OT - Getting rid of induced voltage

A customer of mine has a problem with induced voltage. He has multiple small motors being driven through a multicore cable - I know - should not do it that way or you get the problem he has.

The motors are reversible and even though the induced voltage is only 5 volts, it locks out the motor from being driven in the opposite direction.

I guess that the driving contactor for, say, clockwise direction could ground out the cable for anti-clockwise direction and take the induced voltage to ground but I thought someone my have a better way.

Any help appreciated.

By the way, I have searched Wikipedia without success but may be looking for the wrong thing.

Hi Bob,

What's the rated voltage of the motors? I can't see what a mere 5 volts could do?

If the reason for malfunctioning is induction, then how can the motors run in one direction, and not in the other?

The voltage is only low AC (24). Apparently, the induction is higher until the motor is told to run. The induction reduces to 5 volts when power is switched to run the motor in one direction. The 5 volts appears to be holding the reversing side of the system locked.


By the way, this has been all by phone with a normal but experienced electrician. I may have to go and have a loopk and experiment. At the end of the day, he has to get rid of the 5 volts.

Exactly what kind of motors ?

BobB said:

The voltage is only low AC (24). Apparently, the induction is higher until the motor is told to run. The induction reduces to 5 volts when power is switched to run the motor in one direction. The 5 volts appears to be holding the reversing side of the system locked...

Click to expand...

It's highly unlikely that an induced voltage over a cable would hold enough energy (read amps) to prevent a small motor from running. Even a 10W 24V motor would need several 10mA's to lock up.

It just doesn't make any sense, Bob. There has to be another reason...
Maybe the 5 volts come (indirectly) from the power supply and there's a relay malfunctioning?
Or if the motor is a standard asynchronous type, then there might be a (beginning) shortcircuit between main & starter windings? It could explain why the motor is only willing to run in one direction.

Or, is it possible that the noise is on the drive speed reference. There, 5V noise is a serious problem and could cause these types of problems especially if the direction control is thru +/-10V.

Sounds like you need to nail down just what the problem actually is.

What about some type of shunt or snub on the leads to ground.


Neon bulbs probably wouldn't work, 'specially with the trigger voltage being, what 85 or so.

LED's probably wouldn't have enough drain.

Gimmicks (capacitor) probably (???) wouldn't cut it either.

How about a 28 or 32 volt pilot light to ground on each lead?

There shouldn't be too much current in the induced voltage (READ: Shouldn't).

There are a varitey of line noise filters available that might help. Like the RC networks with MOV's for surge protection. Try one at the source and at the distribution point. I have seen a few that clamped out induced noise.

I do not know what type of motors at this stage. Will have to go and have a look I think. Thanks so far. Will come back when I know a bit more.

I hate these kind of phone calls. They think you can fix anythinbg over the phone - read do not want to pay for a service call to investigate.

BobB said:

I hate these kind of phone calls. They think you can fix anythinbg over the phone - read do not want to pay for a service call to investigate.

Click to expand...

One way to get rid of Freebies, is to have a "900" number.

But, those that did were well disliked.




Tell them they need a "Flyback Varistor" or "Flux Capacitor".

But that you are the only licensed there to install it!




I also love those places that call you in for an interview.

First, you get a grand tour.

Then, several people pick your brain to see what you would improve on.

Later, you find out there was no opening, they just wanted free advice.

i have a similar problem. a small 3phase motor (200W) and four sensors (24V, magnetic proxy sensors) are running through a metal conduit. When the motor runs, all the sensors starting showing output signals at the same frequency even though they are off. These are my observations and solving attempts
1. Removed the counduit and just tied the wires in a bunch .The problem disappears.
2. I connected the counduit body to earth. The problem still persists.
3. Used a shielded cable for the motor and put inside the conduit. The problem disappears.

Unfortunately, in my case, The motor and sensors are supplied along with the conduit by a mass manufacturer so I cannot ask him to make something special for me.

Looks like the conduit is acting as a long magnetic core because of some material quality and this acts similar to a transformer causing induced voltage in the sensor cables when the motor moves.
I cannot try to protect the sensor cables from noise by putting some protection filters because the actual sensing time is also approximately the same so the valid signal also gets blocked. the only way seems to be actually eliminate the noise

Does this seem similar to what you are facing.

vu2gmc2000 said:

Unfortunately, in my case, The motor and sensors are supplied along with the conduit by a mass manufacturer so I cannot ask him to make something special for me.

Click to expand...

This is a very old thread, so it would have been fine to start a new thread.

The manufacturer must have not intended for the motor to be VFD powered, else they'd have shielded it or separated it.

I like shielded VFD cable properly grounded...stops most noise at it's source, and in your case, the signal cables ought to be shielded and drained as well.

To avoid replacing the motor leads, you could try a choke (load reactor), and perhaps a small load on the sensors. Noise tends not to fool relays. Can this application stand the delays and reduced frequency capability of interposing relays as the proximity switch loads? If not, just having them in parallel with the controller input may pull down any voltage spikes that may otherwise be picked up by the high impedance of a PLC input.

firstly, the inverter is part of the motor supply so I cannot use that as an excuse. Your idea of using a relay coil or some such inductor seems good. let me try. my only worry is that this is low frequency interference and I don't know if it can be suppressed thru normal means I may have to used INTs (induction neutralizing transformers) if nothing else works.