×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Switching motor from vfd to contactor while running.
4

Switching motor from vfd to contactor while running.

Switching motor from vfd to contactor while running.

(OP)
I have an application in which it is desireable to switch the feed of a motor between a vfd and a line contactor while the motor is running.  This is desired so that one vfd can be used to ramp more than one motor up to speed.  Does anyone have experience with this?  I am concerned about switching the power source of the motor at full load.  Is there a danger to the vfd or the motor?  
Any insight would be appreciated.

RE: Switching motor from vfd to contactor while running.

You don't want to make a sudden frequency change while the motor is spinning.  If you can run the drive output at line frequency and have it synched to the incoming line, you should be able to switch on the fly.   I would think you would want use isolation contactor to make sure that you are not connecting the drive output directly to the incoming line.  

But I would be cautious about this for larger motors. The concern I have is the possibility of re-energizing the motor out of phase with the residual voltage being generated by the motor during the brief period of time the motor is not energized by either source during the transition.  If the motor slows down much at all, you can have a nasty situation when power come back.

Maybe others have more practical experiences they can share.

dpc

RE: Switching motor from vfd to contactor while running.

3
In theory, yes you can do this but there are a number of things to consider.
1) When switching the supply on spinning motors, there can be a very high current and torque transient. This is due to the motor acting as a generator while it is disconnected from the supply. The reconnection to the new supply requires the resynchronisation of the magnetic field to the supply. If you are very luck and complete the transition while the generated voltage is in phase with the supply voltage, the transients will be minimised, otherwise the transients can be very severe. There are two main ways to reduce this "auto reclose" effect. The first is to synchronise the two supplies, connect them in parallel, then disconnect the first supply. (Closed transition or shunt switching) The second is to mantain the off time between the two supplies for long enough for the rotor field to significantly decay. This can take a significant time (greater than half a second) and in some cases the motor will slow excessively in this time.
Shunt switching is not generaly an option with an inverter supply.
2) Many inverters can be damaged by switching on their output. Before attempting switching on the output of an inverter, check with the manufacturer and ensure that such operation is covered by their waranty. - Output device failure tends to be very expensive!!

Mark Empson
http://www.lmphotonics.com

RE: Switching motor from vfd to contactor while running.

IF you bring it up to line speed with the VFD it's not a problem.  If you buy a "soft start" motor starter that's exactly how there built.  The soft start ( a VFD that is used to accelerate the motor limiting the starting current.) is switched to line voltage with a bypass contactor when it's up to speed.  
See Square D or GE or any motor starter manufacturer.  You may want a Soft Start with a bypass instead of a VFD.

RE: Switching motor from vfd to contactor while running.

this is interesting...

what suddenly comes up to my mind is a UPS operation where in you have the mains and the inverter output..ups nowadays have a static bypass...the inverter freq. and mains freq. are sync...to do a maintenance repair on the UPS you can transfer the load without gap to the mains temporarily...i believe you are looking at the same principle ..you just  have to device a circuitry to do the inverter output switching..

dydt

RE: Switching motor from vfd to contactor while running.

Interesting comments.
Firstly, Soft starters are not the same as inverters. A soft starter is a device that has two or three SCR pairs in series with the supply to the motor and operates by reducing the voltage applied to the motor during start. There is no problem with shorting out SCRs and a number of Soft starters include either controls for switching a bridging contactor and some even include a bridging contactor. The soft starter does not vary the frequency applied and does not control the motor speed. It reduces the voltage applied and thereby reduces the torque produced by the motor.

UPS systems are however inverter based, but they operate at line frequency only and are designed to be synchronous to the supply when it is present and are designed to be switched.

A Variable speed controller is a variable voltage variable frequency device that is not generally designed to be synchronised to the supply, or necessarily to have circuits switched on it's output. This can be done with some units but not all. It is certainly theoretically possible to design and build variable speed drives to enable shunt switching, but not all units are so designed.

Mark Empson
http://www.lmphotonics.com

RE: Switching motor from vfd to contactor while running.

