Starting Time

The voltage drop during starting should be calculated using the speed current data, locked rotor power factor, and the distribution system constants. Speed-torque values at this reduced voltage can be calculated, assuming that the torque varies as the voltage squared. These values are compared to driven equipment curves.

In comparing speed-torque characteristics, driven equipment torque requirement at any speed must not exceed 95% of motor torque at that speed. Also, the driven equipment speed-torque curve for the loaded con dition is used. If less than 5% accelerating torque is available, it is considered doubtful that the motor can successfully accelerate within the allowable time. Theoretically, 1% difference should permit acceleration; however, 5% provides the margin for data and calculation inaccuracies. Because the time for the motor to accelerate through a given speed interval is inversely proportional to the accelerating torque, close approaches of the load-to-motor starting torque may result in excessively long starting times. Even if the motor does not stall at some sub-operating speed, as would be expected if the accelerating torque margin went to zero, the time to accelerate may result in the motor exceeding its allowable starting thermal limit and, at best, experiencing a protection relay trip prior to reaching the operating speed. The loaded curve is suggested for the basic evaluation because in many process applications, unloading may not be feasible. Also, if available, it presents extra margin. It should be pointed out that higher starting torque motors are more expensive. There is a possibility that they will require higher starting currents. To add insult to injury, induction motors with the higher starting torque design will probably be somewhat less efficient. Synchronous motors designed for higher starting torque may be difficult to find.

Starting times for large motors driving high-inertia loads, such as centrifugal compressors, can be 20 seconds or longer. The motor draws locked-rotor current for most of this period. These high currents maintained for such long periods cause winding and rotor temperature to rise rapidly.

If a motor has sufficient torque available at all points along the speed-torque curve, the starting time should be calculated. An approximation of motor starting time can be obtained by summation of starting time increments calculated for several speed intervals. Five or six speed intervals should be used. Use small intervals when accelerating torque changes are large, large intervals when torque changes are small.



At = incremental starting time, sec AN = speed interval, rpm

AT = average accelerating torque over the speed interval (difference between motor and load torque) g = gravitational constant

WR2 = torsional moment of inertia

Before leaving the starting time subject, it should be mentioned that once a reasonable starting time has been established, there is no merit in penalizing the motor by doubling the time. As mentioned earlier, gross oversizing as opposed to a conservative approach leads to an inefficient overall operation.

An early appraisal of motor and driven equipment speed-torque characteristics, particularly at the reduced voltage occurring during starting, is necessary. The following data should be obtained:


1. Speed-torque curve

2. Speed-current curve

3. Moment of inertia (WR2)

4. Locked rotor power factor

5. Time constant for open circuit voltage (when motor control will use delayed transfer to alternate sources on voltage loss). This value must include the effect of any capacitors applied on the load side of the motor controller.

Driven Equipment

1. Unloaded speed-torque curve (zero flow through compressor)

2. Loaded speed-torque curve (compressor design point)

3. Speed-torque curve, intermediate discharge pressure (refrigeration service for restart evaluation)

4. Moment of inertia including gear, at the motor shaft

5. Gear ratio

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