1) Torque Characteristic of NEMA Design B Motor

All AC induction motors will start by producing torque in accordance with a specific speed versus torque curve that is a function of the motor design and applied voltage. The torque generated during starting is divided among the fan load, the inertia loads and friction losses. Fan loads are being changed by the cube of the speed increase. The inertia loads consist of the inertia of the motor, coupling shaft, gear reducer and fan. The total inertia is dominated by the fan inertia. As a result, nearly all the torque generated during a start is passed through the coupling shaft and gear reducer. The most common installation includes a single speed, NEMA Design B motor with an across the line starter. NEMA Design B specifies a minimum locked rotor torque of 150% of rated full load torque and a minimum breakdown torque of 200% of rated full load torque. The motor produces its breakdown torque shortly below full load speed. At full speed, the torque transmitted is determined by the fan load, which is normally less than rated torque. Flexible couplings for cooling tower applications are commonly designed to accommodate 200% of full load torque continuously and to withstand 300% load for shorter periods. This additional capacity allows for an over voltage to 110% of rated voltage. Many gear reducers are designed in similar way. Gear teeth damage begins to appear in the range of 300% rated full load torque. At the breakdown speed, the fan is just beginning to develop its full load when the increased torque causes rapid acceleration. This, in turn, leads to heavy loading at the blade root, gear mesh and coupling.

2) Two Speed Motor

The torque characteristic of this type of two speed motor for the application of the axial flow fan is that the torque requirement increases as the fan speed increases. This is a type of variable torque load, which is proportional to motor speed. Therefore, it is quite important to equip the same type of torque load of motor. If the constant torque motor was used for the application of axial flow fan, the excessive torque is unavoidable at the time of starting the motor to the low speed by about 300% or more torque to the full load torque. Generally speaking, most of two speed variable torque motors produce the breakdown torque in the range of 280-300% of torque to the full load torque when started in the high speed mode. These motors will also generate this peak torque when switched from low to high speed. Torque spikes will result when switching from low to high speed at the situation that there is no sufficient time delay for the motor back EMF (Electrical Magnet Field) to dissipate. It is important to recognize the potential for damage to both electric motors and the connected equipment that will be caused by fast switching or fast bus enclosure. The magnitude of torque spikes generated during these occurrences can be from 6 to 20 times the full load torque rating of the motor. Changing speed or direction without proper time delay presents the possibility of generating damaging torque spikes. Due to the large magnitudes involved, the damage caused can be quite extensive. It is very evident that these torque will bend or break drive shafts; mechanically damage the motor winding; shear coupling keys; break or damage gear teeth; fail fan blades at the root. When the power to the motor is interrupted, some rotor current and flux becomes trapped in the motor. This corresponds to a voltage characterized as the back EMF of the motor and can be measured at the motor terminals. This voltage decreases with time at an exponential rate proportional to the motor open circuit time constant. The phase angle between this residual voltage and the power system voltage also changes with time since the induction motor speed is lower than the synchronous speed of the power system. Thus, the magnitude of the phase voltage difference between the motor and the power system an be much greater just prior to a reconnection than the magnitude of the power system voltage when applied from a zero speed start. The transient electrical torque generated, when an induction motor is connected onto a power system, is dependent upon the speed at which the motor shaft is rotating and by any electrical flux which may have been trapped in the motor as a result of a recent disconnection. If the load inertia is large compared to that of the motor then the transient part of the shaft torque can reach a value equal to twice the magnitude of the change in motor electrical torque. The magnitude of the torque is so high they preclude handling through mechanical safety factors. Instead, circuitry should be used that allows time for the back EMF of the motor to dissipate before line voltage is reapplied. The use if power factor correction capacitors lengthens the time to decay the residual voltage in the motor. The time to wait before a safe reconnection or restart is lengthened when capacitors are used. Proper motor control with sufficient time delays before making any rotational or speed change is necessary to prevent stress failure of mechanical components.

3) Shaft Failure

The following is a check list for determining the probable cause of coupling failure with the two speed motors:    - How many times are the motors switched from stop to low speed, from low to high speed, from high to low speed, or from low speed to stop per hour?     (Generally the frequency of changing the motor speed is limited to 6 in order to prevent the excessive mechanical stress. The electric motor will produce a breakdown torque equal to approximately 250% of full load torque each time the motor is started from the stop, and also torque spikes equal to approximately 600 to 2000% of full load torque each time the motor is switched from low to high unless the proper time delay is used.     Excessive exposure to the such high torque of the motor could fatigue the coupling and cause the premature failure.)   - Motor Control Sequence     There are so many ways to control the motor in accordance with the cold water temp. It is quite essential to make a sequence of motor control, which can limit the frequency of motor speed change. The sequence chart attached is typical for the cooling tower application and the inspection of control panel is required if the system is similar to the enclosed sequence chart.   - Speed-Torque Curve of Motor     Most of NEMA design B motors may be producing a much higher torque than a standard rated motor. Potentially the motor may have been derated and actually operates under the conditions of a much larger motor. Check the speed torque curve at both motor speed provided by Motor manufacturer. It is our experience that two speed motors develop a breakdown torque of approximately 300% full load torque. The excessive torque will make a coupling to break earlier.   - Type of Torque     Inspect the type of motor torque. If the constant torque motor was applied to the cooling tower, a greater motor torque than the predicted torque of motor at the low speed is produced and the shaft can be broken earlier than the normal operation life time.