I have an old boat with a 24 volt reversible hydraulic pump, part of a decommissioned autopilot system. I have been given a newer 12 volt autopilot controller which I would like to retrofit. The autopilot controller has only two wires going to the pump and reverses the polarity to reverse the pump direction. I would like to step up the voltage from the controller to match the 24 volt required. Voltage transformers are easy to buy off the shelf but will this work in this application?
thanks Ian

 

How much current can the new controller supply for the pump?
How much current does the hydraulic pump need?

 

How much current can the new controller supply for the pump?
How much current does the hydraulic pump need?

The new controller can supply 20 amps at 12 volts which should be fine for the pump

 

The new controller can supply 20 amps at 12 volts which should be fine for the pump

But when the voltage is doubled you will need to supply more than double the current (as it will be less than 100% efficient). If the pump needs more than, say, 8A then it cannot be done this way.

Is a separate supply available which is capable of more than double the pump current? If so, this could be used to convert to 24V with the new controller being used to provide control rather than power.

 

You can't simply connect a boost converter to the output from the 12 volt controller, when the polarity changes bad things will happen.

Your better option as Albert points out is to use the 12 volt controller as a signal control and use a separate 24 volt supply for power.

Edit:

If you did want to directly convert the controller voltage…

To maintain the correct polarity into the converter you would need a high power bridge rectifier, then use some kind of switching after the booster to reverse the polarity…say a relay, that ties back to the output of the controller before the bridge.

Edit 2:

There are probably many more ways to do this and probably with less loss than using a bridge, but the basic concept is intact…you must maintain the polarity into the converter and then do switching after the converter.

 

Voltage transformers are easy to buy off the shelf but will this work in this application?

Transformers don't work with DC.
You need a DC-DC converter, as noted.

Does the controller output a varying DC voltage as well as changing its polarity?

 

Good question...didn't even consider that.

 

Thanks for your replies.

I am trying to determine the specs for the 24 volt pump to see the maximum current draw. I can source a 24 volt supply for the pump from another location then it comes down to using the 12 volt controller as a switch to change the polarity. The voltage required for the operation is constant, the controller receives input from a fluxgate compass which determines pump run time.

 

I have been looking at relays to do the switching. I have a red wire and a black wire coming from the controller which is constantly changing polarity. Can I split the the red wire into two and the same with the black wire, put in a normally open relay on each side with one wired in reverse to the other. I can then do the same with the pump motor on the output side of the relay.

I still have to solve the voltage difference though I can source power from elsewhere.

 

You want to use a DPDT relay with contact ratings that can handle the motor voltage and current plus being an inductive load.

Then you wire the contacts the same way you would wire a motor reversing DPDT switch.



Then you place a diode in series with the relay coil, so that when the diode is forward biased the relay turns on and when the diode has a reverse bias the relay turns off.

 

ElectricSpidey that is perfect, really appreciate your help. Its over a $1000 to replace the pump and this work around will be fantastic.

 

You're welcome.

I thought that might get you headed in the right direction, I'm sorry I can't flesh out the circuit diagram any further, I don't have access to my main PC right now.

Good luck with your project.

 

the controller receives input from a fluxgate compass which determines pump run time.

Then you wire the contacts the same way you would wire a motor reversing DPDT switch.

As implied by the TS's post, to control the pump run time there is a third state, where the control signal is zero and the motor off.
A single reversing relay can't do that.

To generate this 3-state control (Right, Left, & Off), you could use two SPDT relays in a bridge circuit (below):

 

As implied by the TS's post, to control the pump run time there is a third state, where the control signal is zero and the motor off.
A single reversing relay can't do that.

To generate this 3-state control (Right, Left, & Off), you could use two SPST relays in a bridge circuit (below):

View attachment 249575

If the relay pull-in time is less than the drop-out time then this would generate a brief short circuit across the supply when switching from one direction to the other.

 

If the relay pull-in time is less than the drop-out time then this would generate a brief short circuit across the supply when switching from one direction to the other.

True.
But I would not expect the controller to do that.
Typically it would give a correction in one direction and then turn off the motor until the boat turns sufficiently to require another correction.
Also I would expect the controller to have a delay between reversing directions, otherwise that could be damaging to the motor.

 

Hi crutschow,

Thanks very much for your diagram I am waiting for the parts to arrive. I am a complete novice. I am struggling to understand the 24 volt side particularly the electricity flow and where the negative is for that circuit. I get the SPDT and the 12 volt side. Do the D5 and D6 go to negative?

 

I am struggling to understand the 24 volt side particularly the electricity flow and where the negative is for that circuit. I get the SPDT and the 12 volt side. Do the D5 and D6 go to negative?

Yes, all the ground symbols () are the 24V supply negative.

Does that help understanding the electricity flow?

My apologies for using Left and Right instead of Port and Starboard in my node labels.

 

Got it! Many thanks. (apology accepted)

 

I shouldn't have said got it. Sorry to keep
bothering you.

A couple of questions.
1. I can't work out what the diodes at D3 and D4 do.
2. I have drawn how I think the 24v electricity goes when the right is activated but does the electricity flow exit out through D6 or through the negative attached to the other SPDT?

Thanks Ian

 

Sorry to keep bothering you.

No problem at all.
Glad to help.

what the diodes at D3 and D4 do

They (along with D5 and D6) provide a path for the transient inductive current from the motor when a relay turns off, to avoid arcing at the contacts.

does the electricity flow exit out through D6 or through the negative attached to the other SPDT?

It flows from 24V through the ON relay contact and the motor to the other off relay contact ground (-24V).

You show current going through D6 in the reverse direction, which it does not.
Diodes only conduct current in the direction of the arrow.