An incandescent light will flicker at 120 Hz, because it gets bright when the current is pushing in one direction, then again when the current pushes the other direction, so twice for every cycle of the power line. That’s why you don’t see light bulbs flicker, even though you can see it with a 60 Hz monitor (I can too, especially in my peripheral vision).
An incandescent light won’t actually flicker all that much, though, at any frequency, because it takes more than 1/120 of a second to cool down.
Napier, yes it’s a 3 phase, but if I read you right you said switching the leads. This is the only machine in my shop that has a switch where I can flip back and forth forward or reverse. Every other machine is on/off and it spins the way it was set up.
Is it all in the switch or is the motor different somehow? Practically speaking is the motor different and did the electrical shop take me for a noob and charge me more?
I’m sure the motor is the same as any other three-phase motor and the switch just swaps two of the three power leads. You should be able to accomplish the same thing by swapping two (any two) of the leads on your other machines.
The switch probably just switching two of the legs.
But the motor may be different. If the motor has thrust bearings being reversible will require double acting thrust bearings. The motor may be more heavy duty incase the operator switches direction to the other without stopping the motor first.
Both good points.
The cooling fan is often directional. So a reversible motor would have to contend with that issue also. Instead of curved or angled vanes for the cooling fan it needs straight ones. Straight vane cooling fans do not move as much air as curved or angled ones do. That is why they are curved or angled. Thus, the motor has to be able to dissipate more heat then a non reversible motor.
Any three phase motor can be a reversing motor. Unless they are designed for reversing, their longevity will be compromised. In its “forward” direction, it will cool just fine. In its “reverse” direction the cooling air flows slower and backwards over the motor. this will cause the motor to get very hot fairly fast. If you are using the motor intermittently, you MAY get by with just any three phase motor. In a machine shop, buy the correct motor. Do not trip over a dollar to save a dime.
IHTH, 48.
OK, I’ve never heard of a directional fan built into a motor. They’d have to sell twice as many motor models if they did this, because whether you consider the motor application one that requires reversing per se, there are plenty of motor applications that require CW rotation and plenty that require CCW.
Blowers in general, yes, unless they’re fairly high pressure ones, have some kind of handedness to the impeller blades. The whole job of the blower is moving air. But every motor with cooling I’ve seen has a radial blade impeller on the exterior mounted to the shaft (if it’s a TEFC or Totally Enclosed Fan Cooled) or it has radial fins molded into the ends of the rotor or a radial blade impeller pressed onto the shaft inside (if it has some kind of open frame). The impeller is a kind of an afterthought not designed for high efficiency – and I bet it consumes much less than 1% of the motor power.
Where could I see an example of a motor whose integral cooling impeller is not symmetric with respect to direction of rotation?
I looked inside the motor in question and it does indeed have straight fins. So I got curious and looked in 6 other motors I can easily get to and they all have straight fins.
I thought of something regarding 3-phase power. For normal 1-phase you can calculate the power used by multiplying voltage by current (ignoring power factor and stuff). For 3-phase, do you have to multiply the voltage by current by 3?
Well the design aim would be to have the same voltage and current on the three legs, but the end result may be that it varies a little.
As Isilder pointed out, that only works if the current through all three phases is identical, which in the real world it often isn’t. In many cases though it is close enough that you can approximate it by multiplying by three.
In large three phase power systems, “per unit” calculations are often done to make things easier to handle. The per unit system is a bit confusing at first, but when you deal with large power systems with lots of generators and transformers, it makes calculating things like power flow a lot easier. Wikipedia has a decent description of it here:
But in general, if say a motor is rated at 400V 10A 3-phase, it’s more likely to use 12kW than 4kW, right?
Power in a 3 phase system is AmpsXVoltsX 1.72
400X10X1.72=6880 watts or 6.88KW
1.72 is the sq rt of 3. The current in each leg is 120 degrees out of phase with the outer legs. Going back 40+ years of schooling that is 1/cosine of 120 degrees.
You forgot the power factor. Its 6.88 Kilo Volt-Ampere. The power factor for this size motor at rated load will be around 0.85. So the power will be 6.88 x .85 = 5.8 kW
nm
Should we attribute this to a brain fart? Your answers are usually so solid I would hate for someone to discount anything else you say because of this.
I shouldn’t post before I’ve had my morning coffee.
Is this assuming Volts and Amps are peak values or RMS?
RMS
In my world I only deal with RMS values.