Drive recommendation

Hi all,

I have an application involving an old Barrett Chip Spinner. The title describes it's function. You load in about 150-200 lbs of steel or brass chips into a basket, load the basket into the spinner and then run the sponner for maybe 5-10 mins which spins the oil out of them. Similar to the spin cycle on a washing machine.

The unit takes about 20 minutes to spin down when you stop it. The customer is looking for a faster stopping method. (I would add a braking resistor)

The motor nameplates data is as follows:

0326 frame
220V 3 phase
F.S. Amps - 15.8
Max amps - 90
1200 RPM

I only have 208V 3 phase to work with. I'm looking at an Automation Direct GS2 7.5HP model. The rated output current is 25A.

The machine is currently started across the line right now - no starters / overloads, etc..

I don't have a mfg. date but I know it's pretty old. I'm thinking it would be a good idea to use a line reactor with this motor.

For the safety portion, I'm going to be solenoid (power to unlock) locking the lid when the machine is running / motor moving. Movement is detected by phase voltage safety relay.

Any thoughts?

If the motor is old, you may have problems with the higher voltages and frequencies generated by the drive. Hopefully you are replacing the motor with a new VFD rated one.

A few more data points:

• How long does it take to accelerate when you bang it across the line?
• When you cut power now, how long does it coast?
• How fast do they WANT to stop it?
• If the drive gets toasted, how long are they willing to wait for a replacement?, In other words, how expensive is down time?

ndzied1 said:

If the motor is old, you may have problems with the higher voltages and frequencies generated by the drive. Hopefully you are replacing the motor with a new VFD rated one.

Click to expand...

Unfortunately I have no way to update this motor. I think the line reactor will help with the output.

jraef said:

A few more data points:

• How long does it take to accelerate when you bang it across the line?
• When you cut power now, how long does it coast?
• How fast do they WANT to stop it?
• If the drive gets toasted, how long are they willing to wait for a replacement?, In other words, how expensive is down time?

Click to expand...

* I don't know the ramp up but I could find out. If I had to guess, maybe a minute or two?

* It takes 20 minutes to coast to a stop from full running speed.

* I'd like to stop it in approx. a minute or two.

* Down time isn't a factor right now. If it turns into an issue, I could recommend they order a spare drive.

I would use Yaskawa there not cheap but worth the extra cost.
You should be able to ramp the load to a stop as fast as you ramp it up to speed.
Just be sure that you pay attention to the duty cycle on the braking transistor and resistor
Most internal brake transistors are only rated for a 10% duty cycle
If the motor is old, it should be replaced as the wire insulation is not rated for inverter duty and it's just a matter of time. If you can't replace the motor then at least have the motor rewound and tell the motor shop to use inverter duty wire.
Don't forget to give some thought to the E-Stop control, the inverter can only control the deceleration If it power up and connected to the motor.

A side note: I have been in the industry a long time and only once did I run across a machine that I couldn't replace the motor. I have had to have a motor base adaptors custom made that’s just part of the job. On the one I could not replace the motor on was made around 1915, it had 5 500VDC motors. The motor frame was part of the machine frame casting. We actually had the taken apart on the plant floor sent the fields and armature out to reworked and then reassembled on plant floor while I was building a new control panel. There's always a solution.

If you plan to use a VFD look very carefully at the maximum decel ramp setting allowed in the drive. A lot of drives do not permit long decel settings. I just looked at a GS3 drive manual (only one I have handy) and the max decel setting was 5 minutes.

adisharr said:

* I don't know the ramp up but I could find out. If I had to guess, maybe a minute or two?

* It takes 20 minutes to coast to a stop from full running speed.

* I'd like to stop it in approx. a minute or two.

* Down time isn't a factor right now. If it turns into an issue, I could recommend they order a spare drive.

Click to expand...

Rules:
1) You cannot stop it electrically any faster than it can start across the line. So if it takes a minute or two to start across the line, it will take at LEAST a minute or two to stop electrically. The only way to make it stop faster is with a mechanical brake.

