Electrical Equipment and Devices

WEEK 7: MOTOR CONTROLLERS

Sections: Motor Controllers | AC Starting | DC Starting | Speed Control

Motor Controllers / Motor Starting Devices: General

Definition. Motor controllers are used for the following functions:
1. Starting
2. Speed Regulating
3. Stopping
4. Protection

1. CONTACTOR
Definition. A device, generally magnetically-actuated, for repeatedly establishing and interrupting an electric power circuit. Rated on continuous carrying current capacity [CCC] and intermittent duty.

Open rating range from 25 to 2500 Amperes. CCC rating is usually 100% of the rated voltage. Interrupting rating is genrally 133-1/3% of open rating.

2. DRUM SWITCH
A device which consists of stationary contact fingers held by a spring-pressured against contact segments on the periphery of a rotating cylinder or section. Ratings range from 40 HP at 115 V to 75HP at 230 V.

3. STARTING-DUTY RESISTORS
A bank of series-, or parallel- or series-parallel connected resistors to achieve the required ohmic values.

Types.
a. Cast Iron / Punched Steel Grid Resistors
Characterized by low in Ohmic values but with high capacity. Continuous rating per grid may range from 0.01 Ohms at 160 Amps to 0.40 Ohms at 20 Amps. Can withstand severe vibration and are resistant to most atmospheric contaminants. Resistance increase at 375°C rise is approximately 6%.

b. Wire-wound Resistors Charaterized by high ohmic values with low capacity. Unit resistance range from 4 to 6400 Ohms with a dissipation rating of about 900 Watts.

c. Edge-wound Ribbon Resistors Intermediate ohmic values and capacity. Ranges from 0.05 to 8.6 Ohms at 1320 Watts.

Selection Guide
1. Determine resistance in ohms using the Ohm's law formula, the total ohmic values and the resistance between steps.

2. Determine the Power in Watts to be Dissipated.

3. Determine proper size resistor based on voltage, current, ohms, watts, altitude, grouping, and circuit conditions. Guide. Most resistor units are designed for a maximum of 600 volts between terminals. For higher voltages connect two or more units in series so voltage drop across any one resistor unit is 600 volts or less. Voltage between resistor terminal and ground should not exceed 250 volts. Where resistors are applied above 250 volts, the resistor units should be mounted on insulated supports. For altitudes between 6000 and 15000 feet derate to 75-percent of the standard watt ratings, or derate to 86-pecent of the current rating. For ambient temperatures above 40 C, derate resistors to approx. 90% for 50C ambient, 80% for 80C and 70% for 100 C of full load watts.

4. Select the most suitable model and the desired mounting.

Sections: Motor Controllers | AC Starting | DC Starting | Speed Control

AC Motor Starting Devices

1. STARTER, MANUAL
A motor controller whose contact mechanism is operated by a mechanical linkage (toggle hand, pushbtton, or on-off switch with overload relays) operated by hand. Advantages. Low cost, simple mechanism, and quiet operation. Disavantages. Contacts remain closed "ON' during power failure and limit application to less than 1 HP-rated motor.

Usage. Applied on small machine tools, fans, blowers, pumps and compressors and conveyors

2. STARTER, MAGNETIC
Starters with contact operated by an electromagnetic circuit.

Types. Magnetic starters are classified according to the type of contactors (2-, 3- or 4-pole), the number of overload relays (1, 2 or 3), the full voltage rating and reversing or non-reversing.

Selection Guide. Starters are selected given the motor HP rating, the number of phases, coil voltage, control power transformer rating, type of enclosure, overload relay heaters specifications, the mode of use: 1-phase or three-phase up to 200 HP at 600 V.

3. AUTOTRANSFORMER STARTER
Usually two autotransformers connected in open-delta configuration to provide reduced-voltage starting. Also called as AC-reduced-voltage starter, used on blowers, conveyors and pumps.

Characteristics. Autotranformer starters offer low line current, low power form and low power factor.

Principles. Line current varies as the square of the impressed voltage. Torque varies as the square of voltage. Fifty percent voltage tap equates to 25 percent starting torque.

Operation. When RUN, starting contactors S energizes connecting transformer to line and motor to taps. Timing relay is energized. Motor accelerates, timer closes, deenergizes S and energizes R. Transformer is disconnected, and motor connected.

