I have recently been working on a Virago with a PMA and stock regulator (thyristor type shunt). I noticed a lot of heat generated at the regulator so have spent considerable time searching for explanations...
I have learnt the following which may come as a surprise to some:
The way in which a thyristor and MOSFET switch on and off is quite different. MOSFET switching is very fast and precise so can provide very good voltage regulation. Thyristor switching is slower and produces a lot of heat. Some estimates I have seen put the heat generation by thyristor shunt regulators at between 40 to 60 Watts. Heat generation by MOSFET switching is quite low but I have no figure to hand.
My Virago's stock thyristor regulator when idling with the headlight off gets hot to the point of being uncomfortable to touch - approximately high 40s Celsius. I have now replaced this regulator with a modern MOSFET and under these same conditions I can barely feel any heating at all. So is it really necessary to run a high load with additional lights to soak up power that might otherwise overheat the stator??
There are many reports of shunt regulation on PMA systems damaging the stator windings. However, this is most unlikely due to the PMA shorted voltage being close to 0 Volts. So what is causing the problem?
It is thought that the heating from thyristor shunt switching stresses the rectifier diodes. This constant heating leads to slow decay of the diodes resulting in increased reverse current leakage and potentially a shorted diode. When a short occurs this can dump significant current back through the stator and damage it. Often the main fuse between the regulator and battery is high so it is often suggested to drop this value close to the max. current output of the PMA. For the Virago 750 - 1100 it is suggested to go to 15 Amp so if a short occurs the fuse will blow before the stator windings get too hot from leaking battery current.
For those in the UK I bought my MOSFET regulator for £72 from Electrex World Limited:
https://www.electrexworld.co.uk
The regulator is nicely finished and has "Made in England " engraved on it. They have a range of MOSFET regulators available but mine is quite basic with 3 phase wires, red and a black earth wire.
On their Technical and Troubleshooting page they provide a detailed procedure for testing rectifier diodes for reverse current leakage. This information may be helpful for those with PMAs when they experience unexplained voltage fluctuations or persistent issues with batteries not holding their charge.
I have learnt the following which may come as a surprise to some:
- A PMA essentially puts out a constant current above say 2000rpm. The impact is that as the revs go up so does the voltage output. Hence the need for a voltage regulator.
- The recommended way to regulate voltage on a PMA is to use a shunt type regulator. These generally come using one of two technologies - Thyristor (SCR) or MOSFET.
- When the voltage output goes too high the shunt regulator shorts out the stator windings so dumps a lot of current.
- The voltage output of a shorted PMA stator is very close to 0 volts. This means the power dumped through the stator is also very low since Power = Volts x Current.
The way in which a thyristor and MOSFET switch on and off is quite different. MOSFET switching is very fast and precise so can provide very good voltage regulation. Thyristor switching is slower and produces a lot of heat. Some estimates I have seen put the heat generation by thyristor shunt regulators at between 40 to 60 Watts. Heat generation by MOSFET switching is quite low but I have no figure to hand.
My Virago's stock thyristor regulator when idling with the headlight off gets hot to the point of being uncomfortable to touch - approximately high 40s Celsius. I have now replaced this regulator with a modern MOSFET and under these same conditions I can barely feel any heating at all. So is it really necessary to run a high load with additional lights to soak up power that might otherwise overheat the stator??
There are many reports of shunt regulation on PMA systems damaging the stator windings. However, this is most unlikely due to the PMA shorted voltage being close to 0 Volts. So what is causing the problem?
It is thought that the heating from thyristor shunt switching stresses the rectifier diodes. This constant heating leads to slow decay of the diodes resulting in increased reverse current leakage and potentially a shorted diode. When a short occurs this can dump significant current back through the stator and damage it. Often the main fuse between the regulator and battery is high so it is often suggested to drop this value close to the max. current output of the PMA. For the Virago 750 - 1100 it is suggested to go to 15 Amp so if a short occurs the fuse will blow before the stator windings get too hot from leaking battery current.
For those in the UK I bought my MOSFET regulator for £72 from Electrex World Limited:
https://www.electrexworld.co.uk
The regulator is nicely finished and has "Made in England " engraved on it. They have a range of MOSFET regulators available but mine is quite basic with 3 phase wires, red and a black earth wire.
On their Technical and Troubleshooting page they provide a detailed procedure for testing rectifier diodes for reverse current leakage. This information may be helpful for those with PMAs when they experience unexplained voltage fluctuations or persistent issues with batteries not holding their charge.
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