Electric motor issue

[jon f mn]

I have 2 electric motors that don't work right. They will both run but don't have much power. Both are 3/4 hp. One you need to spin it to get it started as well. What might cause that?

 

[Bob Bancroft]

I have 2 electric motors that don't work right. They will both run but don't have much power. Both are 3/4 hp. One you need to spin it to get it started as well. What might cause that?

I imagine you're single phase out there. Which means your motors have centrifugal starting switches, starting, AND running capacitors.

 

[used red MN]

I imagine you're single phase out there. Which means your motors have centrifugal starting switches, starting, AND running capacitors.

Bob, won’t a bad cap cause a motor not to start as he has described the one has to be “spun to start”? Does every general farm duty motor in the 1/2 to 2hp range have centrifugal starting switch? Should have paid more attention in my midlife electrical course I took back in 99. I think it is more of a “if you don’t use it you loose it deal.”

 

[jon f mn]

I imagine you're single phase out there. Which means your motors have centrifugal starting switches, starting, AND running capacitors.

So, can I fix them and what will it take?

 

[Bob Bancroft]

So, can I fix them and what will it take?

Motors that size may have only one capacitor. Usually in an obvious housing. Hopefully you can find a number on it. Or find a shop who knows what they're doing and can get you a correct replacement. The centrifugal starting switch should be on the dead end of the motor. Take the end housing off and you can check the action of the switch. If you have a situation where you can hear a good motor winding down after shut off, you'll hear the switch click. Any capacitor should be shorted/discharged before handling, in case it is energized.

 

[Bob Bancroft]

Bob, won’t a bad cap cause a motor not to start as he has described the one has to be “spun to start”? Does every general farm duty motor in the 1/2 to 2hp range have centrifugal starting switch? Should have paid more attention in my midlife electrical course I took back in 99. I think it is more of a “if you don’t use it you loose it deal.”

My single phase motors require capacitors and starting switches. My bigger motors have multiple capacitors. Some for starting, some for running. Some of my bigger motors have a relay which the starting switch activates. This takes the load off the switch so it will last. The biggest motors here are 10 HP. I'm told that's the limit for our electric supplier.

 

[John T]

A single phase split phase motor that doesn’t start unless it’s spun is often due to

1) The centrifugal start switch is bad
2) If it’s a capacitor start split phase motor the start capacitor or it’s wiring is bad
3) If its a capacitor start capacitor run and is weak after starting the run capacitor may be bad
4) The start winding may be bad

These are fairly simple tests and repairs if it’s only a capacitor problem a bit more complex if it’s the switch and more if the start winding is bad

It’s over 50 years since I was an engineer at the century electric motor company so nooooo warranty see what other engineers or electricians have to say and if they can add more to this.

Let us know what you find

John T

 

[Janicholson]

A single phase split phase motor that doesn’t start unless it’s spun is often due to

1) The centrifugal start switch is bad
2) If it’s a capacitor start split phase motor the start capacitor or it’s wiring is bad
3) If its a capacitor start capacitor run and is weak after starting the run capacitor may be bad
4) The start winding may be bad

These are fairly simple tests and repairs if it’s only a capacitor problem a bit more complex if it’s the switch and more if the start winding is bad

It’s over 50 years since I was an engineer at the century electric motor company so nooooo warranty see what other engineers or electricians have to say and if they can add more to this.

Let us know what you find

John T

All good points. I have seen thermal overload devices that have failed when a motor is used on a hard starting application and runs near heat limits. or is so loaded that it slows to start conditions trying to run a load it is not rated for. Jim

 

[Russ from MN]

I remember troubleshooting a water pump once that wouldn't start, I hooked the 2 wires going to the capacitor together, and it started, but not very well. So, I bought a new capacitor, and all was well. Is this an approved way to troubleshoot a failed capacitor?

 

[caterpillar guy]

If they don't have a push button reset for thermal protection I would suspect the Cap on the starting circuit on the one you have to spin to work. On the other if it doesnot start and will not start with as spin I would hazzard the guess to be the starting switch. With the end opposite of the pulley I would look at the switch for dirt and crude causing it to not work freely and see if you have a broken wire connection in the connection box or going to the motor. If all looks and acts good then I would move to the capacitors on it. Not much more I can say about it. Dad's one Air compressor if it sets for longer periods of time the start switch will stick on , tap it with a hammer handle and it will kick off.

 

[wore out]

I have 2 electric motors that don't work right. They will both run but don't have much power. Both are 3/4 hp. One you need to spin it to get it started as well. What might cause that?

This doesn't address the starting issue, but if the motors happen to be wired for 240 Volts and are operated on 120 Volts they won't deliver rated HP.

 

[NoDakInMN]

It depends on the type of motor and what kind of starting components it has. Not all single phase motors are created the same. Just because one guy has a particular type of motor does not necessarily mean that's what you have. Generally speaking, motors that have a semi round cover screwed on top, that houses a start capacitor. Capacitors are rated by a term called microfarads, often abbreviated to mfd. Start capacitors are generally rated in three digits, like 240 mfd, and run capacitors are rated in one or two digits, like 5 mfd or 35 mfd. The rating should be stamped on the capacitor and it should be on the motor nameplate as well. Easiest way to check them is with a multimeter with a capacitance setting. The symbol for it looks a bit like this, -I(-. Capacitors store an electrical charge until it is discharged. Some older ones you will want to touch both terminals with screw driver to make sure it is discharged, with electrical power off of course, most newer ones have an internal bleed resistor that discharges them when not in use. Set the meter to capacitance and the reading should match the rating, if not, it needs replacing. A capacitor start motor will run with a bad start capacitor but will need help getting started. A PSC motor or a capacitor start capacitor run motor with a bad run capacitor can start but will run slowly. But this can also indicate a bad winding. A capacitor start motor needs a device that drops out the capacitor once the motor is up to speed, this usually is an internal centrifugal switch or a external potential relay. If these fail, it can also cause it to run but need help to start.