STARTING THREE-PHASE AC INDUCTION MOTORS [AC/DC Motors, Controls, and Maintenance]

Home | Articles | Forum | Glossary | Books




GOALS:

• state the purpose of an across-the-line magnetic starting switch.

• describe the basic construction and operation of an across-the-line starter.

• state the ratings for the maximum sizes of fuses required to provide starting protection for motors in the various code marking groups.

• describe what is meant by running overload protection.

• draw a diagram of the connections for an across the-line magnetic starter with reversing capability.

AC motors do not require the elaborate starting equipment that must be used with DC motors. Most three-phase AC induction motors with ratings up to 10 horsepower are connected directly across the full line voltage. In many cases, motors with ratings greater than 10 horsepower also can be connected directly across the full line voltage. Across-the line starting usually is accomplished using a magnetic starting switch controlled from a pushbutton station.

The electrician regularly is called upon to install and maintain magnetic motor starters. As a result, the electrician must be very familiar with the connections, operation, and troubleshooting of these starters. The National Electrical Code provides information on starting and running overload protection for squirrel-cage induction motors. A comprehensive study of motor controls is provided in Electric Motors and Motor Controls.

ACROSS-THE-LINE MAGNETIC STARTER

In the simplest starting arrangement, the three-phase AC motor is connected across full line voltage for operation in one direction of rotation. This is referred to as across-the-line (ATL) starting. The magnetic switch used for starting has three heavy contacts, one auxiliary contact, three motor overload relays, and an operating coil. The magnetic switch is called a motor starter if it has overload protection. Older motor starters already in service may have used two overload relays. Three overload relays are required by the National Electrical Code in new installations.

The wiring diagram for a typical across-the-line magnetic starter is shown in FIG. 1.

The three heavy power contacts are in the three line leads feeding the motor. The auxiliary contact acts as a sealing circuit around the normally open start pushbutton when the motor is operating. As a result, the relay remains energized after the start button is released. The four contacts of the ATL magnetic starter are operated by the magnetic starter coil controlled from a pushbutton station, as shown in the schematic diagram in FIG. 2.

FIG. 3 shows a typical pushbutton station. Two pushbuttons are housed in a pressed steel box. The start pushbutton is normally open and the stop pushbutton is normally closed, as shown in the diagram (FIG. 4).


FIG. 1 A wiring diagram for an ATL magnetic starter.


FIG. 2 Elementary diagram of control circuit for the starter.


FIG. 3 Internal wiring and cover of start/stop station.


FIG. 4 A pushbutton station and wiring diagram.

STARTING PROTECTION (BRANCH-CIRCUIT, SHORT-CIRCUIT, AND GROUND-FAULT PROTECTION)

In FIG. 1, a motor-rated disconnect switch is installed ahead of the magnetic starter. The safety switch is a three-pole, single-throw enclosed switch. It has a quick-break spring action and is operated externally. The motor circuit switch contains three cartridge fuses that serve as the short-circuit protection for the motor. These fuses must have sufficient capacity to handle the starting surge of current to the motor. The fuses protect the installation from possible damage resulting from defective wiring or faults in the motor windings. This combi nation of switch and fuse protection and motor starter is available in a single enclosure (FIG. 5). (See NEC Article 430, Part IV.) Briefly, the National Electrical Code provides the following information on starting protection for squirrel-cage induction motors.

The maximum size of fuses per mitted to protect motors are rated at 300% of the full-load current of the motor for nontime-delay fuses, and 175% for time-delay fuses.

Note: If the required fuse size as determined by applying the given percentages does not correspond with the standard sizes of fuses available, and if the specified overcurrent protection is not sufficient to handle the starting current of the motor, then the next higher standard fuse size may be used. In no case can the fuse size exceed 400% of the full-load current of the motor for nontime-delay fuses and 225% of the full-load current for time-delay fuses. (See NEC Article 430, Part IV, Article 430.52(C)(1) Exceptions.) Rotors are constructed with different characteristics. FIG. 6 shows the various types of rotor construction and associated code letters. The applications of motors with these code letters are also indicated. The design of the rotor affects the amount of current needed to pro duce the rotor magnetic field. Code letter A has high starting torque and relatively low starting current. The NEC Table 430.7(B) indicates that a code letter A motor will have less locked rotor kVA than other motors. This calculation indicates that there is less starting current for the same voltage for a code A compared to a code K motor. The chart in FIG. 6 provides some broad categories of motors (A, B to E, and F to V).

