Locked rotor current

Figure 2.8 Starting (locked rotor) currents corresponding to different starting torques...

With a view to achieve yet more standardization in motor design, NEMA Standard MG-1 has also recommended the maximum locked rotor current of single-speed three-phase motors for the various rotor designs A, B, C, and D, for various recommended torque values. These have been derived for a 415 V a.c. system and are shown in Table 2.2. 

Table 2.2 Recommended maximum locked rotor currents for various rotor designs...

HP Approx, maximum Rotor design locked rotor current ... 

Note For motors beyond 200 h.p., NEMA has not covered these data. It is, however, recommended that larger motors may be designed to have even lower locked rotor currents than the above to reduce the starling transient effects on the distribution system as well as on the motor windings. ... 

When the torque is determined by the above method, the voltage during the test should be so adjusted that the locked rotor current is approximately equal to the full load current. After the locked rotor test, the resistance of the stator windings should be measured and may also be considered for calculating the I R losses. 

Locked rotor current of squirrel cage motors with. short- +20 7c of the guaranteed ciirrem... 

Locked rotor current and corresponding hot thermal withstand time... 

No-load running current Checking of current balance Pull out torque Locking rotor current Starting torque... 

Locked rotor current (especially for fractional hp motors). Specify National Electrical Code rating (NFPA). (Should not exceed H, or 7.09 KVA/hp for high-quality motors. Advise if nameplate is to be code-stamped. Large motors usually are so marked.)... 

Design L. A Design L motor is a single-phase integral-horsepower motor designed to withstand full-voltage starting and to develop a breakdown torque as shown in MG 1-10.33 with a locked-rotor current not to exceed the values shown in MG 1-12.33. 

Locked-Rotor Current. The locked-rotor current of a motor is the steady-state current taken from the line with the rotor locked and with rated voltage (and rated frequency in the case of alternating-current motors) applied to the motor. 

Xj, = Total starting reactance (stator and rotor) per phase at zero speed and locked-rotor current. 

KTl OWATTS RPM RATED LOAD CURRENT LOCKED ROTOR CURRENT. ... ... 

RATED LOAD CURRENT LOCKED ROTOR CURRENT... 

INRUSH CURRENT - The current that flows the instant after the switch controlling current flow to a load is closed. Also called "locked rotor current". 

Motors are classified by size, apphcation, electrical type, NEMA design letter, and environmental protection and cooling methods. They are rated for special standard environmental and operating service conditions by performance and mechanical configuration voltage and frequency, locked-rotor kVA, service factor, horsepower, speed, torque, locked-rotor current, performance, temperature rise, duty cycle, and frame size. 

NEMA also rates motors by level of performance, specifically horsepower, speed, torque, and locked-rotor current for each category of motor type and size. Tables specifying acceptable limits for each are too detailed to include here, but may be found in NEMA standard MGl-1993. 

Locked-rotor current Steady-state current taken from the line with the rotor locked and... 

Thermal overloads are hmited from approaching the ideal motor protection curves for these critical conditions. Because they do not directly measure current loading, but instead approximate it through use of heaters within the overload, they are subject to thermal overshoot (continued heat absorption after locked-rotor current is removed), residual heat remaining after reset, mechanical variables, and other difficulties in placing the various curves in optimum relationship with each other. 

A motor is a 3-phase 460 V NEMA Design B induction motor. During testing, the maximum locked rotor current of the motor measures as 290 Amperes, and the full-load current measures as 52 Amperes. The rated horsepower of this motor is expected to be most nearly ... 

The starting torque ciurent is substantially less for an adjustable-frequency controller/motor combination than the locked rotor current for an induction motor started across the line. This results in a soft start for the controller/motor... 

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