End users must consider many variables to choose the better solution.
by James Nanney

An overlap between 460 and 4,000 volts exists where multiple types of motors and starters are readily available. The question, though, is which one is better? Like most engineering questions, not only does the cost need to be considered, but also the differences in each operating voltage in the context of the application.

Low-voltage variable frequency driveImage 1. Low-voltage variable frequency drive (Images and graphic courtesy of Power Electronics)

Even with the small range of motor ratings, there are more than a few variables. Here are a few assumptions to help simplify the discussion: The ratings of 500 to 2,000 horsepower (hp) and 460 volts or low-voltage (LV) versus 4,000 volts (4kV) or medium-voltage (MV) supply voltage will be compared, though larger LV and smaller MV options are available. The motor comparison will be based on 1,800 revolutions per minute (rpm) and totally enclosed, fan-cooled (TEFC) or open drip proof (ODP) because these types are the most common.

When the cost of a 500-hp LV motor is half of its MV cousin and the two are compared, the question of “which is better” is expected. Looking at the motors, it would seem they are similar in frame (size). At 2,000 hp, the LV and MV motor are close in cost. So why is there a cost difference? Are there other differences?

What is not obvious is the performance difference. MV motors typically include copper bar rotors compared with the cast aluminum rotors of LV motors. The copper bar rotor is higher in efficiency, but the copper bars are a major reason for the higher cost. The performance difference in the higher-efficiency copper rotor is expressed in a slightly faster speed at full load (less slip).

The tradeoff for higher efficiency and a higher rpm level is lower starting torque. High starting torque loads or applications that cannot be started unloaded, combined with voltage drop on the line when starting, can lead to a motor not coming up to speed.

Choosing a Starter

The motor starting amps of 600 percent or more and the ability to supply those amps influence the choice of a starter.

A 460-volt motor takes about nine times the current of a 4kV motor for the same rating. For reference, a 500-hp LV motor would be rated for 580 amps versus 70 amps at 4kV.

Understanding what the utility will allow or what the grid can support should be part of the evaluation for determining the starting method.

There are three basic means of starting: across the line, also known as direct on line (DOL); soft starting or reduced-voltage starting; and variable frequency drives (VFDs) for starting only or continuous operation.

An LV mechanical starter, even for a 500-hp motor, is not practical because of cost. Few power grids can handle the inrush of more than 3,500 amps without significant voltage drop. The common option for LV above even 100 hp is a soft starter. The current can be reduced to 250 to 450 percent of the motor full-load amperes (FLA) depending on the motor loading at starting.

LV VFDs are available even at 2,000 hp or larger. They are made by taking the largest single-stage VFD from a vendor, typically around 500 hp, and putting two together to achieve 1,000 hp or three to reach 1,500 hp. With each added block, the price goes up. As a result, the soft starter, which may only be 30 to 50 percent of the VFD cost, can look attractive.

The reduced cost of VFDs in recent years and their ability to improve process efficiency have made them a more common choice. Even when not required, VFDs are often requested because of the familiarity with the technology; their reduced cost and flexibility make them easy to apply.

Cost comparison of starting methodsTable 1. Cost comparison of starting methods

For MV, the options of DOL, soft starter and VFD are available. At 500 hp, there is a large difference between the MV and LV VFD price. The cost narrows as the horsepower increases. The MV VFD amperage is so small, the cost difference from 500 hp to 1,000 hp may increase 20 percent while the LV doubles. For MV, the least expensive is DOL, followed by soft starting and the VFD for starting only.