An AC magnetic starter is shown in FIG. 7. The starter consists of power contacts that are used to open and close the circuit to the motor. As AC is applied to the magnetic coil, the magnet draws the contacts closed and connects the line power to the motor power.

In addition to connecting the line power, the magnetic starter has an add-on block at the bottom to provide for running overload protection. See Guide 14 for detailed operation of the magnetic starter.


FIG. 5 Combination starter with fusible disconnect switch.


FIG. 6 Various types of rotor laminations.

THIS TYPE OF MOTOR HAS A HIGH- RESISTANCE ROTOR WITH SMALL ROTOR BARS NEAR THE ROTOR SURFACE. THIS MOTOR HAS A HIGH STARTING TORQUE AND LOW STARTING CURRENT.

INDUCTION MOTOR WITH CODE LETTER A THIS TYPE OF MOTOR HAS A RELATIVELY LOW-RESISTANCE AND LOW-INDUCTIVE REACTANCE ROTOR. THIS MOTOR HAS A HIGH STARTING CURRENT AND ONLY A FAIR STARTING TORQUE. IT HAS LARGE CONDUCTORS NEAR THE ROTOR SURFACE.

INDUCTION MOTOR WITH CODE LETTERS F TO V THIS TYPE OF MOTOR HAS A HIGH REACTANCE AND LOW-RESISTANCE ROTOR. THIS MOTOR HAS A RELATIVELY LOW STARTING CURRENT AND ONLY FAIR STARTING TORQUE. IT HAS LARGER CONDUCTORS DEEP IN THE ROTOR IRON.

APPLICATIONS:

METAL SHEARS, PUNCH PRESSES, AND METAL DRAWING MACHINERY.

APPLICATIONS:

FANS, BLOWERS, CENTRIFUGAL PUMPS, OR ANY APPLICATION WHERE A HIGH STARTING TORQUE IS NOT REQUIRED AND HIGH STARTING CURRENT IS APPLICATIONS:

MOTOR-GENERATOR SETS, CENTRIFUGAL PUMPS, OR ANY APPLICATION WHERE A HIGH STARTING TORQUE IS NOT REQUIRED AND LOW STARTING CURRENT IS REQUIRED.

INDUCTION MOTOR WITH CODE LETTERS B TO E

------------------

Example 1: A three-phase AC induction motor with a nameplate marking of code letter F is rated at 5 hp, 230 volts. According to the National Electrical Code, this motor has a full-load current per terminal of 15.2 amperes according to Table 430.250. The starting protection shall not exceed 300% of the rated current for squirrel-cage motors with nontime-delay fuses. Thus, the starting protection is 15.2 × 3 = 45.6 amperes.

Because a 45.6-ampere fuse cannot be obtained (see NEC 240.6), the next larger size of fuse (50 amperes) should be used. For motor branch-circuit protection, the motor current listed in the appropriate table of the National Electrical Code should be used. The full-load current, as stated on the motor nameplate, is not used for this purpose.

RUNNING OVERLOAD PROTECTION ( NEC: MOTOR AND BRANCH-CIRCUIT OVERLOAD PROTECTION)


FIG. 7 (A) A magnetic contactor and overload section make up a magnetic starter. (B) AC reversing-magnetic motor starter. The elementary diagram of the starter is shown in FIG. 10.


FIG. 8 Left is solder pot heater, middle is just heater for melting alloy OL, right is heater for bi-metal OL.

Many motor starters installed in the United States use a thermal type of overload assembly. The assembly is normally located beneath the contactor and is directly attached to the magnetic contactor. The overload monitoring system is designed to measure the amount of current flowing to the motor through the contactor. This is done by connecting thermal sensors called heaters in series with the motor current. The heaters are sized to produce a certain amount of heat with a specified current through them. They are calibrated to cause a thermally operated switch to open when there is sustained heat. The extra heat is caused by too much current flow to the motor, which indicates the motor is jammed or is working too hard and is overloaded. The thermal sensors are various types as seen in FIG. 8. The heater sensors with the associated trip-overload relays are pictured. The National Electrical Code requires the use of three overload units as running-overload protection Table 430.37.

Although new installations require three overload relays, electricians work on many older installations that have only two overload relays. These were installed before the three overload relay requirement became effective. The overload relay unit may be either three individual units or a common block containing the three heaters and only one trip switch contact unit reacting from any one of the heaters.

