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DC motor slow in one direction, fast in the other!
2

DC motor slow in one direction, fast in the other!

DC motor slow in one direction, fast in the other!

(OP)
Hi,

Apologies if this is a dense question - my background is mechanical, not electrical!

I've got a 24Vdc motor connected to a 24Vdc, 6A, power supply.
The circuit runs through 2 relays, one switches the circuit on & off, the other reverses the motor direction.
I also have a motor speed controller in there & 2 limit switches.

I can get the motor to run in both directions, however in one direction it is fine, I can adjust a wide range of speeds with the speed controller, no problem.
Running in the other direction, even with the speed controller set to max, it runs very very slowly, on the limit of stalling.

Might this be solved by a higher current power supply? (the green light on the PSU dims & goes out when running in the slow direction!)

Many thanks.
Dave.

RE: DC motor slow in one direction, fast in the other!

(OP)
Hi,

Apologies, have had a go at drawing one here, hope it conveys what I've got!

Essentially two DPDT relays one changes the motor direction, the other switches everything on & off.

S1 & S2 are limit switches (have tested & they work)

The two SPDT relays are part of a PC controlled board, so letting everything be controlled by PC software. (tested & working).

Many thanks,
Dave.

RE: DC motor slow in one direction, fast in the other!

Is the motor designed for a single direction?  Sometimes, to compensate for armature reaction, if a designer knows the motor will only run in one direction, he or she will shift the brushes causing the motor to run well in one direction and poorly in the other direction.

RE: DC motor slow in one direction, fast in the other!

(OP)
Its original application had it driving a much larger load through the same gearing in both directions,(I've cannibalised an old machine to make a new one!)

so guess from that the motor should work equally well in both directions?

RE: DC motor slow in one direction, fast in the other!

(OP)
Sorry, should have been more specific, only the power supply & motor are cannibalised, all the relays, wiring, and the speed controller are new parts.

Will try swapping the wires & let you know!

Thanks,
Dave.

RE: DC motor slow in one direction, fast in the other!

I think the high load indicates a mechanical problem if this is geared.  I was once called in on a drive that loaded down.  Removed the gear lube plug and bronze powder poured out.  The housing had never been filled with oil.  A little speed change may be normal but high currents are not.  A little detective work will sort out this mechanical problem.

RE: DC motor slow in one direction, fast in the other!

There are not many motor speed controllers that are designed to allow their input polarity to be reversed. To reverse the motor you normally switch the wires between the controller and the motor. Do you have any specs on the speed controller?

RE: DC motor slow in one direction, fast in the other!

I couldn't get that diag to download this morning. Controller is definitely the problem.  Place it near the 24v input before the relays.

RE: DC motor slow in one direction, fast in the other!

As the two posts just above indicate, you should reverse the polarity of the motor, NOT THE SPEED CONTROL CIRCUIT.

You should probably assume that the speed controller is fried due to the polarity reversal.

RE: DC motor slow in one direction, fast in the other!

(OP)
Oops, sorry I understand now, you mean as the cricuit switches direction I might have fried the controller - woops.

RE: DC motor slow in one direction, fast in the other!

(OP)
Ok, putting the speed controller immediately after the power supply seems to have resolved it, now have equal speed in both directions!

Many thanks for all the help!

Dave.

RE: DC motor slow in one direction, fast in the other!

Modern current limited PSU  may have saved you from  letting the smoke out,
but adding a fuse between PSU and now suspect controller ,may save the PSU if it all goes horribly wrong !

  Sparky   

RE: DC motor slow in one direction, fast in the other!

a dc motor has "a timed" current injection through the brushes,to maximise the torque,old dc motors had sometimes mechanically adjustable bruches (to adjust the angle).this means that with mechanically fixed brushes,one turning direction would be in favour.  

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