5-208
M60 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
GROUPED ELEMENTS
CHAPTER 5: SETTINGS
5
In this instance, each distinct portion of the thermal limit curve must be known and protection coordinated against the
curve. The relay protecting the motor must be able to distinguish between a locked rotor condition (curve 4) and an
accelerating condition for different levels of the system voltage (curves 2 and 3). Voltage is continually monitored during
motor starting and the acceleration thermal limit portion of the relay overload curve is dynamically adjusted based on
motor voltage variations.
The acceleration thermal limit is a function of motor speed during the start. The dynamically shifted voltage dependent
overload curve inherently accounts for the change in motor speed as a function of motor impedance. The change in
impedance is reflected by motor terminal voltage and line current. This method aids to set dynamically the appropriate
value of the thermal limit time for any given line current at any given terminal voltage.
The
VOLTAGE DEPENDENT FUNCTION
setpoint enables the voltage dependent feature and modifies the locked rotor portion
of the programmed relay overload curve with respect to the acceleration thermal limits. These thermal limits are typically
available from the machine specifications provided by motor manufacturer.
VOLTAGE DEPENDENT MIN MOTOR VOLTS
— This setting defines the minimum allowable line voltage applied to the motor
during the acceleration if the
VOLTAGE DEPENDENT FUNCTION
is enabled. This voltage is expressed as a percentage of the
SYSTEM SETUP
MOTOR
MOTOR NAMEPLATE VOLTAGE
setting. If the measured line voltage drops below this setting
during acceleration, the thermal curve is switched to one based on the programmed minimum voltage thermal limit, as
follows.
Eq. 5-24
VD VOLTAGE LOSS
— This setting is used to address situations when the voltage input into thermal model has been lost. In
this case, the voltage dependent algorithm readjusts the voltage dependent curve to avoid an inadequate thermal
protection response. The VT fuse failure function typically is used to detect a voltage loss condition. If a voltage loss has
been detected while motor accelerates, the thermal curve is switched to one based on the programmed 100% voltage
thermal limit, as follows.
Eq. 5-25
VD STALL CURRENT @ MIN V
— This setting defines the locked rotor current level at minimum motor voltage (I
1
).
VD SAFE STALL TIME @ MIN V
— This setting defines the maximum time that motor is allowed to withstand the locked rotor
current at minimum motor voltage (t
1
).
VD ACCEL. INTERESECT @ MIN V
— This setting defines the starting current level corresponding to the crossing point between
the acceleration thermal limit at minimum voltage and the programmed relay overload curve (I
2
). This value can be
typically determined from motor acceleration curves. The value at the breakdown torque for the minimum voltage start is
recommended for this setting.
Summary of Contents for M60
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