Stator Protection, Repetitive Starts | Siemens MCC-3298 manual

3 Operating the SAMMS-LV Device

3.19 Stator Protection

By selecting the overload class slightly greater than or equal to the motor’s starting time, the motor model defines an energy I2t value. In this value, I equals 600% of the motor’s full load current and t equals the overload class selected.

If the amount of energy exerted by the motor during start, stall or under locked rotor condition exceeds the amount of energy defined by the overload class selected, the SAMMS-LV device will trip. This protects the motor’s winding against excessive heat build up. For example, a typical motor starts within 3 to 4 seconds. By selecting an overload class 4 or 5, you can safely start the motor hot or cold and without nuisance tripping. This also provides protection against stall time exceeding 4 or 5 seconds; assuming the locked rotor current equals 600% of the motor’s full-load current (See Figure 3.10).

3.20 Repetitive Starts

The motor model allows repetitive starts without nuisance tripping while protecting the motor against overload and stall conditions. Repetitive starts can occur as long as the following conditions are met:

1.The energy exerted by the motor during start does not exceed the energy defined by the overload class selected.

2.The motor winding temperature does not exceed the maxi- mum temperature allowed.

3.The SAMMS-LV is not in start inhibit (See Section 3.21).

For example, a motor starting from cold condition could have 4 or more typical starts of 5 seconds without nuisance tripping and without damaging the motor insulation. Figure 3.11 shows a multi-start scenario for an open drip-proof motor.

Figure 3.11 Multi-start scenario for an open drip-proof motor

3.21 Start Inhibit

When the motor temperature reaches the maximum values allowed, the SAMMS-LV device trips. If you attempt a restart, the overload LED flashes and the motor will not start until the motor winding and rotor temperatures cool down to the full load temperature. HHC function F25 displays the motor winding temperature as a percentage of the full-load temperature. HHC function F10 displays the time to enable restart in seconds.

3.22 Cooling Time Constants

The motor time constants vary based on whether the motor is running idle or is stopped. The values listed in Table 3.4 are used in the motor model.

Motor	Delta		Motor FLC
Cond.	Temp.
Cond.	Temp.	0.3-9.0A	9.1-65A	65.1-540A
		0.3-9.0A	9.1-65A	65.1-540A

Stopped	80º C to	1750 s	5500 s	9100 s
	29º C

Running	80º C to	600 s	1800 s	2800 s
Idle	39.4º C

Table 3.4 Motor time constants

3.23Normalized Temperature Rise for Class B and Class F Insulation

Steady state and maximum winding temperature are functions of the motor’s insulation class. In this motor model, normalized temperature rise for both Class B and Class F insulation are used. The motor winding temperature is displayed as a per- centage of the steady-state full-load temperature rise as shown in Table 3.5.

Boundary	Class B	Class F	% Full Load
Parameters	Insulation	Insulation	Temperature
			Rise

Steady State	80º C	105º C	100
Temperature
rise at
full-load
current

Maximum	140º C	185º C	175
allowed
winding
temperature

Table 3.5 Steady state and maximum winding temperature rise

3.24 Ground Fault Detection

In SAMMS-LV devices, HHC function F12 selects ground fault protection or warning.

∙protection = ON

∙warning = OFF

The device detects a ground fault if the fault current exceeds the pickup level for 360 msec. If you have selected protection (ON),