DC Tacho Generator: Will Average Bearing Wear Cause Motor to Speed Up?

[jakbeuk]

The purpose of a tachometer attached to the non-drive end of a brush type dc motor is to supply a feedback voltage to the motor`s drive, telling the drive what speed the motor is running. When the drive starts up it supplies a DC voltage to the motor matching the process required set point, thus motor runs at 10% of max speed the tacho will send back to the drive 10% of its maximum voltage (let's say the tacho's nameplate states 90Volts generated max at 1800 RPM and remains at 90V upto 5000RPM) which will be 9 Volt.

When the brushes aren`t making good contact or the tacho`s signal path back to the drive gets broken, the drive will keep stepping up the dc output voltage since it would like to get some return voltage from the tacho. The result is a brush type dc motor that will ramp up speed wise too quick at initial start up.

We replaced the brushes on the tacho and started the 150HP dc motor up and ramped it up gradually to full speed and after about 3min ramped it gradually down, making sure the drive doesn`t loose control over the motor.

The bearings were checked and average wear was noticed.

Big question!

Will average bearing wear cause the motor to speed off at start up, meaning the tacho attached to the motor failed to generate voltage at start up?
NS: the coupling never snapped nor did the bearings seized on the tacho.


ANY IDEAS?

 

[dlgoff]

I don't see how the bearings can effect the speed control. What is the problem you are experiencing?

 

[jakbeuk]

Tacho service under attack.

On this tacho the brushes were both checked and wear on the bearings was noticed but since no bearings were available at the time of the service, the decission was made to order the bearings the next day. On the day of the service after fitting the tacho back onto the dc motor, the dc motor got run for a while to make sure the tacho is in working order. Four hours later the process personnel started the DC motor and it apparently flew of with them, as they had no speed control over the drive. IS THAT A TACHO FAILURE OR NOT ?

I NEED TO KNOW AS THIS COSTS ME MONEY!

THANKS FOR YOUR INFO

 

[dlgoff]

"...the dc motor got run for a while to make sure the tacho is in working order."

Well if it was working correctly then it must be in the controller. Doesn't the output of the tacho attach to some controller somewhere?

 

[K. NARAYAN]

I assume your DC motor is powered through an electrical analog or digital drive.

This drive will have a speed reference to drive the motor e.g. assume the set speed reference is 550 rpm. The drive system will start by supplying a low voltage to the DC motor and then based on the speed feedback from the tacho regulate the voltage being supplied to the motor so that the motor reaches the set speed. Even after the set speed has been reached , the drive control system continues to use the tacho feedback to keep the motor speed as steady as possible , depending on the kind of drive and tacho , even to within 1 rpm.

In any control system , provision should be made to take into account a loss of feedback , in this case , the tacho signal. This can happen due to several causes :

a) The tacho may fail
b) The tacho coupling may fail
c) The wires carrying the tacho signal to the drive may break
d) There may be a failure within the drive internal circuitry

The provisions can be in two ways :

1. The drive will automatically change from using the tacho feedback to using the motor's armature voltage as a feedback signal.

2. The drive can fault on a "TACHO LOSS" error and the motor will be stopped.

In case neither of these provisions has been made within the drive , then the motor will overspeed , with disastrous consequences.

I hope this helps.