Home |
Search |
Today's Posts |
|
Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
Reply |
|
LinkBack | Thread Tools | Display Modes |
|
#1
Posted to sci.electronics.repair
|
|||
|
|||
How to size capacitor for capacitor run fan motor
I have a small single phase blower motor which requires a capacitor
for running (not starting). The motor nameplate says that I need 7.5 microfarads. I have a pair of 5 microfarad caps in the shop. I hooked up the motor with just a single 5 uF and it didn't start. Then I put the pair in parallel and the motor did start. If I use 10 uF I will draw more current into the directional winding of the motor. So my question is, how closely are these caps sized? Is the difference between 7.5 and 10 uF enough to do damage? thanks |
#2
Posted to sci.electronics.repair
|
|||
|
|||
How to size capacitor for capacitor run fan motor
Hi!
When I needed a start capacitor for an old furnace fan motor, I had to go to an electric motor repair shop to get one. (You'd really think that Lowe's, Home Depot, Farm and Fleet, or a decent hardware store would have these sorts of things, but...) I had to get a capacitor slightly larger than the one I was replacing. However, I was told that the tolerance is 10% either way. A run capacitor might be a little tigher in tolerance, but I don't think you've got enough of a difference there to cause a problem. William |
#3
Posted to sci.electronics.repair
|
|||
|
|||
How to size capacitor for capacitor run fan motor
On Dec 31, 9:01*am, "William R. Walsh" wrote:
Hi! When I needed a start capacitor for an old furnace fan motor, I had to go to an electric motor repair shop to get one. (You'd really think that Lowe's, Home Depot, Farm and Fleet, or a decent hardware store would have these sorts of things, but...) I had to get a capacitor slightly larger than the one I was replacing. However, I was told that the tolerance is 10% either way. A run capacitor might be a little tigher in tolerance, but I don't think you've got enough of a difference there to cause a problem. William Can you hear a click as the motor gets up to speed? If you do, then the centrifugal switch is working and disconnecting the starting capacitor and the extra 2.5 uF isn't going to be a problem. The manfacturer uses the smallest value they can get away with as it saves them a few cents. Happy heating. |
#4
Posted to sci.electronics.repair
|
|||
|
|||
How to size capacitor for capacitor run fan motor
Hi!
Can you hear a click as the motor gets up to speed? I can now. The motor in question was treated badly and left outside in the weather. Rust soon covered the contacts of the centrifugal switch and all of the electrical contacts. It couldn't work properly, so I took it all apart and cleaned it. Even if you helped it, it didn't want to run properly. More often than not it would just wind down and start humming again. Then the switch worked, but the motor almost always needed help to start. After that it would run fine. So I started to look at the cap, and $7 later it was replaced. All problems were then solved. This is a converted fan, set up for standalone use wherever air flow is needed. I have another that uses a run capacitor. It is much fancier, with multiple speeds. William |
#5
Posted to sci.electronics.repair
|
|||
|
|||
How to size capacitor for capacitor run fan motor
On Dec 31, 11:34*am, "William R. Walsh" wrote:
Hi! Can you hear a click as the motor gets up to speed? I can now. The motor in question was treated badly and left outside in the weather. Rust soon covered the contacts of the centrifugal switch and all of the electrical contacts. It couldn't work properly, so I took it all apart and cleaned it. Even if you helped it, it didn't want to run properly. More often than not it would just wind down and start humming again. Then the switch worked, but the motor almost always needed help to start. After that it would run fine. So I started to look at the cap, and $7 later it was replaced. All problems were then solved. This is a converted fan, set up for standalone use wherever air flow is needed. I have another that uses a run capacitor. It is much fancier, with multiple speeds. William So, let's suppose you have a motor, and the nameplate is damaged to the point you cannot see what size of cap is required. Where do you go from there? I'm sure there must be a means of establishing that you do have the correct cap. I found one document which states that "the voltage across the cap should be about 5-10% higher than the voltage across the winding, with the rotor locked." Has anyone any other rules of thumb? |
#6
Posted to sci.electronics.repair
|
|||
|
|||
How to size capacitor for capacitor run fan motor
Hi!
So, let's suppose you have a motor, and the nameplate is damaged to the point you cannot see what size of cap is required. Where do you go from there? I'd start by checking the cap that came with the motor. In the case of the fan I'm talking about, the capacitor was clearly labeled as to its ratings. If that wasn't possible, I'd try looking the motor manufacturer up on the web (either through their web site or from a web site belonging to someone who sells those motors) for more information. You could also try telephoning the motor manufacturer. Still nothing? I'd have to visit an electric motor repair shop at that point. (That's the only way I know of if everything else is missing or does not pan out.) William |
#7
Posted to sci.electronics.repair
|
|||
|
|||
How to size capacitor for capacitor run fan motor
On Thu, 31 Dec 2009 04:39:47 -0800 (PST) Andy
wrote in Message id: : I have a small single phase blower motor which requires a capacitor for running (not starting). The motor nameplate says that I need 7.5 microfarads. I have a pair of 5 microfarad caps in the shop. I hooked up the motor with just a single 5 uF and it didn't start. Then I put the pair in parallel and the motor did start. If I use 10 uF I will draw more current into the directional winding of the motor. So my question is, how closely are these caps sized? Is the difference between 7.5 and 10 uF enough to do damage? Can't answer your question, but Digikey carries 7.5uF AC run caps. For $10 or so, better safe than sorry I always say. |
#8
Posted to sci.electronics.repair
|
|||
|
|||
How to size capacitor for capacitor run fan motor
There are many small motors in use that utilize a low capacitance value
capacitor. The most common type smaller than 1/8 HP is the PSC permanent split capacitor type motor. PSC motors have 2 windings connected in series (3 connections or wires), and don't have centrifugal switches for starting. PSC motors are commonly labeled with; thermal and/or impedance protection. The method used to determine if a motor is a PSC, is to measure the resistance of the windings. A PSC motor's windings resistances will be nearly identical for boh windings. The value of the capacitor affects the torque and speed of the motor. I don' know of any formula for determining the (best) value of the capacitor. Years ago, there was a reference to a capacitor value selection chart, related to PSC motors used in HVAC servicing, IIRC. The chart suggested capacitor values for different sized motors, for increasing torque and/or speed. Many of the small PSC motors that I'm familiar with will operate with different capacitor values, and a couple of them use different capacitor values to make the motors operate at different speeds (low-medium-high). One PSC motor I was using for a machine application actually ran much cooler when the cap value was changed to a higher value (about 3x the manufacturer's specified value). I'm surprised that your motor didn't run with just one 5uF cap connected. Marked values on caps can vary as much as 20% from the actual measured value, but 10% is probably a much more likely worst case situation. The best indicator from a standpoint of harm, is probably going to be the motor temperature. If the motor doesn't run excessively hot, there wouldn't be any reason that the cap's value could harm the motor. -- Cheers, WB .............. "Andy" wrote in message ... I have a small single phase blower motor which requires a capacitor for running (not starting). The motor nameplate says that I need 7.5 microfarads. I have a pair of 5 microfarad caps in the shop. I hooked up the motor with just a single 5 uF and it didn't start. Then I put the pair in parallel and the motor did start. If I use 10 uF I will draw more current into the directional winding of the motor. So my question is, how closely are these caps sized? Is the difference between 7.5 and 10 uF enough to do damage? thanks |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
In need a motor starting capacitor | Electronics Repair | |||
Capacitor Start, Capacitor Run Motor | Electronics Repair | |||
What type of AC motor have I got? capacitor start motor? | Home Repair | |||
Capacitor start motor help please | Metalworking |