Are there any specific standards, that specify the allowable starting motor voltage drops, the behaviour of load torque w.r.t motor torque, behaviour of motor torque and acceleration torque and, variation of speed. I have ran studies for the first time on SKM and couldn’t find much on it.
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Motor Analysis
- Thread starter Kanu_01
- Start date
- Moderator
- #2
That won't be in motor specs as it depends on source impedance.
You will find that information in system power quality standards.
In special cases, users may tolerate a greater voltage drop than specified in the standards.
Sizing software for standby generators allows the default setting for motor starting voltage drop to be altered from the default values.
You will find that information in your motor speed/torque curve and/or motor current curve.
You must calculate the acceleration time from the combined inertia of both the motor and the load and the speed torque curve.
As Bill (waross) said.
The load torque does not "behave" with respect to motor torque. The load torque is the defining criteria - the motor has to meet it, not the other way around. Proper motor design for a given application requires knowledge of the load torque profile. This is because the motor is designed to provide enough torque at all operating points to meet the profile of the driven (load) equipment, including any transient conditions such as momentary overloads. It is ALSO designed to have some margin capability to allow it to accelerate the load to operating speed.
Accelerating torque is the difference between motor torque and load requirements at a given speed point - a larger difference often equates to faster acceleration (in terms of elapsed time).
The load torque does not "behave" with respect to motor torque. The load torque is the defining criteria - the motor has to meet it, not the other way around. Proper motor design for a given application requires knowledge of the load torque profile. This is because the motor is designed to provide enough torque at all operating points to meet the profile of the driven (load) equipment, including any transient conditions such as momentary overloads. It is ALSO designed to have some margin capability to allow it to accelerate the load to operating speed.
Accelerating torque is the difference between motor torque and load requirements at a given speed point - a larger difference often equates to faster acceleration (in terms of elapsed time).
- Moderator
- #4
A clarification on some of Gr8Blu's information.
The torque of a motor depends mostly on the rotor current.
The Rotor current depends on the stator current which develops the magnetic flux.
But I will try to explain the other factors.
With given stator conditions, the rotor current depends on the impedance of the rotor circuit.
The rotor circuit impedance depends on the resistance of the rotor bars, the inductive reactance of the rotor circuit and on the effective or slip frequency.
Inductive reactance is frequency dependent. At locked rotor, the effective or slip frequency is line frequency. at full load the slip frequency is only a few cycles.
Then there is the position of the rotor bars in the iron core.
One design of motor uses hourglass shaped bars, The upper part of the hourglass s more effective at lower speeds, higher frequencies, the lower part of the hourglass is more effective at lower frequencies.
Even this is a very incomplete explanation of the electrical and magnetic interactions in a motor at arious speeds and loading.
But;
IT JUST DOESN'T MATTER.
These are design factors that you will probably never encounter aside from knowing the general characteristics of the common designs.
WHAT DOES MATTER?
The vast majority of integral HP induction motors are designed to one of several standard designs. Hence the design code on a motor.
From the standard curves associated with each standard design.
Check the motor design curve to evaluate whether a given motor is suitable for a given load.
Are temporary overloads anticipated and how does the motor perform when overloaded.
There is a technique called RMS loading which helps to determine whether a motor can handle a given pattern of temporary overloads.
High inertia machines with periodic peak loading.
Here are metal shears, alligator shears, punch press machines.
Large machines of these types typically use the inertia of a large flywheel to complete a work cycle.
A special design D motor is used to accelerate the motor backup to speed following each work cycle.
When a load slows a motor down much below the rated speed, these motors are designed to draw much less current than would other designs at the same low speed.
Another option for these types of loads is a wound rotor motor with permanent resistance in the rotor circuit.
These motors are often referred to as "High Slip" motors.
They are not particularly efficient, but they may spend much of the time idling at high speed and very low load.
The advantages outweigh the poor efficiency for special applications.
Knowing how motors in general and specific designs in particular react to various loads is what you really need to know and you find that in the published motor codes.
And be sure to check out the Cowern Papers for a lot more valuable and easy to read motor information.
https://www.baldor.com/Shared/manuals/pr2525.pdf
The torque of a motor depends mostly on the rotor current.
The Rotor current depends on the stator current which develops the magnetic flux.
But I will try to explain the other factors.
With given stator conditions, the rotor current depends on the impedance of the rotor circuit.
The rotor circuit impedance depends on the resistance of the rotor bars, the inductive reactance of the rotor circuit and on the effective or slip frequency.
Inductive reactance is frequency dependent. At locked rotor, the effective or slip frequency is line frequency. at full load the slip frequency is only a few cycles.
Then there is the position of the rotor bars in the iron core.
One design of motor uses hourglass shaped bars, The upper part of the hourglass s more effective at lower speeds, higher frequencies, the lower part of the hourglass is more effective at lower frequencies.
Even this is a very incomplete explanation of the electrical and magnetic interactions in a motor at arious speeds and loading.
But;
IT JUST DOESN'T MATTER.
These are design factors that you will probably never encounter aside from knowing the general characteristics of the common designs.
WHAT DOES MATTER?
The vast majority of integral HP induction motors are designed to one of several standard designs. Hence the design code on a motor.
From the standard curves associated with each standard design.
Check the motor design curve to evaluate whether a given motor is suitable for a given load.
Are temporary overloads anticipated and how does the motor perform when overloaded.
There is a technique called RMS loading which helps to determine whether a motor can handle a given pattern of temporary overloads.
High inertia machines with periodic peak loading.
Here are metal shears, alligator shears, punch press machines.
Large machines of these types typically use the inertia of a large flywheel to complete a work cycle.
A special design D motor is used to accelerate the motor backup to speed following each work cycle.
When a load slows a motor down much below the rated speed, these motors are designed to draw much less current than would other designs at the same low speed.
Another option for these types of loads is a wound rotor motor with permanent resistance in the rotor circuit.
These motors are often referred to as "High Slip" motors.
They are not particularly efficient, but they may spend much of the time idling at high speed and very low load.
The advantages outweigh the poor efficiency for special applications.
Knowing how motors in general and specific designs in particular react to various loads is what you really need to know and you find that in the published motor codes.
And be sure to check out the Cowern Papers for a lot more valuable and easy to read motor information.
https://www.baldor.com/Shared/manuals/pr2525.pdf
LionelHutz
Electrical
Allowable voltage drop depends on the motor making enough torque to start the load and the controls staying energized. Sometimes, you can tolerate a fairly large voltage drop if you maintain the control voltage.
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