Main Starting Methods.
a. Direct on-line Starting
Direct on-line Starting is simple, umproblematic and economically efficient, Unfortunately, this method of starting is not suitable for all supply networks and driven machine because the locked-rotor current of these motor may reach values up to eight times the rated current and the starting torque may peak at three times the rated torque. The inrush current occuring at the instant of energizing the motor at rest rises to values far higher than the locked-rotor current. The inrush current and the locked-rotor current do not present any problems if thermal overcurrent protection is provided.

b. Part-winding Starting [Increment starting]
A device which imposes limitations on the type of load that can be accelerated on the first point. In rush current approximates to 65% of line current. Usually applicable to light or low-inertia loads such as airconditioning unit's compressors, refrigeration, pumps, fans and blowers.

c. Wye-Delta Starting
Motor windings are connected wye during start [each winding has 58% full voltage] and automatically connected to delta when running which applies full-voltage to each winding. This method is mainly used where the network is not strong enough to take the high locked-rotor current occuring with the direct on-line starting of squirrel-cage motors.

Operation. Contactor S energizes, connecting the motor windings to wye-connection. A normally-open auxiliary contact in S closes, energizing contact 1M, closing its contacts, energizing the motor windings in Wye. After a predetermined time-interval, contactor 1A is energized, connecting R in wye configuration, parallel to wye-connected motor windings. A normally-closed auxiliary contact in 1A opens, deenergizes S, opens contacts at S and Rs are in series with motor winding. Motor is Delta connected. A normally-closed auxiliary contact on S closes, energizes 2M, close contacts and shortens R. Delta-connected motor is energized at full voltage.

d. Smoth Starting
In the case of drives incorporating gears, belts, or clutches, it is often necessary to reduce the inherently high-locked totor torque of three-phase induction motors. This is done by connecting a resistor with bypass contactor or switch in one of the phases of the motor supply line. The locked-rotor torque can be lowered to any required value by appropriate dimensioning of this resistor. The required resistance depends on the ratio of the required locked-rotor torque to the normal locked-rotor torque of the motor. The starting time of a motor must be known, by calculation with accelerating torque and hence motor torque characteristics are known, in order to determine the size of the resistor required.

Sections: Motor Controllers | AC Starting | DC Starting | Speed Control

DC Motor Starting Devices

1. STARTERS, MANUAL
(see AC MOTOR STARTING -- STARTERS, MANUAL)

2. STARTER, MAGNETIC
Methods.
a. Time-Limit Acceleration. The operation of contactors and rate of acceleration, is governed by magentically-operated time relay.

b. Current-Limit Acceleration. Controllers designed to half the starting operations whenever the required starting current exceeds an adjustable predetermined value. The starting operation is resumed when the current falls below this limit.

Operation. Press "RUN", energizes M which closes, conects motors in series with R1 - R3. Motor current clows to SR1 coil. SR1 open, 1A will not close. Motor accelerates to bring current down to set point at SR1. Contacts will close, 1A closes, shorts SR1 coil. Current flows through SR2 coil and 1A contacts. SR2 relay opens, 2A will not close. Motor accelerates, when current setting is equal to SR2 setting, SR2 contacts closes energizing 2A and connecting the motor to line.

Sections: Motor Controllers | AC Starting | DC Starting | Speed Control

Motor Speed Control Methods

General. Motor Speed control methods involve: jog control, variable speed control and reversible speed control.

JOG CIRCUIT
Operation. When JOG is pressed, CR is bypassed, main contactor energized. When JOG is released, contactor releases. When START is pressed, CR closes.

VARIABLE SPEED METHODS
1. Field Control. Adjusting Voltage/Current to Field winding. Increasing the field voltage, motor speed slows down. Advantage: energy efficient due to inexpensive rheostat. Where field current is low due to large field resistance. Disadvantage: to increase speed, decreasing the field current is necessary, thus created weak magnetic field and less torque-ability.

2. Armature Control. Adjusting Voltage/Current to Armature. As armature voltage increases, motor speeds up. Advantage: Torque is better, thus motor starts faster. Disadvantage: Expensive due to large rheostats.

Principles.

Vs = Ec + Ia Ra
Vs = kB N + Ia Ra

where:
Ec = counteremf at armature winding
B = magnetic field at field winding
N = rotational speed
Ia = armature current
Ra = armature resistance

REVERSIBLE SPEED CONTROL
Methods.
1. Reverse the direction of Filed Current, while Armature current remains the same.

2. Reverse the direction of Armature Current, while Field Current remains the same.