Luther (Red) Norris 
Quality Solutions Co. LLC 
Greenwood, Indiana 
Technical Services Committee Member
 
Brushless servo motors ARE electric motors; therefore many of the tools needed to test them are already available in an electric motor service center. In this article, I have listed some instruments and tools that will be needed to service servo motors.

For the purpose of simplifying the instruments and tools needed for brushless servo motor repair, I am going to break them into two groups. 

• Group 1—those usually found in an electric motor service center.
• Group 2—those that may not usually be found in an electric motor service center. 

Group 1 instruments 
Following are instruments that are normally found in an electric motor service center. 

Multi-meters (Figure 1) as the name implies are multiple-function instruments used to measure voltage, current, and resistance. A good quality multi-meter is a basic requirement in servo motor testing. This can be either analog or digital but should of dependable quality. The pictured multi-meter is an Extech Multimaster 560. 

A Megohmmeter (Figure 2) will be used for testing the quality of the insulation and is used the same as it would be used in the testing of other electric motors. Care must be exer­cised that electronic circuits are not contacted during testing of the windings. The test instrument would preferably have selectable voltages available for testing. The voltage level typically used for testing is 500 volts. The pictured unit is an Amprobe AMB-4D with 250, 500, and 1000 volts selectable output voltage. 

High-potential tester (Figure 3) (hipot) is another test unit with which the motor repair technician is already familiar and is used in the same manner for hi-potential testing of servo motor windings as it is used in testing standard induction motors. Features of the tester should include the ability to connect to the circuit being tested and then increase the test voltage to the desired test level.

A unit that will test up to twice rated + 1000 VAC, or 3.4 x rated volts + 1700 VDC. Pictured is a Slaughter model 306-3.0 AC test unit. 

A surge tester (Figure 4) is used to test a servo motor winding in the same manner as it is used on a standard induction motor. All circuits not being tested should be isolated and grounded. The surge test should not be performed with the permanent magnet rotor inside the stator. Cutting oils, dirt, and contamination are common for the machining applications using servo motors. Surge testing may show weak insulation caused by this contamination. Damage caused by loose windings that have suffered damage due to wire vibration may also be found with the surge test. Shown is the Baker ST-103A. 

A Resistance bridge (Figure 5) is used for testing the resistance of windings and other low resistance circuits. Many manufacturers will specify the winding resistance of their units. Testing the resistance will indicate if the winding has been changed (for example, turns, wire size, connection). Resistance measurement of the winding will also indicate unbalance that could be due to open or shorted winding. For consistent readings the temperature of the windings at the time of the test is important. The winding resistance will vary with changes in temperature. 
Shown is a Yokogawa Model 2769 double bridge. 

Group 2 instruments 
Following are instruments that may not normally be found in an electric motor service center. 

Oscilloscopes (Figure 6) are used to observe waveforms and timing differences of these waveforms. A good quality oscilloscope with storage and printout capability is one of the most used tools in the testing of servo motors. It should have a minimum of two channels. An oscilloscope with two isolated channels is very useful in testing circuits that do not have a common reference point. The unit pictured is a Fluke 196 digital scopemeter. 

A variable-speed motor test stand (Figure 7) is used to back-drive the motor being repaired. A variable speed test stand for back driving the brushless servo motor is necessary for working on these motors. The permanent magnet rotors that are used in these motors will cause the motors to become AC alternators when they are driven. The counter-generated voltage will be equivalent to the applied voltage when the motor is running. The alignment of the commutation, voltage level of the counter voltage, and waveform are all three very important to be able to verify. This test stand is used for the purpose of back-driving the servo motor at a desired speed for making these tests. The test stand will need to be me­chanically adjustable to align the shaft of the drive motor to the shaft of the servo motor. The speed of the drive motor needs to be adjustable and there must be an accurate method of reading the rpm. 

Variable regulated voltage supplies (Figure 8) are used for supplying power to the auxiliary devices during test, and for such other applications as releasing magnetic brakes. A variable supply for lock-up of the rotor is also needed. The pictured unit is a Spence Tek Model 6306A. This is a triple output supply for 5.0 volts and two variable outputs 0-30 volts at 6 amps each. 

