Delta MM300 Instruction Manual download pdf (Page 113)

6–26 MM300 MOTOR MANAGEMENT SYSTEM – INSTRUCTION MANUAL

PROTECTION ELEMENTS CHAPTER 6: SETPOINTS

6.3 Protection elements

6.3.1 Thermal protection

The primary protective function of the MM300 is the thermal model. The MM300 integrates

stator and rotor heating into a single model. The rate of motor heating is gauged by

measuring the terminal currents. The present value of the accumulated motor heating is

maintained in the Thermal Capacity Used actual value register. When the motor is in

overload, the motor temperature and thermal capacity used will rise. A trip occurs when

the thermal capacity used reaches 100%. When the motor is stopped and is cooling to

ambient, the thermal capacity used decays to zero. If the motor is running normally, the

motor temperature will eventually stabilize at some steady state temperature, and the

thermal capacity used increases or decreases to some corresponding intermediate value,

which accounts for the reduced amount of thermal capacity left to accommodate

transient overloads.

The thermal model consists of six key elements.

• Unbalance current biasing that accounts for negative-sequence heating.

• Hot/cold biasing that accounts for normal temperature rise.

• RTD biasing that accounts for ambient variation and cooling problems

• An overload curve that accounts for the rapid heating that occurs during stall,

acceleration, and overload.

• Cooling rate that accounts for heat dissipation.

• Thermal protection reset that controls recovery from thermal trips and lockouts.

Each of these categories are described in the following sub-sections.

6.3.1.1 Unbalance biasing

Unbalanced phase currents (that is, negative-sequence currents) cause rotor heating in

addition to the normal heating caused by positive-sequence currents. When the motor is

running, the rotor rotates in the direction of the positive-sequence MMF wave at near

synchronous speed. The induced rotor currents are at a frequency determined by the

difference between synchronous speed and rotor speed, typically 2 to 4 Hz. At these low

frequencies the current flows equally in all parts of the rotor bars, right down to the inside

portion of the bars at the bottom of the slots. On the other hand, negative-sequence stator

current causes an MMF wave with a rotation opposite to rotor rotation, which induces

rotor current with a frequency approximately two times the line frequency (100 Hz for a

50 Hz system or 120 Hz for a 60 Hz system.) The skin effect at this frequency restricts the

rotor current to the outside portion of the bars at the top of the slots, causing a significant

increase in rotor resistance and therefore significant additional rotor heating. This extra

heating is not accounted for in the thermal limit curves supplied by the motor

manufacturer, as these curves assume only positive-sequence currents from a perfectly

balanced supply and balanced motor construction.

To account for this additional heating, the MM300 allows for the thermal model to be

biased with negative-sequence current. This biasing is accomplished by using an

equivalent motor heating current rather than the simple motor terminal current (I

avg

). This

equivalent current is calculated as shown in the following equation.

Eq. 1

In the above equation:

´+´=

kII

avgeq

1 2 ... 108 109 110 111 112 113 114 115 116 117 118 ... 166 167

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Delta MM300 Instruction Manual Page 113

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