250 amp fuse blowing ( 480V 1800 rpm 150hp motor )

[Haji]

Phil Corso:

You are correct.

Only the voltage affects the torque developed by an induction motor: both starting and running.

What I had in mind is this:

The duration and magnitude of motor inrush current on starting is independent of load.

The duration of locked rotor current depends on load and so is the fuse operation.

 

[Phil Corso]

Haji...

Locked-rotor torque, often called break-away torque, has to overcome the machinery-train's static-friction with the load at rest! And, of course, static-friction is considerably greater than dynamic-friction meaning with the train rolling (pun intended!)

Re locked-rotor current duration: I think you will agree that a higher than nominal current will persist for the "duration"of the acceleration period... !

Phil

 

[Besoeker]

The duration of locked rotor current depends on load and so is the fuse operation.

The duration for which high current persists depends on load.
The locked rotor current is the locked rotor current. It's there only when the rotor is at standstill.

 

[Haji]

So some load change on the motor could have caused fuse blowing in the OP's case.

 

[Besoeker]

150 hp 480v ocasionally blows the fuse ( 250 amp ) that feeds it . Motor meggs good . This is on a cold chamber test set-up and it drives a piston pump . Pump not problem as it not bound up . This only ocassionally happens . I did find out they are ocasionally doing up to 10 starts in a hour . If doing this many starts in a hour is causing this would it be advisable to install larger fuses ?

Do the fuses blow on starting or during running?

 

[Davebones]

Test technicians claim that they are running stand and shut down to do adjustments and when they they go to start-up nothing happens .

 

[Besoeker]

Test technicians claim that they are running stand and shut down to do adjustments and when they they go to start-up nothing happens .

Sounds like they are blowing on starting.

 

[Besoeker]

If, as you believe, load affects the torque developed by a motor then there would no reaon to have different NEMA Speed vs Torque characteristics.

Don't agree.
Within the normal operating range, the motor torque will balance the load torque. Steady state operation requires that.
Increase the required load torque and the motor slip will increase a bit to provide that increased torque.

 

[Fordean]

150 hp 480v ocasionally blows the fuse ( 250 amp ) that feeds it . Motor meggs good . This is on a cold chamber test set-up and it drives a piston pump . Pump not problem as it not bound up . This only ocassionally happens . I did find out they are ocasionally doing up to 10 starts in a hour . If doing this many starts in a hour is causing this would it be advisable to install larger fuses ?

Had a similar issue, Someone change the Diameter of the pulleys and it blew fuses. Pulleys must be specific to design.

 

[Haji]

The locked rotor current is the locked rotor current. It's there only when the rotor is at standstill.

No. The locked rotor current does not mean that it is the current when the rotor is locked literally and it is there only when the rotor is at standstill (it derives its name from the testing procedure to determine the starting current). The locked rotor current decays only by a small amount until the induction motor approaches the full load speed.

 

[Besoeker]

No. The locked rotor current does not mean that it is the current when the rotor is locked literally and it is there only when the rotor is at standstill (it derives its name from the testing procedure to determine the starting current). The locked rotor current decays only by a small amount until the induction motor approaches the full load speed.

Yes. Standstill. That's what it means. How it reduces from that value as the motor accelerates depends on the motor design.

Here's one we did earlier.

Starting characteristics depend quite a lot on rotor resistance and sometimes we add external resistors for staring if the rotor has slip rings - wound rotor as opposed to cage rotor.

This is one on a fairly large 6-pole motor:

I sometimes use the Steinmetz equivalent circuit for calculating operating parameters at other than from nominal duty. Is that what you use?

 

[Phil Corso]

Besoeker...

I never mentioned steady-state operation! Just standstill to rated speed!

Now I know you didn't really mean they were balanced during acceleration! But just in case if you did, accelerating-torque would be zero!

Phil

 

[Besoeker]

Besoeker...

I never mentioned steady-state operation! Just standstill to rated speed!

Not in the post to which I responded.

Now I know you didn't really mean they were balanced during acceleration! But just in case if you did, accelerating-torque would be zero.

Given what I do in my day job, I think I just might have already sussed that bit out..................

 

[Davebones]

Engineer is asking if we install soft start will that allow us to increase the starts per hour ?

 

[iceworm]

150 hp 480v ocasionally blows the fuse ( 250 amp ) that feeds it .

... This only ocassionally happens . I did find out they are ocasionally doing up to 10 starts in a hour . If doing this many starts in a hour is causing this would it be advisable to install larger fuses ?

...To this you add motor inrush load and you might have starting current well over 1000 A.

...The trip time characteristic of the fuse is important. If you are using a standard blow fuse, then a slow blow may solve the problem without going to a larger fuse.

...Today it should be possible to create any desired trip algorithm.

... You CANNOT increase the Starts-per-Hour capability of a motor by adding a soft starter. It does nothing to change the physics involved in acceleration nor the energy that goes into it.

...Get a motor protection relay that has a programmable "Minimum Time Between Starts" feature, or make one from a timer. .

Manufacture says 2 cold starts and 1 hot in a hour . They say if you have very low inertia you can increase the starts to 2 cold and 3 hot . Class J fuses were used as they give us more room in the controller cabinets .

This is a classic for burned up motors. At 10 starts per hour that motor should be so hot it will boil water. I'm really surprised it has not burned up already.

I'm generally in favor of setting the OCP right up to the NEC max, and also setting up the overloads to the NEC max. Neither of these two limit the load on the motor. The load is limited by the system design. In this case, the thermal loading (starts + running) is way over any reasonable limit.

