Download Instructions For Rotation Check of Large Motors

Oscilloscope Method for motor phase rotation check
by electricpete
Problem Statement: Commercial motor phase rotation meters require manually accelerating the rotor from rest
very rapidly, in order to clearly distinguish the direction of initial deflection of the meter needle. This works
well for small motors, but does not work for large motors which cannot be rapidly accelerated rapidly due to
high inertia. The commercial phase rotation meter gives ambiguous or subjective results for large motors.
Solution: An alternate method is presented which does not require rapid acceleration, and therefore is suitable
for large motors. This alternate method uses an oscilloscope to monitor the steady-state sinusoidal voltages
generated by the rotor residual magnetism when manually rotating the motor slowly in the normal direction of
rotation. This method has been successfully applied to verify rotation on several large motors including 2
Reactor Coolant Pump motors at our plant.
Theory: The rotor core retains a small amount of residual magnetism which will induce a few hundred
millivolts of voltage in the stator when the rotor is manually rotated. The phase rotation sequence of the
voltage generated during manual rotation of the motor in the normal direction is the same as the phase
rotation sequence of the voltage which would cause the motor to rotate in the normal direction when
energized from the power system. This principle is exploited in the methods below.
Phase-To-Neutral Voltage Method:
If the motor neutral is accessible, then phase-to-neutral voltages could be monitored with an oscilloscope while
manually rotating the motor in the normal direction. Table 1 illustrates the two possible outcomes, which are
described as motor phase rotation sequence T3/T2/T1 or T1/T2/T3.
Table 1 - Phase to neutral voltages while rotating motor forward for T3/T2/T1 and T1/T2/T3 motor sequences
Motor T3/T2/T1 Sequence
Motor T1/T2/T3 Sequence
The motor-to-power-system connection T1-A, T2-B, T3-C can be used in either of the following situations:
 Motor is T3/T2/T1 sequence and power system is C/B/A sequence
 Motor is T1/T2/T3 sequence and power system is A/B/C sequence
For the remaining 2 scenarios (Motor is T3/T2/T1 and power system is A/B/C…. OR motor is T1/T2/T3 and
power system is C/B/A), then motor leads should be relabeled or an alternate connection should be selected (for
example T1-B, T2-A, T3-C).
However, neutral leads are not always available, so a method which relies on phase-to-phase voltages rather
than phase-to-neutral voltages is preferred. The remainder of this whitepaper/procedure will address the
phase-to-phase method.
Phase-To-Phase Voltage Method:
This method is more general since it can be applied regardless of whether neutral leads are available.
A 2-channel oscilloscope is selected since it is more readily available than a 3-channel oscilloscope. As shown
in Figure 1 of the detailed procedure below, oscilloscope channel A will record Vab=Va-Vb and oscilloscope
channel B will record Vbc=Vb-Vc. Phasor Diagrams are shown in Table 2 below. For these diagrams, the
direction of phasor rotation is arbitrarily assumed to be CCW (this need not correspond to direction of motor
rotation). These phasor diagrams prove that if the motor generates voltage in a T3/T2/T1 sequence, then
channel B will lead channel A…. and if the motor generates a T1/T2/T3 sequence then channel A leads channel
B.
Table 2 - Phasor Analysis
(T3/T2/T1 Sequence... Ch B leads Ch A)
Phasor Drawing
Ch A
T2
Ch B
T1
N
T3
Ch B
N
T2
T3
Phasor drawing,
redrawn to
emphasize
relationship of
Channels A and B
(T1/T2/T3 Sequence... Ch A leads Ch B)
Ch B
Ch A
T1
Ch A
Ch A
Ch B
Expected
oscilloscope
pattern
Description of
Channel B leads channel A
Channel A leads channel B
oscilloscope
pattern
Motor sequence is compared to power system sequence in a similar manner to the phase-to-neutral voltage test
above. The entire phase-to-phase voltage method is described in a detailed procedure below which is suitable
for craft instructions.
Detailed Procedure For Rotation Check of Large Motors (Phase-to-phase method)
Equipment: Fluke 199B 2-channel Scopemeter or equivalent isolated-channel instrument. (Almost any
modern battery-powered oscilloscope has isolated channels).
Hookup up test equipment as shown in Figure 1 (T1 to Channel A+, T2 to channel A- and channel B+, T3 to
channel B-). For the Fluke 199B, double-ended probes (Figure 3) should be connected to “input A” and “input
B” ports (Figure 4).
Figure 1 – Connection of 2-channel oscilloscope to motor
T1
Motor
+
+
T2
Channel A
Two Channel
Oscilloscope
Channel B
T3
Set up oscilloscope for monitoring two channels. Suggested steps for Fluke 199B are as follows:
Reset Fluke 199B to factory default settings as follows:
Start with the Unit Off
Press and hold User.
Press/Release On button briefly.
Unit comes on and beeps twice.
Release user.
The unit will now be on, in scope mode, monitoring channel A.
Add channel B trace as follows;
Press B, Press F2, and Press B again
Use the vertical move buttons to separate the A and B traces.
Adjust horizontal time scale to aprox 1 second per division by pressing left side of the “time” button (labeled second) several
times.
Adjust range button (V or mV) to adjust vertical range to approx 100 mV per division.
Energize oil lift pump, if applicable. Rotate motor in normal direction of rotation.
Observe oscilloscope pattern (The “save” button may be used to freeze the display on the Fluke 199B). Compare
results to Figure 2 to determine if the motor voltages sequence is T3/T2/T1 or T1/T2/T3.
Figure 2- Interpretation of Oscilloscope Results
Ch B leads Ch A, signifying T3/T2/T1 motor sequence Ch A leads Ch B, signifying T1/T2/T3 motor sequence
How to connect motor to power system:
The connection T1-A, T2-B, T3-C can be used in either of the following situations:
 Motor is T3/T2/T1 sequence and power system is C/B/A sequence (STP has C/B/A sequence)
 Motor is T1/T2/T3 sequence and power system is A/B/C sequence
For the remaining 2 scenarios (Motor is T3/T2/T1 and power system is A/B/C…. OR motor is T1/T2/T3 and
power system is C/B/A), then motor labels should be relabeled or an alternate connection should be selected
(for example T1-B, T2-A, T3-C). (contact Engineering prior to relabeling or alternate connection).
Figure 3 - Photo of Double-Ended Oscilloscope Probes and Connections
Figure 4 - Fluke Inputs
Figure 5 - B leads A indicating T3/T2/T1 Sequence