That bridge is rated at 600V (420VRMS), 10A ... I very much doubt that was the case.

Agreed, more likely to be D1. Do you think a cap and TVS across the motor/diode would be sufficient? I have a 600V diode IN5406 which is rated at 420Vrms (600V DC). It's only rated to 3A continuous (200A for 8ms) but I'd have thought that should be sufficient for my low power testing

 

Agreed, more likely to be D1. Do you think a cap and TVS across the motor/diode would be sufficient? I have a 600V diode IN5406 which is rated at 420Vrms (600V DC). It's only rated to 3A continuous (200A for 8ms) but I'd have thought that should be sufficient for my low power testing

I don't think the cap is necessary, unless you want to start tackling EMI. I would only use the TVS for now and work on the details later. 420Vrms sounds about right. Just make sure that its working point is below that of the transistors.