These overload sensing units are made of a special alloy. Motor current through these units causes heat to be generated. In one type, a small bimetallic strip is located next to each of the three heater units. When an overload on a motor continues for a period of approximately 1 to 2 minutes, the excessive heat developed by the heater units causes the bimetallic strips to expand. As each bimetallic strip expands, it causes the normally closed contacts in the control circuit to open. The main relay coil is de-energized and disconnects the motor by opening the main and auxiliary contacts. Melting alloy overloads (solder pots) also are commonly used. The heat generated by the overload melts the solder pot to release a ratchet that trips the control circuit contacts.

Before the motor can be restarted at the pushbutton station, the overload contacts in the control circuit must be allowed to cool before being reclosed (reset). When the reset button in the magnetic starter is pressed, the overload contacts in the control circuit are reset to their normally closed position. The motor then can be controlled from the pushbutton station.

The National Electrical Code requires that the running overload protection in each phase be rated at not more than 125% of the nameplate full-load current rating for motors that are marked with a temperature rise of 40°C (104°F) or less (see NEC Article 430, Part III).

Example 2: Using the motor full-load current rating from the nameplate data, deter mine the running overcurrent protection for a three-phase, 5 hp, 230-volt AC induction motor with a rated full-load current of 14.5 amperes and a temperature rise of 40°C (104°F). The running overcurrent protection is 14.5 × 1.25 = 18.1 amperes.

For this motor, heater overload units rated to trip at 18.1 amperes are chosen for the magnetic starter. Where the overload relay so selected is not sufficient to start this motor, the next higher size overload relay is permitted, but not to exceed 140% of the motor full-load current rating [NEC Article 430.32(C)]. Actual motor nameplate currents are used to establish the overload protection.

HOW TO SIZE OVERLOAD THERMAL PROTECTION

The process of determining the correct overload protection to protect a motor from excessive heating due to mechanical work overloads or from failure to start is a matter of following all the rules of the NEC. Also, each manufacturer has methods to determine proper protection.

Article 430, Part III, determines the parameters of protection. Article 430.32 determines the trip point and the percentage of full-load current when the overload must open the power circuit to the motor. Article 430.33 determines the overload protection for intermittent or similar non continuous duty motors. Once you have determined the actual trip percentage point according to the Code, you multiply the actual motor nameplate current times the percentage to determine the actual current trip value.

Most manufacturers use the system of labeling the overload heaters based on the name plate information on the motor. Each manufacturer uses its own numbering system that corresponds to the motor starter used. If the motor is a standard rating with a service factor of 1.15 or greater and the marked temperature rise for the motor installation is 40°C (104°F) or less, then choose the heater catalog number that corresponds to the nameplate rating. This heater will provide a trip point of approximately 125% of the nameplate rating as required by NEC Article 430.32(A)(1). However, if the service factor is less than 1.15, typically 1.0, or the motor has a marker temperature rise over 40°C (104°F), typically 50°C (122°F), then the Code requires closer tolerance, and you choose one size smaller from the manufacturer's table. The one size smaller will yield 115% protection as required.


FIG. 9 Sample motor thermal overload selection table.

In the case of 125% or 115% protection, the Code does allow the electrical personnel to increase the ratings if the original selection does not allow the motor to operate. Article 430.32(C) allows us to increase each category by one size provided the 125% category does not exceed 140%, and the 115% category does not exceed 130%. All the above situations are based on the fact that the controller and the motor are in the same ambient temperature.

When sizing the overload heater from a manufacturer's listing, you must determine the heater catalog numbers. There are standard methods used to determine the heater number.

The electrical technician needs to determine whether the motor controller that contains the overload heater is in the same ambient temperature as the motor it controls. If the controller is in a higher ambient temperature, that means the overload sensors are already warmer than the motor, and not as much heat can be added before they trip and disconnect the motor.

Therefore, choose a heater with a higher number than originally selected. Conversely, if the control is at a lower ambient temperature than the motor, select a heater that is one size less than the original choice. The lower number of the heater produces more heat per ampere of current flow and compensates for the lower ambient temperature.

Electronic overload modules are sized according to the nameplate of the motor and then adjusted from 115% to 140% depending on the circumstances of the motor nameplate and the conditions relating to the location of the motor and controller. In the following description, the overload monitor can also monitor for single phasing of three-phase motors where one phase fails and the motor continues to run on the other two phases, now a single-phase supply. The three phase motor is not designed to operate safely with only one phase and may be damaged if allowed to run too long. The electronic overload monitor causes the controller to disconnect the motor.

