Why a bigger motor doesn't always fix a difficult start
A motor can have enough power for the job at full speed and still struggle to get the machine there. That is because rated power describes the motor at its normal operating point. Starting is a different problem. Before the machine reaches full speed, the motor has to overcome the torque needed to get the load moving, perform the work required by the machine, and accelerate everything connected to the shaft. That requirement can change significantly from standstill to full speed.
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Part 2 of Get Your Motor Running works through the torque requirements of different machines and motor loads, including breakaway torque, work torque, acceleration torque, inertia, and the calculations used to determine the starting current an application requires.
Getting moving is only the first requirement
At the instant of starting, the motor must produce enough torque to overcome the load's breakaway torque. That is the torque needed to initiate movement from standstill. Depending on the machine, it may be relatively small or it may represent one of the most demanding points in the entire start. But clearing breakaway does not mean the job is done. Once the shaft begins to rotate, the motor still has to provide enough torque to perform the work required by the machine while leaving additional torque available to accelerate the load.
That gives us two important components during acceleration:
- Work torque: the torque required to perform the mechanical work of the driven machine and overcome losses such as friction and windage.
- Acceleration torque: the additional torque available to increase the speed of the motor and load.
If the motor only produces enough torque to match the work torque, acceleration stops. To continue gaining speed, motor torque has to remain above the load requirement.
Where is the torque actually required?
This is where looking only at motor kW can become misleading. Different machines place very different demands on the motor during starting. Some applications require relatively little torque at low speed. Others require substantial torque immediately, or need a smaller torque margin sustained for a much longer time.
A centrifugal fan or pump is a useful example. Its work torque is relatively low at low speed and increases as speed rises. The difficult part of the start may therefore occur later in the acceleration rather than at the instant the shaft begins to move.
A high-inertia machine presents a different problem. A large flywheel, for example, may not require much work torque once it is moving, but accelerating its stored mass can require additional torque over an extended period. The motor can therefore break the load away without difficulty and still fail to complete the start.
Inertia changes the time equation
Torque determines more than whether the motor can accelerate the load. It also influences how quickly it can do it. The greater the inertia of the motor and driven equipment, the more energy has to be transferred before the machine reaches operating speed. More available acceleration torque reduces that acceleration time. Less acceleration torque increases it. That matters electrically as well as mechanically. During starting, the motor is operating above its normal rated condition and can be subjected to increased thermal stress. A high-inertia load may therefore require a longer start, a higher starting current, or both.
Starter duty, motor protection, and the capability of the electrical supply all need to accommodate that starting period.
The load curve matters
Consider two machines connected to motors with similar rated power. One might be a fan with work torque that rises significantly with speed. The other might be a machine dominated by a large flywheel, where most of the starting requirement comes from inertia. Both motors may ultimately operate at similar power levels once up to speed, but the torque required to get them there can be completely different.
Compressors provide another example. Depending on the application and how the machine is unloaded during starting, the motor may need to reach operating speed before significant pressure builds, or it may need enough torque to continue accelerating against increasing pressure.
There is no single starting-torque figure that describes every machine.
Start with what the machine needs
Selecting a motor and starting method therefore begins with understanding the driven load.
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How much torque is needed to break the machine away?
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How does its work torque change as speed increases?
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How much inertia needs to be accelerated?
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How quickly does the machine need to reach operating speed?
The motor must initially exceed the breakaway requirement and then continue to provide enough torque to satisfy the work requirement and maintain the acceleration needed throughout the starting period.
Only once that requirement is understood does it make sense to work backwards towards the current needed from the motor and the starting method capable of supplying it.
That is where Get Your Motor Running goes next. Part 3 looks at the principal ways of starting and controlling an induction motor, and how each method changes the current, torque, and acceleration available to the application.
Go deeper with Part 2 of the white paper series
Part 2 of Get Your Motor Running works through the torque requirements of different machines and motor loads, including breakaway torque, work torque, acceleration torque, inertia, and the calculations used to determine the starting current an application requires.