Dc Electric Switch Question

I want to be able to change direction(s) of a DC motor, so my question is: Would I use a DPDT switch or a SPDT switch?

DPDT would be my guess

Hello vicinalvictor,

DPDT is my guess too, as you need to reverse it's polarity,

Guido.

To reverse the direction on a permanent magnet field type of DC motor (if that's what you're talking about??) you have to swap the + and - , so you're switching TWO connections. SPDT has but one Pole, DPDT has two Poles. A tractor starter is typically a wound field so reversing polarity on it DOES NOT CHANGE DIRECTION. For small low current DC motors its cheaper n easier but for larger higher current units Reversers are more expensive.

As you know, but for anyone who may not,
DPDT Double Pole Double Throw
SPDT Single Pole Double Throw

I'm sure someone can post a simple picture of the DPDT switch wiring (you may not even need it), the switch has to have sufficient current, voltage and HP ratings.

John T

Reversing the polarity of power to the motor won't necessarily reverse its direction. Typically it won't. So you first need to figure out what it will take to reverse the motor, then you can buy an appropriate switch.

For reversing applications, car window motors are often used. These are permanent magnet motors with two separate windings, one for each direction. A SPDT switch will work for them.

It depends on the type of DC motor. Plus is needs to be a DC rated switch and not an AC only rated switch. DC arcs and burns up slower acting AC only switches.

I want to switch the polarity back and forth on a 36 volt, 800 watt DC motor.

> I want to switch the polarity back and forth on a 36 volt, 800 watt DC motor.

No, what you WANT to do is to reverse the motor direction. Which may or may not be accomplished by reversing the polarity of power to the motor.

As for reversing the polarity, yes, that can be easily accomplished with a DPDT switch. Just wire the opposite corner "throw" terminals of the switch in a "X" configuration so polarity is reversed when the switch is thrown.

Thanks for the update "I want to switch the polarity back and forth on a 36 volt, 800 watt DC motor."

Based on that I will add: IFFFFFFFFFFFF its a permanent magnet (NOT a wound electromagnetic created field) DC motor and you reverse polarity it (depends on motor design and windings etc etc) can reverse the direction.

YES a proper wired DPDT switch which has sufficient DC Voltage and Current and HP rating can be used to as you state "reverse polarity"

Hope this helps

PS Again, any electrical or legal question always draws out more responses and opinions then any other topic

John T

(quoted from post at 06:50:35 05/20/19) I want to be able to change direction(s) of a DC motor, so my question is: Would I use a DPDT switch or a SPDT switch?

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36 volt, 800 watt DC motor.

I believe the amperage you want to switch will be too high for a typical toggle switch, so unless you are going to use solenoids.

I would look for a line voltage winch control switch.

Similar to the design you see used on small quad winches.

There are heavy duty rotary switches available to handle the load you wish to switch though finding one rated for 36 volt+ might be a little more difficult to locate.

Yet more information...?.

First, a general purpose "switch", say to reverse polarity or whatever, but NOT intended or rated for motor starting, IS NOT NECESSARILY THE SAME THING AND IT MAY NOT BE CAPABLE OF ACCOMPLISHING THE SAME RESULTS AS AN ACTUAL TRUE HP AND VOLTAGE AND CURRENT RATED "MOTOR STARTER"

A 800 watt 36 volt motor draws 800/36 or 22 amps BUT that don't mean a cheap straight "switch" (NOT motor starter rated) even if rated for 30 amps will last very long if used to start n stop a motor. Now if its ONLY used to switch polarity no switched load, no starting or stopping, it can suffice for that purpose. Reason is when a motor starts and when it stops theres a lot more current and/or arcing which is hard on contactors.

An actual motor starter has contacts that are rated for the extra current and arcing when starting and stopping a motor PLUS it has thermal overload protection to shut down in the event of overheating or excess current draw THATS TO PROTECT THE MOTOR

MORAL OF THE STORY A cheap toggle switch rated for say 30 amps and 36 VDC "may" work for a while to start n stop a motor (say its a DPDT Center OFF switch) BUT an actual HP and DC Voltage and Current Rated "motor starter" may be whats needed for the reasons described above. Of course, if there's a heavy duty toggle switch with center off rated for sufficient HP and Current and DC Voltage designed to start n stop a motor GO FOR IT NO PROBLEM. Some motors have their own built in thermal overload protectors. I have seen similar switches used on the small DC motors to say raise n lower an RV travel trailer (they reverse plus start n stop a DC motor!!!) but I have seen them go bad, perhaps they didn't have the necessary ratings ???

DISCLAIMER for those nit pickers or people who love to play "gotcha" NO WARRANTY THIS IS ACCURATE WHATSOEVER. Its been nearly 50 years since I was an Electrical Engineer for the Century Electric Motor Company and my old brain is worn out and I have forgotten things LOL YOU DONT SAY. But hey I try my best to help whenever and the best I can

Best wishes n God Bless Yall

John T

(quoted from post at 08:46:05 05/20/19) I want to switch the polarity back and forth on a 36 volt, 800 watt DC motor.

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ey, why don't you tell these folks what you have and if you have reversed it via wiring and what you did to reverse it, so they will stop guessing what you have and need to do? :idea:

I have taken and reversed the negative and positive wires from the batteries to the DC motor, and yes, this reverses the motor. So a DPDT switch is what I need to change motor direction when I need to. So what kind of switch would I need for a DC 800 watt, 36 volt motor? How much amp switch would I need? 30 Amp?

I have not put a switch on this motor yet, so the only way I can reverse the motor direction right now is to reverse the wiring on the
batteries. My question is: would a DPDT switch achieve the task of reversing the motor when I need to, without changing the wires around on
the batteries? 36 volt, 800 watt DC motor. Would I need the switch to be a 30 amp switch also?

......How much amp switch would I need? 30 Amp?

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800W motor at 36VDC is a bit over 22A at full load. Starting inrush current will be much higher, possibly 4-6 times, but for pretty short duration unless the motor is loaded all the time in both directions.

(quoted from post at 08:46:05 05/20/19) I want to switch the polarity back and forth on a 36 volt, 800 watt DC motor.

Click to expand...

It depends .

> How much amp switch would I need? 30 Amp?

John T gave you a lot of good information.

In general, a 30 amp DC RATED switch should work. The inductive motor load will be a lot rougher on the switch if you're using a single switch to both switch the load and reverse direction.

You ask "Would I need the switch to be a 30 amp switch also"

See my post immediately bellow for an explanation. If the switch is rated for "motor starting" with adequate HP and Volts and Amps ratings SURE some sort of a DPDT switch might suffice. Thermal overload protection for a motor is another topic. I've seen such on RV's to raise n lower the tongue using a 12 VDC permanent magnet motor, but I've seen several go bad grrrrrrrrrr lol. However switching and disconnecting a motor is a far cry from unloaded or low current non inductive switching. A cheap 30 amp DPDT toggle switch not intended for motor control may fail quickly.

John T

you can use a drum reversing switch.
this one is a Square D Class 2601, Type BG-1, Series B.