Blower Fan on VFD High Hrz Low RPM

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Saturn_Europa

Senior Member
Location
Fishing Industry
Occupation
Electrician Limited License NC
Gentleman, I've got a head scratcher.

Its a Trane system with a large blower on the central unit. Its connected to 6 VAVs. Discharge pressure is usually 1.5 psi. It is currently 0.21.

Blower Fan Motor name plate:5 hp . 6.6 FLA. 1750 rpm.

VFD is putting out 60 Hrz but the motor is spinning visibly slower then usual. I am going to put a tach on it first thing tomorrow. One of the guys thought it was free wheeling but when you turn the VFD off it stops. Its just turning really slow. Probably 200-300 rpm. This is why my discharge pressure is so low.

The Hrz are maxed out but the vfd amperage is very low. 2.1 amp. I double checked with a clamp on amp meter and hertz and amps are spot on.

Has anyone seen anything like this before. The unit has been in operation for 2 years so its fairly new and just started acting up today.

I was thinking its the motor. But with a 3 phase induction motor if its bad, its bad and will through a breaker or OCPD, not just turn slower.

One thing I did not do is check volts on the motor side of the VFD. Volts on the line side are 480/277 and well balanced.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Gentleman, I've got a head scratcher.

Its a Trane system with a large blower on the central unit. Its connected to 6 VAVs. Discharge pressure is usually 1.5 psi. It is currently 0.21.

Blower Fan Motor name plate:5 hp . 6.6 FLA. 1750 rpm.

VFD is putting out 60 Hrz but the motor is spinning visibly slower then usual. I am going to put a tach on it first thing tomorrow. One of the guys thought it was free wheeling but when you turn the VFD off it stops. Its just turning really slow. Probably 200-300 rpm. This is why my discharge pressure is so low.

The Hrz are maxed out but the vfd amperage is very low. 2.1 amp. I double checked with a clamp on amp meter and hertz and amps are spot on.

Has anyone seen anything like this before. The unit has been in operation for 2 years so its fairly new and just started acting up today.

I was thinking its the motor. But with a 3 phase induction motor if its bad, its bad and will through a breaker or OCPD, not just turn slower.

One thing I did not do is check volts on the motor side of the VFD. Volts on the line side are 480/277 and well balanced.
Most likely, the motor winding insulation is failing, but the VFD is going into Current Limit to handle it. If there is a lot of turn-to-turn leakage in the windings, which is common on VFD driven motors not designed for that, the current is still flowing so the VFD thinks everything is fine, but the current is no longer producing torque, just heat in the motor. If there was no VFD, the current would jump and the OL or CB would trip, but if the VFD is artificially limiting the current to the motor nameplate level, it "tricks" it into staying running. Seen it many times.

And if you did a megger test on the motor it likely passed, because a standard megger test is connected to ground, so it only picks up leakage to ground. This is leakage within the turns of the SAME WINDING, so nothing is going to ground (yet). It's just basically become a much smaller motor...

Look in the VFD programming for something that looks like a Current Limit setting, or Torque Limit, or Stall Prevention, everyone calls it something different. If you find it and it's enabled, turn it off and you'll likely see the drive trip on OL.
 

Saturn_Europa

Senior Member
Location
Fishing Industry
Occupation
Electrician Limited License NC
Most likely, the motor winding insulation is failing, but the VFD is going into Current Limit to handle it. If there is a lot of turn-to-turn leakage in the windings, which is common on VFD driven motors not designed for that, the current is still flowing so the VFD thinks everything is fine, but the current is no longer producing torque, just heat in the motor. If there was no VFD, the current would jump and the OL or CB would trip, but if the VFD is artificially limiting the current to the motor nameplate level, it "tricks" it into staying running. Seen it many times.

And if you did a megger test on the motor it likely passed, because a standard megger test is connected to ground, so it only picks up leakage to ground. This is leakage within the turns of the SAME WINDING, so nothing is going to ground (yet). It's just basically become a much smaller motor...

Look in the VFD programming for something that looks like a Current Limit setting, or Torque Limit, or Stall Prevention, everyone calls it something different. If you find it and it's enabled, turn it off and you'll likely see the drive trip on OL.

Thank you for the detailed reply. Name plate full load amps are 6.6. The VFD is supplying 2.1 amps.