Example 3: A 5 hp, 230 V, three-phase motor has a nameplate current of 14.5 A and a service factor of 1.25. The marked temperature rise is 40°C (104°F). The controller and the motor are at the same ambient temperature. Use the heater chart in FIG. 9.

Solution: If the controller and motor are at the same ambient temperature, the motor has a service factor of 1.15 or more, and a temperature rise of 40°C (104°F) or less, then simply choose 125% protection and use the nameplate rating of the motor to pick the heater catalog number H37 for NEMA size 00, 0,1 starters. You need three heaters for a conventional overload-heater-style starter for a three-phase motor.

Example 4: A 5 hp, 230 V motor has a nameplate current of 14.5 A and a service factor of 1.0. The marked temperature rise is 40°C (104°F). The controller is in a cooler location than the motor by approximately 10°C (50°F). Use the heater chart in FIG. 9.

Solution: First choose the heater overload as if the controller and the motor were at the same temperature. In this case, because the service factor is 1.0, the NEC requires us to protect the motor at 115% of the nameplate current. Use the chart to determine the heater selection at full nameplate current, and then choose one size smaller to satisfy the 115% requirement.

Now adjust that size based on the ambient temperature of the controller.

Because the controller is already cooler, choose one size smaller than nor mal. In this case, the heater is two sizes smaller than would be indicated by simply using the nameplate current. The heater is H36. If this does not allow the motor to run normally without overheating, the size can be adjusted up one size according to Article 430.32(C).

Many new motor starters are equipped with electronic overload relays. These overloads do not depend on the heat generated by the motor current passing through the heater element, but instead use a current sensor, such as a current transformer, to measure the current and provide a trip contact for the controller. This provides an advantage in that the overload can be adjusted to provide optimum protection without nuisance tripping over a wider range than that available in a thermal overload. The electronic overload can also be used to send operating data to a central location for remote monitoring. Other features include adjustable class of overload from 10 to 30, current imbalance protection, phase loss protection, phase reversal protection, and test function and trip indicators (see FIG. 10).


FIG. 10 (A) Electronic overloads for motor controllers. (B) Voltage monitor to save motor from damage caused by voltage variations.

The class of overload refers to the speed and protection offered by the motor current monitor. Class 10-rated overloads are designed for hermetic refrigeration motors and for quick starting and fast trip on overload. (Trip time is less than 10 seconds, with six trips per hour.) Class 20 overloads are the most common and are designed for standard duty motors with normal time versus temperature curves. (Trip time is less than 20 seconds, with six trips per hour.) Class 30 overloads are used for long-time acceleration motors and extended high-current overloading.

(Trip time is less than 30 seconds, with six trips per hour.) The trip currents are 125% of the minimum full-load current listed in the heater tables when the heater is at 40°C (104°F) ambient.


FIG. 11 Electrical interlocks (auxiliary contacts) switch pilot lights in this circuit.

AUXILIARY CONTACTS

In addition to the standard contacts, a starter may be provided with externally attached auxiliary contacts, sometimes called electrical interlocks (FIG. 11). These auxiliary contacts can be used in addition to the holding circuit contacts and the main or power contacts that carry the motor current. Auxiliary contacts are rated to carry only control circuit currents of 0-15 amperes, not motor currents. Versions are available with either normally open (NO) or normally closed (NC) contacts, or combinations of NO and NC. Among a wide variety of applications, auxiliary contacts are used to:

• control other magnetic devices where sequence operation is desired.

• electrically prevent another controller from becoming energized at the same time (such as reverse starting) called interlocking.

• make and break circuits of indicating or alarm devices, such as pilot lights, bells, or other signals.

Auxiliary contacts are packaged in kit form and can be added easily in the field.

ACROSS-THE-LINE MOTOR STARTER WITH REVERSING CAPABILITY


FIG. 12 Elementary diagram of an ATL magnetic starter with reversing capabilities.


FIG. 13 A panel, or wiring, diagram of an ATL magnetic starter with reversing capability.

The direction of rotation of a squirrel-cage induction motor must be reversible for some industrial applications. To reverse the direction of rotation of three-phase motors, interchange any two of the three line lead connections to the motor terminals.

FIG. 12 is an elementary wiring diagram of a motor starter with a reversing capability. When the three-power reverse contacts are closed, the phase sequence at the motor terminals is different from that when the three-power forward contacts are closed. Two of the line leads feeding to the motor are interchanged when the three reverse-power contacts close.

