Question about electric motors and speed

I have a blower and motor assembly out of an old furnace that I use to circulate air on jobs where I’m working (I rehab houses). I have a grounded plug wired to a switch that turns the blower/ motor on and off (115v). Can I put some sort of speed control on this motor to tone down the air movement. Right now, it seems like a gale force wind comes out of this thing, and sometimes it is too much- say when cutting or sanding drywall. The motor drives the blower via a belt, if that adds anything…

You can control single-phase motor speed using a Variac (a type of variable autotransformer) that can adjust the voltage to the motor from, typically, zero to full supply voltage. Some are even sound to able to supply slightly higher than supply (I had one that you could plug into 120 VAC and get zero to 140VAC out). I’ve used them on fans many times with good results.

Oh, and a word of warning: many fan motors require a certain amount of airflow over them to keep the windings cool. If you adjust the speed TOO low, you run the risk of burning out or shortening the life of the motor. I wouldn’t run it at less that 50% to be on the safe side.

Is this something I can pick up at Radio Shack, or do I need to go to a electric supply house? Does it replace the switch, or get wired between the switch and the motor? Is it three wire, 2 wire, or just to the hot? Any ideas on cost? Thanks in advance…

It depends upon what kind of motor you have. The nameplate should tell you. If it is a universal AC/DC motor you can control its speed via the applied voltage as **QED[/d] says. If it is an induction motor you’re pretty much stuck with the speed you’ve got. In that case you have to juggle pulley sizes or get a variable pulley for the motor from the hardware.

It might be as easy(or cheaper if you can botch it) just to have some kind of shutter to restrict airflow, contrary to what folk might think, when a fan is moving air, and you restrict airflow - it reduces the load on the motor.

Many people instinctively think that if a fan is pushing air against a resitance, such as a shutter, the air pressure must build up and the fan works harder, the reality is that the fan performs work by moving air, the less air it moves, the less work it does, and the less current it draws.

No, you won’t find them in a Radio Shack. Check electrical supply shops and on eBay. They come in both grounded and ungrounded varieties. They’ll usually have a power cord you plug into the outlet, and there will be one or two outlets on the case of the unit itself, which you will plug your fan into. It’ll have a big knob on the top to adjust the output voltage.

Well, there are ways to control the speed of induction motors, but I don’t think it will come to that, since the only use of them in fans I’ve seen are in ceiling fans. I don’t recall ever seeing either a high-volume blower fan or a floor/window fan with an induction motor. They’re probably out there, but how common are they?

Couldn’t you just wire up a wall dimmer or similar rheostat/potentiometer? Sure, this would be ghetto, but it sounds like we’re already dealing with something ghetto (harvested from an old furnace!)

This would be much cheaper than buying a variac, assuming it would work.

I must be missing something here. If a motor spins pretty freely (no real resistance) then it produces back emf which drastically reduces the current going into the motor. The more resistance you add to the motor’s movement, the more you are going to reduce the back emf and the more current will flow into the motor. It’s my understanding that if you reduce the airflow with a shutter that the current in the motor is going to rise dramatically.

A rheostat or potentiometer (I prefer the pots because of their 3 terminal design) would be able to do the same job. You’d wire it inline after the switch and then adjust the resistance up to slow down the fan. This is the same way that the volume control works on your stereo.

You do have to make sure that the pot has the same power rating as the circuit it’s going into (use the maximum voltage multiplied by the current drawn to find out the wattage of the circuit) or you could burn up your pot and cause some serious problems.

If you can’t find any of the parts you need at RadioShack, definitely try an electrical supply store, or possibly Jameco Electronics to find what you need.

One caution: Once Jameco starts sending you catalogs, they never stop.

engineer_comp_geek: CEMF (counter EMF) is produced when a voltage is applied acrpss an inductor (or through the inductors of a transformer not under load) because of the magnetic field generated. It attempts to induce a current which is opposite in direction from the current caused by the applied voltage because the CEMF is opposite in polarity than the applied voltage. What this causes is for the current in the inductor (or the primary current in a transformer) to be ‘lag’ the voltage applied. If the circuit was purely inductive, the current would lag the voltage by 90 degrees. In this case, it will lag by some phase angle between 0 and 90 degrees because it is not a purely inductive circuit.

