Mike Howell
EASA Technical Support Specialist

When possible, it is good practice to perform an uncoupled, no-load run on an induction motor as an incoming diagnostic test. A no-load run should also be performed after assembly, and ANSI/EASA AR100-2020 states that “no-load running tests should be made at rated voltage and rated frequency.” In this article, we’ll discuss some of the reasons why this is important and some things to consider when you cannot meet both requirements.

Dynamic test/no-load run
When an induction motor is operated uncoupled at rated voltage and rated frequency, the speed should be very close to the synchronous speed, which is proportional to the frequency. Typically, the measured rotor speed will be within 1 rpm of synchronous speed. Additionally, the magnetic field in the motor is proportional to the voltage and inversely proportional to the frequency, and this relationship is often referred to simply as volts per hertz. The no-load current is predominantly magnetizing current, and it should be evaluated at rated volts per hertz. Typical values of no-load current as a percentage of full-load current versus rated output power are shown in Figure 1 for 2-, 4- and 6-pole motors. In general, no-load current as a percentage of full-load current will increase as the number of poles increases and decrease as the output power increases.

Motors designed for inverter duty should be operated at rated volts per hertz if a suitable drive is not available with the following exceptions: rated voltage should not be exceeded, and maximum speed indicated on the nameplate should not be exceeded. For example, if using a 60 Hz test panel, a motor rated 400 V 20 Hz (20 V/Hz) with no published safe max speed would have to be operated at 1200 V 60 Hz to reach rated volts per hertz, exceeding rated voltage and base speed significantly (3x rated).

If a motor is operated below maximum line frequency (rotating speed), resonance and shaft critical speeds which might occur at higher frequencies will not be evident. For this reason, conducting vibration tests on motors at lower than maximum speed does not ensure compliance with tolerances when the motor is operated up to maximum line frequency and speed.

If a motor is operated below rated volts per hertz, problems associated with electromagnetic noise and vibration could be masked. The force waves causing electromagnetic vibration are approximately proportional to the square of the volts per hertz. For example, if a machine rated 400 V 80 Hz (5.0 V/Hz) was operated at 230 V 60 Hz (3.8 V/Hz), any electromagnetic vibration would be reduced to approximately 60% of its normal value.

Another issue with operating below rated volts per hertz is that the no-load current will be reduced, and although the relationship will be roughly linear over some range, severe reduction will result in a noticeable slip. The relationship between volts per hertz and current will be less predictable and difficult to evaluate. What’s more, if there are other problems that would have been apparent by a rated volts per hertz test (e.g., winding error, wrong connection) they may not be easy to detect.

Operating more than 10% above rated volts per hertz exceeds permissible limits of most machines. Since the relationship between volts per hertz and magnetizing current becomes nonlinear as the stator and rotor cores approach magnetic saturation, evaluating the no-load current is not feasible. For example, a motor rated 200 V 100 Hz (2.0 V/Hz) operated at 208 V 60 Hz (3.5 V/Hz) might draw closer to locked rotor current than normal no-load current. When this type of testing error is made, the stator winding can be damaged very quickly.

Service centers that regularly repair inverter fed motors should consider investing in variable-voltage, variable-frequency testing capability. For additional information on testing inverter duty motors and sourcing a VFD for your service center, the following Currents articles should be helpful and are available in the easa.com Resource Library:

• “Testing Methods for Induction Motors for Use in VFD-Powered Applications” from November 2014
• “Selection and Use of a VFD for Service Center Testing” January 2016.

Phase balance test
The phase balance test is a non-standardized test and has many names including but not limited to open stator impedance test, wound stator assembly test, ball test, and dummy rotor test. This test is used in some form by many service centers both as a troubleshooting test and a quality control check before winding treatment. The typical approach is to apply a balanced, three-phase voltage to the stator winding terminals with the rotor removed and then to evaluate the resulting current balance and magnitude. Acceptance criteria differ but it is reasonable that the current unbalance should be within 10% of the average current. Additionally, with the lower impedance resulting from not having the rotor in place, nameplate current should be drawn at roughly 12 to 20% of rated volts (volts per hertz). For example, if a motor is rated 460 V 60 Hz (7.7 V/Hz), you would typically obtain nameplate current somewhere between 55 V and 90 V if testing at 60 Hz. If the motor is rated 460 V 200 Hz (2.3 V/Hz), nameplate current would be expected at 60/200 = 30% of that range or 17 V and 27 V, if testing at 60 Hz.

It should be noted that even though the test current will be very close to full-load current when performing this test, the magnetic field developed will be very low. This is because removal of the rotor significantly increases the reluctance of the magnetic circuit. In Figure 2, the top image is a model of one phase of a 40 hp (30 kW) 4-pole stator at rated current (52 A) and close to nominal back iron magnetic flux density (100,000 lines/in2 or 1.6 T). With the rotor removed as shown in the bottom image of Figure 2, the same stator current results in a much smaller back iron magnetic flux density (12,000 lines/ in2 or 0.2 T). This is the reason this circuit arrangement is not practical for performing a core loss test.

This test is very useful for identifying poor connection joints, connection errors, and winding errors when testing at rated volts per hertz, around 75 to 100% of rated current, and supplemented with thermography.

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Categories: Technical topics, Testing, Motor testing

Tags: Variable frequency drives Motor testing

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