motor draws huge current at startup.

[franconara]

Hi. I converted a law tractor to run electric with a PM DC motor at 48V. No controller, just 2 solenoids, one to start with 12V the second one to add the remaining batteries for a total of 48V.

When the motor is stopped it produces no back emf and is pretty much a short circuit. It blows my fuses when I switch it on and it arcs, even at 12V.

How can I prevent the huge current draw and arcing at startup?

Thank you kindly

 

[Peter.bp]

According to your description of the problem, I'd say that magnets lost the flux, or otherwise you have short circuit in armature winding...

Is it easy to turn the rotor when motor is disconnected? If yes, try to short the contacts and try to turn again in order to see is it still easy to turn the rotor.
If it is hard to turn the rotor when motor is disconnected, you have short circuit inside armature.
If it is easy to turn the rotor when it is shorted, stator magnets are out of order.

 

[gogo]

Use a 'power' resistor that drops out of the circuit after initial engagement. This is what Elcar did for each of its voltage steps. The resistor isn't utilized for long enough to overheat.

I think golf carts used the same power resistor scheme before electronic controllers became available. Have you priced an electronic controller?

 

[Peter.bp]

Oops, I again missed the point...

It's better to use controller as it can give soft start and you can regulate the speed...
These controllers are pretty cheap. What is nominal power of the motor? What is max current?

Think that you can find new controller for <1kW/48V motor for less than 20$...

 

[franconara]

Thanks everyone for your replies.

1) The motor is fine. All electric motors are close to a short circuit before they start spinning up.

2) I don't want a controller. I like the simplicity of a non-controller setup and I don't want to spend extra

3) I think what I am looking for is a 'power' resistor that drops out of the circuit after initial engagement". But how does it work? Any schematics anywhere?

4) The motor is a Manta II motor. It's rated at 8kW

Thanks again

 

[gogo]

OK, I'm in over my head. The Zagato Elcars used induction motors and it seems the resistors didn't drop out of the circuit until full voltage was commanded.

Try looking at golf car circuits. My father sold Elcars and Wheel Horse (GE) garden tractors. They all used solenoids and resistors and were jerky. My father got ahold of a solid state controller ($$$$ mid 70's) and we lusted for the future when they would be more affordable. Non-stepped smooth speed and current control is the shiz.

 

[franconara]

The golf cart circuits I found unfortunately are all for controllers (obviously you need one for a golf cart so you can vary the speed). Not sure what to do next.

 

[Peter.bp]

As gogo said, try with resistor...

 

[franconara]

I would like to but I don't know what a "power resistor that drops out of the circuit after initial engagement" is and I have not been able to look up any information. thanks

 

[Gregory]

This is an example of a Zagato Zele wiring diagram.
It shows a shunt wound DC motor.

You could also search for electric forklift or possibly trolley car wiring diagrams (but I suspect they use AC?).




I'll agree with others, a PWM controller seems like a better idea to me.