I’m running a high power reversing DC motor, ships’s bowthruster( 24V, 200 A). Reversal is done by an Albright reversal relay. Issue is with these currents and an X inductive load of the motor I keep burning away my contactors. I’d like to start experimenting with an RC snubber but have no clue where to start with capacitor and resistance values. My first guess would be a very low resistance <10 ohms and a capacitor > 1 microF. Where to start, anyone got some practical experience on this?

 

Does this work for you? https://www.digikey.com/en/articles/resistor-capacitor-rc-snubber-design-for-power-switches

 

Does this work for you? https://www.digikey.com/en/articles/resistor-capacitor-rc-snubber-design-for-power-switches
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Yes usefull albeit a basic setup would require a very very low resistance <0.01 ohm ie negligible. As the parasatic capacitance is unknown the required C remains unresolved…

 

You say required and yet you say negligible, which is it?

 

It maybe that the failure mode is actually due to the very high inrush current at the moment of switch on – and not any back-emf effects.

But assuming that it is some form of back-emf, the simple solution in a dc circuit is to place a reverse biased diode across the inductive load (which acts as a virtual short circuit to any generated back-emf).

The problem is that you cannot use this method as the applied dc voltage is reversed to reverse the thrusters. But a dc snubber network can be formed by two zener diodes in parallel (reversed), each with a blocking diode in the correct polarity. The zener voltage needs to be greater than the supply voltage (30V should give sufficient headroom). The zener diodes need not be high power as they will only be conducting for a very short time when the motor is reversed or switched off.

Given the potential for a high back-emf current, I’d recommend diodes capable of a continuous 5A and 5W 30V zener diodes. The ebay seller below is selling 10 for £4.99.

With the above zener snubber network connected across the motor, any back-emf (in either polarity) should be limited to just over 30V.

https://www.ebay.co.uk/itm/18096051...RmCK9yTQ1w5XiRQzAOTL+xiouusE|tkp:BFBM2N_Q78Ni

 

It maybe that the failure mode is actually due to the very high inrush current at the moment of switch on – and not any back-emf effects.

But assuming that it is some form of back-emf, the simple solution in a dc circuit is to place a reverse biased diode across the inductive load (which acts as a virtual short circuit to any generated back-emf).

The problem is that you cannot use this method as the applied dc voltage is reversed to reverse the thrusters. But a dc snubber network can be formed by two zener diodes in parallel (reversed), each with a blocking diode in the correct polarity. The zener voltage needs to be greater than the supply voltage (30V should give sufficient headroom). The zener diodes need not be high power as they will only be conducting for a very short time when the motor is reversed or switched off.

Given the potential for a high back-emf current, I’d recommend diodes capable of a continuous 5A and 5W 30V zener diodes. The ebay seller below is selling 10 for £4.99.

With the above zener snubber network connected across the motor, any back-emf (in either polarity) should be limited to just over 30V.

https://www.ebay.co.uk/itm/180960512634?hash=item2a22164a7a:g:RJAAAMXQAF5RhIj6&amdata=enc:AQAIAAAA4I+r7EASzVURiJDdE28kD6tjDe6wawcvzNCORhje6jm9sddHy8JsjTa6UCzHy+HEM/U/7QtR3yZa0x3RZEs0/YhL1AWK09pVn1bQElwX9LfLttiPSBjKNtOt3QuQT9MJCtA8zEuUMs0ie7ho6Ny2dqKHmRXgLAzXvgmMKHPn0QCPXOnPWC2SVxyL6xsHyPn6uhrkPH33AdmoZJHjK0wQbq+NOlWAoueISl6TQJizMo5Ige429YjUHeRNpdEMM1LmRq/75HX0HmF4PozRRmCK9yTQ1w5XiRQzAOTL+xiouusE|tkp:BFBM2N_Q78Ni

Thanks Hymie, bare in mind this is a serial- field wound motor reversed by an Albright sw202-2 double throw relay, where would the diodes need to be placed accordingly, over the relevant contacts which are broken upon stopping the motor?

 

If necessary you could construct more than one zener diode snubber network and place them wherever you believe a back-emf might occur; especially if the reversing contactor is performing some complex switching that prevents a single snubber network doing what you want.

The advantage of the proposed circuit is that you can place it where you like, as it will be effectively open circuit to any voltages less than 30V, but present a short circuit to emf voltages above this.