The control circuit has a pushbutton station with forward, reverse, and stop push buttons.

The control circuit requires a mechanical and an electrical interlocking system provided by the pushbuttons. Electrical interlocking means that if one of the devices in the control circuit is energized, the circuit to a second device is open and cannot be closed until the first device is disconnected. Mechanical interlocks, shown by the broken lines in FIG. 12, are used between the forward and reverse coils and pushbuttons.

Note in FIG. 12 that when the forward pushbutton is pressed, it breaks contact with terminals 4 and 5, opening the reverse coil circuit, and makes contact between terminals 4 and 7.

As a result, coil F is energized and the forward contacts close. The motor now rotates in the forward direction. If the reverse pushbutton is pressed, it breaks contact between terminals 7 and 8 and opens the circuit to coil F. This causes all forward contacts to open. As the reverse pushbutton is depressed further, it closes the contact between terminals 5 and 6 and energizes coil R. All reverse contacts are now closed, and the motor rotates in the reverse direction. If the stop button is pressed, the contact between terminals 3 and 4 is opened, the control circuit is interrupted, and the motor is disconnected from the three-phase source. The National Electrical Code requirements for starting and running overload protection that apply to the ATL motor starter also apply to this type of motor starter.

Figures 12 and 13 are actually the same motor controller. FIG. 12 is drawn in an elementary diagram. It has the control circuit in a schematic style, which shows the electrical relationship of the components. It shows the power contact of the magnetic starter below the schematic, and the electrical relationship of the motor control components. FIG. 13 shows the same components, but in the approximate physical location of the components. This style of drawing is called a wiring diagram. Many electricians find it is easier to wire a panel from the wiring diagram as it shows physical location as well as general wire routing. Many electricians find it easier to troubleshoot from a schematic, or elementary, diagram, as it shows the electrical sequence of operation more clearly. It is important that you know how to read both types of drawings and be able to transfer from one to the other.


FIG. 14 (A) Reversing-drum switch. (B) A bakelite section of a drum switch. (C) Bakelite section with cover removed.

DRUM-REVERSING SWITCH

A drum-reversing switch, shown in FIG. 14(A), can be used to reverse the direction of rotation of squirrel-cage induction motors.

The motor is started in the forward direction by moving the handle of the drum-reversing switch from the off position to the forward (F) position. The connections for this drum controller in both the forward and reverse positions are shown in FIG. 15. In the forward position, the switch connects line 1 to motor terminal 1, line 2 to motor terminal 2, and line 3 to motor terminal 3.

To reverse the direction of rotation, the drum switch handle is moved to the reverse (R) position. In the reverse position, line 2 is still connected to motor terminal 2. However, line 1 is now connected to motor terminal 3, and line 3 is connected to motor terminal T1. When the handle of the drum switch is moved to the off position, all three line leads are disconnected from the motor.


FIG. 15 Connections for a drum-reversing switch: (A) forward; (B) reverse.

SUMMARY

Many squirrel-cage motors are started with across-the-line motor starters. The motor and branch circuit should include short-circuit protection such as fuses or circuit breakers.

The motor must also have running overload protection. This protection is usually found with the starter and is in the form of thermal-overload heaters or current-sensing overloads, and the associated overload relay. The overload relay is designed to open the control circuit to the motor starter in the event of a sustained overload on the motor. Motors can be automatically controlled by using a magnetic starter or may be manually controlled by using a drum type of controller. In either case, a three-phase motor can be reversed by interchanging two of the three line connections to the motor.

QUIZ

1. What is the purpose of starting protection for a three-phase motor?

2. What is the purpose of running overload protection for a three-phase motor?

3. What is meant by the code letter markings of squirrel-cage induction motors?

4. List some of the industrial applications for squirrel-cage induction motors with the code letter classification A.

5. List some of the industrial applications for squirrel-cage induction motors with the code letter classifications B to E.

6. List some of the industrial applications for squirrel-cage induction motors with the code letter classifications F to V.

7. A three-phase motor (code letter J) has a full-load current rating of 40 amperes, and a temperature rise of 40°C (104°F).

a. What is the maximum size of fuses that can be used for branch-circuit protection?

b. What size of heaters would be used for running overload protection?

8. What is the maximum starting protection allowed by the National Electrical Code?

Top of Page

PREV.   NEXT   Guide Index HOME