Model Railway Forum

Hopefully a basic, If not silly, question -

Building the first layout, I have fitter 7 pm2 motors.
I have an issue with one motor that keeps heating up that the point of the plastic covering the coils melting

I checked the wiring and it is all good, and consistent with the other motors.

I then took another motor, held it my my hand and switched it a few times and noticed it gets warm to the touch.

The motor that melted is one where the point gave me some grief , so I kept switching it to find out what is the problem, I think the repeated switching made it heat too much.

Two questions -
1. Are they meant to get warm at all?
2. Assuming not - what might be the cause?

In terms of wiring - I am using a 15v 10amp PSU I bought - the negative goes directly to the motor and the positive goes through a momentary switch and from either side of the switch to the motor

Thanks

Surely they shouldn’t have time to warm up if the switch is making contact for only a fraction of a second.

Does the switch have a toggle that returns to the centre position when you release it? If not then you do not have a (on) - off - (on) the brackets indicating momentary on. If it is I would have a look at that switch as it is probably malfunctioning. Check your switch’s current rating solenoid switches draw about 3 - 4A momentarily which can cause arcing internally in underrated switches and is a switch killer.

If it turns out to be standard changeover toggle switch all is not lost just put a good quality push switch upstream of the present switch between it and power supply use the toggle switch to set point direction followed by a press on the push button to activate it.

Consider using a CDU, these eliminate a number of problems.

Richard

thank you both -

Yes, the switches are momentary switches and return to centre upon release so current is only flowing briefly.
The switches are rated for 5A (https://www.amazon.co.uk/MinticeTM-Mome ... r=8-5&th=1)

I also thought that they should not warm up, but they do, although only if activated repeatedly in short order and I'm trying to find out if that's normal and if not - what might be the cause...

Are you flicking the switch left or right then immediately letting it return to the centre.

Why are you repeatedly switching the motors?

Are you 100% sure none of the momentary switches are remaining On even when the switch lever is in the sprung to centre Off position? A simple multimeter voltage check on each motor will quick prove this. Holding the switches lever over in either direction for longer than 1/2 second is too long

It may be beneficial to install a CDU in between the power supply and the first momentary point switch as this delivers the Beefy pulse of power needed but also virtually cuts off all power to the motor(s) as soon as the pulse has been sent.

The PSU is probably be the cause if its unregulated and the ouput voltage applied to the point motors will higher than 15 volts The 15 volt rating is what it will be delivering with a 10 amp load

These switches are only rated at 5A for AC operation, they have no DC rating at all because they are not designed to break arcs and are slow in operation.
It is quite likely you are experiencing contact welding.

A CDU may help if the motor has enough inductance to prevent welding on the make but will only reduce the current on the break if the capacitor has discharged before you attempt to restore the switch to normal.

As Tricky Dicky said a single robust momentary switch (with a DC current rating) in the power supply line would solve the problem by ensuring these small AC rated toggle switches are not making/breaking high currents, another alternative is to change your point power supply to 12-16V AC.

In any case point motors are not designed for repeated switching nor being powered for more than is required to throw the point.

I used this type of switch for decades with no problem. Get a CDU.

Switch contact arcing occurs when the contacts open (break) due to the Bemf from the solinoid coil voltage collapsing as the switch opens. A CDU will eliminate this as its discharged into the coil long before the switches contact opens. A pair of diodes connected in inverse to the feed polarity will also help stop the Bemf getting to the switch contacts, 1N4001 or 1N4002 will be adequate.

As I said previously in #6 and WTD has stated this too..... Obtain a CDU.