Reducing the amount of rotation of the fan doesn’t increase applied voltage (applied EMF), and the magnetic field is not increased, so why would CEMF be increased?

I don’t know how common they are. I live in an area where evaporative air cooling is the common mode and they use induction motors. I’m pretty sure my furnace blower is driven by an induction motor. It’s true that most small appliances use universal motors but a furnace blower isn’t exactly a small appliance. In addition my bench grinder and drill press are both capacitor start/run induction motors. I would check before I bought a lot of parts on the assumption that the motor is a universal.

This site onfurnace blowers lists various typical motor speeds. I quote: “The most common RPM is 1200 (1050 or 1075) also 900 (850, 825) and 1550 are used too. Most motors are single speed but can have as many as 4 speeds (3 is typical for direct drive indoor blowers on most furnaces).”

Those speeds sound like induction motor synchronous speeds to me. I assume the 1550 is a mistake and it should be 1750. When 2, 3, or 4 speed motors I immediately think induction motor because you don’t ordinarily talk about specific speeds for universal motors.

The clincher is the next paragraph about typical wiring colors:
“Wiring colors for most motors are White = Common, Black = High, Yellow = Medium, Blue = Medium Low, Red = Low. Brown w/white tracer is for the run capacitor the same as white. Brown is for the run capacitor. If you can place the run capacitor in the equipment cabinet use the brown with the white tracer and tape off the white wire because both brown wires have 1/4” quick connectors on them."

So it would seem that many furnace blower motors are capacitor start and run induction motors. In that case the simplest speed change would be to get an adjustable pulley for the motor. When the pulley is adjusted to the slowest speed (widest gap on the adjustable pulley) it would be a good idea to have it give an effective diameter of about half the current pulley. If that still gives too much air then a bigger blower pulley could be installed.

If it’s a regular brush-type motor (“universal” or whatever) then a “phase-fired proportional controller” should work. In other words, a regular 'ol light dimmer might work, assuming it can handle the wattage. Just don’t turn it down too low, else the motor will stall.

If it’s a “brushless” motor (induction, synchronous, etc.) then speed control doesn’t come easy. Perhaps a Variac[sup]TM[/sup] would work, but it’s certainly not a convenient option.

If it’s a brushless motor, then perhaps a smarter option would be to swap out the motor for a regular brush-type motor. There would be two advantages; for the same amount of power the brush-type motor should be lighter than a brushless motor, plus you’d be able to control the speed with a light dimmer. I guess I should also mention two disadvantages w/ a brush-type motor: 1) Brushes wear out, and 2) They create tiny sparks, which could be a problem if flammable gas is in the vicinity of the motor.

I would buy one at All Electronics . I don’t work there but have bought many things from them, and they always have great service and shipping and great prices. Here are their variacs. Get one that can handle the load of the motor (they are rated in amps, so find out the max your motor will draw).

As I remember it,
CEMF is caused by the windings of the armature or spinning part cutting through the lines of flux generated when current flows through the windings of the not moving parts or the moving flux lines cutting through the stationary windings.

Current is determined by voltage = pressure sorta / resistance = size of the hole sorta.

So, speed of rotation affects the amount of flux line cutting going on so speed of rotation has direct effect on induction which is directionally related to CEMF, so …

Transformers don’t work so good on DC ( add a vibrator and they do OK ) and are not motors and don’t have moving windings… but motors do and stuff and such and the principals are pretty much the same as you need to look at what happens during ½ a cycle of AC to see what is going on, sorta.


No, casdave is correct. When you close the shutter, the fan actually speeds up. Try it with your vacuum cleaner. Listen to the pitch of the motor noise, then put your hand over the hose. The pitch increases, because the motor is spinnning faster.

This will only work if it’s a “Type F” motor, designed to run with high slip.