2) If you think you can violate rule #1, you are wrong.

3) Before you get to duty cycle, you have to look at the braking power limits. The cheaper the drive, the cheaper the components. So a cheap drive like that Automation Destruct unit is going to be limited to 125% of the drive rating. So if you COULD stop it in the time it takes to start it, you would require equal power, so 600% of motor FLC. Since the braking transistor is limited to roughly 20% of that, that means it will be roughly 5x the time it takes to start it. So you two minute becomes 10 minutes to stop it.

4) Then as mentioned, duty cycle is CRITICAL in the design. Duty Cycle is defined as the time it takes to go from full speed to full stop, divided by the total run time of a cycle (start, stop, re-start). So if your braking time is 10 minutes at BEST, and your duty cycle is 10%, that means 10% = 10min/Xmin, where Xmin = the total cycle time. Basic math then tells you that the total cycle time is now 100 minutes, which means, minus the 2 minutes to accelerate and 10 minutes to brake it, you must leave it OFF for a minimum of 88 minutes, basically 1-1/2 hours of Off time after each time you brake it. Any faster than that and you fry the braking transistor, which means throwing out the drive (noting is repairable in those).

Thoughts

There are multiples things to consider when contemplating putting a drive on this application.

First : If the load takes 20 minutes to slow down ( Because of the inertia of the metal) Then a 5 min Deccel would be a good boost in production. The wear and tear on the mechanicals of the system could out weigh the benefits of the quicker turn around.

Second: If the motor is very old, Motors were over built as a practice in the earlier days to hedge there bets against warranty and durability issues. If a replacement motor was an option, then it would be wise to oversize the motor by at least 50 % with the correct drive to match.

Third: if the unit acts like a washer, Drives on Industrial washers have specific software to deal with the overhauling load. If the load is unbalanced then it will go into regen mode and potentially trip the drive out on DC Bus overvolatge, so they speed up the frequency of the motor temporarily until the motoring action is needed again. Otherwise , if a DC Brake was used then it would burn up the resistors for the DC Brake because of the heat build up from the Constant intermittent braking. Overhauling loads are not problem on units across the line. If you have multiple baskets , then multiple drives can run on a Shared DC Bus System, when one is in regen, the other might be in motor and the power from the regen is used for the motoring action of the other. This is common in big automotive plants where multiple stamping presses are running.

Fourth: For an E-Stop Case there is a case where you could bring the motor to the fastest stop possible by using a Mechanical Brake and Plug reversing the Motor( Dropping out the Drive , and engaging a Contactor with reversed , or swapped phases, WARNING!!! This is not good for the Motor , It will draw MASSIVE amounts of current, The Motor will Generate huge amounts of Heat, If the Breaker, Wire, and Contactor is not sized according it will trip the breaker , Burn the wire Open, or weld the contactor. If the Breaker trips before the motor is fully stopped, it will free wheel from that point. I have seen this in a old rubber mill , where there was a dead man's rope safety that plug reversed the motor in case some one fell into the huge dutch mill. ) The mechanical brake will be powered open, so safe state it will close, and the contactor can be tied to a centrifugal switch on the shaft in series with a NC set contacts on the Mechanical brake and NO on the E-Stop. Therefore when the mechanical brake is closed and the E-stop is closed , and the motor stops turning the contactor will drop out. two more parallel sets of contacts are needed on the E-stop. One for the Drive run/Ouput contactor control. E-stop is pressed, Drive run drops, and the output contactor opens. The other set of contacts on the E-stop ,NC, will be in series the brake coil and drive run (separate output relay from drive as first, contacts. So the Drive starts, Closes it Run contacts, brake coil is energized, mechanical brake is released. If the E-stop pressed, the mechanical brake is de-energized, closes, once closed; the NC contacts of the brake close, since the NO E-Stop is now Closed, the Motor will Plug reverse until it stops rotating , and the centrifugal switch drops out the plugging contactor.

