The starting function of the motors is often misunderstood, impacting Motor performance and compromising energy efficiency.

L&S Electric and AuCom are delivering a series of technical white papers to introduce the theory of starting motors, based on the work of electronics design expert Mark Empson, one of AuCom’s founders in 1978.

Reduced Voltage Starting of Three-Phase Induction Motors

Reduced voltage starting of three-phase AC induction motors can be used to reduce the starting current drawn by the motor.

Reduced starting current is often required by electrical supply authorities to reduce current surges and the resulting voltage fluctuations in the supply system.

Full voltage starting (direct-on-line or DOL) creates a starting current surge equal to the locked rotor current (LRC) of the motor. The LRC typically ranges from 500% to 800% of the motor full load current (FLC). The LRC depends on the design of the motor; a value of 600% FLC is common.

The full voltage starting torque is equal to the locked rotor torque (LRT) of the motor.

Reduced voltage starting reduces the available starting torque of the motor. It cannot be used for some applications because of the load’s starting torque requirements.

Induction Motors

The induction motor performs two primary functions in industry:

1. To convert electrical energy into mechanical energy, to accelerate the motor and load to the operating speed. This is the starting function.
2. To convert electrical energy into productive work output from the machine. This is the operating or work function.

The starting characteristics and the full load characteristics are both very important in the selection and specification of motors. The starting function of the motor is poorly understood; many motors are misapplied and so exhibit very poor starting characteristics. The operating function’s full load characteristics are easy to specify with motor speed, torque, and efficiency is the major selection criteria.

Motor Designs

Motors consist of two major sections: the stator and the rotor.

The stator consists of magnetic poles and stator windings within the frame of the motor. The full load characteristics are determined by varying the winding configuration and the contour of the stator laminations. The motor speed is determined by the number of poles.

The rotor consists of a cylindrical, short-circuited winding around iron laminations. The rotor winding is often referred to as a squirrel cage.

The cage is constructed of some bars running parallel to the motor shaft, near the surface of the rotor. These rotor bars are short-circuited by shorting rings at each end of the rotor. The shape, material, and position of the bars within the rotor determine the starting characteristics of the motor.

In operation, the motor performs as a transformer with current induced in the rotor by the flux from the stator. When the rotor is stationary (locked rotor conditions), the effective series impedance of the rotor and stator limit the motor current.

At very low speeds, the dominant impedance is the rotor. At high speeds, the stator impedance becomes influential. Thus, the rotor determines the starting characteristics of the motor and the stator influences the full speed characteristics.

The torque developed by the motor is a function of the rotor current, the effective rotor resistance, and the rotor slip (the difference between rotor speed and synchronous rotor speed). During starting, the current is limited by both the rotor resistance and the rotor leakage reactance. Motors, which exhibit a high LRC, tend to have a low LRT, while motors with a low LRC have a high LRT.

A high starting torque is generated by using a high resistance rotor, but this results in an increased slip at full load. One compromise is to use a rotor consisting of two cages: a high resistance outer cage, giving a high starting torque; and a high resistance inner cage giving a low slip operation.

This double cage motor sometimes is more limited in starting capacity than single cage motors and so is not always suitable for multiple start applications. Typical full voltage starting torques (LRT) are in the range of 120% to 220% of full load torque (FLT). It is often possible to increase the LRT by over 50% by utilizing a different rotor design.

The designs of AC induction motors fall into four main categories, each exhibiting different starting and operating characteristics. Select the motor design by the machine manufacturer to suit the mechanical load of the machine.

• Design A motors have a shallow rotor bar design resulting in low rotor inductance and usually low rotor resistance. Design A motors exhibit a high LRC and a low LRT. They have a good operating efficiency and a high pull-out torque. The full load slip of these motors is low.

Typical LRC = 650% to 1000% FLC

Typical LRT = 100% to 140% FLT

• Design B motors have a higher rotor inductance and rotor resistance than Design A motors. Design B motors have a lower LRC and higher LRT than Design A motors. The efficiency is like Design A, but pull-out torque can be lower and slip higher.

