solid state relay transcient suppression calculation - Need help

[northbound]

I curently building a solid state driver to be used on a boat autopilot to drive a 12 volt DC reversing motor (30 amps).To achieve, I use four DC output solid state relays rated 40 amps, 100 VDC.Since the electric motor will be energized with polarities which alternate according to the vessel's heading, I am concerned about the precautions to be taken to protect the solid state relay from transcient voltage at turn off . The SSR data sheet suggest to use a reversed-biased rectifier diode in series with a zener diode (opposite direction) or a varistor (MOV) to be installed accross the output terminals of the SSR. Does someone could help me determine the value of using diodes (voltage and amperage) or suggest me the best system to be used ?

 

[KeepItSimpleStupid]

What I would use is a TVS or )transorb brand or similar) unidirectional diode. One possibility is this one: Digi-Key - 1N6278HE3/54-ND (Manufacturer - 1N6278HE3/54) lately, I've noticed that nearly all sources to transint voltage supression has dried up and i would not be surprised if it has to do with the Japan Earthquake. Your SSR is rated to 100 V, so I would consider using anything from 18 to 50 V.

For a more robust design, I;d consider a 18 or so volt TVS combined with a 50 V MOV. The TVS will be much faster.

 

[crutschow]

Are the 4 relays connected in a bridge type configuration to drive the motor?

If so, then you just need a rectifier across each relay (cathode to positive) which will conduct the motor inductive current when the relays turn off. 5A diodes should be sufficient for the task.

Edit: The transient current is carried by the opposing switch's diode. Thus if the relay connected to ground turns off, the inductive current will flow to V+ through the diode connected across the upper switch connected to V+.

 

[northbound]

Thank you for your reply,
About the TVS, do you know another sourcing were this item is stock ? Would you place the TVS across the SSR output terminals and the MOV across the load ?
Regards,

 

[northbound]

Carl,
Yes they are configured in a bridge to allow the motor to reverse rotation.My concern is that the rectifier will blow off as the polarity switch.
Regards,

 

[crutschow]

Carl,
Yes they are configured in a bridge to allow the motor to reverse rotation.My concern is that the rectifier will blow off as the polarity switch.

Well, you have to provide a delay in the relay switching so that one set of relays is completely off before you turn the other pair on. This will prevent current shoot-through and damage to the relays when you change polarity to reverse the motor rotation. Do you have such a delay included? If so, then the diodes will not see any unusual current.

 

[northbound]

Carl,
The drive signal come from the output of an existing boat autopilot limited to 5 amps. As the signal comes from smaller SSR,I take for granted that delay has been provided in the software by the manufacturer. What do you think ?

 

[KeepItSimpleStupid]

What is the drive signal? +0 to 12, -0 to12? With or without braking? What is the required drive signal for the SSR? I'm imagining (no braking case) turning on 2 SSR's when > some value and vice versa.

In the more complicated case, the drive is PWM.

For protection, it's customary to sense the current using, say, an Allegro hall effect current sensor that could enable/disable the proper drive SSR.

It would be simpler if the drive and not enable signals are available or the two bits that represent fwd, rev, brake and coast.

Some amount of delay is probably built into software.

 

[northbound]

The autopilot driver was designed to drive a small 12 V DC, 3 A motor and would like to upgrade the output to drive a reversing hydraulic actuator rated 12 Vdc, 25 A. The actual drive output signal is an alterning +0 to 12, -0 to12 according to the change in direction. My guess is also that a delay had been provided.

 

[crutschow]

The autopilot driver was designed to drive a small 12 V DC, 3 A motor and would like to upgrade the output to drive a reversing hydraulic actuator rated 12 Vdc, 25 A. The actual drive output signal is an alterning +0 to 12, -0 to12 according to the change in direction. My guess is also that a delay had been provided.

It would be good if you could determine the delay with an oscilloscope.

 

[KeepItSimpleStupid]

Check and see if the drive has any AC which could indicate PWM modulation for speed. You get crappy torque at low voltages compared to motor rating. Hopefully, you can find the signals coresponding to brake, not enable (PWM), coast (optional) and brake (optional).