Overloaded fused spur, opinion please

davezawadi (David Stone):

No Adrian the slip is a secondary effect. The magnetic field in the rotor is developed by the slip and is related to the torque, BUT the motor current is directly controlled by the load torque. The change in slip up to the design maximum load is quite small. The voltage change Graham is talking about due to a fuse is tiny, and even if it had a 1% effect on the running current would not make much difference to the heat. However, with a single-phase motor things are a bit different because the capacitor in one of the windings provides phase shift, and in capacitor start motors the phase shift is much more complex and not very efficient. In order to run they have windings and poles which are not symmetric to provide an artificial second phase shift and this is a problem. Similarly, in capacitor run motors, the capacitor phase shift depends on running current which is also problematic in terms of efficiency. However, the fuse in the circuit has a tiny effect, which is the point I was making.  Yes, domestic appliances are run close to the limits by design because of cost considerations, but this should not cause direct failure unless the prototypes are not tested fully (which is common). Once a few manufacturing tolerances are added it is very easy to make an unreliable product, but not because of the fuse volt drop! In all of these cases it is very easy to blame all kinds of things and manufacturers Engineers often do, because it reduces blame, and the bosses tend to believe them for some reason I don't understand, although I have been there.

David,


Consider the following scenario and you will understand the failure mechanism at play. It's very simple, and due to a lack of tolerance in design (for cost purposes).


The voltage across the capacitor in series with the "start" winding is very high as a result of the phase shift - usually well over 350 V, but can be more. Sometimes, the voltage (with the machine loaded - say full of wet washing in the case of a tumble dryer) is greater - sometimes getting close to the rated working voltage of the capacitor. At this point, a sub 1 % change in VD is sufficient to tip the voltage over the rated capacitor voltage.


Now, the capacitor is self-healing, but over time the capacitance drops. This causes repeated overheating of the windings - typically the "start" winding. This in turn causes higher resistance over time due to bad connections etc.


So, the capacitor eventually pops, over a period of say 12-18 months. If it is simply replaced, you find the new capacitor only lasts about 3 months because of the increased resistance posed by the "start" winding having been damaged by heat.

davezawadi (David Stone):

No Adrian the slip is a secondary effect. The magnetic field in the rotor is developed by the slip and is related to the torque, BUT the motor current is directly controlled by the load torque. The change in slip up to the design maximum load is quite small. The voltage change Graham is talking about due to a fuse is tiny, and even if it had a 1% effect on the running current would not make much difference to the heat. However, with a single-phase motor things are a bit different because the capacitor in one of the windings provides phase shift, and in capacitor start motors the phase shift is much more complex and not very efficient. In order to run they have windings and poles which are not symmetric to provide an artificial second phase shift and this is a problem. Similarly, in capacitor run motors, the capacitor phase shift depends on running current which is also problematic in terms of efficiency. However, the fuse in the circuit has a tiny effect, which is the point I was making.  Yes, domestic appliances are run close to the limits by design because of cost considerations, but this should not cause direct failure unless the prototypes are not tested fully (which is common). Once a few manufacturing tolerances are added it is very easy to make an unreliable product, but not because of the fuse volt drop! In all of these cases it is very easy to blame all kinds of things and manufacturers Engineers often do, because it reduces blame, and the bosses tend to believe them for some reason I don't understand, although I have been there.

David,


Consider the following scenario and you will understand the failure mechanism at play. It's very simple, and due to a lack of tolerance in design (for cost purposes).


The voltage across the capacitor in series with the "start" winding is very high as a result of the phase shift - usually well over 350 V, but can be more. Sometimes, the voltage (with the machine loaded - say full of wet washing in the case of a tumble dryer) is greater - sometimes getting close to the rated working voltage of the capacitor. At this point, a sub 1 % change in VD is sufficient to tip the voltage over the rated capacitor voltage.


Now, the capacitor is self-healing, but over time the capacitance drops. This causes repeated overheating of the windings - typically the "start" winding. This in turn causes higher resistance over time due to bad connections etc.


So, the capacitor eventually pops, over a period of say 12-18 months. If it is simply replaced, you find the new capacitor only lasts about 3 months because of the increased resistance posed by the "start" winding having been damaged by heat.