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Power/Grounding Requirements
The following information covers the grounding requirements of 3-phase servo amplifiers
manufactured by MTS Automation. It has been found when an amplifier has been connected
to a transformer with an ungrounded secondary, premature amplifier failure will occur.
The 3-phase MPA amplifiers require the AC power (L1, L2, L3, and Ground) be derived from a
transformer which has it's secondary intentionally bonded to earth ground. This means that
some point on the secondary must be connected to an earth ground with no exceptions (see
examples A1, A2, A3). Do not assume just because there are three power leads with a ground
available at an installation, that this is a valid configuration. Some facilities are supplied with
13,200 volts AC which is reduced to 460 volts AC via a transformer. However, the secondary
of this transformer usually is not grounded as in an ungrounded delta secondary (Example
U3). Each installation or facility is unique and the power distribution must be inspected or
measured to make sure the transformer secondary is, in fact, tied to earth ground. A machine
or system built and tested at one facility, may fail at another site due to incorrect transformer
configurations.
There are two common transformer secondary configurations. They are the Wye and the Delta
secondary. Most problems are found with an ungrounded Delta secondary connection. The
examples show acceptable (A1, A2, and A3) and unacceptable (U1, U2, U3) configurations.
If it is not possible to visually inspect the transformer configuration, you can electrically
measure the line voltages to verify a correctly grounded transformer secondary.
A properly grounded secondary (wye or delta) will have certain voltage characteristics when
measured with an AC voltmeter:
•
A properly grounded wye secondary will read the same voltages when measuring all three legs,
phase to ground (A1).
•
A properly grounded wye or delta secondary will read the same voltage when measuring all three
legs phase to phase (A1, A2, A3).
A properly grounded delta with high leg (A2) and delta with grounded leg (A3) show different
characteristics when measuring phase to ground.
•
In example A2 (Delta with high leg), the two low legs (L1 and L2) must be the same voltage when
measured phase to ground.
•
In example A2 (Delta with high leg), the high leg (L3), when measured phase to ground, will read
twice the value of L1 or L2 to ground.
•
In example A3 (Delta with grounded leg), L1 and L2 must be the same voltage when measured
phase to ground.
If the measured voltages at the installation do not correspond with the above, or the
transformer secondary is, in fact, ungrounded, one of the following steps must be done:
A) Ground the secondary of the transformer if it is electrically and mechanically possible.
B) Add an isolation transformer and ground the secondary per acceptable connection.
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If unsure, ask a licensed electrician to perform the above steps.
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