Gold Resistor exploded?

[Avulon]

Originally Posted by berkshirelad

Because the water will rapidly turn to steam and the fact that steam is not a good conductor means that the resistor will be out of full contact with its heat-sink.

The only way to create a current 'surge' is to reduce the resistance (i.e. a short circuit) - which water in the resistor might conceivably do - but then it will only surge to maximum that it can. Given that the fan motor is still in the circuit all that will happen is that it will be as if the resistor was replaced by a solid wire - the fan will run at full speed - The associated wiring is more than capable of handling the current. (steam is a fairly good conductor btw (being that it's water - never try to insulate your electric heater by placing it on the edge of the bath or in the shower ok?)

[Arctic]

Quote:
Originally Posted by Arctic
I would say yes myself if the join to the resistor as not been heat shrank tight, it is always best to make sure the heat shrink is pushed fully home up against the wall of the resistor.

It's a worthwhile precaution, but there's no way to demonstrate how much it contributes to resistor longevity. These failures happen so infrequently, it's just as likely to be within normal manufacturing tolerance. Everything has a failure rate. Jules has said that 0.3% or so of his resistors fail, so it's pretty rare. So, when it does happen it attracts a lot of attention and invites various theories about the cause.

I'm more concerned about the lack of fuse protection. It's obvious that the 80amp fusible link in the motor supply circuit isn't doing what it should.

TC

I have just found photo's of when I did a members fan with gold resistor & my own on the MGZT 5 years on and they are still working as they should, note which resistor was used back them on my MGZT

The heat shrink is most important and always make sure it is fitted correctly to give your self the best chance of it not failing, the quick quid washes can do lots of damage when they spray water through the front grill with the jet wash,

[Avulon]

Originally Posted by Arctic

Quote:
Originally Posted by Arctic
the quick quid washes can do lots of damage when they spray water through the front grill with the jet wash,

I'm pretty sure they can with the localised high pressure of the lance, but then that's where the rain goes in at 70mph anything getting blasted by that needs to be 100% waterproof or not care if it get's wet.

On another note why would any 'sane' person aim the hp end of a pressure washer lance straight through the front grills - bet they do the same to hot brake disks as well....

[T-Cut]

Originally Posted by Avulon

The only way to create a current 'surge' is to reduce the resistance (i.e. a short circuit) - which water in the resistor might conceivably do - but then it will only surge to maximum that it can. Given that the fan motor is still in the circuit all that will happen is that it will be as if the resistor was replaced by a solid wire - the fan will run at full speed - The associated wiring is more than capable of handling the current. (steam is a fairly good conductor btw (being that it's water - never try to insulate your electric heater by placing it on the edge of the bath or in the shower ok?)

Exactly. So why do some reports of resistor failure involve fried wiring and nearby stuff melting or even catching fire? This has to be due to dead shorting of the motor from a cause independent of the resistor. In other words the resistor is a casualty of the short and not the cause.

TC

[Mike Trident]

Originally Posted by T-Cut

Exactly. So why do some reports of resistor failure involve fried wiring and nearby stuff melting or even catching fire? This has to be due to dead shorting of the motor from a cause independent of the resistor. In other words the resistor is a casualty of the short and not the cause.

TC

Which still suggests:- why didn't the fuse blow before the resistor got fried?

[Avulon]

Originally Posted by T-Cut

Exactly. So why do some reports of resistor failure involve fried wiring and nearby stuff melting or even catching fire? This has to be due to dead shorting of the motor from a cause independent of the resistor. In other words the resistor is a casualty of the short and not the cause.

TC

I hadn't actually seen these reports of fried wiring, but if thats the case then...

Originally Posted by Mike Trident

Which still suggests:- why didn't the fuse blow before the resistor got fried?

Either the fuse is over-rated or too slow to blow.


There is one possible way in which the resistor could fail then cause burnt wiring - but I don't think it's possible. If the resistor comes apart and the live end now exposed contacted the metalwork it would cause a short ... which should blow the fuse: but if it doesn't could cause meltdown in the wiring.

I am left wondering if the fuse is always in circuit...(grasping at straws really) I'll double check the schematics when I get a chance.

[HarryM1BYT]

Originally Posted by Mike Trident

Which still suggests:- why didn't the fuse blow before the resistor got fried?

The problem Rover faced, was to specify a fuse which would not blow under normal circumstances, yet would blow under fault conditions - difficult when (I think) there is only the one type of fuse available to specify.

