motor speed controller

I recently built a motor speed controller from a 20A kit from Oatley. It
works except for a few minor bugs:

1. the variation is only over the first half of the 5k pot, after that it's
at max.

2. the longer the wire between the controller and the motor, the slower the
motor goes. When it's connected with 150mm of wires it works at almost the
full RPM it's capable of, with about 600mm of wire (barely practical for the
purpose I want it for as a water pump) it works at about half it's
capability.

Does anyone know why it runs like this?

Paul

 

"Paul" wrote

I recently built a motor speed controller from a 20A kit from Oatley.
It
works except for a few minor bugs:

1. the variation is only over the first half of the 5k pot, after that
it's
at max.

2. the longer the wire between the controller and the motor, the slower
the
motor goes. When it's connected with 150mm of wires it works at almost
the
full RPM it's capable of, with about 600mm of wire (barely practical for
the
purpose I want it for as a water pump) it works at about half it's
capability.

Does anyone know why it runs like this?

***** To state the bleeding obvious,Oatley Electronics will know!

Brian Goldsmith.

 

Are you sure you've got the right value components?

What sort of cable are you using? Is it 240VAC wiring that you're playing
with??

Ken

 

I think so. It came with 2x 4.7k resistors but the circuit diagram and the
PCB both said it needed 2x 4R7 (4.7 ohm?) resistors. I went and bought 2x
4.7ohm resistors and used them.

I've tried it with different types of cable all with the same results. I
am amased at how much difference there is with the length of the cable. I
measured some of the cable I was using and it was about 0.4 ohm.

I just thought it was something to do with the pulse modulation and the
value of the capacitor on the motor terminals I put on.

 

Yepp,
your wire is too high a resistance. If a wire of 150mm gives the speed
you want, but you need 600mm, then you'll have to increase the cross
sectional area by a factor of 4. This can be done by using a thicker cable
(try some car jumper leads to get the maximum practical effect here), or by
using 4 of the same diameter wire in parallel.

You have a huge reistance (0.4 ohms is mentioned somewhere) that indicates
that you are either using very thin (less than 30 gauge) wire, something
other than copper, or (most likely) a crappy multimeter to measure this.
Better off measuring the current throug and voltage across the wire and
using Ohm's law.

HTH,
Alf

www.micromagic.net.au

 

All my text books are packed up at the moment, so I cant remember what
aluminium resistance is... I think it's aluminium, it's about 2.5mm
stranded stuff I had lying around. I will try with thicker stuff.

Yeah you are right it is a cheap n nasty digital multimeter, wire resistance
is the only explanation unless it's something to do with the capacitor that
I put across the terminals of the motor.

 

"Paul" <

** What was it - 10,000 uF??



......... Phil

 

"Paul" <

** What was it - 10,000 uF??



......... Phil

 

I fixed it. I forgot a capacitor that goes to one of the terminals to the
chip. I knew it was a bit strange... The 2 mosfets were getting really hot
which I thought was strange since they are rated at 10A each, and then I
realised the 2 transistors driving the mosfets were even hotter. After I
replaced the capacitor it works at full speed no matter how long the wires
are, and the mosfets dont even get warm which you would assume since I
measured the little water pump motor was drawing 550mA... no where near 20A
that it is rated to.

 

Usually its "silly mistakes" like this that cause the most frustration
later

 

Yep and I've done it so many times I knew it was a silly mistake
Probably what threw me was that I was under the impression that Oatley has
put in some wrong value components and there was an extra capacitor. But in
reality I needed the capitor and the 4.7k resistors probably would have
still worked, since they are used to drop the voltage down going to the
transistors that drive the mosfets (even after a years study I still have no
idea what sort of transistors they would be and what the centre terminal is
called... back to school one day when I can afford it.


Thanks for your suggestions, but it definately must have been a capacitance
issue... that much wire (600mm) was dropping it about 6v!