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Author Topic: Switching on/off large DC Motor  (Read 4314 times)

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Switching on/off large DC Motor
« on: November 01, 2009, 02:18:22 PM »
Hi All,

This is not exactly a Mach question, but thought someone might be able to help.  I am having trouble with switching my DC turret motor on/off without killing whatever is doing the switching.  I cannot find any mechanical relays that are rated for DC at the current/voltage I want to switch, and the super-expensive DC solid state relay I bought from http://www.power-io.com turned it on about 3 times before failing on.  The guy I talked to there said it would work fine, and I installed the inductance-spike diode as required, etc...

The motor draws 5 amps @60V while running, and a whole lot more when starting up.  The DC supply is 60V 1500VA with a 40,000mfd cap, and also runs a Viper 200.

Thanks for any suggestions,
Dustin

Offline Hood

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Re: Switching on/off large DC Motor
« Reply #1 on: November 01, 2009, 02:25:04 PM »
Probably better looking at a contactor rather than a relay and using a relay to switch the contactor.
Hood
Re: Switching on/off large DC Motor
« Reply #2 on: November 01, 2009, 03:33:09 PM »
Hi Hood,

Thanks for the post, I searched for "DC contactors" and found a place in no-time, http://www.dccontactors.com/contact.html.  I will give them a call tomorrow and place an order, hopefully I can get my money back for the fried semiconductors....  For some reason Automation Direct does not do DC contactors, I called and talked to their support and everything, they sent me down the solid state relay path..

Anyway, thanks,
Dustin

Offline Hood

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Re: Switching on/off large DC Motor
« Reply #3 on: November 01, 2009, 03:39:14 PM »
You can often get DC contactors from motor factors (not sure if you call them that? ) In fact with you being in marine side of things they are quite common on boats, or at least in this country they are, not sure what voltage rating they are though as usually they are just switching 24V..

Hood
Re: Switching on/off large DC Motor
« Reply #4 on: November 01, 2009, 03:45:58 PM »
You should also consider using either a DC motor controller that will ramp up the current, or switch in a series resistance for the first second or two, to limit the inrush current.  When first switched on, the motor looks like almost a dead short.  Those 40uF caps can provide an enormous amount of current for a short time - FAR in excess of the supplies continuous rating.  A smaller dedicated power supply with less capacitance would also help a LOT.

Regards,
Ray L.
Regards,
Ray L.
Re: Switching on/off large DC Motor
« Reply #5 on: November 01, 2009, 04:10:22 PM »
Hi Ray, Thanks for the thought.  It does draw a huge current at start, about 100A based on measuring a current sense resistor in series with the load with my digital o-scope.  I am going to be using a 5ohm resistor for braking, maybe I can somehow get that in series to start and still use it for braking?  Will need to draw that up...

Dustin
Re: Switching on/off large DC Motor
« Reply #6 on: November 01, 2009, 05:19:17 PM »
You are better off to switch the AC side, relays that switch very high DC currents are designed with contact arc quench methods, such as made by P&B,  which use magnet arc blow out.
An ordinary relay can look like a Plasma torch when breaking high DC loads, which ends up either melting the contacts or fusing them together.
You could use the braking resistor in series for a certain time or until the current drops, a better method might be a simple Triac circuit with ramp up in the AC side.
Although if using the supply to feed a  Viper drive,  it may be time to look for a method of its own supply!.
Nosmo.
 
Re: Switching on/off large DC Motor
« Reply #7 on: November 01, 2009, 10:43:57 PM »
Thanks Nosmo,  I know normal relays cannot do the trick, I am looking at some contactors that are inductive DC rated that run ~$35 per pole, that is sounding better than rigging up another supply.

Dustin
Re: Switching on/off large DC Motor
« Reply #8 on: November 04, 2009, 05:45:02 PM »
I hate to mention this but we use regular automotive starter solenoids for 48 and 72 volt pump motors that start at 140 amps and run at 40 amps. 12 volt coil is the only drawback and you may want to use one with a timer for your brake as the brake currents can be brutal. Our pumps run for hours at a time and the solenoids heat up but don't fail.
Re: Switching on/off large DC Motor
« Reply #9 on: November 04, 2009, 06:00:08 PM »
I will keep it in mind, good suggestion.  For now I have ordered P&B part number PRD-11DH0-24 (mouser 655-PRD-11DH0-24) for $34.  They are 1/3 HP rated at 125VDC with magnetic blowouts and DPDT contacts.  24VDC coil on the one I ordered, but there are lots and lots of options. 

As for the braking, I am going to have a 2-ohm resistor in series with the braking circuit to limit the current to 30A max.  I am seriously considering adding a 2ohm resistor in series with the motor all the time also to limit the inrush to something reasonable (30A) as I can handle the voltage drop and power consumption, and this is an intermittent application.

Thanks for the suggestion, I will let you know how my relay stands up.

Dustin