 

As an alternate to my proposed zener snubber circuit, you could use a metal oxide varistor (MOV), which are available with breakdown voltages suitable for a 24V system.

https://en.wikipedia.org/wiki/Varistor

 

bare in mind this is a serial- field wound motor

"Bear in mind" ?

The features of "series" field motor is very high current, very high torque, under load.
I don't think a simple snubber is going to cut it.
You may have to resort to variable voltage/current control.

 

"Bear in mind" ?

The features of "series" field motor is very high current, very high torque, under load.
I don't think a simple snubber is going to cut it.
You may have to resort to variable voltage/current control.

Exactly, given the sparks I‘m seeing it‘s tough. Voltage/ Current control hard to realize in the setup, need to stick to the relay. Back to the RC snubber? Could use a high capacity condensor used for running 3 fase motors on 220V.
That should absorb the spike. Any clue which cap and resistance to start with?

 

The high capacity capacitors used in motor start are dry electrolytic bi-polar versions and can only be in circuit for a few seconds before destruction, you would need the Run versions, which are often paper oil-filled versions and go up to ~15μfd.
Maybe 10Ω res to start with?
That electrical arc will posses quite a bit of energy.!

 

The high capacity capacitors used in motor start are dry electrolytic bi-polar versions and can only be in circuit for a few seconds before destruction, you would need the Run versions, which are often paper oil-filled versions and go up to ~15μfd.
Maybe 10Ω res to start with?
That electrical arc will posses quite a bit of energy.!

Ok, thanks for the advice. Will give it a go shortly and reply my findings back in this forum. Given the cost of these capacitors I‘d probably just buy various types and expiriment.

 

Whilst a snubber made up of a 15µF with a series resistor might provide some voltage spike reduction on a mains circuit, it is unlikely to have any effect on an inductive circuit of 24V @ 200A. To give some context, the energy that a 15 µF could absorb, at 24V the capacitor will be storing less than 10mJ.

 

I wonder what the effect of arc blow out magnets may offer, there are some Very strong 1-1/4" neodymium on Amazon etc, place a stack of opposite poles across the contacts.

https://www.amazon.ca/DIYMAG-Powerf...cientific/dp/B06XD2X45M/ref=asc_df_B06XD2X45M

 

I drew this up for you. As long as your prop isn't huge, it's probably not a big deal to leave the snubber in-circuit full-time because once the motor is turning again the current through the cap should go to zero as long as your speed controller isn't PWMing it. If you have an electronic speed control, I'm not sure why you'd need to slam it into reverse anyway ;-)

Your situation is a little different from a typical motor reversal because for a brief moment, the prop is still pushing water while it's winding down and the whole boat is pushing water through it as well. There's an enormous amount of inertia being converted back into electricity.

Regardless, the snubber doesn't have to be in-circuit full time. Just when changing direction. To accomplish this, you would want to employ two additional relays like we use when plug-stopping a large motor with a high inertial load - which is what you are doing - you're just keeping the power on after the stop. A couple of other relays can control the timing if you cannot find a marine-grade timer-relay. The snubber can be brought in parallel with the motor just before the direction reversal and disconnected afterwards by the use of a timing capacitor. This is something you'd usually do with a PLCs but relays are simple, reliable, easy to t/shoot, available everywhere, etc.

A 2-pole stopping relay connects the snubber to the motor. The second pole opens power to the main contactor, the second relay which only needs to be a single pole. Direction control is handled by a third relay which has two poles. This one doesn't need to be as robust as the one you currently have because it isn't being switched under load. It only needs to be able to handle 200 resistive amps.

Two additional relays take the place of a timer/delay one and another two lock-out the timer so it doesn't click continuously while you are underway. The idea is that either direction signal will hold the stopping contactor closed for the time it takes for the prop to stop turning. You may have to change out the capacitor values to tune (bigger == longer delay), but these are cheapo electrolytics - a couple bucks each. C9 is deliberately smaller - it should close K2 while the snubber is activated. The precise values of those caps will vary with the relays you select.

The snubber should be rated for AT LEAST 100V and the capacitors should be bipolar. For power resistors, something like the HSA25R50 would be a good choice. The series-parallel arrangement shown spreads the heat out and adds redundancy. The two 1k resistors act as bleeders. At only 24V, I'm not sure they're entirely necessary but it's good practice in an environment where fuel vapors may collect.

