# What determines AC Motor RPMs

I had a random thought today.
If I feed 120 volts AC (60Hz) into a motor, what determines it's RPMs? Obviously the 60Hz comes into play, however, various motors have different
RPMs at the same voltage, correct?
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If you're referring to AC induction motors, it's the frequency and the number of poles.
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And to a lesser extent the amount of load on the shaft, and the shape of the torque curve of the motor. There are variations in induction motor design that have the same number of poles and the same no-load RPM but different behaviour under load.
Dave
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face=Arial size=2>...<BR>&gt;&gt;I had a random thought today.<BR>&gt;&gt;<BR>&gt;&gt; If I feed 120 volts AC (60Hz) into a motor, what determines it's RPMs?<BR>&gt;&gt; Obviously the 60Hz comes into play, however, various motors have different<BR>&gt;&gt; RPMs at the same voltage, correct?<BR>&gt;&gt;<BR>&gt;&gt;<BR>&gt; <BR>&gt; If you're referring to <STRONG>SYNCHRONOUS</STRONG> AC induction motors, it's the frequency and the <BR>&gt; number of poles.<BR>&gt; <BR>&gt; <BR>&gt;</FONT></BODY></HTML>
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As James Sweet said, it is the frequency and the number of poles. The synchronous speed in rpm is given by 120*frequency/poles. Poles come in pairs. At 60 Hz, these speeds are 3600, 1800, 1200, 900,.... rpm fro 2,4,6,8,...poles and are independent of voltage. Generators and some motors run at synchronous speed and induction motors slip and run at some (generally less than 5% less) slower speed under load -hence a 4 pole induction motor may have a rated full load speed of 1756 rpm.
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Don Kelly snipped-for-privacy@shawcross.ca
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| If I feed 120 volts AC (60Hz) into a motor, what determines it's RPMs? | Obviously the 60Hz comes into play, however, various motors have different | RPMs at the same voltage, correct?
That depends on the type of motor. At least one type of motor (called the universal motor) has a speed independent of the frequency (it can even run on DC). Syncronous motors have a speed that is locked to the frequency at some whole number ratio based on the number of poles in its design. The induction motor will have a speed somewhat less than syncronous and it can have multiple poles for a slower design, too.
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wrote:

------------------ The universal motor is actualy a DC series motor which will run on AC It's AC performance is limited by its inductance so it runs better on DC than AC.
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Don Kelly snipped-for-privacy@shawcross.ca
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| The universal motor is actualy a DC series motor which will run on AC It's | AC performance is limited by its inductance so it runs better on DC than AC.
My father used to have a nice little "motor" that would reach some screaming high speeds. It ran on air pressure, not electricity. Electricity was used to compress air into a tank to run it from. It was a dental drill.
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It's almost a DC motor. However, a real DC-only motor could have solid steel pole pieces in its field, since they would be energized with DC. This might have significant eddy-current losses if powered from AC. So "universal" motors have laminated steel pole pieces for both the armature and field, giving low eddy-current losses on AC - and it still works fine on DC too.
Of course, if you have an appliance with a universal motor and you want to run it on DC, you need a power switch that's actually capable of interrupting DC. Some better power tools advertise that they do have DC switches for just such use. I'm not sure what produces 120 VDC - perhaps an engine-driven arc welder.
Dave
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writes:

------------------------ You are right in that a DC machine doesn't need laminated poles-and this is a modification to improve its efficiency (and likely it is cheaper to stamp out pole shaped laminations rather than cast or machine the poles) when run with AC. However, the basic operation is that of a series DC machine and it will still run better on DC. Both AC and DC machines have laminated armatures in any case.
There were 120V and 240V DC machines in use in the not so distant past. Some that I know of were donated to a university (Alberta) EE department when a mine closed and these were then converted to dynamometers.
Like you I would really avoid the typical 120V AC switch for 120V DC. Go back not all that far into the past and household switches could handle DC. Note that in the 30's and 40's many rural homes had windchargers and battery banks at 120V DC for household service and lamps needed to be switched. Compared to modern AC switches, these took more effort to operate and were noisy.
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Don Kelly snipped-for-privacy@shawcross.ca
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| |>The universal motor is actualy a DC series motor which will run on AC It's |>AC performance is limited by its inductance so it runs better on DC than AC. | | It's almost a DC motor. However, a real DC-only motor could have solid | steel pole pieces in its field, since they would be energized with DC. | This might have significant eddy-current losses if powered from AC. So | "universal" motors have laminated steel pole pieces for both the | armature and field, giving low eddy-current losses on AC - and it still | works fine on DC too. | | Of course, if you have an appliance with a universal motor and you want | to run it on DC, you need a power switch that's actually capable of | interrupting DC. Some better power tools advertise that they do have DC | switches for just such use. I'm not sure what produces 120 VDC - | perhaps an engine-driven arc welder.
10 car batteries in series (someone was mentioning car batteries in series for computer PSUs elsewhere).
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writes:

Back in the sixties, there was a product called Tote A Volt, it was simply a DPDT swich that would switch the output of your car's alternator from the battery to a duplex outlet. There was about 130 v pulsating DC available under a 10 to 20 amp load. Our electric drills ran well on it, and the circular saws actually had a lot more power. We had a shim we would put on the engine's throttle idle stop screw to boost the idle speed up to about 1200 rpm, worked very well. I had a 100 watt lamp connected at all times to make sure the alternator had a load.
Tom
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writes:

Some (all?) tower cranes and offshore oil rigs have DC power.
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James Sweet wrote:

The tower cranes I have seen have wound rotor AC motors. They use braking resistors connected through contactors to the rotor circuit for speed control & stopping.
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Benjamin D Miller, PE
www.bmillerengineering.com
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I've also seen wound-rotor used on the two rail drive motors. With the two tower-crane rails many feet apart, and the overall length of each 'truck' about the same as the distance between rails, 'cocking' on the rails can be a problem. So they use two wound-rotor motors, one in each side and they tie the rotor circuits together before connecting to the resistor bank (through some contactors). In this way, the two rotors turn in locked-step with each other and the crane moves down the track in a nice straight manner.
daestrom