New 5HP Motor Blowing Fuse

[Marco Nunzio]

Hello everyone, I recently replaced a 5HP motor, 208v 3ph 60hz, A - 14.3amps (exact specs as the old one) which runs a large return fan. Replaced the motor and pulleys, good alignment and belt tension, ensured correct 3ph voltage and started up motor. A fuse blows (Ferraz Shawmut Fes30 - a 30 amp fuse - believe it is not time delay). I replaced the fuse and removed the load from the motor and started it up again. Motor inrush amps on 1 phase to 15 amps and then dropped to 6 amps running (w/correct rotation for my application). Confirming my motor is okay. Put the belts back on the pulleys, which adds the load back to the motor. Flipped the switch and motor inrush amps jumped to 90amps and immediately blew a fuse again.

On the old motor, the inrush would be around 60 amps but would not blow a fuse, and then drop and stablize at about 15 amps full load running (just slightly overamping).

I'm thinking time delay fuses would be the fix to the issue as the motor inrush is blowing the fuse. And I need an electrician to change the fuse holder in the panel as they do not have time delay fuses that would fit the existing screw in type fuses now.

I've ensured continuity and similar resistance values across windings, ensured no direct shorts to ground and ensured proper wiring for 208v 3ph.

But before I go through all this work, something doesn't make sense to me as to why the new motor is blowing a fuse and would need time delay fuses and the old one did not, any explanation?

I have images of my motor nameplate, the motor starter, motor overload, fuses, and setup which I can send through email to give you guys some more info, can't figure out how to drop them from my phone gallery to the thread.

Any help/explanation is greatly appreciated, this one has me going crazy.

 

[ptonsparky]

The age difference in motors may be part of it. Newer high efficiency motors take longer to start than older ones.

I wasn't aware they made screw in type fuseholders for 3 phase but that means little.

Close all the access doors before starting your new motor. The load will be higher if it is a centrifugal fan with those doors open.

 

[retirede]

Presuming everything is ok mechanically with the load, your new motor likely has higher inrush than the old one. That’s not uncommon with newer, higher efficiency motors. According to Fusetek, if the inrush is more than 6X FLA ( which yours apparently is) you should be using a 40A class C fuse for this application.

https://www.fusetek.com/staff/assets/uploadifive/uploads/1875067471006FusetekSpecProdInside_opt.pdf

 

[texie]

It seems to me that the OCPD is not large enough. Not sure about CEC but NEC would permit a 50 nontime delay fuse, 30 amp time delay fuse or a 45 amp breaker for this motor.

 

[GeorgeB]

Your bolt-on 30A fuse is to a Canadian standard. It is fast blow. It is not surprising that it would clear on currents of a motor with FLA half its rating. What is surprising is that it worked reliably with the earlier motor, but Tom's higher efficiency issue is real. A larger (40A) fuse is not dimensionally compatible, so you have to solve it with reduced startup current (less air load) of the addition of a soft start / VFD.

 

[Jraef]

15A starting current on a 14A motor is WAY too low of an expectation. When you are measuring the starting current with the motor unloaded,your meter is not fast enough to see the high spike. But with the load attached, it increases the acceleration time so then your meter can see it. It was always much higher than 15A, but unloaded, you just can't see it with your meter.

 

[hillbilly1]

Hello everyone, I recently replaced a 5HP motor, 208v 3ph 60hz, A - 14.3amps (exact specs as the old one) which runs a large return fan. Replaced the motor and pulleys, good alignment and belt tension, ensured correct 3ph voltage and started up motor. A fuse blows (Ferraz Shawmut Fes30 - a 30 amp fuse - believe it is not time delay). I replaced the fuse and removed the load from the motor and started it up again. Motor inrush amps on 1 phase to 15 amps and then dropped to 6 amps running (w/correct rotation for my application). Confirming my motor is okay. Put the belts back on the pulleys, which adds the load back to the motor. Flipped the switch and motor inrush amps jumped to 90amps and immediately blew a fuse again.

On the old motor, the inrush would be around 60 amps but would not blow a fuse, and then drop and stablize at about 15 amps full load running (just slightly overamping).

I'm thinking time delay fuses would be the fix to the issue as the motor inrush is blowing the fuse. And I need an electrician to change the fuse holder in the panel as they do not have time delay fuses that would fit the existing screw in type fuses now.

I've ensured continuity and similar resistance values across windings, ensured no direct shorts to ground and ensured proper wiring for 208v 3ph.