I'll try to change that current limiting setting tomorrow.

I've got an empty bucket in an MCC. I could unmount the motor and bench test it with an across the line starter.
 

Ingenieur

Senior Member
Location
Earth
1.5 psi is huge
40" wc
it's usually 1-2" wc on something like this

I think your idea to by pass the vfd to test is a good one
just make sure the vav are open or open an access panel
 

kwired

Electron manager
Location
NE Nebraska
Thank you for the detailed reply. Name plate full load amps are 6.6. The VFD is supplying 2.1 amps.

I'll try to change that current limiting setting tomorrow.

I've got an empty bucket in an MCC. I could unmount the motor and bench test it with an across the line starter.

If it is shorted like Jraef mentioned and you hit it with across line full voltage and frequency it likely lets some smoke out, possibly even in a violent manner depending on available fault current, maybe not a big deal because your motor is bad anyway just beware that is a possibility.
 

Besoeker

Senior Member
Location
UK
Most likely, the motor winding insulation is failing, but the VFD is going into Current Limit to handle it. If there is a lot of turn-to-turn leakage in the windings, which is common on VFD driven motors not designed for that, the current is still flowing so the VFD thinks everything is fine, but the current is no longer producing torque, just heat in the motor. If there was no VFD, the current would jump and the OL or CB would trip, but if the VFD is artificially limiting the current to the motor nameplate level, it "tricks" it into staying running. Seen it many times.

And if you did a megger test on the motor it likely passed, because a standard megger test is connected to ground, so it only picks up leakage to ground. This is leakage within the turns of the SAME WINDING, so nothing is going to ground (yet). It's just basically become a much smaller motor...

Look in the VFD programming for something that looks like a Current Limit setting, or Torque Limit, or Stall Prevention, everyone calls it something different. If you find it and it's enabled, turn it off and you'll likely see the drive trip on OL.

If the 2.1A is VFD output current as stated by Saturn_Europa, the VFD should not be close to current limit.
 

Besoeker

Senior Member
Location
UK
Any chance the VFD is bad?
Yes. That's a distinct possibility.

But another thing you might like to think about is the driven load.
If the motor is taking just 2.1A, that's about 30% of FLC. Typically, in my experience, that would be about the level for no load current. If the load is a centrigul fan (cube law) and running at 300rpm instead of 1780 rpm that is, to all intents and purposes, no load.

Are you sure it's the motor running at that speed? Could there be a slipping/loose coupling/drive belt between the motor and the fan and it is fan speed rather than motor speed you are observing?

Another thought. If the motor really was running at 300rpm and the drive is puting out rated 60Hz frequency, that would represent a huge slip frequency - the difference between motor synchronous speed and operating speed. At the design speed (1780) in your case, that would be around 1% which is quite typical in my experience. A huge slip frequency would result in huge currents of possibly six times motor rated current and that's not happening.

One final point. Have you checked the VSD output voltage when the speed is low?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
If the VFD is current limiting to 2.1A on a motor with a nameplate rating of 6.6A, then the problem might simply be a configuration setting. Perhaps someone unauthorized changed settings....

-Jon
 

Besoeker

Senior Member
Location
UK
If the VFD is current limiting to 2.1A on a motor with a nameplate rating of 6.6A, then the problem might simply be a configuration setting. Perhaps someone unauthorized changed settings....

-Jon
But then you'd expect the frequency to drop to keep the V/f right wouldn't you?
 

Saturn_Europa

Senior Member
Location
Fishing Industry
Occupation
Electrician Limited License NC
Yes. That's a distinct possibility.

But another thing you might like to think about is the driven load.
If the motor is taking just 2.1A, that's about 30% of FLC. Typically, in my experience, that would be about the level for no load current. If the load is a centrigul fan (cube law) and running at 300rpm instead of 1780 rpm that is, to all intents and purposes, no load.

Are you sure it's the motor running at that speed? Could there be a slipping/loose coupling/drive belt between the motor and the fan and it is fan speed rather than motor speed you are observing?

Another thought. If the motor really was running at 300rpm and the drive is puting out rated 60Hz frequency, that would represent a huge slip frequency - the difference between motor synchronous speed and operating speed. At the design speed (1780) in your case, that would be around 1% which is quite typical in my experience. A huge slip frequency would result in huge currents of possibly six times motor rated current and that's not happening.

