Switching motor from vfd to contactor while running. 4

[mcdou]

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.

 

[dpc]

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

 

[Marke]

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

 

[BJC]

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.

 

[dydt]

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

 

[Marke]

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

 

[electricuwe]

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.

 

[dpc]

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

 

[fareast]

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.

 

[cbarn24050]

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.

 

[Marke]

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

 

[electricuwe]

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.

 

[mcdou]

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.

 

[cbarn24050]

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.

 

[TheDOG]

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!!

 

[Marke]

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

 

[cbarn24050]

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.

 

[electricuwe]

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.

 

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

 

[BJW]

Danfoss has a Cascade Controller option for their VLT 8000 model VFDs;

http://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.