Signal generators (Figure 9) are used in the test out of auxiliary devices such as resolvers. To test resolvers, you will need the ability to generate a variable frequency sine wave. A frequency range from 1,000 Hz to 20,000 Hz with an adjustable output voltage from 2 volts to 10 volts is ideal. The unit pictured is a CSI/Speco Model SS-1. 

A shop built dual unregulated variable voltage DC power supply (Figure 10) is shown above. This supply was built from two 0 to 90 VDC power supplies that were designed to supply power for perma­nent magnet DC motors with a 90 volt armature circuit. This makes available a dual DC voltage of 0 – 90 volts at 10 amps.

 

Encoder/Resolver test equip­ment (Figure 11) is used for pulse counts on encoder outputs and testing of pulse width and the relationships of these pulses to each other. Pulse type commutation signals are tested and set. The testing of serial encoder signals requires special decoding equipment to be able to read these signals. The angular position of a resolver may be determined at ninety-degree increments with an oscillo­scope. (See “Brushless Servo Motors: How Are They Different?” in the March 2005 issue of CURRENTS.) However, resolver to digital interface equipment will allow the angular position to be determined for all positions of the resolver. Pictured here is the Mitchell Electronics TI­5000R test unit. 

Servo motor drive amplifiers (Figure 12) are used for test running of motors after repairs. The ability to test run a motor up to operating speed is useful to checkout the dynamic 
operation of the motor. Bearings, vibration, noise, rubs, and overall operating condition of the motor can be checked. Shown here is the Mitchell Electronics TI-3000R test unit that will test run many different servo motors by processing the commutation signals and converting them to match a standard amplifier.

Signal breakout boxes (Figure 13) for looking at various feedback and counter generated voltages. The use of a signal break out box will give quick, easy and safe availability to the voltages and feedback signals for oscilloscope test probes. The unit pictured is the Mitchell Electronics TI-5250. 

Flux meters (Figure 14) are used for measuring flux densities. Since permanent magnets are present in servo motors, the ability to measure and compare flux densities is very useful. The flux density in the air gap is difficult to measure because of accessibility. Unless you are involved in the design or redesign of a motor measuring the air gap flux density is not necessary. If the magnetic material used is Ferrite (sometimes called ceramic) then the air-gap flux density will typically be 3 to 4 kilogauss. If the magnetic material is rare earth then the flux density would usually be about 8 to 10 kilogauss. The measure­ment of torque, BEMF voltage, and waveform will give better information about the magnets. However the ability to measure the flux balance between poles when a motor is disassembled will sometimes be helpful. The flux meter shown is an F. W. Bell Model 5080 Gauss/Teslameter. 

Other instruments and tools will be needed for these repairs. The above outline of instruments and tools is not all-inclusive and does not include all facilities and equipment that may be needed in the repair of servo motors. Physical location, electrical power, compressed air, and amount of floor space will all need to be determined. Tools such as those pictured here of a 
tachometer and torque wrench (Figure 15) are normal tools that will also be used in the testing of servo motors. Additional equipment such as vibration analysis instruments for testing vibration and bearings may be shared with the other areas of the service center. 

A dynamometer for testing of servo motors under load may be of some value for heat cycle testing or re­design. A dynamometer is not a requirement for repairing these motors. 
The fact that the brushless servo motor develops full torque at stall allows measurement of the stall torque with a torque wrench at zero speed. This test will indicate if the motor falls within the designed torque. 

Magnetizing equipment for re-magnetizing of permanent magnets is necessary only for tachometer generators or disc motors that use Alnico magnets. 

Brushless servo motors use ferrite, neodymium-Iron-boron (Nd-Fe-B), or samarium, cobalt (SmCo) magnets. 

These magnets are not affected by disassembly and removal of the rotor. None of these motors use Alnico magnets; therefore, they are resistant to de-magnetization and rarely require re-magnetization. Magnetizing equipment is not a requirement for repairing these motors. 

Separation of servo motor repairs from the standard motor repair areas is recommended for any servo repair operation. The dedication of facilities, tools, instruments, equipment, clean working environment, and personnel are essential to building a successful servo repair service offering. 

Categories: Service center equipment, Servo motors, Testing, Motor testing

Tags: Servo motors

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