Scale Model electric train - motor control

[Echinos]

My uncle has a 7 1/2" guage train track. He currently has 1 gas and 1
steam engine. We would like to make an electric train that would be
very simple and safe to operate, such that an average 10-year-old
could manage it.

I am planning to use a 12V car battery/batteries for power, and I have
an extra 2000-lb winch motor I can use. I have taken the gearing out
of it, so the shaft spins quite fast. I will make a PWM circuit for
it, with either a set speed (or a couple of them), and basic on/off
and forward/reverse. If it has enough power, I would have a normal
throttle for the big kids (adults ) to use.
The body I am going to use for the train weighs about 150lbs.

I'll use mosfets to link the PWM to the motor, but I'm unsure of a few
things:

- How do I measure/calculate how much current my motor will draw, so I
can select the proper mosfets?

I have hooked the motor straight to a battery, and if it's not more
than the 10A my meter can handle, I'd measure that, but I'm more
concerned about surges and variations due to load etc.

- How can I avoid burning the motor out if it stalls (too much weight/
uphill/stuck)

Being a winch motor, I don't know if I have to worry about it.
Wouldn't winch motors have that kind of thing built in?

- Is there a problem if the train goes downhill? Will it feed back and
melt stuff? If I use a controller with a current feedback/current set-
point, will that keep the motor from going too fast?

And, of course, is there anything else I should consider? (Yes, I
will be using fuses!)

I have considered buying a prebuilt controller, and I'm quite happy
to, so long as I know it will handle the current. It would be fun to
make my own though

 

[[email protected]]

- How do I measure/calculate how much current my motor will draw, so I
can select the proper mosfets?

Ideally, you would read the nameplate data, but I'm assuming there is
no nameplate.
I built a drive using HRF3205 MOSFETs with a HIP4081A MOSFET driver.
It can handle up to 75A with heat sinks. Your 12VDC winch motor
probably has much less current draw than that.

I have hooked the motor straight to a battery, and if it's not more
than the 10A my meter can handle, I'd measure that, but I'm more
concerned about surges and variations due to load etc.

That's right. In my experience, a typical DC motor will only draw 20%
- 40% of its full-load current under no load.

- How can I avoid burning the motor out if it stalls (too much weight/
uphill/stuck)

If your motor doesn't already have thermal overload protection, one
way is to limit the current to the motor with fuses or a circuit
breaker. A better way is to use commercial motor overload protection
relays (e.g. Allen-Bradley)

Being a winch motor, I don't know if I have to worry about it.
Wouldn't winch motors have that kind of thing built in?
Maybe.

- Is there a problem if the train goes downhill? Will it feed back and
melt stuff? If I use a controller with a current feedback/current set-
point, will that keep the motor from going too fast?

If the torque applied to the motor shaft is great enough, it will
become a generator. The generated energy will be fed back to the
battery or dissipated as heat in the drive. This can be bad if the
voltage gets too high. Dynamic braking resistors are sometimes
required to brake the motor in these situations - the motor's energy
is dissipated as heat in the DB resistors.

And, of course, is there anything else I should consider? (Yes, I
will be using fuses!)

I have considered buying a prebuilt controller, and I'm quite happy
to, so long as I know it will handle the current. It would be fun to
make my own though

It's a personal choice. Building your own drive can be very
frustrating, but ultimately rewarding. It could be cheaper too,
depending on how many components you fry .

 

[Tom Biasi]

My uncle has a 7 1/2" guage train track. He currently has 1 gas and 1
steam engine. We would like to make an electric train that would be
very simple and safe to operate, such that an average 10-year-old
could manage it.

I am planning to use a 12V car battery/batteries for power, and I have
an extra 2000-lb winch motor I can use. I have taken the gearing out
of it, so the shaft spins quite fast. I will make a PWM circuit for
it, with either a set speed (or a couple of them), and basic on/off
and forward/reverse. If it has enough power, I would have a normal
throttle for the big kids (adults ) to use.
The body I am going to use for the train weighs about 150lbs.

I'll use mosfets to link the PWM to the motor, but I'm unsure of a few
things:

- How do I measure/calculate how much current my motor will draw, so I
can select the proper mosfets?

I have hooked the motor straight to a battery, and if it's not more
than the 10A my meter can handle, I'd measure that, but I'm more
concerned about surges and variations due to load etc.

- How can I avoid burning the motor out if it stalls (too much weight/
uphill/stuck)

Being a winch motor, I don't know if I have to worry about it.
Wouldn't winch motors have that kind of thing built in?

- Is there a problem if the train goes downhill? Will it feed back and
melt stuff? If I use a controller with a current feedback/current set-
point, will that keep the motor from going too fast?

And, of course, is there anything else I should consider? (Yes, I
will be using fuses!)

I have considered buying a prebuilt controller, and I'm quite happy
to, so long as I know it will handle the current. It would be fun to
make my own though

See if you can find specs on the motor. Look for stall current. That would
be you minimum specs for controls.

Tom

 

[Echinos]

Ideally, you would read the nameplate data, but I'm assuming there is
no nameplate.
I built a drive using HRF3205 MOSFETs with a HIP4081A MOSFET driver.
It can handle up to 75A with heat sinks. Your 12VDC winch motor
probably has much less current draw than that.

Yeah, from what I've seen by poking around with google, I will
probably be OK with 20 or 30 amps. The motor should be about 1hp.. I
might be able to calculate something using that.

If your motor doesn't already have thermal overload protection, one
way is to limit the current to the motor with fuses or a circuit
breaker. A better way is to use commercial motor overload protection
relays (e.g. Allen-Bradley)

Is there a way to make a controller hold the motor at a certain speed
& current, even though it is going downhill?

If the torque applied to the motor shaft is great enough, it will
become a generator. The generated energy will be fed back to the
battery or dissipated as heat in the drive. This can be bad if the
voltage gets too high. Dynamic braking resistors are sometimes
required to brake the motor in these situations - the motor's energy
is dissipated as heat in the DB resistors.

I'll look into this - if I get ambitious, I might try regen braking
too.

It's a personal choice. Building your own drive can be very
frustrating, but ultimately rewarding. It could be cheaper too,
depending on how many components you fry .

Well, the controller itself is not really what I'm trying to save
money on, it's more the motor. It seems that this may be made much
easier if I were to buy a motor, so I know all the specs etc, but this
winch motor seems like it should do the job nicely, if I can get the
controller part worked out.