Electrical Equipment and Devices

WEEK 8: MOTOR STOPPING / PROTECTIVE DEVICES

Sections: Brakes | Fuses | Overload Relays | Discharge Resistors

Selection Guide.
To determine the specifications of the brake to use, employ the following formula:

T = 5252 x HP / rpm

where:
T = Full load motor torque, pound-feet
HP = Horsepower rating of motor
Rpm = Speed of motor shaft

Definition. Devices which is composed of a linkage of moving parts and a friction member which allows motor to make several revolutions. Electric braking is also defined as a retarding force exerted on a conductor moving ina magnetic field and carrying induced current.

Methods.

a. Dynamic Braking. - the kinetic energy of the motor and load is absorbed in the resistance of the motor winding or in a resistor bank. See example below:

Operation: When motor is running A1 and A2 are open, A3 is closed. When motor stops, A1 and A2 are open and A3 is closed. To determine the ohmic value of the braking resistor, use the formula R = (E - Ia * Ra) * I.

b. Plug Braking. - also referred as reversal braking, is the most popular and simplest method. Interchanging any two phase connnections reverses the direction of the rotating field and cause the motor to brake with a mean torque slightly higher than the locked-rotor torque. A speed monitor is required to prevent the motor from running up in reverse direction immediately after stopping.

Operation.
Starting: Pushbutton S1Q actuates contactor K1M. The self-holding contact of K1M closes and the motor is started. The contact of speed monitor F3V changes positions.
Stopping: Pressing the OFF pushbutton S0Q causes contactor K1M to be deenergized by the NC contact and contactor K2V to be energized by the NO contact. The ON command for contactor K2V does not become effective until the NC contact of K1M is closed. The self-holding contact of K2V closes. The motor is stopped by pug braking. At a low speed preset on the speed monitor, the contact F3V returns to its original position, causing contactor K2V to be deenergized and plug braking is terminated.

c. Regenerative Braking. - energy is returned to the supply line using solid-state converters, thus the roles of the motor and the driven machine is revered temporarily in the supersynchronous stage. If the braking energy is returned to the system, driven machine cannot be braked to less than synchronous speed. To permit braking to continue at lower speeds, the motor can be switched onto a capacitor bank, which maintains regenerative operation for a certain time down to values below synchronous speed by supplying reactive current.

d. Short-circuit Braking. - the stator terminals are disconnected from the supply and are subsequently short-circuited. It is important that the arc of the line switch be extinguished before the motor is short-circuited because otherwise a line short-circuit would occur. Short-circuit braking is very simple but does not permit prediction of the time required for brakinf to stand still. This method cannot be used where high moments of inertia are involved because the magnetic flux in the motor decays rapidly, as does the braking effect.

e. DC Injection Braking. - the stator is disconnected from the line and is excited with a direct current.The resulting braking torque curve closely resembles the mirror image of the motor torque curve; its height depends on the direct current injected.

Operation.
Starting. The ON pushbutton S1 actuates motor contactor K1, which is sealed in a self-holding contact K1. The NO contact of K1 actuates auxiliary contact K4, which is sealed in by self-holding contact K4. The NC contact of K1 locks out the DC voltage (contactors K2 and K3). The NC contact of K4 locks out the motor contactor during subsequent braking.
Stopping. The OFF pushbutton S2 opens contactor K1 and thus disconnects the motor from the system. The NC contact of K1 actuates line contactor K2, which in turn actuate contactor K3. The DC voltage is applied to the motor only after the inherent delays of K1 and K2. The NO contacts of K2 and K3 start the timer K5, which cuts off the DC and cancels the motor interlock after stopping

Types.
a. DC Brakes. - usually referred as the DC Spring Set Shoe Brakes -- a two-shoe, spring set brake operating on a wheel mounted on the shaft of a motor or other machinery and released by means of solenoid or a direct operating magnet. Used as holding, stopping, or retarding brake on cranes, hoists, conveyors, movable bridges, rolling mill drives and others.

Operation. When the magnet is energized, shoe will clear the wheel. When deenergized, shoes are pressed against the wheel by compression springs.

Connections.
Series Brakes. - designed to pick up (release brake) at 40-percent motor full-load current or less and dropout (set brake) at 10-percent motor full-load current or less. The low dropout point keeps the brake from resetting on light motor loads.