Typical LRC = 550% to 650% FLC

Typical LRT = 140% to 240% FLT

• Design C motors are often known as double cage motors because of two windings on the rotor. One winding is low resistance, as found on Design B motors, and the outer winding has a high resistance. The low resistance inner winding is designed to have a high reactance. Double cage motors have a low LRC and high LRT, typically greater than 200%.
• Design D motors have a high reactance squirrel cage winding. They exhibit a high LRT (up to 300%) and a low LRC. The high resistance rotor results in a high full load slip and low efficiency.

Increasing the motor size or rating does not always increase the starting torque. When it is difficult to start a machine, the motor is often incorrectly replaced with a higher rated motor. In fact, an equivalent rated motor of a different design is often more effective and costs less.

Example One

A 75-kW motor with 180% LRT has a higher starting torque than a 100-kW motor with 120% LRT. The starting current for the 75-kW motor is less than the 100-kW motor.

Incorrect motor selection may use an oversized motor to achieve the required starting torque, with associated increased motor and starter cost, and a higher starting current.

With a reduced voltage starting, the torque is reduced by the square of the current or voltage reduction. High current motors tend to have low starting torque, so any reduction in the start voltage results in a greater difference in starting torque between the high and low starting torque motors.

In many applications, the starting current is required to be less than 300% FLC. A reduction from 600% FLC to 300% FLC is a 2:1 reduction, resulting in a torque reduction of 4:1. A reduction from 900% FLC to 300% FLC is a 3:1 reduction in current, resulting in a 9:1 reduction in starting torque.

Example Two

Motor A has LRT 180% and LRC 600%, so at 300% FLC the motor produces 45% starting torque. Motor B has LRT 120% and LRC 900%, so at 300% FLC produces 13% starting torque. This is a torque differential of over three times for two motors that appear to be very similar and would be sold in direct competition.

The higher torque motor may be a little more expensive, but this is insignificant compared to the available torque. The increase in torque means some machines can be successfully started at 300% FLC with Motor A, but not with Motor B. To develop 45% FLT; Motor B requires 520% FLC. This results in a much more expensive starting equipment and in many cases, the start current would be unacceptable.

In many situations, it is best to use a high starting torque motor that has a low LRC. This may result in a higher motor cost, but the cost of the motor and starter combination will often be reduced.

Example Three

The first table shows that for motors of similar rating, the realizable torque covers a wide range of reduced voltage (or reduced current) starting conditions.

For the nine motors surveyed, the initial start torque at 300% FLC ranges from 66% to 24%, a span of greater than 2:1 at the same current. The motors have very different starting efficiencies, despite the very similar full load characteristics.

Example Four

The second table shows that with an initial starting torque of 50% FLT, a standard duty starter can be used with Motor 1, but with Motor 9, a more expensive heavy-duty starter is needed. The lower starting efficiency motors also suffer a high level of heating during start. So the number of starts per hour must be lower.

The start current for an initial start torque is calculated as follows:

The speed/torque curves are unique to each motor design type. When engineering a motor and starter for an application, plot the speed/torque curve for the motor and starter against the machine speed/torque curve. Some motor manufacturers show the curve as a single line, while others illustrate the curve as a shaded band. The speed/torque characteristics are not smooth but have many peaks and troughs. The manufacturers’ curves are averages only.

To ensure the motor starts satisfactorily, there should be a good differential between the motor torque and the machine torque requirement at all speeds. If the start torque is marginal, the motor noticeably changes in acceleration as it increases in speed. The flat spots are usually very audible. Particularly severe torque flat spots occur for motors that have a rotor that is ‘off round,’ causing an uneven air gap between the rotor and stator.

Get Your Motor Running white paper series

This is the first in a series of technical white papers AuCom has published as an introduction to the theory of starting motors. L&S Electric has received permission to feature them on WATTS New.

The future white papers will focus on:

• The differing start torque requirements of machines and motor loads
• Methods of motor starting
• An overview of solid-state soft starters
• Variable frequency control.

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