# Contactor or no contactor?



## MadDawg (Jun 12, 2012)

I have two projects coming up and I am looking to use an ABB PSR30-600-70 soft start for a pump, and the other project involves replacing a 3/4HP single phase 240V motor and starter with a 3 phase motor and an AB Powerflex 523 drive.

My question is, after reviewing existing soft start and VFD applications at various locations, I would like to know what the reason would be to keep a contactor in the circuit?

I don't see any rhyme or reason for the different schematics which use the control circuit to power the A1 and A2 of the drive which I think would cause the drive to have to power up each time.

I am simply looking to remove the existing motor starter controls, and replacing with the drive.

Thanks.


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## jacoblipi (Mar 31, 2015)

A contactor can ON/OFF the pump whereas the drive can vary the speed along with ON/OFF control.


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## MadDawg (Jun 12, 2012)

I guess I should have been more clear. The contacor is installed before the soft start, and the control power for the soft start is controlled through the start circuit.

Here is the circuit.
*Start selector switch to 'RUN'.
*Control voltage to 'A1 and A2' on the drive.
*At the same time CR1 coil energized, closing L1,L2,L3 to input of drive.
*The drive ramps up motor to speed.

I don't see how this would be any benefit in the design???


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## KennyW (Aug 31, 2013)

Most soft starts have current leakage, even when they are "off". What this means is that they are, in a way, never actually "off". So, it is common practice to install an isolation contactor on the input of the soft start, so that the motor leads are actually disconnected from the power source when there is no run command active.

It is good practice. Check the specs of your particular soft start, some have integrated isolation and/or bypass contactors.


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## jacoblipi (Mar 31, 2015)

It seems you are right. But the CR1 is also energized by the same start circuit????may be there is any other interlock????


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## John Valdes (May 17, 2007)

You don't need the contactor for either control. You do need a disconnect thought for each.
I agree with you regarding dropping out the power as this could lead to problems with braking or ramping to stop. 
You want the soft start to control what the motor does. If you put a contactor in front of the control, you will defeat the control and allow the motor to coast to a stop rather than ramping it down.
Could be a safety concern as well?

Note" ABB used to sell a line side contactor with there DC drives. They used the contactor as an E-Stop. I never really understood why they used the contactor?


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## MadDawg (Jun 12, 2012)

After looking at the ABB PSR soft start, I noticed that this drive does not include motor overload protection. So in this case I would need to have the contactor in the circuit to de energize if the motor overload trips.


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## MadDawg (Jun 12, 2012)

KennyW said:


> It is good practice. Check the specs of your particular soft start, some have integrated isolation and/or bypass contactors.



The AB SMC-3 does show that a contactor ahead of drive is optional.

The units I am looking at all have bypass contactors built in.


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## bill39 (Sep 4, 2009)

Call ABB's tech support.

Some applications require a contactor to physically open the circuit for safety concerns. Depending on the drive it either needs to be on the line side or the load side. Get the correct answer from ABB.


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## JRaef (Mar 23, 2009)

First, stop referring to soft starts as "drives", or you will get yourself all messed up, especially when talking to tech support people. So let's separate out the "drive" from the soft start issue. A "drive" is a VFD when talking about AC systems.

Don't put a contactor ahead of the drive (VFD) unless it is there for some sort of safety reason. Cycling power on and off to a VFD puts stress on what is called the "pre-charge" circuit components and they wear out, which leads to VFD failure. Once in a while is fine, EVERY time you want to run the motor is not.

So on to the soft starts. There are actually two flavors of them; Soft Start _*Controllers*_ (often called an *SSC*), and Soft _*Starters *_(often called a Reduced Voltage Solid State starters, or _*RVSS*_). The difference is, the contactor and overload. An SSC is MEANT to retrofit down stream of an EXISTING across-the-line starter, so the intent is that the existing starter takes care of the On-Off control and Overload Protection, all the SSC does is ramp the voltage to the motor. There are some that are a hybrid version, an SSC that also has the ability to accomplish On-Off control, but still rely upon something else to protect the motor. That is what the ABB PSR 30 series is. The INTENDED use is for wiring it down stream of a MANUAL Motor Starter (MMS), which provides the Short Circuit and Overload protection, as well as a disconnecting means. So it isn't enough to just have a contactor ahead of it, you need the SC and OL protection as well. These devices are easy to retrofit and are relatively low cost. But if you want to take advantage of Soft Stopping, for example to reduce water hammer on a pump system, then they get more complicated to use with an existing starter, as you have already discovered. Another major downside to that device, as it is with other low cost soft starters, is that it is a "two-phase" design, meaning that it only controls 2 of the 3 phases, L3 to T3 is just a piece of bus bar going straight through the unit (you can see that depicted right on the front cover). This means that when it is soft starting (or soft stopping) the motor, it is PURPOSELY creating a severe phase current imbalance, heating the motor up disproportionately to the load on it. In addition, ONE failed SCR and you have unrestricted current flowing to one winding of the motor, because that 3rd pole is constantly hot. So while they INTEND for it to be used with a MMS, it really REQUIRES you to use the contactor so that you have a way to actually remove power if there is a shorted SCR. That, like I said, makes it more complex to use. The reason they do it is because they can sell it cheaper, since it only has 2/3 of the power devices inside.

An RVSS, like the SMC-3, is a true motor starter in it's own right, it needs nothing else to make it work. It has the On-Off control and the OL protection built-in so it can truly REPLACE the contactor type starter. it also uses 6 SCRs, 2 per phase, so it is not as susceptible to damaging the motor if one SCR shorts (it would have to be two, and in opposing phases). 

The only time I ever recommend a line isolation contactor is if you are in an area where lightning strikes are a common occurrance. ALL solid state devices are subject to damage from surges cause by nearby lightning hits, so if you want extra reliability of a soft starter, use a line isolation contact ahead of it so that there is an air gap that any surge would have to jump to get at the soft starter.


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