How to select the right motor starter

Engineers should consider various factors and different types of units when selecting a motor starter in a commercial building project.

03/14/2011


Motors abound in different sizes and many different uses in new construction applications. Specifying engineers have several types of motor starters to choose from, ranging from sophisticated to bare-bones units. How does the engineer decide which to choose? There are many factors involved, and different types of units are available depending on the requirements of the application. Let’s start by breaking motor starters into a set of global product categories.

Starter types

VFDs: At the high end of the motor starter capability spectrum are variable frequency drives (VFDs). VFDs are typically used for motor speed control, but they are also used on small motors where they serve as motor starters only. In these cases, the VFD will operate at full speed while providing several benefits, including reduced current starting, communications to a central building management system, and easy interface for automatic control. These benefits, of course, come at a cost of increased complexity, increased installation costs, and sensitivity to the environment in which the VFD is installed. Additional equipment is often required to support VFDs, such as filters and surge protection, which further increases cost.

Figure 1: Specifiers should consider motor starters that provide external control and monitoring capability, avoiding the need to open the enclosure for regular verification and maintenance. Courtesy: Cerus Industrial

Soft starters: Similar to VFDs in that they are capable of varying the speed of a motor, the role of soft starters (sometimes referred to as reduced voltage solid state starters) is to smoothly ramp up the speed of a motor to avoid causing large current surges and to minimize the amount of wear on electrical contacts in the system. Given this functionality, soft starters are typically used with on/off devices that are cycled often, such as compressors or conveyer belts. While this technology is capable of continuously running a motor at partial speed as with VFDs, soft starters are generally inefficient in partial speed control. Like VFDs, soft starters incorporate electronics and are sensitive to the environment and quality of power supplied. Thus, using soft starters involves complexity and costs that are similar to those for VFDs while providing less capability.

Across-the-line starters: The most basic type of motor starter, across-the-line starters simply connect and disconnect power to the motor. While not limited in the size of motors they can control, these on/off starters are typically used in commercial buildings with motors sized at 15 hp and below that are intended to be run continuously. Typically, they provide no feedback to building-level control systems by themselves beyond simple contact closure confirmation, so contractors must augment them with other devices such as current or power monitors to integrate with the latest building control systems for proof of flow (air or liquid) and for monitoring energy consumption.

Figure 2: The typical across-the-line starter is very basic and can require a lot of add-ons, which can result in a crowded enclosure that's prone to safety hazards and mainenance problems over time. Courtesy: Cerus Industrial

Smart starters

Recently, a new type of motor starter was developed with an understanding of the strengths and weaknesses of the traditional starter types listed above, and the goal of combining features to deliver the best cost/performance for a broad range of applications.

This new type is the smart starter. Smart starters are basically across-the-line starters that incorporate many of the features of the electronic starters but without a lot of the complexity. They are designed for use with motors that run at a single speed, but they provide built-in communications and safety features that enable them to function as part of automated building environmental control systems without requiring additional hardware. They are designed to include many of the features that are typically installed as add-ins to other motor starter devices, such as built-in power metering, advanced motor protection, and helpful human interfaces, which are great improvements over traditional across-the-line starters. Overall, these new smart starters reduce installation and maintenance costs while maintaining the same robust and reliable simplicity of across-the-line starters.

Courtesy: Cerus Industrial

Improving protection

Regardless of the starter type, there are a set of capabilities that must be provided in any motor starter. One of these is protection for the controlled motor.

A traditional starter will have overload protection that is mechanical. If a motor is started too often and too quickly, it will overheat and potentially damage the motor windings permanently, so some type of thermal overload protection is required. Traditionally, this is supported by installing a thermal overload relay with an internal heater on the motor circuit. The overload relay heats a bimetallic strip that opens a contact when its temperature reaches a desired limit. It’s generally not practical or cost-effective to actually monitor the temperature of the motor itself. It’s easier to design the contact heater to model the thermal characteristics of the motor. The higher the current flowing into the motor, the faster the overload switch will trip. Being a mechanical device that operates on heat, the thermal overload retains heat from repetitive starts or continuous operation at full load, which leads to faster trip times or an inability to start if the restart occurs too quickly from the last overload trip.

Rather than traditional bi-metal thermal switches or thermal modeling that tracks only the current to the motor, specifiers should look for starters that actively monitor current and look at voltage information to track power consumption. By measuring power, the starter can tell if the motor is running correctly and can protect against low voltage and overload conditions, such as if one of the power phases is lost. A single-phase condition can damage a multi-phase motor, is difficult to detect with current or thermal monitoring alone. Other low-power conditions, such as would be caused by powering a dry pump or a broken fan belt, can be detected, enabling protection of the motor and connecting equipment, reducing maintenance costs, and increasing the operating life of the whole system. Systems also need to detect and protect against repetitive on/off commands produced by bad control signals and tripping the system to avoid overheating or damaging the starter or motor.

Typical current sensors provided as add-ons to motor starters require adjustment or provide only on/off readings to verify current flow. An easier and safer to maintain solution is to select a motor starter with a current sensor that has adjustable thresholds that can be set using the operator interface, without the need for opening the starter enclosure.

