Single VFD, different HP motors

[ptonsparky]

Customer with new equipment. They were supposed to all come with VFDs. Two of the three came with Manual Motor Starters ahead of contactors for control. The third came with a 5hp vfd feeding a 2 hp and a 3 hp motor with the MMS on the load side of the VFD. Good enough if you ignore the fact that they aren't even sized close to providing OC protection for the motors.

I have done and seen multiple motors on a single VFD but always with the same style/hp motors. We are in the process of installing drives on the other two pieces of equipment and were just going to copy the third. Now that we see some of the details of their work, 240v fuses on 480v, The OC protecton and misc other, I am not sure if this is a correct application or not.

We are installing PowerFlex 4s.

 

[Jraef]

OC or OL? If they used IEC style MMS devices, they have thermal OL and magnetic OC (Shor Circuit) trips built-in. The thermals are adjustable but the mag trips are not and although the trip levels are not always clearly visible, they will fit a motor that's within the thermal range.

If not, i.e. they used the old style NEMA MMS devices, you may still be OK because most modern UL Listed digital VFDs, the PF4's included, have to provide short circuit and GF protection on the load side that usually falls within the requirements even for multiple smaller motors. It works because it has a trip threshold that is very low by comparison to a CB since its primary mission is actually to protect the transistors.

One thing to watch out for though is that you have an aux. contact on the MMS that is wired back to a VFD input programmed to disable it if either MMS trips, and look at the timing chart on them to make sure they will change state slightly ahead of the main contacts opening so that you don't have a power circuit opening on the output side of the VFD. Not a really big deal since it's not an every day thing, but people sometimes see the MMSs and use them as manual switches instead of just OL devices, and that's where you get in trouble.

 

[ptonsparky]

Looks like equip was originaly built for 240v 3ph operation. IEC style MMS were sized for that. Somewhere...dealer I am guessing...changed the equipment over to 480v and ignored the fact that the MMS were now oversized. We made sure all the new load side MMS aux contacts were interlocked with the drives and added Class CC fusing on the line side. Vfds appear to be working fine with the different sized motors but we have only ran them long enough to ck rotation.

I did not check the timing on the opening of the Aux contact. Never thought of it. Don't know why, I use overlaping contacts on my Safety Switches. :slaphead:

Cool and damp. Just right for digging spuds so we will know soon.