TEMPERATURE RISE AND CLASS OF INSULATION FOR AC MOTORS
ISSUED BY C.J.HEWETSON
10/1/03
The correct class of insulation is essential in terms of a satisfactory operational life of AC motors, particularly in Middle East high ambient operation. The motor operational life is dependant upon a number of factors
a) The winding insulation class.
b) The ambient temperature.
c) The motor efficiency.
d) The Copper/Iron mass of the stator and windings.
e) Voltage fluctuation & phase imbalance.
f) Winding temperature.
g) Bearing operating temperature and lubrication specification.
h) The load on the motor.
The winding operating temperature directly effects the life of winding insulation materials plus the bearings. Winding temperature rise is directly related to the efficiency of the motor, which is the power output at the shaft divided by the power input at the supply side. In general, motors are 85% efficient unless high efficiency motors are requested, in which case the efficiency would be 90-95%. High efficiency motors have more Copper/Iron by design; therefore the losses and the equivalent heat generation will be less. The difference between power input and output is called the loss and this power loss is transformed into heat, which warms up the motor windings and must be expelled from the motor to avoid excessive temperature rise. If the efficiency of the motor is 85% then 15% of the KW input is generated in winding heat with an equivalent winding temperature rise based upon the mass of Copper and Iron and the specific heat thereof. The hottest part of the windings is in the centre of the slots where the heat is generated as a result of the losses.
The heat generated is dissipated to the ambient air through the external surfaces of the motor and assistance of air forced over the surface in the case of TEFC or through the stator/rotor air gap in the case of ODP motors, via a fan mounted on the rotor shaft end. For every 10degf (5.5degC) winding temperature rise above the insulation rating of the motor, the motor operating life will be reduced by 50%. Motor insulation temperature rise allowable for each class of insulation is always based upon an ambient of 72degf (40degC) which is the air on temperature to the motor for cooling purposes. The ambient design should be taken as 50 deg C for the Eastern Province (unless the motor is located indoors in an air conditioned or ventilated environment in which case the design ambient may be lower). As motor insulation class is based on 40DegC ambient, and design ambient is 50DegC, the winding temperature will be 10degC (18degf) above the allowable temperature rise and likely to exceed the maximum temperature for the windings. For TEAO or TEFC motors the bearings provided will relate to the expected motor winding temperature. As most of the York product designs are based on N.American markets, the insulation class specified will generally be class B and the bearings fitted will reflect this. For Saudi Arabia the winding temperature is likely to fall into class F or H and if you consider that lubricants begin to break down and lose their lubricating properties at 85-90degC, it can be readily understood that the bearings may well fail due to a reduction in grease viscosity and leakage or vapourisation before the motor windings. Indeed as the bearing lubrication is gradually lost, the bearing elements will pick up, resulting in higher current draw which in itself may cause the motor windings to fail. Most motors supplied by York have no service factor or thermal reserve as described by NEMA or IEC standards, which allows for overloading of the motor for short or continuous periods without serious damage of overheating. The service factors that should be specified are 1.10, 1.15 or 1.2 which provides a 10,15 or 20% overload; sales people should not accept a service factor of 1.0. It is very easy to overload motors especially during pull down, increased air flow on AHU's due to lower ESP, changes in air density/humidity or due to power supply voltage fluctuations or phase imbalances. Under such conditions a thermal reserve will allow the motor to operate without exceeding their class rating by using some or all of their thermal reserve. The temperature rise for motors with 1.0 service factor is as follows.
INSULATION CLASS / A / E / B / F / H / F WITH CLASS B RISETEMPERATURE RISE OF WINDING / 60 / 75 / 80 / 100 / 125 / 80
AMBIENT TEMPERATURE / 40 / 40 / 40 / 40 / 40 / 40
ALLOWANCE FOR HOT SPOTS / 5 / 5 / 10 / 15 / 15 / 15
THERMAL RESERVE / 0 / 0 / 0 / 0 / 0 / 20
TOTAL WINDING TEMPERATURE / 105 / 120 / 130 / 155 / 180 / 155
For all motors be they single or 3 phase for operation in Saudi Arabia to ensure trouble free operation and a normally expected life of 25,000hrs, the winding class should be class F with a service factor of 1.1 as a minimum especially in view of the likelihood of voltage imbalances and voltage reduction general to SA. In the Eastern Province we are frequently faced with changing condenser fan motors on rooftop packages, split condensing unit condensers and air cooled chillers or to change the bearings. Motor failure due to incorrect specification of winding insulation class causes customer dissatisfaction with our products even though he may not appreciate the reasons for the failure. This in turn may result in lost sales in the future where customers do not perceive our units as being reliable or suited for high ambient operation.