Electric Motor Controls

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MAGM 225

Mr. Conrado

Electric Motor Controls


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Motor ControlElectric motors present some specialproblems from the standpoint of control. An electric motor will try toprovide the power required by a load,even if it results in self-destruction.

Therefore a motor must be protectedfrom overloads.A motor draws up to six times as muchcurrent when the rotor is not turning asit does when operating at full speed.If a motor should stall the controldevice and the disconnect switch mustbe capable of handling the highlocked-rotor current.


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Types of Motor ControllersA controller is simply a means of closing the circuit supplying power toan electrical motor.The most simple control device is acord and plug. (1/8hp or less)

A branch-circuit protective device, iebreaker, may serve as the controller for stationary motors of 1/8 HP or lessthat normally operate continuouslyA time delay circuit breaker is alsoallowed as a control device.A fusible knife switch may act as amotor controller provided it is rated for the horsepower rating of the motor supplied.


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Magnetic Motor Starter The main contacts are located insidethe unitRepair kits are availableWhen current is supplied to theinternal solenoid the motor-starter

contacts are closed.An overload relay section is added tothe motor starter to sense motor current.


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N EMA SizesN EMA has developed a standardnumbering system for motor starter sizes.All manufactures use this systemThe larger the rating number the

greater the horsepower rating of thestarter.Are rated according to the current theywill be expected to carry continuously,and interrupt the if the motor shouldstall.Motor starters come in 2 or 3 poles. 2pole is for single phase and 3 pole for 3 phase.A 3 pole motor starter will handle asingle phase motor with a horsepower rating half as large as 3 phase motor.


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EnclosuresThe motor-starter housing is selectedfor the environment in the area.N EMA has developed a standardnumbering system for enclosures andrequire they be marked by the

manufacture. N EMA 1 - dry and free from dust and

flammable materials N EMA 3 weather resistant suitable

for outdoors but not dusty conditions. N EMA 4 water and dust tight. N EMA 4x water and dust tight but

also corrosion resistant. N EMA 7 Explosion proof. N EMA 9 Dust Ignition proof


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Motor-Full load currentThe motor full-load current is used todetermine the minimum size of circuitcomponents.The name plate current rating must beused for selecting the maximum size

motor running overload protection.Wires supplying a single motor shallhave an amperage rating of not lessthat 1.25 times the full-load current of the motor.


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Short-Circuit ProtectionMotor circuits must be protected fromexcessive current, ground faults, andshort circuits.Over current protection can be fusedor circuit breaker a circuit beginning.

Short circuit and ground fault must becapable of carrying the starting currentof the motor.A single-phase, 5-hp, 230v motor witha nameplate of 28amps drives anormal load. Determine the overloaddevice. Full load current = 28A Fuse multiplier = 125% 1.25x28A=35A Use a 50-A fuse.


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Service supplying several motors

The feeder wires are required to havea minimum rating equal to the sum of the full-load current of all motorssupplied, plus 25%. Motors of 5-hp 28a , 3-hp 17a, 1-hp 8a,

and 2-hp 12amp.=72amps.

N o. 4 AWG minimum (from wire charts)The disconnect must be selected for ahorsepower rating of not less than thesum of all HP ratings. 5+3+1+2=11hp The disconnect closest is likely 15 hp If a breaker is used it must be a

minimum of 115% of the sum of thefull-load current for all motors served.

(28+17+8+12)1.15=75Amps


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Feeder TapsA common practice is to tap motor branch circuits from a motor feeder.This is permitted as long as thebranch-circuit wire is terminated at anover current of proper size.


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Motor Control CircuitA magnetic motor starter is operatedwith an electric solenoid coil.Here we have a typical wiring diagramfor a magnetic motor control. Thethick lines are the power flow to the

motor, the thin lines belong to thecontrol circuit and generally are pre-wired by the manufacture.


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Control DevicesCommon wiring schematics use basicsymbols and terminology.Switches and contacts are the basiccontrol components, and they takeseveral different forms.They can be operated mechanically,by timer, change in condition liketemperature, pressure, flow, liquidlevel, or humidity.Switches and contacts are describedby the number of pole and number of throws. Poles the number of paths into a

switch or contact. Throws the number of paths leaving

each pole.It must also be known if the switch isnormally open or normally closedwhen inactivated.


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Control DevicesThe common types of switches andcontacts are incorporated into varioustypes of control devices.


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Reversing Motor StartersElectric motors may be called upon todrive a machine which must bereversible.A reversible motor starter is requiredto select the desired rotation direction.

3-phase only two of the three leadsneed to be reversed.Single phase the start windings mustbe reversed with respect to the runwindings.Drum switch has forward, reverse, andstop. N ote the phase rotation at themotor for forward and reverse.

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Electric Motor Controls