Inverter Question

Now, I know this has been on the forum before, and i hate to ask this again, But here it goes. What issues will I have if I put a contactor before an inverter?

DickDV and others will have a better idea on this. But I think the biggest issue is that an input contactor too often stresses the precharge circuit and may ultimately lead to precharge circuit failure.

In case you don't know, modern inverters operate by rectifying the AC line voltage into a DC bus, often with a straightforward 3-phase diode bridge, and switching the DC bus to the motor using semiconductors. The DC bus has a certain design capacitance to it to allow some short-term output overload capacity and to maintain a smooth DC bus. The AC input current would be very high when the drive is first powered up and the bus capacitors are charged. To prevent this the drive input circuit contains a precharge circuit that limits the current flow through the rectifiers until the DC bus gets to a certain voltage level. But by limiting the current flow many precharge circuits develop heat and will fail if used too often.

Hope this helps.
Keith

a better question would be why would you want to put a contactor ahead of an inverter?

We put contactors in front of inverters quite often here. We don't specifically put them in front of individual drives but in front of all the drives in general. This is part of our e-stop functionality. Before the relatively recent advent of drives with safe inputs, this was the easiest way to isolate a drive on an e-stop.


Keith

Where would be a could place to get info on this.

There is nothing wrong with putting contactors in front of an inverter. In fact there are good reasons for doing so. kamenges mentions e-stop operation which is a very common one.

The problem comes where this contactor is used for routine stopping or any other purpose where frequent power-up/down cycling occurs. Variable Frequency Drives (inverters) are not designed for such frequent power cycling and will fail prematurely if exposed to this kind of cycling.

Since this is on a waste treatment pump I don't think I will not need an estop. I just want to control my flow better than we are right now. I wonder, can I just use the +10v power supply on the inverter and run it to a NO contact on a relay to control the start and stop of my inverter? I should be able to set my parameters for the 10v analog signal.

Which inverter manufacturer and model are you using? It sounds like all you want to d ois control the start and stop on the inverter. All inverters I have ever seen have hardware control terminals for start and stop. Most also provide a logic voltage so you just need to provide contact closures between two terminals.

Post the inverter info and we'll see what we can do.

Keith

There are many threads here concerning inverters, several on inverter bypass panels. They always have a contactor on the input, killing power to the inverter.

A basic inverter bypass is a contactor on the input, and on on the output, which is interlocked to another that supplies power to the drive motor directly from the line. The output contactor is generally just a rewired reverser starter.

In theory, there is a control switch with run and bypass, an on-off switch, and a test switch.

The test switch will power up the inverter, without closing the output contactor, so it can be tested with the drive motor off or the drive motor being powered by the line.

I thought these were pretty common ten years ago when I was designing them for GE/Baldor. When I went back out into industry in general, no one seemed to have them. But they are gaining in popularity.

I do not recommend e-stopping the drive input, many inverters have an input to ramp the drive down.

Depending on the application, naturally, estopping is aceptable.

regards.....kc

agentinfoil, you are exactly right. Do your start/stop from the low voltage control terminals and your speed control from the analog 0-10V or 4-20ma input.

And, just for the record, many if not most inverters are installed with branch circuit fusing or circuit breaker and NO input contactor. If there is an input side contactor, it is part of the system control circuit and not really an essential part of the drive system. Hopefully, as I stated earlier, it also has a very low cycle rate.

Your drive should have a terminal strip for logic and speed control. Use a dry contact or NO in your control to start the drive. Most inverters will take a 2 wire 0 to 10 volt signal from your source for speed control. Most also can use 4-20 ma directly.

You should power the drive on with the supply voltage, 230/480 and have it remain on.

I have seen drives that were powered on and off via input contactors on HVAC that would not last 6 months due to the constant powering of the drive.

I have seen drives last 6 years with the power constantly on and the start/stop cycled many times per day.

The drive I choose is a model GS2-42PO, I purchased it through automation direct. You all have given me some great input, Thanks for all the help.

Travis

See page 2-11 in your manual. It should show you about everything you need.


Keith

kamenges said:

We put contactors in front of inverters quite often here. We don't specifically put them in front of individual drives but in front of all the drives in general. This is part of our e-stop functionality. Before the relatively recent advent of drives with safe inputs, this was the easiest way to isolate a drive on an e-stop.
Keith

Click to expand...

We use a simular solution without problems.

G2

We have several of the G2 drives with a contactor (for E-stop) and then use dry contacts/PLC, for start/stop functions. We have never had any problems with this setup