To BJC:

I guess you should have a closer look on soft starters and VFDs:

A soft starter consist of a set of thyristors, connected as an AC-Controler gated in a way that the voltage is reduced during starting

A VFD, Variable Frequency Drive includes some more components  and changes voltage and frequency and is of course bigger and more expensive for a given rating.

There are some devices on the market using AC-controllers for speed control of small motors, but these devices are only VSD (Variable Speed Drive)

But back to mcdous original post:

The operation you describe is possible and it is used to put large hydro-electric generator sets into pumping operation. As far as I remember the machines are ramped up slightly above synchronous speed using  an VFD and synchronisation is done after disconnecting the VFD during decelleration.

RE: Switching motor from vfd to contactor while running.

Marke has a good point regarding solid-state reduced voltage starters.  If all you want to do is "soft-start" the motor, then the SS starter should be much less expensive than a VFD.  If you need speed control, then you will need the VFD.

If a solid-state starter will meet your needs, I'd advise buying a separate on for each motor, along with its own bypass contactor.  By the time you get the switching system worked out to run multiple motors from one starter, you will probably have spent more than the additional starters would cost and have a much less reliable system.

Actually, I'd advise this even for VFDs as well.  

dpc

RE: Switching motor from vfd to contactor while running.

Without shunting , just try and stick a contactor on the output of a VFD and see how long it lasts . From my ltd experience , I have found that VFD's don't like their output disconnected whilst running , and if you try to do a controlled stop with ramp down and then change over to DOL , you will find that the changeover contactor lasts about 5 mins.

RE: Switching motor from vfd to contactor while running.

Hi, you cannot use a standard vfd for this job, you can use a thyristor soft starter but you must make sure it is large enough to handle repeated start currents, approx 3 times FLC as they are normaly rated for occasional starts.

RE: Switching motor from vfd to contactor while running.

If you want to look at soft starters, try the AuCom units at http://www.aucom.co.nz They have software to select units for any start duty.
The start current is dependant on the motor characteristics and load torque requirements.

Mark Empson
http://www.lmphotonics.com

RE: Switching motor from vfd to contactor while running.

When you compare the starting of an motor with an Soft Starter and an VFD you should consider that there are some situations where a Soft Starter is not able to start the motor due to reduced torque at reduced voltage. Soft Starters are usually only suitable for applications like fans or pumps where torque increases with  the square of speed.

RE: Switching motor from vfd to contactor while running.

(OP)
For clarification on the original posting - the vfd is required for speed control on the pump and therefor a soft start is not an option.

Thanks for all of the insight.

cjm.

RE: Switching motor from vfd to contactor while running.

Hi, can you clarify what you are trying to do. If you want speed control then you cannot switch the motors over to the line.

RE: Switching motor from vfd to contactor while running.

The only way you will be able to acheive this is by using Marke's suggestion (posted Feb 25) ie. you will require a 0.5~1 second delay on switchover.  The problem then is the inrush current of the motor, due to:
1.  The magnetizing current.
2.  The reduction in motor speed (depending on the loading of the motor) will lead to an increase in current draw.
You will also require seperate overload protection for each motor.
If you are trying to limit the input current, seperate VFD's will be required.


Good luck!!

RE: Switching motor from vfd to contactor while running.

I take it from mcdou's last comments that he wants to vary the speed of the motor in order to alter the flow. I would guess that the theory is that when the motor runs at full speed and more flow is required, that you switch that motor to the supply and speed control the second pum to increase the flow even further. If this is the logic, then that is fine and is certainly one solution provided that you use a VFD that is capable of switching motor on it's output. There is a very high cost in the inverter though, and the cost of all the switching equipment and interlocking is also high. You also have the problem of total system failure when one inverter fails.

Another way of achieving a similar result, is to us a VFD on one pump and soft starters on the rest. When the flow exceeds that provided by one pump at full speed, soft start a second pump and reduce the speed of Number one. Continue to control the speed of number one to atain the correct flow / pressure. Likewiase, you can bring in many pumps individually using soft starters and use the one pump on the VFD for fine control. This has been done many times before very effectively.