One final point. Have you checked the VSD output voltage when the speed is low?

I put the tach on it first thing this morning. 1750 at the motor shaft. The fan has a slightly bigger pulley sheave and was turning at 325 rpm.

The first thing I checked yesterday was the belt the drives the fan. It was tight and looked brand new. It wasnt. The width had been worn down to were it was slipping perfectly inside the sheave and not making a bit of noise.

Replaced the belt and the blower shaft speed shot up to 650 rpm. Motor amps went to 5.5 and the hrz went to 45.

I talked our mechanics and that belt was less then 6 weeks old. Probably going to go to a double sheave pulley system.

So it turns out that the laws of physics are still in place and a century's worth of electrical principles and proofs are still valid:)

Thank you everyone for your suggestions!
 

Besoeker

Senior Member
Location
UK
I put the tach on it first thing this morning. 1750 at the motor shaft. The fan has a slightly bigger pulley sheave and was turning at 325 rpm.

The first thing I checked yesterday was the belt the drives the fan. It was tight and looked brand new. It wasnt. The width had been worn down to were it was slipping perfectly inside the sheave and not making a bit of noise.

Replaced the belt and the blower shaft speed shot up to 650 rpm. Motor amps went to 5.5 and the hrz went to 45.

I talked our mechanics and that belt was less then 6 weeks old. Probably going to go to a double sheave pulley system.

So it turns out that the laws of physics are still in place and a century's worth of electrical principles and proofs are still valid:)

Thank you everyone for your suggestions!
Good. A resolution compatible with the symptoms.
 

tkb

Senior Member
Location
MA
I put the tach on it first thing this morning. 1750 at the motor shaft. The fan has a slightly bigger pulley sheave and was turning at 325 rpm.

The first thing I checked yesterday was the belt the drives the fan. It was tight and looked brand new. It wasnt. The width had been worn down to were it was slipping perfectly inside the sheave and not making a bit of noise.

Replaced the belt and the blower shaft speed shot up to 650 rpm. Motor amps went to 5.5 and the hrz went to 45.

I talked our mechanics and that belt was less then 6 weeks old. Probably going to go to a double sheave pulley system.

So it turns out that the laws of physics are still in place and a century's worth of electrical principles and proofs are still valid:)

Thank you everyone for your suggestions!

It always seems that the perceived problem is always electrical until we prove it as a mechanical problem.
It's easy to blame any problem as electrical.
 

Besoeker

Senior Member
Location
UK
The least understood technology is usually the first to be blamed.
Yes, you and tkh are on the money with that.
This moves the topic a little - mods, be kind, it does have some relevance.

I had a sort of similar problem on a 275kW DC variable speed drive system some years ago.
Interesting application. It was on winches for a tin mining barge in Malasyia. The barge isn't powered. It moved by winches on the vessel anchored to the banks of the pond where the operation takes place. To move fore and aft, one winch pulls in and the one at the opposite end lets out or renders. Their roles reverse to move in the opposite direction. I mention that because that's how they were tested.

Two were fitted on a test rig pulling in opposite directions, one winching in, the other rendering. This was done over a period of days with the roles being reversed. I was called one Saturdy morning because one of the drives was making a "thumping" noise. Sure enough, there was this noise. It sounded for all the world like a semiconductor misfire so my first shot was to test for that. No fault.

I then had a look at the motors and saw what was happening. The transmission from the DC motor shaft to the winch gearbox was by an internally toothed (Morse) belt. It wasn't quite tight enough and slipped a tooth now and again. But, like Saturn_Europa, the drive was being blamed.

The irony was that the guy who called me to fix the "drive" problem was the mechanical engineer who designed those winches......
It's a longish post but it's another example that makes your point about the least understood technology usually being the first to be blamed.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
...

The first thing I checked yesterday was the belt the drives the fan. It was tight and looked brand new. It wasnt. The width had been worn down to were it was slipping perfectly inside the sheave and not making a bit of noise.

Replaced the belt and the blower shaft speed shot up to 650 rpm. Motor amps went to 5.5 and the hrz went to 45.

I talked our mechanics and that belt was less then 6 weeks old. Probably going to go to a double sheave pulley system.
I wonder if the new belt was actually the wrong width or insufficiently tensioned to wear so quickly?
 
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