True. Could he also use a VFD? Though last I checked they were kinda pricy…

When a current flows through a wire because there is an EMF (commonly expressed as Voltage, which is a measure of the potential difference of two points), it creates a magnetic field around that wire. In an inductor, the wire is coiled, and due to the properties of the flux lines of the magnetic field being perpendicular to the current flow following the right hand rule, the flux lines (and thus the magnetic field) are ‘concentrated’ down the center of the inductor, will exit the end of the inductor, and wrap around to the other end of the inductor where they will enter. Each end will have an oppsite pole magnetically, just as it will have an opposite polarity in terms of voltage.

The right hand rule is demonstrated by placing your palm facing the conductor and your thumb pointing in the direction of current flow, the flux lines will travel in the direction of your curved fingers. In an inductor, the magnetic flux lines are still perpendicular to the flow of current, but because the wires are coiled, this will mean that the perpendicular direction to each coil is through the center of the coil and out around the outside for the return path.

In a transformer, current flows through the primary, inducing a magnetic field. The secondary is placed within this magnetic field, and as the windings of the secondary cut the magnetic flux lines, an EMF is induced in the secondary proportional to the ratio of the windings of the primary and the secondary (for example of the secondary has 1/2 the number of coils as the primary, the EMF induced in the secondary will be 1/2 that of the primary and you would have a Step-Down Transformer). The induced EMF in the secondary (secondary voltage) will be present whether or not there is a load on the transformer, but a current will not exist in the secondary unless it is placed under load (there must be a complete path for current to flow). If the voltage of the secondary is 1/2 that of the primary, and their resistances are the same, the secondary will have twice the current of the primary. A Step-Down transformer lowers the voltage from primary to secondary, but increases the current because V = IR.

In a generator (also called an alternator for AC) this is true. The armature is rotated inside a permanent stationary magnet and is connected to slip rings and brushes. The ends of the armature freely rotate inside the slip rings, and the brushes are used to collect the electrons that flow through the armature. The output voltage will be determined by the strength of the magnetic field, the length of the wire in the armature, and the angle at which the armature cuts the flux lines. A 90 degree angle causes the most flux lines to be cut by the armature and results in the highest voltage. A 180 or 0 degree angle causes no flux lines to be cut and results in a 0 voltage. Maximums are reached at 90 degrees (positive maximum) and 270 degrees (negative maximum) and your generator produces an output voltage that follows a sinusoidal pattern. The maximum is referred to as a Peak voltage, and is used to calculate the voltage that we are most familiar with as coming out of the wall, which is the Effective or RMS voltage. The RMS voltage can easily be approximated by taking .707 x Peak Voltage. The frenquency of the output sine wave is a direct result of the speed of rotation of the armature. For an AC generator to be effective, it must maintain a constant speed.

DC never changes polarity, thus the current flow never changes direction, and the transformer setup doesn’t work as well because induced voltages and CEMF only really becomes an issue when current (and voltage) are changing. Once they reach a stable level, there’s really nothing to induce the CEMF or a voltage in the secondary.

Sure he could use one. But, as you said, they’re kinda pricey. I’m not sure why that’s still the case though, given the way that the price of 12 DC -> 110 V 60 Hz / 240 V 50 Hz inverters has plummeted over the past couple of years. A quarter-horse VFD costs about US$150. A variac costs about US$60, and a straight inverter about US$40.

(Note for the bystanders: “VFD” means “Variable Frequency Drive”. The incoming AC power is rectified to DC, and then inverted to AC again, but at a variable frequency, lower than the incoming 50 or 60 Hz mains frequency. Brushless motors are most efficient spinning at speeds close to the AC frequency they’re supplied with. If you want to change the speed of the motor, the best (but not necessarily the cheapest) way to do it is to change the frequency of the AC supply, not the voltage.)

Look, catsix, I’m sure you mean well, but any potentiometer you’re going to find in an electrical supply store isn’t going to handle the voltage or the current, and it’s dangerous advice to suggest that unqualified people should fiddle around with mains voltage wiring.