Any electrical geniuses here ?

[Vagrant No2]

I got a non related Surf problem . At work we have a towbar mounted gritter machine that has to be available to work 24hrs a day in Winter. I have to maintain this to ensure it works when needed. Just got it ready for this Winters work when the control unit in the cab has gone faulty again . This box of tricks basically allows you to regulate the spead of spreader motor so you can control the throw of the grit. These control boxes keep failing at a cost of £345 a go ! Inside is a tiny circuit board and a couple of relays.
We can not see how this can cost £345 . So today I made a panel up using a 3 speed heater fan switch and the resistor pack out of a Transit van . It worked but the resistor pack got very hot and smoked a lot . It did however give the 3 speeds we wanted .
Question is .... How can I build a better unit then this which has 5 or more speeds and wont smoke due to the load . I recon the rating on the resistor pack I used was about 25 or 30 amp , which is a bit low I think . I would estimate the spreader motor is about 45 or 50 amp draw.

Can anyone help with parts supplier or any advise .

This is quite urgent as temps are to drop next week .

[Apache]

Rick, can you post a photo up of the inside of your original controller box?

I am an electronics genius, and by inference therefore, an electrical genius too

[Vagrant No2]

Rick, can you post a photo up of the inside of your original controller box?

I am an electronics genius, and by inference therefore, an electrical genius too

I dont have the unit here . But if you have any ideas they would be greatfully recieved . We bascically want to cut out any fancy electronics and keep it basic and Vagrant friendly to repair .

[TonyN]

....... and Vagrant friendly to repair .

Clockwork?



Hi pa......

[Vagrant No2]

Clockwork?



Hi pa......

Issue yourself a yellow card .

[Apache]

Is it a 24v system Rick?

If so, and the motor is 50A, then you need resistors capable of handling 1.2kW! Thats a serious bit of kit! Even if it's 12V, they are still pretty huge at 600W.

What you would normally do is put several resistors in series calculated to give voltage taps you require to run the motor at different speeds, and 'tap' off them to change the speed.

Have a look here - there's some variable power resistors too. They ARE expensive though.

[Vagrant No2]

It is a 12 volt system . I just think the resistor block I used was a little bit light duty for the motor. If I knew the amps of the motor it would help I think . I know the alternator takes a good load when you switch the motor on .

[Apache]

Sounds like you're on the right track then. Bigger resistors required!

If you can get the bolt-down ones, screw them to metal to heatsink them.

[cabbage]

being the polar opposite from genius i cant really help much on the circuitry side, but i have had bother with ford resistor packs melting. if the resistor pack is the way forward it might be worth finding a none ford one!

[blackpoolsparks]

If you take the rated wattage of the motor and divide it by the voltage, that gives you the amprage.

Electrical genius, Andy?
Hows about Mr. Modesty instead

thats like B.Obama wining the Nobel Peace Prize

[Predictable Bob]

Surely there's a better way of controlling the motor speed than using resistors ???

[lord lucan]

the heater resistor pack is always in the vent pipes from the fan to keep it cool, andy is half right about heat sink blocks but the would have to be big

[blackpoolsparks]

The speed of a DC motor is directly proportional to the supply voltage, so if we reduce the supply voltage from 12 Volts to 6 Volts, the motor will run at half the speed. How can this be achieved when the battery is fixed at 12 Volts?
The speed controller works by varying the average voltage sent to the motor. It could do this by simply adjusting the voltage sent to the motor, but this is quite inefficient to do. A better way is to switch the motor's supply on and off very quickly. If the switching is fast enough, the motor doesn't notice it, it only notices the average effect.

When you watch a film in the cinema, or the television, what you are actually seeing is a series of fixed pictures, which change rapidly enough that your eyes just see the average effect - movement. Your brain fills in the gaps to give an average effect.

Now imagine a light bulb with a switch. When you close the switch, the bulb goes on and is at full brightness, say 100 Watts. When you open the switch it goes off (0 Watts). Now if you close the switch for a fraction of a second, then open it for the same amount of time, the filament won't have time to cool down and heat up, and you will just get an average glow of 50 Watts. This is how lamp dimmers work, and the same principle is used by speed controllers to drive a motor. When the switch is closed, the motor sees 12 Volts, and when it is open it sees 0 Volts. If the switch is open for the same amount of time as it is closed, the motor will see an average of 6 Volts, and will run more slowly accordingly.

As the amount of time that the voltage is on increases compared with the amount of time that it is off, the average speed of the motor increases.

This on-off switching is performed by power MOSFETs. A MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) is a device that can turn very large currents on and off under the control of a low signal level voltage. For more detailed information, see the dedicated chapter on MOSFETs)

The time that it takes a motor to speed up and slow down under switching conditions is dependant on the inertia of the rotor (basically how heavy it is), and how much friction and load torque there is. The graph below shows the speed of a motor that is being turned on and off fairly slowly:

[Apache]

Goran, you could have just said 'look up PWM on wikipedia' ;-)

[Predictable Bob]

And if you're in the mood for doing research try "Rhetorical question"