Mark Empson
http://www.lmphotonics.com

RE: Switching motor from vfd to contactor while running.

Hi, I agree with Marke his solution of 1 vfd plus softstarters is the best solution apart from vfd's on each pump.However the motor with the vfd will need to be larger than the, others at least 50%, and you will need a complex control system if you need a smooth changeover from 1 mode to the next.

RE: Switching motor from vfd to contactor while running.

I have just found an intresting document on the topic described:

http://www.robicon.com\library\acrobat\synctrans.pdf

Robicon calls this type of operation synchronous transfer. Maybe you will find other sites if you search for that term.

RE: Switching motor from vfd to contactor while running.

ABB drives are available with built in software to bring additional fixed speed motors as required. they call it PFC (pump & fan control), hope this helps

mike

RE: Switching motor from vfd to contactor while running.

Danfoss has a Cascade Controller option for their VLT 8000 model VFDs; www.namc.danfoss.com/products/8000/cascade.html.  The cascade option coordinates the control of one VFD controlled pump with up to 4 fixed or variable speed pumps to maintain constant pressure.  The option's software minimizes pressure distubances as pumps are switched on and off.

RE: Switching motor from vfd to contactor while running.

I will assume that you have two or more pumps, equally sized, on common suction and discharge manifolds.  Be careful to set your VFD minimum speed for the worst case operating condition, all pumps running and the VFD at minimum speed due to demand.  The VFD driven pump will make steam without some flow.

If you have energy consumption as a consideration, my experiance is that usually running all pumps on VFD's will be the lowest operating cost. You can run a cost analysis to justify use of VFD's on each pump.

RE: Switching motor from vfd to contactor while running.

Suggestion: Since various drives have efficiency less than 1, power factor less than one, and lower reliability than electromagnetic contactors, some consideration can be given and was given in literature to have the drive bypass by a contactor at a proper time. Then, the drive is free for another motor control, e.g. start-up. The clue appears to be in achieving an optimal motor state at which the transition to its contactor power feed is smooth with the minimum harm to the motor.

RE: Switching motor from vfd to contactor while running.

We switch from vfd to bypass on our drives quite frequently, but we make sure the drive gets shut down at this transition so if the switch is thrown back to vfd the drive will have to go through the start sequence to come on. Our drives can start on the fly or into a spinning motor. our contactor scheme is set up to completly isolate the drive at bypass(input and output)

RE: Switching motor from vfd to contactor while running.

Suggestion: If the drive has an intermediate DC section, i.e. AC-DC-AC converter, then the output AC frequency synchronization may be somewhat off from the input AC that is actually a side the contactor is using. Any mismatch in frequency during the transition is somewhat affecting the motor in terms of speed adjustment, current adjustment and mechanical stress adjustments. It would be preferable to have the output AC synchronized with the input AC of the converter (vfd).

RE: Switching motor from vfd to contactor while running.

Actually, this application is done all the time with VFDs, in fact many now have canned "application macros" that do the sequencing control in the VFD processor. But you WILL have a transition spike when connecting the motor across-the-line. You MUST isolate the VFD output from the line power, so there is no (economical) way to avoid an open transition. Robicon VFDs offer a "synchronous transfer" option, but it is very expensive and usually applied to large motors.
Going the other way, from X-Line back to VFD control is also problematic unless your VFD has frequency matching, a.k.a. "speed search" or "flying restart" which detects and matches the VFD output to the rotor frequency. Look for this feature for this application.
Another solutionoften used is to put the lead pump on the VFD permanently. When it gets near full speed, switch on a lag pump full speed and slow down the VFD. Pressure surges can be a problem, but using soft starters on the lag pumps can help mitigate that as well.

That which does not kill me, makes me stronger... and pissed off!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

White Paper - PLM and ERP: Their Respective Roles in Modern Manufacturing
Leading manufacturers are aligning their people, processes, and tools from initial product ideation through to field service. They do so by providing access to product and enterprise data in the context of each person’s domain expertise. However, it can be complicated and costly to unite engineering with the factory and supply chain. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close