You don't need more than 50V diode rating for a 28V supply as the transient voltage never gets to be more than about 30V from the diodes as the transient is conducted through the forward direction of the diodes.
The diodes do need to momentarily carry the full motor current until the transient subsides, which is why the diodes need a surge rating of at least 200A.
The transient only lasts as long as it takes the relay contacts to transfer.
After the transfer, the current then continues through the contacts (even if in the reverse direction for awhile).
Do you have a spec on the relay?

One like this found here, should likely work.

https://www.albrightinternational.com/wpcms/wp-content/uploads/2018/11/SW200-Series-Catalogue.pdf

 

https://www.albrightinternational.com/wpcms/wp-content/uploads/2018/11/SW200-Series-Catalogue.pdf

Based upon the time below from the relay data sheet, the diodes should only need to carry the 200A for a maximum of 14ms, so a diode surge rating of 500A for 8.3ms I would think should be adequate.

 

Based upon the time below from the relay data sheet, the diodes should only need to carry the 200A for a maximum of 14ms, so a diode surge rating of 500A for 8.3ms I would think should be adequate.

View attachment 301145

Pretty slim pickings in that size range. I'd think a safety factor of around 2 might be worth considering as any contact arcing at all may reduce the changeover time considerably. One of the downsides to diodes in this size range is that they're awefully slow. A bank of diodes from eg a diode pack from a suitably large alternator might make more sense and be simpler to source should a replacement part become necessary in future.

 

One of the downsides to diodes in this size range is that they're awefully slow.

Large diodes may have slow recovery times due to stored charge, but turn-off time is not a factor here.
The turn-on time is what's important, and that's generally quite fast, even for large diodes.

But certainly you could put more than one diode in parallel, if you like.

 

Pretty slim pickings in that size range. I'd think a safety factor of around 2 might be worth considering as any contact arcing at all may reduce the changeover time considerably. One of the downsides to diodes in this size range is that they're awefully slow. A bank of diodes from eg a diode pack from a suitably large alternator might make more sense and be simpler to source should a replacement part become necessary in future.

I would need to see how to setup the bridge with a dpdt relais and a serie- field motor (see earlier drawing) , but challenging. Any suggestions on diode type or any pick 50 V, 200A?

 

https://www.albrightinternational.com/wpcms/wp-content/uploads/2018/11/SW200-Series-Catalogue.pdf

After all I’ve read and been advised I’m going for a bridge rectifier for my dpdt relay. I am however reading some opposite opinions on the forward current it should be able to handle. Some say it should be able to handle the full forward current ( ie 200A). Others just suggest a multiple of x on the surge rate (Ifrs ~500A) ? What to follow?

 

A 200A continuously rated diode will be able to withstand a higher repetitive peak forward current. As an example a 1A IN4001 diode can typically withstand around a 5A peak current – but it depends for how long and how often.

That said, I would expect a 200A rated diode to withstand likely peak currents into a nominal 200A rated motor. If you check out the spec for your chosen diode, it should list the peak current – your problem is that you don’t know the peak thruster current draw.

 

A 200A continuously rated diode will be able to withstand a higher repetitive peak forward current. As an example a 1A IN4001 diode can typically withstand around a 5A peak current – but it depends for how long and how often.

That said, I would expect a 200A rated diode to withstand likely peak currents into a nominal 200A rated motor. If you check out the spec for your chosen diode, it should list the peak current – your problem is that you don’t know the peak thruster current draw.

Just to give some background on the usage: the thruster on an event is used in 1 direction, so no quick changing of direction. (1 pole of the dpdt relay excited) The thruster will be in action usually between 5 and 10 seconds. Intervals between actions will be between 3 and 10 seconds. Max usage all intervals added up will be 2-3 minutes. The thruster itself oveheats upon continuous usage over 90 seconds.

 

The peak current draw will occur when the thrusters are starting from stationary and when suddenly reversed. The limiting factor (besides the absolute peak current) on current draw is the heating effect on the diode – given that you don’t (normally) suddenly reverse the thrusters and only operate them for around 10 seconds, with at least a 3 second pause, I would expect a 200A diode to comfortably handle these conditions.

 

I should add, if the diodes are acting a snubbers (reducing the back-emf voltage) and not conducting the 200A motor supply, then I would think a much lower current rating would suffice. The snubber diodes will only be conducting the generated back emf at the instant the thrusters are reversed or switched off.

 

I should add, if the diodes are acting a snubbers (reducing the back-emf voltage) and not conducting the 200A motor supply, then I would think a much lower current rating would suffice. The snubber diodes will only be conducting the generated back emf at the instant the thrusters are reversed or switched off.

But the way the diodes snub the voltage is by conducting the motor current when the contacts open, which then decays as the inductive energy is dissipated.

 

Show us the circuit with the relay and thrusters, and how you will connect the diodes.

 

Show us the circuit with the relay and thrusters, and how you will connect the diodes.

I’ll connect the diodes as earlier suggested. For this I’ll use 2 x vishay vs-vskds400/045. If 200A and 45Vrrm.