 

But assuming that it is some form of back-emf, the simple solution in a dc circuit is to place a reverse biased diode across the inductive load (which acts as a virtual short circuit to any generated back-emf).

The problem is that you cannot use this method as the applied dc voltage is reversed to reverse the thrusters.

You can by using a diode bridge across the motor, such as below:
The diodes should have a voltage rating of ≥50V and a surge rating of at least 200A.
A 50V, 50A bridge rectifier should likely work.

Edit: Technically, for the circuit below, you only need D2 and D4 since only one set of contacts moves from On to Ground at a time.
Thus, for example, when U2 opens when going from On to Ground, the motor inductive current will through D4 and the U1 ground connection during U2's switching interval.
If one DPDT relay is used for the reversal than the bridge would be needed.

 

I drew this up for you. As long as your prop isn't huge, it's probably not a big deal to leave the snubber in-circuit full-time because once the motor is turning again the current through the cap should go to zero as long as your speed controller isn't PWMing it. If you have an electronic speed control, I'm not sure why you'd need to slam it into reverse anyway ;-)

Your situation is a little different from a typical motor reversal because for a brief moment, the prop is still pushing water while it's winding down and the whole boat is pushing water through it as well. There's an enormous amount of inertia being converted back into electricity.

Regardless, the snubber doesn't have to be in-circuit full time. Just when changing direction. To accomplish this, you would want to employ two additional relays like we use when plug-stopping a large motor with a high inertial load - which is what you are doing - you're just keeping the power on after the stop. A couple of other relays can control the timing if you cannot find a marine-grade timer-relay. The snubber can be brought in parallel with the motor just before the direction reversal and disconnected afterwards by the use of a timing capacitor. This is something you'd usually do with a PLCs but relays are simple, reliable, easy to t/shoot, available everywhere, etc.

A 2-pole stopping relay connects the snubber to the motor. The second pole opens power to the main contactor, the second relay which only needs to be a single pole. Direction control is handled by a third relay which has two poles. This one doesn't need to be as robust as the one you currently have because it isn't being switched under load. It only needs to be able to handle 200 resistive amps.

Two additional relays take the place of a timer/delay one and another two lock-out the timer so it doesn't click continuously while you are underway. The idea is that either direction signal will hold the stopping contactor closed for the time it takes for the prop to stop turning. You may have to change out the capacitor values to tune (bigger == longer delay), but these are cheapo electrolytics - a couple bucks each. C9 is deliberately smaller - it should close K2 while the snubber is activated. The precise values of those caps will vary with the relays you select.

The snubber should be rated for AT LEAST 100V and the capacitors should be bipolar. For power resistors, something like the HSA25R50 would be a good choice. The series-parallel arrangement shown spreads the heat out and adds redundancy. The two 1k resistors act as bleeders. At only 24V, I'm not sure they're entirely necessary but it's good practice in an environment where fuel vapors may collect.

Thanks Ryan, for the suggestion. Albeit I agree a PLC would easily resolve the issue as would your multiple relay suggestion. I’m quite limited in space however as the whole machinery is quite compact AND moves up and down ( retractable). Also the thick cabling going to it Is quite inflexible —> I’d like to stick to the “integrated” relay (sw202-2 Albright). The operation basically means usage in one direction for max. 15 secs. But this multiple times (<10). It is not so much changing direction as it is the repetativeness which wrecks the relays. Small picture of the unit which, as a whole, moves up and down.

 

You can by using a diode bridge across the motor, such as below:
The diodes should have a voltage rating of ≥50V and a surge rating of at least 200A.
A 50V, 50A bridge rectifier should likely work.

Edit: Technically, for the circuit below, you only need D2 and D4 since only one set of contacts moves from On to Ground at a time.
Thus, for example, when U2 opens when going from On to Ground, the motor inductive current will through D4 and the U1 ground connection during U2's switching interval.
If one DPDT relay is used for the reversal than the bridge would be needed.

View attachment 301115

Thanks Crutschow, yes a DPDT is used. This idea is indeed the simplest. It would have to be serious powerdiodes to handle the inertia. Would 100V suffice ? I’m seeing serious arc-ing.

 

It would have to be serious powerdiodes to handle the inertia. Would 100V suffice ? I’m seeing serious arc-ing.

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.