Back to Basics

By combining the two elements we have discussed so far, the contactor and overload protection, we have a new device: a starter.

Figure 13. A Starter is Made Up of a Controller (Most Often a Contactor) and Overload Protection

A starter lets you turn an electric motor (or motor-controlled electrical equipment) on or off, while providing overload protection. It represents another evolution in control. Now, we have a power control device that offers more than just a manual on/off control, such as a knife blade switch. The manual starter also provides a means to protect the motor from burnout: overload protection.

There are two main types of starters: the manual starter and the magnetic motor starter.

Manual Starter

As the name implies, a manual starter is operated manually . Operating a manual starter is fairly simple and straightforward: you press a button or toggle (mounted directly on the starter) to start or stop the connected electrical equipment. Mechanical linkages from the buttons or toggle force the contacts to open and close, starting and stopping the motor.

Figure 14. Toggle Operated Starter

Figure 15. Pushbutton Operated Starter

Often, a manual starter is the best choice for an application because it offers:

Low Voltage Protection (LVP), which prevents automatic restarting of equipment after a power failure, is usually not possible with a manual starter.

This means, if the power fails, the power contacts remain closed (toggle or button in ON position). When the power is restored, the motor automatically restarts itself. This could create a dangerous situation, depending on the application.

Because of this, manual starters are generally used on smaller loads where low voltage protection is not needed . On applications like pumps and blowers, where the motor should run continuously, and restart automatically, this is actually an advantage.

Inside this saw mill, a high-speed saw quickly reduces logs to construction beams.

The saw uses a starter on the motor to allow it to get up to speed to stop it and to protect it from overload damage. If the saw were to hit a knot or a nail, and the motor was not able to turn at its synchronous speed, it would attempt to draw more current in order to do so. The resulting overload would cause the saw to stop and prevent permanent damage to the motor.

Magnetic Motor Starter

The other main type of starter is the AC magnetic motor starter. These are so commonly used, that when we use the term “motor starter,” we mean “AC magnetic motor starter” unless specifically stated otherwise.

We will begin with a look into what they are and how they work, and finish up with an overview of specific types of motor starters.

Motor starters offer some additional capabilities not available in a manual starter, most importantly, remote and automatic operation. In other words, the magnetic motor starter did for manual starters what the magnetic contactor did for manual controllers: it removed the operator from the immediate area.

Like the magnetic contactor, the motor starter depends on magnets and magnetism for its operation. These additional capabilities are due, for the most part, to the motor starter’s electromagnetic operation and the control circuit.

Starter Circuitry

Before we go any further, we need to discuss starter circuitry.

The motor starter has two circuits: the power circuit and the control circuit. Figure 16 shows a three-phase, full voltage, non-reversing magnetic starter. The thick lines are the power circuit, and the thin lines are the control circuit.

Figure 16. Control and Power Circuits

The power circuit runs from the line to the motor. Electricity passes through the contacts of a starter, the overload relay and out to the motor. The power (main) contacts carry the motor current.

The control circuit operates the contactor (on/off). As shown in Figure 16, the contacts that interrupt or allow the main current to flow to the motor are controlled by opening or closing the contacts in the control circuit. The control circuit energizes the coil creating an electromagnetic field that pulls the power contacts closed, thereby connecting the motor to the line. The control circuit makes remote operation possible.

The control circuit can get its power in one of two ways. If the control circuit gets its power from the same source as the motor, this is termed Common Control. Figure 16 depicts a common control circuit configuration.

The other type is Separate Control. This is the most common form of control. In this arrangement, the control circuit gets its power from a separate source, usually lower in voltage than the motor’s power source.

In addition, there are two ways to wire the control circuit. One common method of wiring the control circuit is known as Two-Wire . It uses a maintained contact type of pilot device -- such as a thermostat, float switch, or presence sensor. This circuit provides for an automatic operation (start - stop) of the load.

Figure 17. Two Wire Control

The other common method of wiring the control circuit is Three-Wire control. It uses momentary contact pilot devices and a holding circuit contact. The holding circuit contact is commonly an auxiliary contact on the starter or contactor. If circuit power is interrupted, the circuit must be restarted by an operator or other intervening logic.

Figure 18. Three Wire Control

Features of Motor Starters

All motor starters share the following power control functions:

Types of Motor Starters

Four particular varieties of motor starters are Across-The-Line, the Reversing Starter, the Multispeed Starter, and the Reduced Voltage Starter.

Across-the-line or full voltage non-reversing (FVNR) is the most commonly used general purpose starter. This starter connects the incoming power directly to the motor. It can be used in any application where the motor runs in only one direction, at only one speed, and starting the motor directly across the line does not create any “dips” in the power supply.

The reversing starter (FVR, for full-voltage reversing) reverses a motor by reversing any two leads to the motor. This is accomplished with two contactors and one overload relay. One contactor is for the forward direction and the other is for reverse. It has both mechanically and electrically interlocked sets of contactors.

The multispeed starter is designed to be operated at constant frequency and voltage. There are two ways to change the speed of an AC motor:

The multispeed starter uses the latter option to change speed.

Reduced voltage starters (RVS) are used in applications that typically involve large horsepower motors. The two main reasons to use a reduced voltage starter are:

Power companies often won’t allow this sudden rise in power demand. The reduced voltage starter addresses this inrush problem by allowing the motor to get up to speed in smaller steps, drawing smaller increments of current. This starter is not a speed controller. It reduces the shock transmitted to the load only upon start-up.

We will look at reduced voltage starters in much more depth in Module 21, Reduced Voltage Starters.

Standards and Ratings

You will undoubtedly run across two acronyms when dealing with contactors and starters: NEMA and IEC . These are two organizations that recommend design and testing standards for electrical devices such as contactors and motor starters . It is important to note that neither organization performs actual testing of equipment.

NEMA and IEC

NEMA is the National Electrical Manufacturers Association. It has its headquarters in Washington D.C., and is associated with equipment used in North America . NEMA devices are built to a high level of perfection, for use in a variety of applications.

NEMA devices, because of their conservative ratings, can be used in almost any application. Being less application-sensitive and more durable , NEMA devices tend to be larger , and therefore more expensive than IEC devices.

IEC is the International Electro-technical Commission . With headquarters in Geneva, Switzerland, it is associated with equipment used internationally . IEC devices are commonly used in OEM machines, where specifications are known and not likely to change.

Because of their greater application sensitivity , IEC devices sometimes require more care in selection than NEMA devices.

As we expand into a global economy, an increasing number of control products are manufactured to IEC standards and conventions. Consequently, there is also more confusion in understanding the differences the IEC and NEMA products. One standard is not necessarily superior to the other, they are just different.

UL and NEC

In addition to IEC and NEMA standards, there is UL (Underwriters Laboratories, Inc.). UL provides product standardization and testing. Their goal is to verify (through testing) that equipment will not pose a hazard to personnel or property when properly installed. They are chiefly concerned with safety issues .

NEC (National Electrical Code) is a standard for applying electrical equipment in the U.S. In the case of motors and starters, one of NEC’s requirements is that a motor must be protected from destroying itself under overload conditions . And that is where overload relays come in. The code is adopted and enforced by local electrical inspectors.

CE and CSA

Finally, there are the CE and the CSA . CE relates the European market. CSA (Canadian Standard Association) is very similar to UL. The CSA mark is required on products for sale in Canada. Products receiving their CE/CSA approval are marked as such, serving as an “entrance visa.” These marks indicate compliance with harmonized European Standards.