AC Motor:  All the newer machines have AC motors. Every maintenance person comes across a question whether the motor is bad or the drive is bad. How do you tell if the motor is bad? Can you even tell if the motor is bad? Absolutely. The following tips will walk you through in a simple step by step process.

AC motor is cheaper compared to DC motor and needs less maintenance. It can be single phase (2 wires) or three phase (3 or 4 wires) for the armature. It has a solid rotor unlike DC motor.

Heat check: When the motor gets hot that is an indication either the load is high or the motor is going bad. Heat is generated due to loss of current inside the motor either by really working on the load or leaking inside.  First line of check is to measure the current.

Current Check: Check the current using clamp on meter. Use a true RMS clamp on meter. The new generation of motors will have a high frequency also on the lines. If the meter is not a true RMS meter it will give erratic readings. It is not abnormal for a motor to draw up to 12% of its rated current (See the name plate), at standstill or no load. If the no load current is higher than 25% further checks are necessary. If the load current is in the range of 50% and higher the motor will get hot which is OK.

Resistance check: Next check is to check the resistance across armature. Use a Multi meter to check the resistance. When you check the resistance make sure the motor is disconnected from the drive.  The resistance should be in single digits. Bigger the motor the lower the resistance. It is not unusual for a 50KW motor to have a .3 ohm resistance. Second resistance check is to check the resistance between the ground and one terminal of the  armature (ground resistance). It should theoretically be fully open (Infinity). But it will be Meg ohms. This check gives a quick idea of whether there is problem.  This means the motor is leaking current to the ground. The third more thorough check is to do a Meggar check. You can pump in a higher voltage into the motor and check ground resistance. There are several companies sell Electronic Meggars. When doing a Meggar check there is a chance to damage the circuit to which the motor is connected. Always disconnect the motor from rest of the circuit to have a good reading. There is a good chance there is more than one motor of the same kind in the machine. It is always a good idea to compare it with another good motor. Some motor manufacturers would want the resistance to be in the 200 Mega Ohms. Anything below 10Mega ohms is a problem.

Cable check: If the readings are not high with Meggar check, make sure the cable is disconnected and the check is performed on the motor terminals. The cable, which carries current to the armature, should be completely open without motor or the drives disconnected to it. If the resistance is 100 Mega ohms or less the cable should be replaced.

Mechanical Check: Make sure the bearing are Ok. The air gap between the stator and rotor is 1mm. There cannot be much play in the bearings.

If you decide to send the motor to a shop, make sure you take readings of ohms before you send it out and check it when it comes back in.

If you would like to learn more about CNC maintenance tips go to http://cnconestop.com/CNC.htm

Next month the planned topic is DC Motors. If you have any suggestions on what topic you would like to see in the next issue please email the author. Author: Ven Swaminathan. Email - Ven@cnconestop.com