With some intelligence built into the smart starter, it is also able to do alarm logging—keeping a record of alarms to help service technicians debug problems more quickly and easily—and it is possible to provide different programmable options for restarting the motor after a power failure.

Enabling remote monitoring and control

Building-level automation and energy management systems need complete control over HVAC systems. They also need to gather information regarding how the systems are functioning. Over the years, different means have been developed to support this effort.

Remote controllers typically activate interposing relays to turn on motor starters. The reason for this is that controllers are designed to handle only low current and low voltage through their output contacts, while motors operate at high voltage and current. Often, it is up to the controls contractor to supply the relay components.

Once an installation is complete, contractors use different means to verify that the device connected to the motor is operating, proving that the motor starter has done its job. For example, the system can tell if a motor is running by monitoring fan output with a pressure switch, or using a current sensor to monitor the current going to the motor.

Typically, current sensors are mounted inside the motor starter enclosure and can also be used by maintenance personnel or building control systems to detect loss of load (e.g., from fan belts breaking). These are commonly referred to as proof of flow or status sensors, and often require calibration while the motor is running.

Other controls may need to be installed inside some motor starter boxes, for example damper controls. If dampers are installed in the HVAC system’s ducting, the appropriate damper(s) must open before the motor starts in order for the system to run. This requires additional control components—sometimes a transformer, maybe a relay or two. In the worst case, there’s a relay, a current sensor, and damper controls, all interfaced inside the starter box.

Alarms may need to be provided to alert maintenance personnel that the system is not functioning correctly. Often more relays are added, which can be difficult as the number of available contacts is limited. With all of the add-ins, the environment inside the starter enclosure can become very tight and messy (see Figure 1) and costs can quickly increase. The costs for these components, including installation, can range from $150 to $250 per starter, depending on the local market labor rates.

If power monitoring is built into the motor starter, and the motor starter has a building network interface, then it is possible to save the cost of adding a power meter at each individual motor. (Installing a single power meter at the starter can cost $750 to $1500 for each motor.)

For interfacing to building control systems, specifiers should look for starters that connect directly to an industry standard network such as BACnet. BACnet (building automation and control network) is the term commonly used to refer to the ANSI/ASHRAE Standard 135-1995, adopted and supported by the American National Standards Institute (ANSI) and the ASHRAE. A nonproprietary communication standard conceived by a consortium of building management, system users, and manufacturers, BACnet is becoming the accepted alternative to the proprietary communications solutions that to date have been used in most HVAC controls installations.

Avoiding the arc flash hazard

The fact that the typical motor starter has been a nucleus for adding on devices and mixing high-and low-voltage connections can also make it a point of concentrated danger for installation and maintenance personnel. The risks of working with high-voltage electricity are obvious to everyone who installs electrical gear, yet accidents resulting in injury and death still occur. The risk of injury is higher since motor starter systems mix low-voltage controls (below 120 Vac) with line voltage inputs and outputs in the same enclosures. To reduce the risk of catastrophic events resulting from unprotected workers coming into contact with high-power contacts in electrical gear, the National Fire Protection Assn. has established a standard, NFPA 70E, that provides guidelines for maintaining electrical safety in the workplace. This standard is consistent with the National Electrical Code (NEC) and supports the Occupational Health and Safety Administration (OSHA) requirements for the use of protective equipment when working where a potential electrical hazard exists (29 CFR 1910.335(a)(1)(i)).

NFPA 70E requires that employers conduct a flash hazard analysis and provide clothing to workers that is designed to protect against the level of risk associated with each task. For installers of typical motor starter devices, which need to be tested and verified for proper operation by opening the starter case to take current readings, protection can require an elaborate full-body, fire-retardant suit and insulating gloves.

A better option is for electrical specifiers to select a motor starter that makes its control and monitoring functions accessible via a control panel that doesn’t require opening the starter enclosure and exposing the electrical components and high-voltage connections, thus completely avoiding the arc flash hazard.

As a bonus, selecting a motor starter that incorporates the monitoring functions standard reduces installation work required and decreases the chance of installation errors, hence increasing reliability. Also, when maintenance is needed, more information on the status of the system is provided to streamline the maintenance process while making it safer.

Saving energy

Beyond streamlining maintenance functions, motor starters with built-in support for monitoring can promote energy savings as well. For example, power monitoring is required for U.S. Green Building Council LEED certification, and verification of power consumed contributes more points to a building’s LEED score. No longer do HVAC motors need to run continuously for long periods of time. Occupancy sensors and timers can be simply integrated with the starter controls, delivering service on an on-demand basis and reducing overall energy usage. And easy interfacing with BAS can support building-wide energy reduction strategies.

So the next time that you look for a motor starter, consider whether you want technology that dates back to the 1950s, or whether you want to equip your building to optimize cost and energy savings and improved maintenance and safety for the future.


Perra is Cerus Industrial’s co-founder and president. Prior to Cerus, he was president of Veris Industries, a Schneider Electric acquisition, and marketing vice president at Square D.