VFD load side contactor

[ptonsparky]

I am thinking of using one VFD to run either of two motors. Same HP with fairly close loads. Augers. I can't think of any occasion that they would run at the same time so to cut some costs am thinking of putting a pair of contactors under it to select which auger I want to run. Proper interlocking would ensure either contactor is pulled in before the drive starts and drop out after it stops. What other considerations besides HP and voltage rating do I need to worry about? 480v, 10 HP.

 

[GoldDigger]

I am thinking of using one VFD to run either of two motors. Same HP with fairly close loads. Augers. I can't think of any occasion that they would run at the same time so to cut some costs am thinking of putting a pair of contactors under it to select which auger I want to run. Proper interlocking would ensure either contactor is pulled in before the drive starts and drop out after it stops. What other considerations besides HP and voltage rating do I need to worry about? 480v, 10 HP.

If the motor controls are just attached to the VFD control inputs, I would worry about someone starting the wrong auger by not being aware of the position of the contactor. If you have a good way to positively and visually interlock that with the operator's controls I do not see a problem. Maybe running the start button from each control station through an auxiliary contact set?

 

[petersonra]

A 10 HP VFD is not horribly expensive.

I think by the time you figured everything out to make this work right you might well be better off just buying another VFD.

A reversing contactor with the line jumpers removed might be a good choice. It already has the mechanical and electrical interlocks.

Get early break aux contacts from the contactors to wire into the enable circuit of the VFD.

 

[ptonsparky]

A 10 HP VFD is not horribly expensive.

I think by the time you figured everything out to make this work right you might well be better off just buying another VFD.

A reversing contactor with the line jumpers removed might be a good choice. It already has the mechanical and electrical interlocks.

Get early break aux contacts from the contactors to wire into the enable circuit of the VFD.

The outdoor 3R Hoffman enclosure with fan, etc is more than the drive. Plus labor for guys that only build a few a year. Interesting but slow, for us.

 

[kwired]

I am thinking of using one VFD to run either of two motors. Same HP with fairly close loads. Augers. I can't think of any occasion that they would run at the same time so to cut some costs am thinking of putting a pair of contactors under it to select which auger I want to run. Proper interlocking would ensure either contactor is pulled in before the drive starts and drop out after it stops. What other considerations besides HP and voltage rating do I need to worry about? 480v, 10 HP.

I have seen similar where a belt type conveyor had a motor on each end. Which motor runs depends on which way the conveyor is running. Both were driven by same drive and interlocked contactors on output side. Can't recall the little details though it was several years ago that I encountered this. The conveyor needed drive motor on the end in which direction of travel was, you don't want the slack portion of the belt to be the portion that has the load on it.

 

[Jraef]

I think your approach is fine, with the added advice of the mechanically interlocked contactors. I just went through the pain of a customer who ASSumed that using only electrical interlocks would be fine. Murphy's Law prevailed...

One other word of advise based on experience though. Use a NEMA design contactor, or over size an IEC contactor by at least 50%. IEC contactors have a harder time with the added heat flowing through them due to the high harmonic content on the VFD output to the motor. I've never seen them admit it and I used to work for Siemens where they put them on the output side of VFDs when used with SIL 3 machine safety systems, but I've seen cooked contactors too many times to ignore it.

 

[Besoeker]

I think your approach is fine, with the added advice of the mechanically interlocked contactors. I just went through the pain of a customer who ASSumed that using only electrical interlocks would be fine. Murphy's Law prevailed...

One other word of advise based on experience though. Use a NEMA design contactor, or over size an IEC contactor by at least 50%. IEC contactors have a harder time with the added heat flowing through them due to the high harmonic content on the VFD output to the motor. I've never seen them admit it and I used to work for Siemens where they put them on the output side of VFDs when used with SIL 3 machine safety systems, but I've seen cooked contactors too many times to ignore it.

We routinely use VFD output contactors. This is frequently a safety issue to have a hard break between the VFD and the motor.
Unless otherwise stated in the specification, we use AC1 rating which is thermal. Not ever had a problem.
The harmonic content of the motor current is normally pretty low - machine inductance takes care of that.

I've seen the odd case where it was higher than might me anticipated but that was when a rather low switching frequency (circa 1kHz) was used.

 

[wallypiper]

We do multiple motors per vfd fairly often, usually two motors but sometimes more. One thing I see NOT mentioned above anywhere is that each motor branch has to have individual overload protection sized to that motor. The vfd's built-in protection has to be set to cover both motors although the situation described, with the contactors interlocked so that only one will run at any one time, might exempt you. But the overload protection requirement is in 430.124(C). The multimotor applications we use are all simultaneous run, typically multiple fans in some flavor of ventilation system. You have to make decisions about load side overload protection (required) and load side contactors (application driven). If you have two or several motors, each with its own overload protection, do you interrupt the drive enable or run control circuit for ANY overload trip or just when the last overload trips?

I have been told by a VFD engineer that the risk associated with opening a load side circuit interruptor is not THAT great, but does exist. So it may come down to potential cost of a replacement VFD vs. added cost and complication at installation. On the other hand, I encounter more and more OWNERS that require that any disconnect or contactor that turns off power to a load be interlocked with the power source so that it is de-energized any time any of those switches is opened. That protects maintenance people from assuming (yeah, we all know) that the line side of a remote disconnect is safe just because the motor has been turned off. All that control wiring can add up when the motors are far from the drive or starter.