Shunt Brakes. - designed to pick up at 80 percent or less of rated voltage and will operate satisfactorily at 110-percent rated voltage. These brakes are furnished with partial-voltage operating coils to give good speed of response. The partial-voltage coil in circuit with a series resistor reduces the ratio of inductance to total impednace and gives a faster pickup than would occur with a full-voltage coil alone. The simplest connection for a shunt brake is one in which the brake is energized by the same contactor that starts the motor. It is important that both sides of the motor line be broken to prevent delay in brake setting due to counter emf of the motor.

b. AC Brakes. - spring-set, electrically released brakes operated by solenoids in smaller sizes and thrustor mechanisms in larger sizes. They provide reliable low-cost operation on varied industrial applications. They are used on cranes, bridges, turn-tables, conveyors, machine tools, elevators, printing presses, hoists, locks and dams, overhead doors, and deck machinery.

Types.
1. Solenoid-Operated. - weather-resistant brakes for outdoor locations subject to occasional rain, sleet or snow. Solenoid inrush V/A ratings ranges from 1570 to 31200, with Holding V/A ratings from 100 to 1760, respectively.

2. Torque-Motor Type. - rotary motion of armature is transformed into a straight line motion through an anti-friction jack, with brake fully released. Torque motor is stalled. Motor is disconnected. Spring applies the brakes.

3. Thrustor-Operated. - consists of a self-contained motor-driven centrifugal pump, oil chamber and a piston which produces a straight line movement to release the brake. These brakes are available at 230 and 460 V applications and at respective motor currents.

Sections: Brakes | Fuses | Overload Relays | Discharge Resistors

Motor Protection: Fuses

Classification.
1. Low Voltage Fuses. - usually rated less than 600 VAC (voltage class), amperage, interrupting rating and current-limiting. Interrupting rating is the highest rms symmetrical AC which the fuse can interrupt at rated voltage. These fuses have a minimum IR of about 10,000 amperes and a maximum of 100,000 A DC, or 200,000 A AC.

These fuses are further classified into:

Class
Voltage
AC Amp.

G
300
0 - 60

H
250
0 - 600

K, J
600
0 - 600

L
600
600 - 6000

S
600
0 - 30

CC
600
0 - 20

Types.
a. Cartridge Fuses
b. Knife Blade
c. Bolted
d. Plug

Guide. AC fuses should not be used for DC applications

2. High Voltage Fuses. - used by electric utilities to protect distribution class equipment and by large industrial complees which have thier own electrical distribution systems.

3. Cut-outs, Expulsion Types.
a. Open - rated 20 kA assymetrical maximum interrupting current at 5 to 35 kV. Maximum continuous current rating is at 200 A.

b. Enclosed - rated 8 to 10 kA assymetrical maximum interrupting current

c. Open link - rated 1200 A assymetrical maximum interrupting current with 50 A maximum carrying current rating. Usually used on rural lines and small transformers.

3. Current Limiting Fuses [CLF]. - usually used on distribution systems. These are rated by a) continuous current rating, the maximum current that the fuse is designed to carry continuously, and peak-arc voltage, the maximum voltage genreated by the CLF. Guide: Fuses should not be applied to circuits with a voltage less than 50% of the fuse-voltage rating to avoid excessive peak arc voltage.

Classes.
a. General Purpose CLF. - will interrupt currents which will melt the fusible element in one (1) hour.

b. Backup CLF - have a definite minimum interrupting rating usually specified by the manufacturer.

4. Power Fuses. - usually used on power system grids, nominally rated at 20 kA interrupting current.

5. Oil-immersed link. - rated at 12.5 kA interrupting current when used with SF6, and 8 to 10 kA using other liquids.

RULES ON FUSES.

1. High voltage fuses should be mounted in enclosures. Especially the following: a. Industrial service entrance switchgear b. Pad-mounted switchgear and transformers for underground circuits. c. Sub-surface applciations.

2. Power fuses should be fitted with mufflers to reduce the intensity of exhaust the ingress of fluid when applied under oil such as in transformers.

3. Fuse cut-outs are not recomended for use in enclosures.

Sections: Brakes | Fuses | Overload Relays | Discharge Resistors

Motor Protection: Overload Relays

1. Magnetic Overload Relays. - series coil connected in motor circuit and a plunger pulled up in coil center by direct magnetic action of the motor current. Time delay is adjustable so that overload will not trip on starting-current inrush but on small sustained overloads.

2. Thermal Overload Relays.

a. Bimetallic - bends under heating of overload current releasing the latches and opening the contacts.

b. Fusible-alloy - tube and shaft are joined by a special low-melting eutectic alloy on overload, the increased current melts the alloy, turn shaft and open contacts.

Sections: Brakes | Fuses | Overload Relays | Discharge Resistors

Motor Protection: Discharge Resistors

Rating. Usually rated at 1 to 3 times the field ohmic resistance.