# how calculate the energy use for starting a 3 phase motor until return to normal work?

#### dashhossein

Joined Jul 30, 2020
49
hello everyone , for a project i must calculate the exact energy that used for start of motor until the motor return to normal work
in other words how much energy use for normal work of this motor equals the starting energy ?
the motor is 3 phase 20 Ampere 9KW
unfortunately i have no complete details but overestimatly could you tell me will 10 second of normal work equals the starting energy ?

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Joined Jul 18, 2013
25,636
Depends on the motor construction and the nature of the load, a 3ph motor is generally fitted with fuse/overloads that are 1.5x the rating of the motor, but this O/L protection would operate after a time delay of ~10 sec.
The motor initially appears as a very low (winding) resistance to the AC supply until the motor starts to rotate, where it will then decrease, dependant on the first part of the answer.
Max.

• dashhossein

#### dashhossein

Joined Jul 30, 2020
49
Depends on the motor construction and the nature of the load, a 3ph motor is generally fitted with fuse/overloads that are 1.5x the rating of the motor, but this O/L protection would operate after a time delay of ~10 sec.
The motor initially appears as a very low (winding) resistance to the AC supply until the motor starts to rotate, where it will then decrease, dependant on the first part of the answer.
Max.
for a guess if 5 second last till motor reach normal speed could you tell me this 5 second equals how much of starting work ?

thanks for your useful information

#### GetDeviceInfo

Joined Jun 7, 2009
2,106
view this from the perspective of the load. What energy is required to accelerate a given load to a given speed within a given time. Time will be dependant on motor type, but can be generalized. A motor, as opposed to other forms of energy transmission, won’t change the loads requirement.

#### dashhossein

Joined Jul 30, 2020
49
view this from the perspective of the load. What energy is required to accelerate a given load to a given speed within a given time. Time will be dependant on motor type, but can be generalized. A motor, as opposed to other forms of energy transmission, won’t change the loads requirement.
as a guess could you help me to estimate that the start energy of three phase motor equals how much of normal work ?

#### Hymie

Joined Mar 30, 2018
1,088
In my experience an electric motor will reach operating speed within a very short time, typically less than a second.

During this time the motor will be drawing a greater than the normal current, mostly due to the lower inductance (circuit impedance). But this inductance will be increasing (increasing the circuit impedance) with time as the motor increases speed – making any such calculation impossible without knowing the initial impedance and the rate of change in impedance over time (until the motor reaches operating speed).

As a guestimate, the starting energy (over the first second), is likely to be less than three to four times the normal operating energy – otherwise the motor protection circuit (fuse/circuit breaker) is likely to operate/trip.

If the start-up energy was averaged over the first 10 seconds, the difference between this and the normal running energy would be less than 30%.

Joined Jul 18, 2013
25,636
In my experience an electric motor will reach operating speed within a very short time, typically less than a second.
That depends a lot on circumstances, for example when a 3ph motor is permanently connected into a heavy load, methods such as star-delta starting is employed.
Also the size of the motor, i.e rotor inertia.
Max.

#### GetDeviceInfo

Joined Jun 7, 2009
2,106
as a guess could you help me to estimate that the start energy of three phase motor equals how much of normal work ?
As a guess, say your motor drew 4x current initially, decaying to name plate, linearly over the start period. It would take 1.5 of those periods at nameplate to equate.

#### Just Another Sparky

Joined Dec 8, 2019
234
Calculating peak starting power consumption is easy with the right nameplate data. Locked rotor code letter value * horsepower * 1000.

Calculating starting energy is much more involved. I'm no mechanical engineer but you'll probably have to figure the moment of inertia for both the motor and load, figure how long it will take for the load to accelerate to full speed based on the motor's torque curve and then account for power factor and inefficiency.

If you want a quick-and-dirty estimation for most low-inertia loads, take the motor horsepower * 746 * 6 * 10. That will give you a very generous result in joules. (746 watts per horsepower times 600% FLA during startup for 10 seconds... which is the nominal trip rating for a class 10 overload heater if I'm not mistaken.)

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#### crutschow

Joined Mar 14, 2008
30,455
For accurate results you will need to do as JAS recommended and determine the moment of inertia of the motor rotor plus any load, and use that to determine the rotational energy at the running RPM.
You then divide that by a factor equal to the estimated efficiency of the motor during startup to get the total start-up energy.

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