CAPACITOR IN 110 1HP1PH MOTOR BLOWS

[SCMR2242]

HELP!!! MAYBE CSR FOR ME AT MY AGE BUT ....I HAVE A 1HP GRINDER MOTOR THAT RUNS FINE ONCE IT STARTS WITH ITS 24MFD 130VAC CAP....BUT IN 2 MINS IT BLOWS THE TOP OUT OF THE CAP....IT IS SINGLE PHASE 115 VAC INDUCTION MOTOR WITH START RUN CAPACITOR ...WHAT IS CAUSING IT ????????
I RAN FINE FOR YEARS AND NOW IT STARTS THIS ..I HAD THREE CAPS INSTALLED AND THEY LAST 2 TO 5 MINS ....PLS HELP!!!!!!!

 

[awright]

Blown Motor Capacitors

Sound like your centrifugal start switch is not opening as the motor comes up to speed, leaving the start winding and capacitor in the circuit until it blows.

The start capacitor and start winding are designed to be in the circuit for a few seconds, at most, just to get the motor up to a high enough speed that the reactive currents in the rotor will generate enough torque to keep the motor running and drive its load. At that point the centrifugal switch is supposed to open the start circuit. If it doesn't open the start circuit, heavy currents continue to flow in the start circuit until something gives - in your case, your capacitors.

Does the motor sound like it is laboring or just not running as it should before the cap burns out? If so, that is the effect of having the start winding still active after it should have been cut out.

Listen to the motor after it is turned off. As it coasts down, there should be a very noticeable click at 20 to 30 % of running speed as the centrifugal switch closes to be ready for the next start. If you don't hear the click then the centrifugal switch never opened during startup. If that is the case, you may be able to repair it by disassembling the motor and cleaning the switch. Check the start contacts while you are there.

The startup sequence it usually too quick for you to be able to sense the opening of the start switch, although a grinder with heavy grinding wheels and low starting torque may accelerate slowly enough for you to be able to hear the click of the switch opening.

If you want to prove to yourself that the start switch is definitely defective before chucking the grinder or opening it up for repair, measure the current in the start circuit during startup with a clamp-on ammeter. If the current doesn't go from many amps to zero amps very quickly (less than a few seconds) after switching the grinder on, the switch is not opening.

If the centrifugal switch is not repairable and you are handy with electrical wiring, you could substitute a current-operated start relay for the centrifugal switch. Unfortunately, such a start relay (available from appliance parts suppliers) has to be matched pretty well to the motor starting current and has to be wired to the running winding also, so even though it is feasible and inexpensive, it is not a slam dunk.

If you "...had 3 caps installed...", instead of installing them yourself, the repairman should have checked the start switch, especially after the first replacement failed.

Have fun.

awright

 

[Scarr]

Motor the needs help starting

Hi,

I didn't even think of asking questions about an old motor I have that does not work!!!

It's a great forum this

OK, the motor needs a helping hand to start (turnning the pully), if you don't it trips the fuse after a while, once it's running it looks fine (no load on the pully tho) as it needs a hand starting I cannot connect it to the lathe to make sure it does work fine once started.

Any ideas?

Looking at getting a new / second hand one from ebay as we speak.

 

[awright]

Motor Caps

Scarr, the fact that you have to give the motor a spin confirms that you don't have an operating start circuit. (By the way, hand starting a motor by pulling on the belt is an excellent way to lose fingers.)

The fact that when you spin the motor up it runs normally just means that you are doing manually what the start circuit should do automatically, i.e., get the rotor turning fast enough to induce phase shifts between the reversing magnetic field of the stator (which does not normally have any rotational component) and the rotating magnetic field of the rotor. With no start circuit in operation, there is no tendancy for the rotor to start turning. In fact, you can manually start and run the motor in either directioin the absence of a start circuit.

An induction motor is a lot like a transformer with a shorted secondary. If you look at the rotor, it consists of steel magnetic laminations with aluminum bars inset into slots running the length of the lamination stack and with solid aluminum rings at each end. The bars and end rings form shorted turns of extremely low resistance. With the rotor not turning (including at the instant of switching on), the transformer action of the motor causes very high currents in the rotor shorted turns. This is why there is a spike in the motor current at the instant of switching on.

With a normal start, the start circuit gets the rotor turning very quickly by creating a magnetic field that is displaced 90 degrees from the main magnetic field AND SHIFTED IN TIME BY THE ACTION OF THE WINDING INDUCTANCE AND THE CAPACITOR. This time and angularly displaced magnetic field adds a rotational component to the magnetic field of the stator, dragging the rotor along with it and starting rotation. When the rotor reaches about 1/6 to 1/5 of synchronous speed (3600 RPM for a 2-pole motor, 1800 RPM for a 4-pole motor), the reaction between the rotating rotor and the oscillating magnetic field of the main windings causes both a reduction in the currents circulating in the shorted turns of the rotor and a phase shift in those currents sufficient to cause torque that accelerates the motor to running speed and drives the load. That is the point (about 1/5 synchronous speed) at which the properly working centrifugal switch mounted on the rotor (or sometimes an external current-sensing relay) switches the start winding out of the circuit because at that point the start circuit is a burden, not an aid, to motor operation. Additionally, the start winding and capacitor are not designed for continuous operation and will overheat, often with disastrous consequences, if left in the circuit.

Scarr, the reason your motor trips the fuse if you don't get it running quickly is that it is acting like a big transformer with a shorted secondary. As I told SCMR2242, you should check the start capacitor and the centrifugal starting switch mounted on the rotor shaft inside the case. You can check the capacitor most easily because it is outside the case under that metal shell (assuming we're not talking about a split-phase motor that has no capacitor). But, if you find that replacing the capacitor without opening up the case restores normal starting, be sure to also check the functioning of the centrifugal switch to avoid the prompt failure of the new capacitor that SCMR2242 experienced. After all, something caused the capacitor to fail.

Good luck.

awright