Tolerances of Armature Gaps



In large motors and generators, the air gap needed to separate the revolving rotor from the stator should be as small as possible as this reduces the magnetizing power requirement. The gap also needs to be large enough to prevent contact between the two, despite manufacturing tolerances on their dimensions or movement resulting from mechanical deflection or looseness in supporting bearings. The gap should also be uniform; a non-uniform gap causes increased noise and vibration.

large motor generator Since large generators and motors are usually quite expensive, it’s advisable to monitor the gap to prevent catastrophic failure should the rotor or stator expand due to heating and operating conditions. Automated shut down or load reduction can save millions of dollars and possibly even save lives.

The inquiry

A large multinational company asked MTI to provide a rotor-to-stator gap monitoring system in their large power generation systems. The probe had to be very flat to fit in the gap but needed to be robust as access was impossible after installation. It had to be able to withstand several Tesla (magnetic field), which of course required it be non-magnetic and finally, it had to be able to withstand high heat conditions. A challenge!

The solution

MTI designed a non-magnetic probe (made of Kapton insulation and copper electrodes) that could be bonded in place on either the stator or rotor components. A Digital Accumeasure capacitance amplifier was mated with the probe to provide a low drift gap measurement (< 100PPM) with a very high MTBF.

air gap flat probe

The probe was capable of measuring the gap (Max ~ 10mm) with a linearity error and resolution that was below 100um. Of particular importance was that MTI also designed a flat coaxial cable to allow the probe to be routed outside of the armature gap before transitioning to conventional round coax. MTI also worked with the company to help provide unique and specific mounting instructions and test instructions as the probes could not be accessed once the generator was fully assembled. The Digital Accumeasure is capable of providing both digital gap information over Ethernet and/or simultaneous analog gap information for local control.

Flat Kapton probes are installed on the stator poles as the generator is assembled.

stator probe


It worked! Since then, numerous systems have been assembled and deployed around the world to provide real time maintenance and operating information.

MTI Instruments enjoys making a difference.