Can you identify the motor type? Shaded pole? Split-phase induction? Capacitor start-induction run? Capacitor start-capacitor run? How much work or expense is it worth to restore the motor? 1/4 hp motors have rather limited applications. Around my shop, motors of 1 to 5 hp see lots of use.

Hello Bus. Thanks for your response.

Here's the information from the plate:

A.O.Smith Type S 1725 rpm SF 1.35 115v PH 1 A 5.0 SFA 5.4 60 Hz AMB 40C FR 48 Code N INS B Time Cont.

Only part of that makes sense to me. I did not see a capacitor on the motor (big lump on top of housing, right?)

It's not worth a lot to restore the motor except for the experience. I have other 1/4 hp motors available and don't really need another; it's just that I hate to see a functional piece of equipment go to scrap. Still, I wouldn't mind learning what's involved, and I could always take it back to the shop and keep it in reserve for the next time we have the problem. (I'm a baker, and we have two of these ovens. It appears to be a weak spot in their design, although they're otherwise great. Probably costs us $500 or so to have this motor replaced by Hobart.)

Thanks again. I appreciate your time, but if you think it's better left alone please don't hesitate to say so.

M

I believe that is a split phase motor. It is for certain a 48 frame. Service Factor 1.35- good. 40C ambient, not bad, higher is better. Rated for continuous operation. A very common motor, but minimal starting torque. Starting amperes is always high for this type motor. If it had LRA posted, that would be the maximum starting amperes, typically for very brief time. Does it drive the fan directly or by belt? Has anyone checked the actual ampere draw while running? This very important when a new motor is installed.

Amp draw while driving the fan was 4.9. It is a direct drive. I believe the tech said the starting amperage should have been less than double the rated amperage, not sure, but it drew about 24A on start up. The squirrel cage it was driving simply wasn't pulling enough air through. Identical motor on the other oven was replaced a few weeks earlier. Both ovens several years old now- one five, the other four. I suspect we'll be adding another soon.

Thanks again for your time, Bus. I guess what I'm trying to learn is how to tell whether motors that aren't in use are any good, and this is a roundabout way of getting there. A little hands-on experience for a non-electrician.

Regards,

M

Obviously, a part number would provide the key to a lot of information. Lacking that, however, you can make a few assumptions.

LRA, or locked rotor amps, is the current the motor draws when the rotor is stopped - either at the instant power is applied or if sufficient load is applied to stall the motor. This figure can vary a bit but is typically 2-3 times running current. There isn't much a loose connection in the motor could do to increase this unless it created a short (by allowing one terminal on a wire to touch another, for example))

The possibility exists that a loose connection might have allowed the motor to overheat and short windings, and the extra heat would eventually damage the bearings, especially if they are sleeve type (the oil cooks). It is much more likely that the internal start switch failed in some fashion and may have fried some windings, both in the start circuit and the run windings. Start windings are only to get the motor spinning initially and will burn if left in circuit too long. The fact the motor was running hot and not providing sufficient power to run the blower is a good indication the motor is damaged. While a bad connection isn't good for the motor, at the currents the motor should be running at the likely effect of a bad connection would be that the motor would start and stop (or possibly slow and speed up) as the connection would make and break - if this was the problem.

If your business depends on the motor running correctly it hardly makes sense to even consider trying to use it again if a service tech pulled it. Take it apart and see what you can learn. If it's full of flour or other stuff your next replacement should be an enclosed motor (as opposed to a vented one)

UNK

Thanks, Unc.

It's definitely full of flour! The interior of the housing is coated with it. Given its history I won't try to save the motor. But I do think I'll put it back together, wire it up, and plug it in just to practice using my new dmm.

Thanks again to Bus also.

Regards,

Mark

In theory, the LRA could be 8 times the running amperes- or even more. The duration of the 24 amp starting draw would interest me. If less than 1 second, I would not be concerned at all. You say the squirrel cage was not pulling enough air through. Is it doing so with the new motor? If the speed is the same the air flow should be the same-unless the blower was dirty before and now is (temporarily) clean.

Bus- good questions.

The new motor solved the problem. We went from very low draft to what appears to be full draft. (The two ovens are side by side, so the missing draft on the second oven was covered by the sound of the working draft on the first oven.)

The original problem with the oven was that the unit wouldn't fire. It's a gas convection oven. After repeated attempts we'd finally get it going. Tech said that a vacuum switch in the vent hood overrode the start sequence, and after two tries the unit would lock up until we turned off the breaker or opened the oven door for a reset. Apparently, after several tries we'd get enough draft for long enough that the vacuum switch would operate. (It was also necessary to clean flour out of the vacuum switch, but that in and of itself didn't solve the problem.)

Wasn't there when the tech relaced the motor, so I don't know how bad the squirrel cage was, but I can imagine. Makes me wonder what my longs look like.

:-)M

If in fact the old motor is a split-phase induction run, it will run at full speed or it very quickly will fail to run at all. And if it fails, a spontaneous recovery is completely out of the question.