Why slip is more in low HP induction motor?

[srinaath]

am doing a comparative study of induction motors (both single and three phase) based on cost, size, efficiency,etc... while analyzing 1/4Hp and 5 HP motors, i read that 1/4HP motor has high slip than 5HP motor because 1/4HP motor has greater rotor winding resistance than 5HP...so efficiency is low for lower HP motors and high for higher HP motor...my query is (1)why do 1/4HP has higher winding resistance than 5HP, is it so for economical constraint??
(2) 3600rpm motor is more efficicnt than 1200rpm motor, why and how? (is it because 1200rpm requires more poles than 3600rpm, so there will be increase in size and conductors used which increases the reactive current ?)

 

[Andrew Mason]

am doing a comparative study of induction motors (both single and three phase) based on cost, size, efficiency,etc... while analyzing 1/4Hp and 5 HP motors, i read that 1/4HP motor has high slip than 5HP motor because 1/4HP motor has greater rotor winding resistance than 5HP...so efficiency is low for lower HP motors and high for higher HP motor...my query is (1)why do 1/4HP has higher winding resistance than 5HP, is it so for economical constraint??

I expect that there are many reasons that lower power motors are generally less efficient. Motors < 1 HP tend to be single phase or split phase. Higher power motors will be three phase which are inherently more efficient (more average torque with less current because of the rotating field, so less I^2R heating losses).

(2) 3600rpm motor is more efficicnt than 1200rpm motor, why and how? (is it because 1200rpm requires more poles than 3600rpm, so there will be increase in size and conductors used which increases the reactive current ?)

Since the power of a motor is the product of torque and speed (F x rω) motors of the same power require greater torque with slower speed. The greater torque requires greater current which results in greater heat losses. Greater torque also requires greater slippage.

AM