# Induction motor Acceleration/starting time

#### Mahmoud-Diaa

Hello Fellows,

Does anybody experienced the impact of increased fan starting torque on the acceleration time of the motor? and has it an effect on the starting current?Is there an equation which relates the "starting" torque and current? Is there a practical method for measuring the starting time/current to validate the acceleration curve and the torque-speed-current curves?

Thanks and Regards,
Diaa

#### ControlsGuy25

Hello !

would you plz tell us more on :

- Configuration of the motofan group and drive also ...
-OEM model/ type

Regards,
James

#### Mahmoud-Diaa

Here is attached the motor datasheet, I don't have the documents for the whole set at the moment, but I can tell you that it is a D.O.L start motor coupled to a high inertia fan via a gear box

#### ControlsGuy25

Is that motor driven by VFD or not as i see that note on the document saying that must be used with VFD...

Or DOL starter is the configuration on site...

Since Torque & Speed ( ramp up /limitations )can be much easily controled /managed by VFD

Yes there are method like rotor locked for squirrel cage induction motor like this one... OR OTHER METHODS for validating OEM curves at least...

It clearly depending on the load characteristics....

A good search on the web will higlight you some points you are looking for..

#### ControlsGuy25

Starting Current

When a three-phase, squirrel-cage induction motor is connected across the full line voltage, the starting surge of current momentarily reaches as high a value as 400% to 600% or more of the rated full-load current. At the moment the motor starts, the rotor is at a standstill. At this instant, therefore, the stator field cuts the rotor bars at a faster rate than when the rotor is turning. This means that there will be relatively high induced volt ages in the rotor which will cause heavy rotor current. The resulting input current to the stator windings will be high at the instant of starting. Because of this high starting current, starting protection rated as high as 300 percent of the rated full-load current for non-tune delay fuses is provided for squirrel-cage induction motor installations.

Most squirrel-cage induction motors are started at full voltage. If there are any questions concerning the starting of large sizes of motors at full voltage, the electric utility company should be consulted. In the event that the feeders and protective devices of the electric utility are unable to handle the large starting currents, reduced voltage starting circuits must be used with the motor.

#### ControlsGuy25

Speed Regulation and Percent Slip

The squirrel-cage induction motor has very good speed regulation characteristics (the ratio of difference in speed from no load to full load). Speed performance is measured in terms of percent slip. The synchronous speed of the rotating field of the stator is used as a reference point. Recall that the synchronous speed depends on the number of stator poles and the operating frequency. Since these two quantities remain constant, the synchronous speed also remains constant. If the speed of the rotor at full load is deducted from the synchronous speed of the stator field, the difference is the number of revolutions per minute that the rotor slips behind the rotating field of the stator.

Percent Slip = [(synchronous speed — rotor speed) / synchronous speed] x 100

Example 2. If the three-phase, squirrel-cage induction motor used in Example 1 has a synchronous speed of 1,200 r/min and a full-load speed of 1,140 r/min, find the percent of slip.

Synchronous speed (Example 1) = 1,200 RPM

Full-load rotor speed = 1,140 RPM

Percent slip = [(synchronous speed - rotor speed) / synchronous speed] x 100

Percent slip = [(1200 - 1140) / 1200] x 100

Percent slip = 60 / 1200 x 100 = 0.05 x 100

Percent slip = 5%

#### PhilCorso

Responding to Mahmoud-Diaa

Starting-current is the current drawn by a motor during its runup, or acceleration to normal operating speed. Its value is in Amperes. Ignoring the transients of the first 2 or 3 cycles, then starting-current increases (as shown in the motor’s spec sheet) from the time the motor is first energized (breakaway) until the motor nears full speed... often defined as starting-time.

In your case the nameplate gives the start-to-rated current ratio, Ia/In, as 6.3.

Starting-current magnitude is determined only by the motor's design parameters. Neither load torque, nor system inertia, influences the motor’s nameplate rating. However, they are extremely important for calculating starting-time. Three caveats:
• Starting time is very dependent on the Short-Circuit Duty at the source… typically the secondary of a step-down transformer.
• Feeder Cable size between the source and the motor’s terminal.
• Starting-time cannot be determined without knowing total inertia of the both the motor and driven-machine, especially if a gear-box connects them.

Starting-current, starting-torque, and starting-power-factor, are all independent of load! Run-up or starting- time is dependent on the available accelerating torque (that is produced by the motor minus that required by the load) and the moment of inertia of both the motor and load.

I stress the importance of inertia, which is not on the motor spec, and may not be easily obtained from the motor mfg!

Repeating… the starting current, as well as breakaway torque, are functions of only the motor's design parameters. Net torque is the motor’s capability minus driven-machine’s requirement!. Closing a fan's inlet or outlet louvers, or closing a pump's inlet or discharge valves, or the unloading of a compressor, will reduce the torque required by the driven-machine. These operations performed on the driven-machine serve to increase net torque, not change the motor's electrical starting current, nor its starting (breakaway) torque, nor its run-up torque.

Slip, for the motor shown, is given as 100*(1,500-1,485)/1,500 or 1.00 %

If you need additional information or specific equations do not hesitate, just ask !

Regards, Phil Corso

#### PhilCorso

Oliverm...
Attached is an example of a Starting-Time Calc using Excel !
Regards, Phil Corso

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#### Carl

I would guess that the question is related to jolts on the system that you are interpreting as torque/current issues.
Usually, on large fans, you use the s curve acceleration as opposed to the trapezoidal acceleration. The trapezoidal acceleration causes a jolt in the system whereas the s curve is a much smoother transition to your set point.