Single-Phase Electric Motor Characteristics and Applications

7/29/2019 Single-Phase Electric Motor Characteristics and Applications

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Single-phase Electric Motors: Characteristics & Applications

Single-phase AC motors are as ubiquitous as they are useful -- serving as theprime power sources for a seemingly limitless array of small-horsepowerapplications in industry and in the home. Knowing how to apply the various typesis the key to successful design.

Where three-phase power is unavailable or impractical, its single-phase motors tothe rescue. Though they lack the higher efficiencies of their three-phase siblings,single-phase motors -- correctly sized and rated -- can last a lifetime with little

maintenance.

Occasionally a manufacturing defect can result in early motor failure. However,most failures result from inappropriate application. Pay careful attention to theapplication requirements before choosing a motor for replacement of a failed oneor for a new design application. Not choosing the correct motor type andhorsepower can cause repeated motor failure and equipment downtime. Obviously,you don't want to specify a motor too small for the application, thus resulting in

electrical stresses that cause premature motor failure. But neither should youspecify a motor too powerful -- either because of its power or its inherent designcharacteristics. It can also have serious effects. For example, a motor with highlocked-rotor and breakdown torques can damage the equipment it drives. Also,

running a motor at less than fullrated load is inefficient, costingyou money for power wasted.

The key: First, size the motor tothe application but, just as

importantly, understand thecharacteristics of the majortypes of single-phase motors --characteristics that go right tothe heart of matching a motor

to an application.

In general, an ac polyphasesquirrel-cage motor connectedto a polyphase line will develop

starting torque. A squirrel-cagemotor connected to a single-phase line develops no startingtorque, but having been startedby some external means, it runs

approximately like a polyphase motor. The many types of single-phase motors aredistinguished by the means by which they are started.


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Split-phase

The split phase motor, also called an induction-start/induction-run motor, is probably the simplestsingle-phase motor made for industrial use, thoughsomewhat limited. It has two windings: a start anda main winding, Figure 1. The start winding is madewith smaller gage wire and fewer turns relative to

the main winding to create more resistance, thusputting the start winding's field at a different anglethan that of the main winding, and causing themotor to rotate. The main winding, of heavier wire,

keeps the motor running the rest of the time.

A split-phase motor uses a switching mechanismthat disconnects the start winding from the mainwinding when the motor comes up to about 75% ofrated speed. In most cases, it is a centrifugal switch

on the motor shaft.

The split-phase motor's simple design makes ittypically less expensive than other single-phasemotor types made for industrial use. However, italso limits performance. Starting torques are low,typically 100% to 175% of rated load. Also, themotor develops high starting currents,approximately 700 to 1,000% of rated.Consequently, prolonged starting times cause the

start winding to overheat and fail; so don't use thismotor if you need high starting torques.Other split-phase motor characteristics: Maximum running torques range from 250to 350% of normal. Plus, thermal protection is difficult because the high locked-rotor current relative to running current makes it tricky to find a protector with triptime fast enough to prevent start-winding burnout. And, these motors usually aredesigned for single voltage, limiting application flexibility.Good applications for split-phase motors include small grinders, small fans andblowers, and other low starting torque applications with power needs from 1/20 to1/3 HP. Avoid any applications requiring high cycle rates or high torques.


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Capacitor start/induction run

Here is a true wide-application, industrial-dutymotor. Think of it as a split-phase motor, butwith a beefed-up start winding that includes acapacitor in the circuit to provide a start "boost",Figure 2. Like the split-phase motor, the

capacitor start motor also has a startingmechanism, either a mechanical or solid stateelectronic switch. This disconnects not only thestart winding, but also the capacitor when themotor reaches about 75% of rated speed.

Capacitor start/induction run motors haveseveral advantages over split-phase motors.Since the capacitor is in series with the startcircuit, it creates more starting torque, typically200 to 400% of rated load. And the startingcurrent, usually 450 to 575% of rated current, ismuch lower than the split-phase due to thelarger wire in the start circuit. This allows highercycle rates and reliable thermal protection.

The cap-start/induction-run motor is more expensive than a comparable splitphase design because of the additional cost of the start capacitor. But theapplication range is much wider because of higher starting torque and lowerstarting current. Use them on a wide range of belt-drive applications like smallconveyors, large blowers and pumps, as well as many direct-drive or gearedapplications. These are the "workhorses" of general-purpose single-phase industrialmotors.

