I have a few questions about VFD's. First, I would like to know that if using a VFD removes the need for an overload device. I do not think that it would, but a colleague told me that it does. I would think that I would still need an overload even if the motor is running from a VFD. Also, should I use the NEC tables to size the overload or should I use the motor nameplate data? My last question is just to make sure that it is OK to use a single VFD to drive separate motors that will operate at the same time. Are there any precautions that I should take when setting up multiple motors on a single VFD? Thanks in advance.
VFD FOR MULTIPLE MOTORS
- Thread starter fifty60
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I don't know what code implications there are but, from a practical perspective, you would need to provide overload protection for each motor.
FWIW, we never fit fuses on the output of a VFD even if it is a multi-motor application.
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- Illinois
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- retired electrician
Most current VFDs have built in overload protection for a single motor. If you are feeding multiple motors, then you need to provide overcurrent protection for each motor on the load side of the drive. Note that not all overload relays are suitable for use with drives, so check before you purchase.
We use thermal O/L units this kind of application.
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- Illinois
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- retired electrician
I believe that I read that some of the electronic ones don't correctly see the actual motor current when used with VFDs.
I have used P&B Golds on larger single-motor variable speed drives - typically upwards of 500kW. They are electronic and work just fine. However, they provide a lot more functions than than just overload status and it would be prohibitively expensive to use them on multi-motor applications where, on some on roller tables and textile lines, there might be a few dozen small motors.
- Location
- Illinois
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- retired electrician
I have used a number of small Allen Bradly overload relays. One of their electronic overload relays does not say that it is suitable for use with VFDs. Their bi-metallic and their more expensive electronic overload relays say they are suitable for use with VFDs.
I think bi-metalic is a good choice. It responds to rms current and that's what heats motor windings.
But, one other point/thought/consideration. Most of the single drive, multi-motor systems we've put in have run above base speed, usually by some margin in normal operation. Shaft mounted fans, the usual arrangement, ensure adequate cooling under such circumstances.
If the OP needs to run the motors at reasonably heavy load below base speed, self cooling may an issue. We had a recent application where running at rated torque at about 1Hz for extended periods was a process requirement.
A cooling fan at fixed speed, rather than shaft powered, was an obvious requirement. A single motor drive so no biggie. One motor. Another cable, another small starter. Make it a few and it might need a different approach.
Just some Tuesday night ramblings...
mike_kilroy
Senior Member
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- United States
You received complete replies on MULTIPLE motors on ONE vfd but answer to above first question may not be obvious yet...
Your colleague was 100% correct if one motor one vfd no OL required.
scottmarston
Member
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- Meridian, ID
+1...the overload value is set inisde the VFD via software or the HIM unit on the front when protecting only ONE motor. If you're unsure on the individual motor protection on the load side of the VFD, use eutectic style overloads.
- Location
- Illinois
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- retired electrician
Good point...I look past that part of the question. As far as I know all of the currently available VFDs have a listed overload function.
Many years ago the drives did not have a listed overload function and we were often required to install an external overload device on the load side of the drive.
petersonra
Senior Member
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- Northern illinois
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- engineer
The VFD supplies the overload protection as part of the VFD.
The overload is sized from the nameplate.
VFds can supply multiple motors but you will need to supply separate overload protection for each motor. You might also potentially get into a situation where the short circuit protection feeding the VFD is not adequate for the motor size, although this would be a highly unusual situation.
We use self protected type E starters for protecting individual motors where there are more than one motor on a VFD. It would probably be cheaper to just buy a second VFD for how we are using them.
weressl
Esteemed Member
...and all these comments are valid for Variable Torque aplpication but all things become different if Constant Torque loads are applied.
Hence my comment:
Not altogether likely to happen with a variable torque load.
Validity validated?
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
The E1 SSOLs cannot be used behind VFDs because they are self powered, meaning they derive all of the power to run the electronics from the current passing through them, and on a VFD, that is too "ugly" for use in that way. As a general rule, any SSOL that is self powered like this is not a candidate.
The E3 SSOLS can be used behind a VFD, but require a separate power source, either 120VAC or 24VDC, and they need a network connection or a hand held programmer for initial setup.
AB does not make bimetal OLs, but they do have eutectic melting alloy OLs that take heater elements. They are fine for VFD use as well and need no programming.
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- Location
- San Francisco Bay Area, CA, USA
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- Electrical Engineer
VFDs that are UL listed as "Motor Controllers" are required to provide BOTH the running overload protection and the motor short circuit and ground fault protection now, but only for one motor down stream of the VFD. But unfortunately some of he smaller Asian cheap VFDs are either not UL listed at all, or they are UL listed as "Power Conversion Equipment", just like solar inverters, so they do not need to have motor protection built in. You must read the manual to discover this.
But if you have multiple motors, the better route is to use separate IEC style Motor Protective Switches (like manual motor starters) between the VFD output and each individual motor. They provide the running OL protection as well as the SC protection required for each motor.
The motor FLA tables in article 430 are only for sizing conductors, you should NEVER use them for selecting OL heaters or settings, unless you have no other info. The NEC tables are typically much higher than reality, so you can damage a motor using them. You should ALWAYS use the motor FLA from the nameplate for protection issues. Then you should ALSO read the instructions from the mfr of the OL, some say to set/select based on the actual value x 1.25, but others already have the 1.25 or some other pickup value built into their selection charts. If you assume always adding the 1.25, you can end up with a setting that is way too high and can lose the motor.
weressl
Esteemed Member
AB does not have bi-metallic overload relays for NEMA starters, but http://ab.rockwellautomation.com/Ci...n/Motor-Protection/193CT-193T1-Overload-Relay, whic certainly can be used for this application.
scottmarston
Member
- Location
- Meridian, ID
I've used the E3's in this application and they worked great. You run in to this situation a lot when you are designing a way to control evaporator fans. If you are going to use the E3's, I urge you to slap in a DeviceNet card so you can retain the full functionality of the E3. This allows you to monitor the amperage and indicate it on your SCADA. That way, the client can be proactive when the bearings are going bad on the shaft.
As it happens, I had our local Moeller (might be Klockner-Moeller in NA) rep come to see me today. A guy I'd known from from previous contact in different businesses.
There ought to be synergy between the businesses. We buy quite a lot of electrical control gear that Moeller could supply. Thermal and magnetic overloads, contactors, soft starts, VSDs and a lot more besides that we could use. And we are encouraged to buy internally. Pricing ought to be competitive with inter-company trading.
He'll get a shot at the next project.
A bit off-topic, I know.
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