Ruud 5-ton 10 SEER heat pump compressor turns off...

Question

My Cruud, I mean Ruud heat pump shuts off the compressor after 15 to 20 minutes in the heat mode. I have not tried it in A/C mode. The condensor and evaporator fan continue to run but the compressor stops. The contactor stays engaged and there is 240 volts at all 3 compressor connectors. Could I have a bad compressor, or maybe low Freon? any help would be appreciated.