[sliskapa]

Greetings.

Let's see if I found the right place for this.

I'm going to build a small electro-hydraulic loader. I'm restoring an old 67 V, 7.5 kW (100kg ) series DC motor that will drive a hydrostatic pump. Since it will run on a constant max rpm, I don't really see the need for a controller, but I need to limit inrush current.

Do you think it would be possible to use only a precharge resistor and a big contactor?


/Robert

 

[frodus]

You can't limit inrush current without a controller of some sort. As soon as you close that switch, the motor is going to demand whatever amps it requires to get to the RPM it wants to get to.

The precharge resistor is to charge up the Capacitors inside the controller before the Contactor shuts, which helps increase the life expectancy of the contactor. Without one, as soon as the contactor shuts, it would arc, because the capacitors inside the controller aren't charged. By precharging, the capacitors are at the same potential as the battery, and there is little or no arcing.

Using a precharge resistor won't help your motor inrush.

 

[sliskapa]

Ah, yes of course. A controller would start PWM:ing from zero, and thus limit the inrush current of the actual motor. I was thinking the resistor would keep the winding magnitized.

May I could use thermistors until the emf builds up.

 

[Coulomb]

Resistors have been used for starting DC motors for over a hundred years, e.g. in older streetcars. But they typically have at least 2 intermediate speeds between full off and full on. So you might be able to get away with say three contactors (one to turn on/off, the other two initially off and short the resistors one by one as the motor comes up to speed).

The resistors are typically very large; most of the underside of a streetcar is taken up by the starting resistors. The total energy dissipated isn't huge, but the peak power is (comparable to the power of the motor at starting load). In this sense, they are similar to pre-charge resistors. But they have to take full load current, not just enough current to charge some capacitors. So they will need to be much larger than a typical pre-charge resistor.

 

[major]

Hey Robert,

Even if you get the motor started you'll have trouble using a series motor running on fixed voltage driving a variable loading pump. RPM will vary significantly with load. And unless you keep an idle load, the motor will overspeed and cavitate the pump. So if you can get it working I'd think you'll find it inefficient causing rapid battery drain and hot oil.

Regards,

major

 

[sliskapa]

Thanks for the feedback. Most probably I'll start looking for a more suitable motor type. Shunt?

Now for speculating: I wonder what kind of behavior I would get if I would use this series DC motor but measured and limited the RPM with a controller. I'm also thinking of the possibility to automatically turn down the speed of the hydrostatic pump if the DC motors speed gets too low.

Also: How does a series DC motor behave when operating above the rated RPM? Would that be a purely mechanical issue with bearings/temperature or would some other aspect cause a waste of power?


/R

 

[major]

Higher RPM won't be a problem unless you get carried away. Using a controller with series motor to run constant speed gets complex and expensive. Shunt, compound or PM DC motors would do. You may need a starting resistor for inrush limiting.

major

 

[sliskapa]

I didnt explain properly - Since this DC motor would only power the drive, it wouldn't need to run at constant speed as I said first. Between 600 and 3600 rpm is ok, with the actual driving speed set by the hydrostatic pump.

 

[major]

When you run a DC motor across a battery it becomes "constant" speed. Yes, that "constant" speed value is load dependent, but you have no control over it. It is governed by the battery voltage, motor winding and load. So unless you have a motor speed controller (which varies motor voltage), the DC motor gives you more or less constant speed.

How the DC motor speed changes with load is called speed regulation and depends on the type of DC motor. The series wound motor has such poor regulation it will cause problems with the type of varying load in your application. PM and shunt motors have good regulation and speed will just vary slightly with load and is referred to as "constant" speed. Once you switch the battery on, the motor will spin at that speed until you turn it off.

major

 

[sliskapa]

Yes, I got that a series wound motor rpm varies a lot when loaded.

I was just thinking about letting it slow down and instead use the pump to "downshift" between the electric motor and hydraulic drive motors. As long as the electric motor delivers its power, I think it should work. After all it was used in a similar fashion in its earlier life, without hydraulics.

Regardless of motor type, I would somehow like to monitor the current, and thought that a situation where the hydrostatic pump is in its strongest/slowest setting at the same time as the electric motor rpm is to low could mean its time to back up.

 

[major]

It's not the loaded condition, but unloaded condition which will kill you. Series motors have an infinite no-load RPM.

 

[sliskapa]

Thats where I would use a controller to keep a lid on the rpm

Despite my obvious lack of knowledge concerning electric motors, I'm quite confident that I could measure the rpm and regulate the throttle signal to a controller. If it works fast enough, maybe it could save power

 

[major]

Thats where I would use a controller to keep a lid on the rpm

Despite my obvious lack of knowledge concerning electric motors, I'm quite confident that I could measure the rpm and regulate the throttle signal to a controller. If it works fast enough, maybe it could save power

Famous last words

You end up with 2 speed control systems back to back. Good luck.

major

 

[sliskapa]

But the controller would only be used to keep the rpm under 3000ish and over 600.

If it works, Ill post it here

If I dont post within a few months, lets assume it went to hell

 

[Duncan]

But the controller would only be used to keep the rpm under 3000ish and over 600.

If it works, Ill post it here

If I dont post within a few months, lets assume it went to hell

Post and show us especially if it blows up

Always post your mistakes - they are the BEST learning