I work with many EEs who ignore 430 Part X, which requires UPSTREAM circuits to be based on the full INPUT amp rating of the VFD, not the HP of the connected motor. Since most VFD's seem to be built in common KW steps rather than HP steps, it is very common to have a VFD with a very high input amperage rating. For example, an Allen-Bradley PF400 25 HP 460/3 VFD has an INPUT amperage rating of 40 amps compared to 430.250, which lists 25 HP at only 34 amps. That can add up if you have many VFDs on the same feeder, possibly pushing you to a larger wire size or larger breaker frame size.

 

[Besoeker]

We do multiple motors per vfd fairly often, usually two motors but sometimes more. One thing I see NOT mentioned above anywhere is that each motor branch has to have individual overload protection sized to that motor.

It's a dood point to mention.
We've done a few VFDs for multiple motors. One of the first was a suite of them (five from memory) for a steel mill roller table application. Each VSD had up to 80 small motors and each had its own thermal overload protection.
Similar systems went into textile applications. Many motors - one VFD.

I have been told by a VFD engineer that the risk associated with opening a load side circuit interruptor is not THAT great, but does exist.

Yes, but appropriate control sequencing can avoid that risk. Operating a stop command first shuts off the VFD output then drops the contactor.

 

[kwired]

We do multiple motors per vfd fairly often, usually two motors but sometimes more. One thing I see NOT mentioned above anywhere is that each motor branch has to have individual overload protection sized to that motor. The vfd's built-in protection has to be set to cover both motors although the situation described, with the contactors interlocked so that only one will run at any one time, might exempt you. But the overload protection requirement is in 430.124(C). The multimotor applications we use are all simultaneous run, typically multiple fans in some flavor of ventilation system. You have to make decisions about load side overload protection (required) and load side contactors (application driven). If you have two or several motors, each with its own overload protection, do you interrupt the drive enable or run control circuit for ANY overload trip or just when the last overload trips?

I have been told by a VFD engineer that the risk associated with opening a load side circuit interruptor is not THAT great, but does exist. So it may come down to potential cost of a replacement VFD vs. added cost and complication at installation. On the other hand, I encounter more and more OWNERS that require that any disconnect or contactor that turns off power to a load be interlocked with the power source so that it is de-energized any time any of those switches is opened. That protects maintenance people from assuming (yeah, we all know) that the line side of a remote disconnect is safe just because the motor has been turned off. All that control wiring can add up when the motors are far from the drive or starter.

I work with many EEs who ignore 430 Part X, which requires UPSTREAM circuits to be based on the full INPUT amp rating of the VFD, not the HP of the connected motor. Since most VFD's seem to be built in common KW steps rather than HP steps, it is very common to have a VFD with a very high input amperage rating. For example, an Allen-Bradley PF400 25 HP 460/3 VFD has an INPUT amperage rating of 40 amps compared to 430.250, which lists 25 HP at only 34 amps. That can add up if you have many VFDs on the same feeder, possibly pushing you to a larger wire size or larger breaker frame size.

If motors are not run simultaneously, any overcurrent protection built into the drive can protect them - if they are the same motor rating anyway.
I don't think the risk of damage to the drive is all that great from opening the output, but to close a switch to a motor into the drive output while it is energized has greater risk of damage.

 

[Jraef]

If motors are not run simultaneously, any overcurrent protection built into the drive can protect them - if they are the same motor rating anyway.
I don't think the risk of damage to the drive is all that great from opening the output, but to close a switch to a motor into the drive output while it is energized has greater risk of damage.

The risk of damaging the transistors was much greater in days gone by. There is a rapid increase in voltage that occurs as the contacts separate and strike an arc, which acts like a capacitor to increase the voltage across the gap. This is made worse by the high speed pulses making up a PWM output, which could cause harm to the old darlington transistor based systems and possibly cause them to misfire due to dV/dt. But modern IIGBTs now incorporate systems in the firing circuits that will detect a contact opening on the output and turn off all of the transistors very very quickly. It's still not great and should not be a preferred method of stopping a motor, but the risk of transistor damage is much much lower than it used to be, to the point where most mfrs have removed the restriction and just warn against it like I just did above.

It was, for a while, the only way to achieve the highest Safety Integration Levels in machine safety systems that used VFDs; you had to have one or in some cases two contactors in series with the output of the VFD. Now with Safe Torque Off that is rarely still necessary, but the potential problems that were created by having them forced the drive mfrs to solve the transistor damage issue years ago.

 

[wallypiper]

It's a dood point to mention.
We've done a few VFDs for multiple motors. One of the first was a suite of them (five from memory) for a steel mill roller table application. Each VSD had up to 80 small motors and each had its own thermal overload protection.
Similar systems went into textile applications. Many motors - one VFD.


Yes, but appropriate control sequencing can avoid that risk. Operating a stop command first shuts off the VFD output then drops the contactor.

I think a common situation is multiple motors on a conveyor system connected to one drive. They all run simultaneously but each motor has a local safety disconnect. For us, a very common situation is a single motor on a VFD with a local safety disconnect, especially when the motor is remote from the VFD. We always include an auxiliary switch in that disconnect and usually design the control circuit to interrupt the run signal first. Most of the auxiliary switches available for safety switches work that way inherently, breaking before the main contacts when you open the switch.