But before I go through all this work, something doesn't make sense to me as to why the new motor is blowing a fuse and would need time delay fuses and the old one did not, any explanation?

I have images of my motor nameplate, the motor starter, motor overload, fuses, and setup which I can send through email to give you guys some more info, can't figure out how to drop them from my phone gallery to the thread.

Any help/explanation is greatly appreciated, this one has me going crazy.

If it’s Edison base type fuse holders, you should be able to install type S adapters, and get TD fuses for it.

 

[retirede]

If it’s Edison base type fuse holders, you should be able to install type S adapters, and get TD fuses for it.

A Shawmut FES is not an Edison base fuse.

 

[hillbilly1]

Oh, it’s a Canadian thing. Someone mentioned a screw in fuse. In the states that’s an Edison base.

 

[Marco Nunzio]

Thank you for all the feedback, I am going to go through with having the electrician change the fuse holder, and put time delay fuses in. Will update how it works out. Definetley some interesting information on older motors and now


EDIT: Thank you for all the feedback. I am going to go through with having the electrician change out the fuse block so I can put in time delay fuses. Will ask him first about upsizing to 40amp fuses and see his opinion and if its Canadian code complaint. (Dimensions of 40 amp should fit existing block if my measurements are correct).

Interesting info on the old motor compared to the now high efficiency motor. Definetley learned something new there. Will post an update with what happens with the new fuses in.

 

[Jraef]

To be clear, the issue with newer energy efficient motors is the magnetizing inrush current being significantly higher, but that typically only lasts a fraction of a second when you first turn it on and the motor windings are essentially a short circuit. The STARTING current, meaning the current that is creating torque and accelerating the motor, is generally either not different or in some cases slightly LOWER than older motors. But because of that initial magnetizing inrush, it pretty much eliminates the possibility of using non-time delay fuses any more, where you might have gotten away with it on an older motor.

 

[ptonsparky]

To be clear, the issue with newer energy efficient motors is the magnetizing inrush current being significantly higher, but that typically only lasts a fraction of a second when you first turn it on and the motor windings are essentially a short circuit. The STARTING current, meaning the current that is creating torque and accelerating the motor, is generally either not different or in some cases slightly LOWER than older motors. But because of that initial magnetizing inrush, it pretty much eliminates the possibility of using non-time delay fuses any more, where you might have gotten away with it on an older motor.

That STARTING current may be lower, but it does seem to be there longer.

 

[Jraef]

That STARTING current may be lower, but it does seem to be there longer.

Yes, because the actual starting TORQUE is (sometimes) lower, so it can take longer to accelerate the load.

 

[Marco Nunzio]

Hello everyone, I recently replaced a 5HP motor, 208v 3ph 60hz, A - 14.3amps (exact specs as the old one) which runs a large return fan. Replaced the motor and pulleys, good alignment and belt tension, ensured correct 3ph voltage and started up motor. A fuse blows (Ferraz Shawmut Fes30 - a 30 amp fuse - believe it is not time delay). I replaced the fuse and removed the load from the motor and started it up again. Motor inrush amps on 1 phase to 15 amps and then dropped to 6 amps running (w/correct rotation for my application). Confirming my motor is okay. Put the belts back on the pulleys, which adds the load back to the motor. Flipped the switch and motor inrush amps jumped to 90amps and immediately blew a fuse again.

On the old motor, the inrush would be around 60 amps but would not blow a fuse, and then drop and stablize at about 15 amps full load running (just slightly overamping).

I'm thinking time delay fuses would be the fix to the issue as the motor inrush is blowing the fuse. And I need an electrician to change the fuse holder in the panel as they do not have time delay fuses that would fit the existing screw in type fuses now.

I've ensured continuity and similar resistance values across windings, ensured no direct shorts to ground and ensured proper wiring for 208v 3ph.

But before I go through all this work, something doesn't make sense to me as to why the new motor is blowing a fuse and would need time delay fuses and the old one did not, any explanation?

I have images of my motor nameplate, the motor starter, motor overload, fuses, and setup which I can send through email to give you guys some more info, can't figure out how to drop them from my phone gallery to the thread.

Any help/explanation is greatly appreciated, this one has me going crazy.

Time delay fuses for the win, the motor works without issue. Thank you everyone for all the insightful feedback and interesting info on motors. Definetley learned a few things, and enjoyed my first thread on this forum. Maybe you electricians aren't so bad after all (just some light humour from your friendly hvac tech). Cheers and thank you everyone, really appreciate it.