assuming that your high inertia load also has some kind of brake - if the present brake is tied to the motor T-leads, you will have to rewired and add a contactor. You will also need a dynamic braking resistor to shunt energy from the drive when decelerating and logic to manage the brake contactor. Not difficult to do, but does require some forethought.

motion guru said:

Assuming that your high inertia load also has some kind of brake - if the present brake is tied to the motor T-leads, you will have to rewired and add a contactor. You will also need a dynamic braking resistor to shunt energy from the drive when decelerating and logic to manage the brake contactor. Not difficult to do, but does require some forethought.

Click to expand...

No brake on the load. The motor is only spinning at 810 rpm. Will that effect the use of a vfd?

dirk said:

No brake on the load. The motor is only spinning at 810 rpm. Will that effect the use of a vfd?

Click to expand...

Is that the motor speed or the gantry speed? If that is the motor shaft speed, that means it is an 8 pole motor with about 10% slip. So don't assume a standard 5HP drive will work, that motor will have a higher than normal FLA. VFDs must be sized by the motor FLA, HP is just a convenience.

Jraef said:

Is that the motor speed or the gantry speed? If that is the motor shaft speed, that means it is an 8 pole motor with about 10% slip. So don't assume a standard 5HP drive will work, that motor will have a higher than normal FLA. VFDs must be sized by the motor FLA, HP is just a convenience.

Click to expand...

That is the motor shaft speed. The rated amps on the motor are less than on the VFD. I read that somewhere before, and double checked it. If motor speed isn't an issue, than I'll hook up this thing up and see if it works.

Side topic, since there are three pendant stations, and they will be controlled by the voltage from the VFD, are there any control blocks that would clean up the wiring?

I think this is a perfect application for a VFD.

When you program the VFD, pay attention to the 'ramp' rates... bring it up gently, allow it to slow gently... that way when you 'jog' it, you won't be getting surges, and you'll have the ability to be very precise in positioning the gantry. You might even wanna modify your pendants to give you a high or low-speed choice, or mebbie even find a way to put speed control potentiometer on the pendant.

As for multiple pendants, you'll have to re-think how the controls work... it may be best to re-use the entire control scheme... just disconnect the prior motor starter/controller from the pendant system, and use the pendant system's connections as-is, to signal the VFD's direction and speed controls.

I don't think your motor will be under any dramatic stress from the VFD's output... mainly because your slow-speed motor will have plenty of it's own inductance to knock out any HF switching noise generated by the VFD... but adding the inductor is certainly a worthy consideration. I run 'old' motors in my stuff all the time, and have not had any suffer problems from VFD switching.

My next major VFD project will be similar to yours... a 32ft long 10-ton Shepard bridge crane from 1917... running the original 3-phase 480v motors for both travel axis AND... the hoist motor. Each axis will have it's own VFD and brake contactor, as each has a dedicated electric brake. My control pendant will be wireless... using a RC aircraft transmitter. All the controls and drives will be mounted in the footwell of the Sheperd's original 'cab', and instead of a trio of busbars running the length of track, there'll just be a travelling cable on rolling track to follow it from end-to-end.