Self-Protecting Starters

Nov. 15, 2002
Self-protecting starters (SPSs) were first introduced to the U.

Self-protecting starters (SPSs) were first introduced to the U.S. in 1987. However, SPSs have been used internationally for several years. Recently, several manufacturers and private labelers have introduced new products worldwide. These starters are expected to be widely used in the U.S. during the next two years.

Several features make the SPS preferred over traditional motor controls. A self-protected starter combines contactor, overload, and short-circuit protection in one package. It is sized according to motor load current and horsepower. Generally, a small interchangeable module protects against both thermal and magnetic overload. The self-protected starter can be used in single or multiple installations and satisfy Article 430 of the National Electric Code (NEC) which addresses the safe installation of motors, circuits, and controllers.

Power-control equipment comprises motor control, overload and short-circuit protection, and isolation. Before SPSs, no one device could perform all these functions. They were handled by a motor starter (contactor plus overload relay) wired to either fuses or circuit breakers.

Depending on the use, location, and control sophistication required, three options were available for group or multiple motor installations. The first choice was a NEMA-rated starter. It was selected by a particular size classification that was suitable for switching motors and other kinds of loads such as capacitor banks. The second was an IEC (International Electrotechnical Commission -- European Standard) starter rated by horsepower. The third choice was a horsepower-rated definite-purpose starter.

To add to the complexity, there are over six different classifications available for fuses including H, J, K, RK, RK-1, and 5. Furthermore, short-circuit protection must be coordinated with the overload relay and the contactor to protect personnel and equipment. To ensure coordinated protection, an engineer must determine the available fault current, the corresponding fuse class, and the need for a single or dual-element device.

Choices for circuit breakers include thermal-magnetic, magnetic-only, solid-state, current-limiting, and standard or high interrupt capacity units. Thermal-magnetic devices were originally designed to protect wiring between circuit breakers and motors according to code. But they frequently had to be oversized to handle high inrush current upon start-up. The magnetic-only circuit breaker was designed to protect motors, not wire. Thus, this device provides protection more in line with the overload relay and contactor ratings.

Solid-state circuit breakers are more commonly used on motors above 100 hp for economy. Special fuses used for motor protection have built-in time delays called dual-element time-delay fuses. It is a common practice to oversize short-circuit protection for fuses or breakers. This cuts down on frequent nuisance blowing or tripping. Even the NEC allows oversizing within prescribed limits.

Self-protected starters are sized for the horsepower and full-load current of the motor. A typical inrush current to start a motor is six to eight times normal running current. Some new high-efficiency motors have inrush currents of eight to 10 times their running current. SPS devices eliminate nuisance tripping with adjustable protection of two types. First, adjustable overload protection for full-load current is provided. In any case, the overload setting should not be greater than the actual full-load current. Second, a magnetic-only trip coil, which is adjustable from six to 12 times full load, compensates for a variety of inrush currents.

When sizing motors, service factor rating is a necessary consideration. Most U.S.-made motors have a service life of 1.15, meaning they will handle 115% of normal running current indefinitely without damage. A service factor of 1.0 indicates the motor will tolerate nameplate running current only. This type of motor is being used more frequently, and needs overload protection that trips faster than the traditional Class 20 overload relay.

Class 10 overloads (trips within 10 sec at six times full-load motor current) for small motors work well with a 1.0 or 1.15 service factor. Some special applications with over 5 sec start-up times can cause early tripping.

However, the Class 10 overload used on self-protecting starters is the preferred method of motor protection.

At full locked-rotor current, the Class 10 overload relay trips in 10 sec or less, protecting motors without creating nuisance tripping. Relays do not trip because most motors reach full operating speed in less than 5 sec. This is particularly important with the trend toward smaller motors with short start-up times. Another advantage is that overload adjustments can be sealed or locked to prevent tampering.

Another factor to consider is phase imbalance during brownouts or phase loss. In the event one phase drops out in IEC devices like SPSs, there is a 57% increase in current across the other phases. Time is a critical factor in protecting motors from thermal damage, so these devices contain a differential bar that increases the speed at which the overload trips. This reduces the likelihood of motor damage or burnout.

For complicated installations using PLCs or similar controls, it is best to make sure the starter can handle accessories such as shunt trips and auxiliary contacts. If field upgrades and expansions are expected later, it is also wise to make sure that spare parts will be available and that installed units can accept accessories. For example, it is easier to add auxiliary contacts to a fusible switch in the field than to add a circuit breaker which must be factory installed.

Also important is the available space devoted to motor starters, both in the panel and on the plant floor. If either is severely restricted, the choice should favor the device that saves space. The SPS will generally cut panel space by about 50% because a single device replaces separately mounted controls. This, in turn, reduces the size of the enclosure.

For example, one large control system was originally designed to house 50 motor starters. Nine motor control-center vertical sections would have been needed. But only three small panels were needed with self-protecting devices. These panels sat on an overhead walkway instead of on the shop floor, making service easier.

By comparison, another application where space was critical used 30 SPSs instead of traditional NEMA-style devices and circuit breakers. SPSs reduced the required panel size by 64% over the NEMA approach.

Communication or dialog with automated systems is an important feature of any control device. In most systems, this responsibility has typically been left to a programmable logic controller (PLC). Until now, many PLCs had to send full 120-V control power to starters located remotely. Status indicators only appear on the contactors and breakers themselves.

In contrast, screw-in modules enable self-protected starters to both receive controls signals from PLCs and transmit status on trip conditions or signal normal operation. The SPS can also receive direct control signals from proximity, photoelectric, limit, or pushbutton switches.

In fully automated systems, an additional communication function enables the device to be reset from a remote panel or central control location. A shunt-trip enables tripping by external commands.

SPS starters can cut installation time by 33% compared to the time needed to mount NEMA-style starters. Self-protected starters are DIN rail mounted which eliminates installation of additional protection components such as fuses and circuit breakers. And using SPSs in multiple motor-starter applications can reduce installation time as much as 80%.

The operational design of SPSs provides a clear visual indiction of contact operation and trip status. A visual indicator on the face of the device confirms that the unit is operating. In overloads, the handle rotates to a trip position. In a short circuit, the device provides an additional optical indication of the trip.

Auxiliary contacts can provide PLCs with feedback from three independent signals: contactor status , short-circuit status, and overload status. PLCs can then trigger alarms in the event of trips.

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