Why do AC motors pull high amps when they go bad?

[timmeister37]

TL;DR

Why do Alternating Current motors pull high amps when they go bad?

I used to work in HVAC. There are three main types of alternating current (AC) motors in residential air-conditioners and heat pumps: outdoor fan motors, compressors, and indoor blower motors. All three types of AC motors have the maximum number of amps stated on the nameplates of the units. When any of these three types of AC motors used in residential HVAC go bad, the AC motors start pulling higher amps than the maximum number of amps on the nameplate of the unit.

Why do AC motors start pulling excessively high amps when they go bad?

 

[alan123hk]

Why do AC motors start pulling excessively high amps when they go bad?

I am not familiar with AC motors.

Is it possible that the rotation speed is reduced or even stopped due to excessive rotational friction, which causes the current to rise as the back EMF decreases?

 

[jrmichler]

Some possibilities:

Disassemble a failed AC motor and look closely at the stator winding for signs of a short circuit. The stator winding for each pole consists of a number of coils placed in successive stator slots. These coils are connected in series, and overlay each other. One coil can short to the next coil. This reduces the total number of turns for that pole winding, which increases the current.

A single phase motor can have a starting switch or starting winding fail. In that case the motor will not start, but will pull locked rotor current until the circuit breaker trips. Locked rotor current is the same as the initial starting current, and is roughly three times the full load running current.

Bearing wear or failure causing the rotor to drag on the stator, overloading the motor.

A normal AC induction motor will deliver up to about three times its rated full load power, but only for a short time until it overheats. The full load power rating is the maximum power at which it will not overheat.

I had a summer job out of high school in an electric motor rebuild shop and saw many failed motors.

 

[timmeister37]

I am not familiar with AC motors.

Is it possible that the rotation speed is reduced or even stopped due to excessive rotational friction, which causes the current to rise as the back EMF decreases?

That sounds plausible. I wish an EE here would verify it though.

 

[russ_watters]

Photos and explanations of many of the types of failures @jrmichler was talking about:
https://electricmotorwarehouse.com/winding-failures/

 

[Baluncore]

The most common failure is a short circuit between two turns of one coil. That changes a winding designed to be an electromagnet, into an autotransformer with a short circuited secondary.

Since the resistance of the shorted secondary is very low, a large current flows, that heats the wire and causes more shorts to occur. The current drawn by the motor rises rapidly once the first shorted turn occurs.

The number of primary turns and the inductance of the coil is reduced by the short, so the primary current increases. That is even without the primary current needed to support the shorted secondary current.

 

[timmeister37]

I am not familiar with AC motors.

Is it possible that the rotation speed is reduced or even stopped due to excessive rotational friction, which causes the current to rise as the back EMF decreases?

Would the decreasing of the back EMF directly cause the current to increase?

 

[Baluncore]

Would the decreasing of the back EMF directly cause the current to increase?

That would be true of any AC winding.

These AC motors are induction motors. They employ fixed field coils that generate a rotating magnetic field, which drags the rotating conductive armature around with the field. Those motors are designed to have a slip under load, of about 5% below synchronous speed.

Excessive rotor friction due to bearing problems, will increase the power consumption. But the extra power consumed will be dissipated in the bearings leading to mechanical failure. Where bearing failure allows the poles of the field to contact the rotor, the field can be heated through friction with the rotor. The mechanical vibration can then damage the field windings.

A shorted conductive turn in the field allows an equal and opposite current to flow in that turn. That current causes an equal and opposite magnetic field that cancels the wanted field. The reduction of magnetic field reduces the available motor power and heats the field windings further.

As motor current begins to increase, a cascade of catastrophic failures follow, that quickly destroys the motor.