Just guessing:
The existing fuse curve is just inside of the overload trip. Or as gar said, the fuses are mechanically damaged on each start and eventually fail. Pushing up the fuse size (to NEC max) will probably stop blowing fuses. I would then expect the overloads to come into play. After tripping the overloads three times in an hour, I would expect the fourth start to murder the motor.

There is a system design problem. Changing the fuses or the overloads is not going to change the thermal loading on the motor.

A thought:
Likely no one is going to want to spend a bunch of money changing the installation to lower the thermal loading. But they want the fuse blowing to stop and the motor to not burn up.

Adding to jraef's comments, consider an electronic overload relay like an AB E3+. It can model and monitor the thermal loading. It can be set to not allow a start when the thermal loading is such that there is not enough headroom for another start. Now, the tests have to stop until the model resets to where is safe to start the motor.

It might require larger fuses. As was already discussed, you will just have to lay out the starting and trip curves.

Note on the engineer:
Why is he asking you if a soft start is a solution? Get a sharp stick and tell him to tell you if it is a solution. However, I'm pretty sure jraef has the right answer on that.

ice

 

[iceworm]

This is a classic for burned up motors. At 10 starts per hour that motor should be so hot it will boil water. I'm really surprised it has not burned up already.

A couple of other thoughts:
Considering he motor has not yet burned up, I suspect it is a really high end (read expensive) motor.

Consider glueing/bolting a thermocouple to the motor case. Run the wire back to where the readout is easy to monitor. I suspect it will get hot enough to burn one's fingers.

Be interesting to know. And should serve as evidence thermal modeling is a good choice.

ice

 

[iceworm]

Note on the engineer:
Why is he asking you if a soft start is a solution? Get a sharp stick and tell him to tell you if it is a solution. However, I'm pretty sure jraef has the right answer on that.

Another though - probably best answered by jraef and bes:
If the load (air pump?) can stand it, perhaps a VFD will limit the thermal loading from multiple starts per hour.

ice

 

[Besoeker]

Another though - probably best answered by jraef and bes:
If the load (air pump?) can stand it, perhaps a VFD will limit the thermal loading from multiple starts per hour.

ice

Most of the VFDs I dealt with in at least the last decade have had an input for a motor thermistor.
Excerpt from a recent job spec:


1 off 4-pole door interlocked off load fuse-switch, padlockable in the off position.
1 off Output line contactor AC3 rated and with provision for external interlocks.
1 off Control circuit, fuses etc.
1 off Motor anti-condensation heater supply
1 off Motor current ammeter driven from a VSD analogue output
1 off Thermistor relay (in VSD)
1 set Control relays
1 set Lamps, pushbuttons etc including key-release Emergency Stop.
1 set Operator interface unit, door mounted
1set Power and control terminals
1 off Panel anti-condensation heater, thermostatically controlled
1 off Cubicle cooling fan, interlocked with the output contactor
1 off 6-pulse Inverter module. See next page for ratings

Actual spec. Current in production project.

 

[Jraef]

Most of the VFDs I dealt with in at least the last decade have had an input for a motor thermistor.
....

Besoeker, most high end VFDs do now come with a thermistor input, or have an analog input that can be configured as a thermistor input, that's true. Unfortunately what is also true is that here in the US (and probably Canada), most standard motors are sold with no embedded thermistor or other type of thermal device. They are generally special order so nobody waits that long. Inverter Rated motors tend to come with thermistors, but if he doesn't have a VFD on there now, chances are that this is not an Inverter Rated motor.

Other issues raised by ice worm:
You usually can accelerate a motor more often using a VFD, as long as you can live with a long acceleration cycle. Might be with considering here, in fact the longer acceleration time may preclude the need to start and stop so often, but its hard to tell because we dont really know the nature of the process. But that may also mean changing the motor eventually.

Attaching a thermal sensor to the outer case may be problematic in that there can be a 30deg C difference between the outer case temperature and the winding temperature, so without at least some empirical data from which to factor it, the information is not all that useful, except as a way to determine eminent failure. Kind of like the "Temp" light on your car; by the time it goes off, it's already too late.

Re: increasing the fuse size and letting the OL trip. I agree that is likely what will happen if the fuse size in increased. But also I think that although this would give a better indication as to what is really going on, in my experience it tends to set up a new form of failure. The operators will immediately now complain that the overloads are too sensitive and either demand that they be turned up, or do it themselves, moving the problem right down to the motor windings and rapid failure of the motor.

So I too think this really is a process change issue. Determine whether or not that motor was de-rated for the higher starts per hour and if it was, put in a way to limit the minimum time between starts and the maximum starts per hour that this is rated for. If it was not de-rated, well, good luck with that, better have a spare motor handy...

 

[Besoeker]

Besoeker, most high end VFDs do now come with a thermistor input, or have an analog input that can be configured as a thermistor input, that's true. Unfortunately what is also true is that here in the US (and probably Canada), most standard motors are sold with no embedded thermistor or other type of thermal device. They are generally special order so nobody waits that long. Inverter Rated motors tend to come with thermistors, but if he doesn't have a VFD on there now, chances are that this is not an Inverter Rated motor.

I agree with all of that. Mostly we are dealing with the industrial sector and it isn't uncommon for the specification to call for two PTC thermistors per phase in the stator (warning and trip) and one each for the DE and NDE bearings.