(µMOTOR) Technical Manual - PBSI Group Ltd

mMOTOR TECHNICAL MANUAL
In order for this protection to operate successfully a
motor is assumed to have a “thermal capacity” or
TC. In its simplest terms, a motor start from
standstill using a DOL method will absorb 6x its full
load current (FLC) in order to magnetise and
establish a magnetic field. Once the Motor
overcomes its initial inertia and begins to rotate, the
inrush current decreases until the motor reaches its
nominal rotating speed and draws a continuous
current, FLC. Once the TC reaches 100% a trip
occurs.
From standstill, with a thermal capacity of zero
(referred to as the Cold Condition) a motor has
100% of its TC available to start. If the inrush of 6X
FLC remained steady (stalled rotor) the time taken
to absorb 100% TC and a trip would be equal to the
t6x setting. This is called the basic thermal
characteristic.
The graph, right, shows multiples of FLC against the
operating time. The Thermal model curve remains
DT above 6x to allow for fuse discrimination.
Assuming t6x = 1: If a 2x inrush, from cold (0%) occurred the resultant trip time would be
10s.
If the Pre-Alarm setting is Enabled an Alarm is called when the Thermal Capacity
reaches 95%. When it reaches 100% a Trip is called. The Pre-Alarm can be disabled,
where as the Trip cannot.
When a motor successfully starts and maintains normal a FLC its TC increases to a
steady state condition which is determined by the Hot / Cold ratio (H/C). If an overload
occurred from this steady state condition the trip time is faster as the motor has already
absorbed some thermal capacity up to its steady state (referred to as the Hot Condition).
How much it has absorbed is dependant upon the withstand times of the motor or, its
H/C ratio.
Hot/Cold Ratio.
The H/C ratio is calculated from the motor manufacturers’ data, overload times are
quoted both from a stand still (cold) overload and for a running (hot) overload.
The Hot withstand time divided by the Cold withstand time gives us a ratio. This ratio,
expressed as a percentage, then equates to a steady state running condition.
i.e.
% Hot / Cold Ratio Setting 80 70 60 50 40 30 20
Steady State
% Thermal capacity Running At 100% FLC
17 26 35 44 52 61 69
Available % Thermal Capacity
For Overloads
83 74 65 56 48 39 31
P&B Engineering Issue 6 25/04/2005 Page 43