Permanent split capacitor

A permanent split capacitor (PSC) motor, Figure 3,has neither a starting switch, nor a capacitor strictlyfor starting. Instead, it has a run-type capacitorpermanently connected in series with the startwinding. This makes the start winding an auxiliarywinding once the motor reaches running speed.Because the run capacitor must be designed for

continuous use, it cannot provide the starting boostof a starting capacitor. Typical starting torques ofPSC motors are low, from 30 to 150% of rated load,so these motors are not for hard-to-startapplications. However, unlike split-phase motors,PSC motors have low starting currents, usually lessthan 200% of rated load current, making themexcellent for applications with high cycle rates.Breakdown torque varies depending on the designtype and application, though it is typically somewhat

lower than with a cap start motors.

PSC motors have several advantages. They need nostarting mechanism and so can be reversed easily.Designs can be easily altered for use with speed


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controllers. They can also be designed for optimum efficiency and high powerfactor at rated load. And they're considered to be the most reliable of the singlephase motors, mostly because no starting switch is needed.Permanent split capacitor motors have a wide variety of applications depending onthe design. These include fans, blowers with low starting torque needs, andintermittent cycling uses such as adjusting mechanisms, gate operators and garagedoor openers, many of which also need instant reversing.

Capacitor start/capacitor run

This type, Figure 4, combines the best of thecapacitor-start/induction-run motor and thepermanent split capacitor motor. It has a start-typecapacitor in series with the auxiliary winding like thecapacitor-start motor for high starting torque. And,like a PSC motor, it also has a run-type capacitorthat is in series with the auxiliary winding after thestart capacitor is switched out of the circuit. This

allows high breakdown or overload torque.

Another advantage of the capacitor-start/capacitor-run type motor: It can be designed for lower full-load currents and higher efficiency. Among otherthings, this means it operates at lower temperaturethan other single-phase motor types of comparable

horsepower.

The only disadvantage to a cap-start/cap-run motoris its higher price -- mostly the result of more

capacitors, plus a starting switch. But it's a realpowerhouse, able to handle applications toodemanding for any other kind of single-phasemotor. These include woodworking machinery, aircompressors, high-pressure water pumps, vacuumpumps and other high torque applications requiring1 to 10 hp.


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Shaded-pole

Unlike all the previous types of single-phase motorsdiscussed, shaded-pole motors have only one mainwinding and no start winding, Figure 5. Starting isby means of a design that rings a continuous copperloop around a small portion of each motor pole. This"shades" that portion of the pole, causing themagnetic field in the ringed area to lag the field inthe unringed portion. The reaction of the two fieldsgets the shaft rotating.

Because the shaded pole motor lacks a startwinding, starting switch or capacitor, it is electricallysimple and inexpensive. Plus, speed can becontrolled merely by varying voltage, or through amulti-tap winding. Mechanically, shade-pole motorconstruction allows high-volume production. In fact,

these are usually considered "disposable" motors,meaning they are much cheaper to replace than torepair.

The shaded pole motor has many positive features, but it also has severaldisadvantages. Its low starting torque is typically 25 to 75% of full load torque. Itis a high slip motor with running speed 7 to 10% below synchronous speed. Also, itis very inefficient, usually below 20%.Low initial cost suits shaded pole motors to low-horsepower or light-dutyapplications. Perhaps their largest use is in multi-speed fans for household use. Butlow torque, low efficiency, and less sturdy mechanical features make shaded-pole

motors impractical for most industrial or commercial uses where higher cycle ratesor continuous duty are the norm.

The preceding information establishes guidelines for determining the proper motortype for your application. However, there are always special cases and applicationsin which it is acceptable to vary from these guidelines. Make it a point to checkwith your motor manufacturer for technical support in these areas.

Capacitor Details

Start Capacitor: The electrolytic start capacitor helps the motor achieve the mostbeneficial phase angles between start and main windings for the most locked-rotortorque per locked-rotor ampere. It is disconnected from the start circuit when the

motor reaches about 75% of full-load speed.

The start capacitor is designed for short-time duty. Extended application of voltageto the capacitor will cause premature failure, if not immediate destruction. Typicalratings for motor start capacitors range from 100 to 1,000 microfarad (uF)capacitance and 115 to 125 volts AC.

However, special applications require 165 to 250-Vac capacitors, which arephysically larger than capacitors of lower voltage rating for the same capacitance.Capacitance is a measure of how much charge a capacitor can store relative to the


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voltage applied.

Run Capacitor: These are constructed similarly to start capacitors, except for theelectrolyte. They are designed to serve continuously in the run circuit of acapacitor-start /capacitor-run motor. They withstand higher voltages, in the range

of 250 to 370 Vac. They also have lower capacitance, usually less than 65 uF.

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Single-Phase Electric Motor Characteristics and Applications