When the motor first starts, it draws considerably more current, than when it has settled to a speed. The fuse has to be able to allow for that initial massive start up current, yet protect the system when there is a fault condition. For mains wiring and electronics you can specify, slow to trip MCB's and surge resistant fuses, to allow for motor start up loads. Cars only have a normal, all purpose fuse available. Fit one too large and it will not blow, fit one too small and sometimes it will fail due to the motor start load.

T-Cuts idea to replace his fan fuse with a 40amp fuse is fine, so long as he is aware that the starting load will make it more liable to blow and he keeps a spare 40amp to hand. Better would have been to fit a 40amp surge resistant fuse, where such fuses available.

[murphyv310]

Originally Posted by Mike Trident

Which still suggests:- why didn't the fuse blow before the resistor got fried?

Simply because the circuit is wrong. The fuse has to be rated high enough to allow high speed running of the fan without blowing the fuse. When on low speed the current is lower so a baulked fan or a short downstream of resistor will take it out rather than the fuse.
This way of doing the job is penny pinching and quite simply that. With a car like 75 with sophisticated electronics a PWM driver would have been sensible.

In fact the old 50 watt resistor may have been less prone in a fault condition to have instigated a burn up than the 100 watt resistor that will get hotter before it lets go!!!

On low speed the resistor will also act as a surge limiter for the initial start of the fan and they always start on low speed initially.

[Avulon]

Originally Posted by Arctic

Quote:
Originally Posted by Arctic
I would say yes myself if the join to the resistor as not been heat shrank tight, it is always best to make sure the heat shrink is pushed fully home up against the wall of the resistor.



I have just found photo's of when I did a members fan with gold resistor & my own on the MGZT 5 years on and they are still working as they should, note which resistor was used back them on my MGZT

The heat shrink is most important and always make sure it is fitted correctly to give your self the best chance of it not failing, the quick quid washes can do lots of damage when they spray water through the front grill with the jet wash,

Bit of quick math here, caveat that I don't know the motor power rating. But do know that a 100W resistor survives when dropping half the voltage. So looking at highest possible current at which that could happen.

Main voltage (engine running and alternator working) = 14.5V
Resistor drops half available voltage = 7.25V
Resistor power dissipation = (steady state) = 100W

Current through resistor = 100/7.25 = 13.79A

Given that is half speed, is it reasonable to assume twice the current for full speed?

full load fan current = 27.58A Max - allowing that always starts on slow then the surge can't be more than this (regulated by the resistor) however if the resistor fails then it will start on high speed, with the motor initially static then the current surge could be much larger (limited only by inductance of motor windings) but is extremely brief, I can't see why it would exceed 40A for long enough to blow a 40A fuse, but repeated cold starts like that may weaken a fuse over time.

Remember the 27.58A calculation is for absolute maximum possible current, I suspect it's somewhere between half and two-thirds of that. So why would the normal fuse by rated above 40A? Conceivably it would continue to work with a 30A fuse (with a healthy resistor).

[murphyv310]

Originally Posted by Avulon

Bit of quick math here, caveat that I don't know the motor power rating. But do know that a 100W resistor survives when dropping half the voltage. So looking at highest possible current at which that could happen.

Main voltage (engine running and alternator working) = 14.5V
Resistor drops half available voltage = 7.25V
Resistor power dissipation = (steady state) = 100W

Current through resistor = 100/7.25 = 13.79A

Given that is half speed, is it reasonable to assume twice the current for full speed?

full load fan current = 27.58A Max - allowing that always starts on slow then the surge can't be more than this (regulated by the resistor) however if the resistor fails then it will start on high speed, with the motor initially static then the current surge could be much larger (limited only by inductance of motor windings) but is extremely brief, I can't see why it would exceed 40A for long enough to blow a 40A fuse, but repeated cold starts like that may weaken a fuse over time.

Remember the 27.58A calculation is for absolute maximum possible current, I suspect it's somewhere between half and two-thirds of that. So why would the normal fuse by rated above 40A? Conceivably it would continue to work with a 30A fuse (with a healthy resistor).

Yes the theory stands up but the loading on the fan increases with speed so at double speed the load on the fan increases in a logarithmic way so it could well be drawing over 40amps, or the resistor value takes that into account, best way is to measure the voltage across the resistor while the fan is running. I'd be inclined to pull the fuse and do a real test with an ammeter.

One other factor is the pulsing you get of the fan if the resistor is O/C when the aircon is on. The pulsing is actually going from slow (that is not working) to fast, this is to cool the Aircon system down fast in a heat soak condition where the ambient temperature is high or the system is over-heating.