Fifth: Drive manufacturers make an external brake unit, and DC Braking resistors can be arranged to have a higher duty cycle , and shorting stopping times. The unit monitors the DC Bus, When the bus voltage exceeds a programmed level, it will turn on the DC Brake. The braking units can be sized according to load as well. If the external brake is sized correctly, the braking IGBT and resistors can handle frequent use, and high current from shorter stops.

These are just some places to start. Please do not take my word as gospel, but applications dictate the equipment and control sequence.

The max decel time is really not that important you really want the motor to stop in much less time 60-120 sec. plenty of time.
I don't agree that you can't stop a motor any faster than you can start it. I have seen it done and have done it myself. Client kept wanting a quicker stop he finely backed off when we stripped the gears on a stop never hurt the drive. It cost him plenty to repair the machine. The limitations on a VFD are dumping the energy without destroying the VFD. You have to control the energy.
Most VFD's have a duty cycle of less than 5% using the internal transistor. The limitation is on the transistor if you add an external DBU you can go 100% or higher depending on the components selected.
Did I see this correctly that there is no motor starter or overload on this motor. If that the case then you defiantly need to redo the controls you are risking a major failure.

If you don't need all the features of a VFD then maybe an electronic motor brake will work for you.
Check this link it may be a better choice for you.
I have used a different brand previously they work well in your application
http://www.motortronics.com/pages/motorbrakes_b.html
As with anything duty cycle and proper selection of system components you will be surprised what you can do.

Thanks for all the replies everyone. This gives me some things to think about. I'll have to get back with the customer and see what our options are regarding the budget. I suspect that if this machine goes down they may invest in a newer model. Surprisingly they look very similar and they are build like a Sherman tank.

GaryS said:

The max decel time is really not that important you really want the motor to stop in much less time 60-120 sec. plenty of time.
I don't agree that you can't stop a motor any faster than you can start it. I have seen it done and have done it myself. Client kept wanting a quicker stop he finely backed off when we stripped the gears on a stop never hurt the drive. It cost him plenty to repair the machine. The limitations on a VFD are dumping the energy without destroying the VFD. You have to control the energy.
Most VFD's have a duty cycle of less than 5% using the internal transistor. The limitation is on the transistor if you add an external DBU you can go 100% or higher depending on the components selected.
Did I see this correctly that there is no motor starter or overload on this motor. If that the case then you defiantly need to redo the controls you are risking a major failure.

If you don't need all the features of a VFD then maybe an electronic motor brake will work for you.
Check this link it may be a better choice for you.
I have used a different brand previously they work well in your application
http://www.motortronics.com/pages/motorbrakes_b.html
As with anything duty cycle and proper selection of system components you will be surprised what you can do.

Click to expand...

I'm liking the idea of the motor brake as I have no real need to alter the speed of the motor. The main goal was to reduce the stopping time. I think I'll pursue this avenue along with some safety contactors w/ MOL. Thanks for the idea. We used to sell them years ago but I never did anything with them myself.

For those mentioning E-stops, I'm going to lock the lid when it's running and require it to be locked before they can start it.

I would recommend a LOAD reactor also for the older motor, more than a LINE reactor.

NetNathan said:

I would recommend a LOAD reactor also for the older motor, more than a LINE reactor.

Click to expand...

Oh yeah, forgot that!
A line reactor protects the drive, a load reactor helps protect the motor. If the motor is unique, I'd recommend upgrading to a dv/dt filter on the output.

Re: stop time. You cannot violate the laws of physics. A motor cannot brake a load any faster than it can accelerate it. But notice I said accelerate it ACROSS THE LINE, not accelerate it with the VFD.

Actually for a motor that is not VFD wound, I would HIGHLY recommend a "Sine-Wave" (or as stated by jraef...dv/dt) filter.

TCI makes a very good one..and also other filters and reactors.
http://www.transcoil.com/Products/MotorGuard-Sinewave-Filter.htm

I have done VFD all the way to 600HP (at 240Volts!!)